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Further Thoughts on Sound Masking

Robert Chanaud, Ph.D.

A comprehensive book on sound masking entitled "Sound Masking Manual" has beenavailable on this website for some time. Since then several advances have been made inunderstanding masking and some uses have strayed from the well accepted rules of application.This document contains eight interrelated articles on several of these subjects. The last articlesuggests the need for research to define the impact of sounds other than speech

Acoustical Privacy and Sound Masking in Offices……………………………….……..….…3The relationship between sound masking and acoustical privacy is not simple. Installing

a masking system does not guarantee good privacy unless this relationship is defined. Modelingoffice acoustics with software helps to define that relationship.

Spatial Uniformity of Sound Masking………………………………………….…..………….7Spatial uniformity of sound masking helps to improve the possibility of good privacy in

open offices for many employees. Creating speaker arrays with software programs helps.

Tuning of Sound Masking Systems………………………………………………… ……… 10Tuning of masking systems is a critical function and cannot be done automatically.

There is no easy way to match sound masking spectra to acoustical privacy needs.

Under Floor Sound Masking…………………………………………………….…………….12Placing masking speakers beneath a raised floor provides the most acceptable masking.

Whenever possible, this location should be chosen.

Adaptive Sound Masking…………………………………………………………….………. .15Although the concept of having masking sound levels change automatically to

accommodate changing office sound levels is attractive, evidence suggests the extra cost isnecessary only under special conditions.

Direct Field Sound Masking.......................................................................................................19Direct field masking speakers are speakers that are suspended face-down in a suspended

ceiling. They do not meet accepted standards, are less acceptable to listeners and do not performas well as speakers in a ceiling plenum or under a raised floor. They are not recommended.

Radius and Area of Distraction in Open Offices without Furniture Systems ………....…25In open offices without separating panels of significant height, surrounding persons must

be a critical distance from a talker to be free of acoustical distractions. The Radius of Distractionand Area of Distraction can be used to define that distance.


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Distraction Potential…………………………………………………………… ………….....30Analysis of distractions caused by all office sounds points toward separating the severity

of distractions from their duration. Distraction Potential incorporates both factors but thepercentage of the workday that distractions occur may be as an important as the severity.


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ACOUSTICAL PRIVACY AND SOUND MASKING IN OFFICES

IntroductionPersons desiring to add sound masking to their facility implicitly assume that the masking

system installer will achieve the degree of privacy desired by his employees. Most often,however, the installer has been tasked only to add sound masking at some specified level. Thefacility person must ask a basic question: How much acoustical privacy will my people havewith sound masking?` In most cases, an honest answer by a contractor would be: I don't know. The primaryreason for this is that the installer does not have control of all the factors that result in acceptableprivacy. The relationship between sound masking and acoustical privacy is not straightforward.

Specifications by consultants often go into detail requiring a specific masking spectrumover a broad range of frequencies but do not specify a required degree of acoustical privacy (seeSpatial Uniformity).

Sound masking in open offices is meant to cover a number of people, but acousticalprivacy is a very personal interaction between two people; a person making sound, and a personinadvertently hearing it. There are three factors that determine the degree of privacy betweenthem:

The loudness and direction of the talker's voice and his or her activitysounds.

How much of that sound is reduced enroute to a listener. How much of the sound reaching the listener is covered by the existing

background sound at the listener (masking).

Without knowledge of the first two factors it is not possible to claim a priori that a maskingsystem will provide acceptable acoustical privacy with reasonable levels of sound masking.

In short, the first two factors must be known before the third factor can answer the mainquestion with any degree of precision. But even this addresses only speech privacy (seeDistraction Potential).

Degrees of Speech Privacy` Prospective owners seldom state what degree of privacy they want, so the contractor stillhas a problem even if he were able to define all of the above factors. Standards have beendeveloped that divide speech privacy into several degrees.Secret Privacy - speech is intelligible only to those for which it is intended, despite the attemptby deliberate persons to use listening devices.Confidential Privacy- speech is intelligible only to those for which it is intended, excludingunintentional casual listeners.Normal Privacy - speech is partially intelligible to unintentional listeners, but not enough tocause significant distraction.Transitional Privacy- speech is partially intelligible to unintentional listeners, but is enough tocause distraction.No Privacy - speech is fully intelligible to listeners, and enough to cause complete distraction.


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Determining Speech Privacy with MeasurementsOne approach to determining speech privacy is to use vocal speech intelligibility tests

between two people. The impracticality of them for an office environment is clear. A secondapproach is to make several field measurements between workstation or closed office pairs witha broad band sound source and a high quality one-third octave band sound level meter. Thisdetermines the sound loss factor. Fortunately, the speech spectrum of people in an officeenvironment is well known, so the voice level factor can be determined (see Radius ofDistraction). With knowledge of the first two factors, one can computer model with varioussound masking spectra to determine the desired masking. Calculations use the ArticulationIndex or Privacy Index to determine the degree of privacy. There are some weaknesses to thisapproach:

The listener should have normal hearing.A statistically significant number of samples must be taken and suitably averaged.It implicitly presumes that the talker is speaking all the time (a worst case scenario).It does not address loss of privacy by other sounds.

See Distraction Potential for discussion of the last two items. It is clear that fieldmeasurements entail considerable effort to clearly define the relationship between soundmasking and acoustical privacy.

Early Guidance for Achieving Acoustical Privacy with Sound MaskingOver the last forty some years, measurements have been made of what was called

interzone attenuation; the sound loss from one workstation, or one office, to another. Withthose measurements and estimates of normal speech spectra it was possible to define a maskingspectrum through the use of the Speech Privacy Potential (SPP). It was defined in the GSAPublic Buildings Service document PBS-C.1, 1972. That method used the Noise CriterionPrime (NC') rating to define and constrain the masking spectrum. Experience with this methodsuggested that levels higher than 47 dBA in open offices were not considered acceptable to mostlisteners, although some exceptions have been found. Levels higher than 45 dBA in closedoffices were not considered acceptable. Levels lower than 42 dBA in open offices wereconsidered non-performing. Since masking is not always used in closed offices, there seems notto be a lower limit for masking levels there; it depends critically n the wall and doorconstruction. This early guidance at least defined the range of acceptable masking levels. It hada requirement for an SPP greater than 60 to provide what is now called Normal Privacy but notother degrees.

More recently, the earlier spectrum contour requirement has been replaced by a standardcalled Room Criterion (RC) and Noise Criterion Balanced (NCB) (ANSI S12.2-2008). Thisstandard covers frequencies in the speech range and defines what is called a neutral spectrum,one that does not have too much low frequency or too much high frequency. The contourdecreases at 5 dB per octave in the speech band of frequencies. The word "neutral" implies anacceptable spectrum. This part of the standard places a restriction on the frequency distributionof the masking spectrum contour.

These developments have placed useful constraints on the level and spectrum of soundmasking but still require field measurement of the sound loss factor if the masking is to achievethe desired degree of speech privacy. Since it is unlikely that measurements will be part of a


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specification for sound contractors, most facility managers now must wait until employeescomplain or make use of a computer modeling program.

Modeling Speech Privacy to determine the Sound MaskingWith the publication of extensive data on the acoustical properties of office equipment,

CCR Associates has a modeling program Called SCOPE (Sound Conditioned Open Planenvironment) that can be downloaded. It includes a database of ceilings, furniture panels,carpets and other surfaces aswell as various voice levels formen and women. A widevariety of open and closedoffices can be modeled. Thefigure on the right shows anexample of an open officeworkstation pair. Theoccupants can be placed in anyposition, and side walls can beadded as well as a number ofother factors, shown in theseveral boxes.

Once the design iscreated, the program analyzesthe sound loss for up to 46possible sound paths and creates a list starting with the weakest path (the critical path). The nextstep is to choose voice characteristics, background sound level, desired degree of privacy, and amasking spectrum from a data base of commonly used spectra. The program then designs amasking spectrum and level that satisfies the privacy criterion. If the level is unacceptable high,the critical path must be improved. A sample of workstation types can be examined in shortorder.

The program has several advantages:It can be used prior to the final design of an office space.It can readily determine whether masking will create the desired degree of privacy withacceptable levels or where the weaknesses are, if it does not.It can be used to rapidly analyze a number of office designs.It can be used facility managers as a means of separating legitimate from illegitimatecomplaints.

One weakness of modeling is that it presumes the person is talking continually (the worstcase scenario.). Fortunately, the level chosen is likely to be an upper bound on the requiredmasking and initial installation recommendations for lower levels can be used to handle that.


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The Final Privacy TestDespite good estimates of privacy with a given sound masking spectrum, the final test is

whether the environment is acceptable to the occupants. On initiation, a sound masking systemshould be set lower than final levels and then the levels should be slowly increased over days. Itcan be done automatically (Atlas Sound ASP-MG24-TDB). This process can be used toadvantage by the facility manager to point out that the privacy will improve with time. Theremay be a level that is acceptable to the employees that is lower than the design level.

The final test of success is lack of legitimate complaints. Legitimate complaints resultfrom cumulative distractions that finally result in annoyance and then to a complaintComplaints can be based on several factors. A legitimate one is from persons with hearing lossthat has been given more privacy than others. Turning off a local speaker is not a solution; theperson should be moved to another area. Persons with vision loss use acoustical cues to navigatein offices; masking can destroy those cues; they should be moved also.

Another compliant is damage to a person's hearing caused by alleged excessive masking.There is no evidence that reasonable levels of masking cause hearing damage. Other complaintscan be based on non-acoustical factors, such as unhappiness with the temperature, themanagement, etc. These are not indications of masking system failure.

Recommendations Owners should define the desired degree of privacy for contractors. Contractors should be given access to office plans prior to design of the

sound masking system. Contractors should be responsible for operation and training of the

masking system, and not the privacy results. When possible, modeling should be done to determine the value of

sound masking.


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SPATIAL UNIFORMITY OF SOUND MASKING

The uniformity of sound masking levels can be important in offices, although it is onlyone of several factors that contribute to successful systems. There is one important question:Will uniformity of sound masking provide uniformity of acoustical privacy? See AcousticalPrivacy and Sound Masking for the relationship between them.

Uniformity implies that a person moving from one area to another will notdetect changes in the background sound. To a lesser extent, uniformity within aworkstation is also desirable. Deviations from uniformity might occur for variousreasons; they are generally localized and randomly distributed. There aresituations where deliberate non-uniformity is introduced in order to match thesound level in one area with that in another; it is called soundscaping.

Uniformity is generally not an issue in closed offices, but uniformity in open offices canbe. ASTM E1573-09 addresses uniformity of sound masking in open offices. Consultantspecifications most often require that levels be uniform over an area. For example, they mayrequire a specific masking spectrum contour and may allow only limited spatial level variations,such as +/- 2 dB, in each frequency band or in overall A-weighted level. If these requirementsare met, masking sound levels should be essentially uniform over the measurement area. Oftenin specifications the area is not defined. However, there are further requirements:

During office design, areas of very similar office structure should be designatedas separate zones.

The frequency spectrum contour and the desired overall level for each zoneshould to be defined (see Acoustical Privacy and Sound Masking).

Level measurements over a zone should be made with 1/3 octave band filters todetect sharp spectrum deviations.

A-weighted levels should be measured in aisles for spectrum uniformity betweenworkstations by an extended walk though at standing height.

Masking level measurements should be made at seated height in workstations forlevel uniformity.

The A-weighted levels should all be within +/- 2 dBA of each other. Persons with vision or hearing handicaps should be placed in separate areas.

If these requirements are met, spatial uniformity of masking is generally achieved anduniformity of acoustical privacy is likely to be achieved if the workstation arrangements areconsistently the same in a zone.

Assisting masking uniformity with proper speaker arraysProper speaker spacing helps to insure reasonably uniform masking levels in open

offices. Proper spacing is a compromise between close spacing resulting in excessive uniformityand excessive costs and more open spacing with less uniformity and less expense. There are anumber of factors that determine the best spacing. These are listed below.


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Speaker Location Important FactorsAbove Suspended Ceiling Suspended ceiling height

Plenum heightCeiling materialPresence of plenum sound absorption

In an Open Ceiling Structural ceiling heightPresence of plenum sound absorption

Under a raised floor Cavity height

CCR Associates has a program calledSPEAKER LAYOUT that expedites the creation ofspeaker arrays. Entering the room geometry and thefactors listed above, results in a speaker arrayconsidered an optimum balance between cost andperformance. The user chooses the shape anddimensions of a zone to be masked, then choosesthe vertical location of the masking speakers. Theprogram then quickly creates a speaker array for thespace that can be printed with location details foreach speaker. An example is shown in the figure onthe right for an office with a central elevator core. Amanual layout for an office this size one would takeconsiderable time. This is just one tool to assist inspatial uniformity.

Examples of spatial uniformity in Open OfficesPlenum MaskingThe figure on the right shows an example of the

A-weighted masking level measured in an openoffice aisle area. The speaker array in the plenumwas a 15 foot square and the ceiling height was 9feet. A person passing through that aisle wouldexperience less than a 2 dB variation. None of theemployees that used that aisle said they detected nolevel changes when questioned. Measurements forthis case were made at seated height.

Open Plenum MaskingThe figure on the right shows examples of the A-weighted masking level at seated height when themasking speakers are placed in an open plenum withno suspended ceiling. The speaker array was a 14foot square. The speakers were pointed up and theirbottoms were 12 feet high. The heavy solid line isthe level with no panels; the other lines represent thelevel for three heights of workstation panels. These panels were absorptive so the levels near the


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panels was reduced. Typically, people are three feet from the panel so the level reduction is lesssignificant.

Under Floor MaskingThe figure on the right shows two examples ofthe A-weighted masking level at seated heightwith no panel systems. The speaker array in thecavity was a 16 foot square. The cavity heightwas 6 inches and low profile speakers wereneeded (see Under Floor Masking).

Direct Field MaskingSee Direct Field Masking.

Recommendations Follow the requirements listed above. Use software to create speaker arrays.


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TUNING OF SOUND MASKING SYSTEMS

Correct tuning or equalization of a sound masking system is just as importantas the products used.

Early systems had two deficiencies in this regard. Generators were sent to installers witha pink noise electrical spectrum with little guidance on the tuning process. Often installers wereunfamiliar with proper tuning of the masking system, resulting in rejected systems. It isimportant to ask the question: Does the installer have experience in the correct tuning of amasking system?

Correct tuning is a direct benefit to the occupants of the office but requires additionaltime for the installer. There are several steps to make the tuning process generate acceptablesound masking:

The product should be capable of being equalized in 1/3 octave bands.The installer must be familiar with how tuning is done and have the proper sound meters.The masking spectrum should meet the privacy objectives of the owner.

Most, but not all, modern systems meet the first requirement (see Direct Field Masking).Any system that does not have the capability of spectrum and level changes in 1/3 octave bandsshould not be used. Good systems have the capability to set different spectrum contours andoverall levels in multiple zones. There are methods to assist with the second requirement. Thethird requirement is more difficult (see Acoustical Privacy and Sound Masking).

Assisted Manual TuningManufacturers can oversee the installation, or perform the equalization themselves. They

can provide training for installers. Both methods presently are being used by the majormanufacturers, but many smaller systems are sold directly to installers with no support. Majorproducts should not only meet the first requirement, but also have built-in initial spectra andlevels for the three major speaker locations (above suspended ceilings, in open ceilings, andunder raised floors).

Pre-Tuning and Automatic TuningThere are systems that provide only one masking spectrum for open offices. Levels may

be changed, but the spectrum contour remains the same despite the large variations that occur inopen office furniture systems (see Direct Field Sound Masking). Claiming these systems arepre-tuned, bypasses the task of equalization, so is attractive to installers. This concept may beapplicable in smaller open offices or closed offices, but in larger open offices with a variety offurniture systems and workstation sizes, pre-tuned systems should not be used.

There are products that have been claimed to be capable of performing tuningautomatically to an optimum spectrum and level. Optimum here must be interpreted to meanacceptable acoustical privacy. One company claims that their software has a self-tuning techniquethat takes into account the specific acoustic characteristics of the workspace and the existingbackground sound. Without knowledge of the specific relationships between each employee pair,measurement of reverberation time and background sound level cannot provide the desiredacoustical privacy, but only an approximation to the desired masking spectrum.


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The General Problem with TuningAll of the above tuning methods can create a specific masking spectrum, but they do not

address the third requirement. An installer may be required to meet a masking specification and arequirement for uniformity of level (see Spatial Uniformity). Unfortunately, the responsibilityfor matching the masking to the privacy goals of the owner is often left with the installer and notthe specification writer or the owner's representative. Too frequently, the matching is not done.

Recommendations Tuning a masking system is a critical operation for a successful system. A knowledgeable installer is required to provide accurate tuning Pre-tuning or automatic tuning does not guarantee acoustical privacy. There is no substitute for on-site tuning of a masking system. The system should be tunable in 1/3 octave bands.


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UNDER FLOOR SOUND MASKING

When given an option for locating sound masking speakers, the prospective owner shouldask the question: What is the best location for the speakers to provide the most acceptablemasking?

Possible locations are: In the plenum above a continuous suspended ceiling'. In the plenum above a discontinuous suspended ceiling ("clouds"). In an open plenum without a suspended ceiling. Face-down in a suspended ceiling. Under a raised floor.

IntroductionFor many years sound masking speakers have been placed in the plenum above a

suspended ceiling. In some designs, the suspended ceiling was discontinuous so the tiles werereferred to as "clouds". In some offices, the suspended ceiling was completely absent, so thestructural ceiling was visible and speakers were placed in the open plenum. In each of thesecases, rules for positioning the speakers and tuning the sound spectrum and level weredeveloped. (See the Sound Masking Manual). These three locations are commonly available andare used to create successful sound masking systems. More recently, face-down maskingspeakers in suspended ceilings have been used. (see Direct Field Sound Masking).

In the 1980's, raised floors migrated from computer rooms to office spaces, providinganother possible location for sound masking speakers. To the author's knowledge, the first suchinstallation was done in the early 1980s at a government office in Florida. The speakers wereplaced on the structural floor under a twelve inch high raised floor, prior to the raised floor beingcompleted. The installation process was much quicker and simpler than for placement above asuspended ceiling. Because the sound transmission loss of a raised floor is much higher than thatof a suspended ceiling, higher levels of sound within the cavity were required to achieveacceptable masking levels in the room above. The masking spectrum contour used for plenummasking supplications was found not to be applicable so it was modified to achieve the desiredlevel and spectrum at the employee level above.

The remarkable and unexpected result was that the source of the masking sound wasimpossible to locate. As a result, the masking levels above were more uniform than that for allother masking speaker locations. Also, the sound field was exceedingly diffuse. A diffusesound field is one where the sound comes from so many directions it is not possible to locate thesource. For a fuller discussion of sound diffusion see the section below.

Cavity Depth IssuesAlthough most ceiling plenums are sufficiently deep to accept standard sound masking

speakers, there are some that are quite shallow and require low profile speakers. The same istrue for raised floors. When the cavity is 12 inches or greater, usual plenum rated speakers workwell. For smaller cavities, low profile speakers must be used.


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The Masking SpectrumThe masking spectrum desired in workstations above a raised floor is determined by the

sound spectrum under the floor. Unlike plenum speakers, the change in level and spectrumcontour is so large that a different base acoustical spectrum is required. To handle under floorapplications, the Atlas Sound ASP-MG24 and ASP-MG24TDB masking generators store a baseelectrical spectrum for under floor applications that provide an office acoustical spectrum that isnear that recommended by standards. The initial spectrum is approximately correct and onlysmall changes in spectrum and level are required.

Spatial UniformityThe spatial uniformity exceeds the uniformity of masking speakers located in any other

position. The results of tests done by Dynasound are given below. The standard deviation forthe several important spectra was determined over a number of positions in each test and in anumber of tests. The deviations were exceedingly small.

Freq 500 630 800 1000 1250 1600 2000 2500 3150 40001 0.5 0.3 0.3 0.2 0.6 0.5 0.6 0.3 0.3 1.52 0.6 0.7 1.0 0.5 0.7 0.4 0.3 0.7 0.4 0.13 0.9 0.8 0.5 0.4 0.3 1.0 0.6 0.8 0.4 1.04 0.4 1.3 2.0 0.3 0.9 0.6 0.3 0.5 1.3 1.45 0.7 0.5 .5 0.3 0.3 0.6 0.6 0.3 0.1 0.66 0.9 0.5 1.1 0.6 0.2 0.5 0.6 0.7 0.5 0.87 1.2 0.7 1.0 0.4 0.3 0.4 0.6 0.7 0.8 1.7

A graphical example of this uniformity is shown in the figure on the right. There was a16 foot square foot speakerarray. Low profile speakerswere placed in a shallow cavity.A traverse was made, 48 incheshigh, before the furniture systemwas installed. It was made bothlaterally and horizontally. The 8foot position was the center ofthe array for both directions.Such uniformity is very difficultto achieve in speaker arrays atother locations. Comparisonwith other locations is given inthe companion article entitledSpatial Uniformity.

DiffusionDiffusion of sound masking is seldom mentioned in the literature, but has a significant

influence on both performance and acceptability. It is best appreciated by reference to lighting


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system design. Engineers use Equivalent Sphere Illumination and Visual Comfort Probability todefine the acceptability of lighting designs. The more diffuse the lighting, the less the glare, andthe more acceptable the visual environment. This concept applies also to sound. An example ofa nearly diffuse sound field is the outdoor ambient weexperience every day. It is not normally possible to identify adirection to a specific source and most of us are not evenaware of its existence. In the office environment, under floormasking is highly diffuse so it is not possible to identify thesource of sound. The figure on the right below shows asimplistic sketch of the directions masking sound impinges ona workstation occupant. With under floor masking, it appearsthat the higher sound levels in the cavity can causeundetectably minute vibrations of the furniture panels andeven the room walls which then reradiate sound from anumber of directions. Plenum masking is less diffuse; alistener can usually determine that the sound is coming fromabove but cannot identify a specific source (if the system iswell designed). Sound shadowing is minimized. Direct fieldsound masking has no diffuse characteristic and is equivalentto a glare light source. A listener is perfectly capable ofpointing at the source.

Subjectively, diffuse masking sound in an office alwaysappears subjectively quieter and thus is more acceptable at thesame overall level as masking from other locations.

RecommendationsWhenever possible, use under floor masking in preference to any otherspeaker location.


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ADAPTIVE SOUND MASKING

Activity sounds, both voice and non-voice, will vary in level throughout the workday inan office. A prospective owner should ask the question: Is it beneficial for the level of a soundmasking system to continually adapt to the changing activity sounds?

Handling Activity Level ChangesThe three important factors in achieving acoustical privacy are: (1) the sound level of

talkers or their activity sound; (2) the sound loss enroute to a listen; and (3) the sound level of themasking or background (See Acoustical Privacy and Sound Masking). Item 1 can vary withtime, but item 2 cannot, so in order to maintain the desired degree of privacy, item 3 has to bevaried in time if item 1 varies.

Most activity sound is related to office occupancy. Most often, activity levels arereasonably constant during the workday when occupancy is high, but in some offices it can beintermittent. Activity levels are always lower before and after work hours when occupancy islower. During work hours, employees desire acoustical privacy to do their tasks while outsidework hours they need a sense of community and less privacy. (See the Sound Masking Manualon this site) Since work hours are reasonably well defined, it is possible to separate the workdayinto two parts: work hours and non-work hours. During work hours activity levels are likely tobe unpredictable so the requirements to maintain privacy would be variable. During non-workhours, activity levels diminish and privacy may not be of as much concern.

The initial means of separating the workday into two parts was to manually turn themasking system off after work and back on in the morning. Later, a timer performed the On/Offfunction. The abrupt level change in sound level was highly noticeable since not all employeescame and went at the same time. The need to vary masking levels slowly was recognized and aUS patent for a programmed level controller was issued in 1977 (4,052,720). The output of themasking generator was controlled by an internal clock. Early versions allowed the level to riseslowly from a low nighttime level over a specified number of hours early in the work day.Masking levels usually remained constant during the majority of the workday and then decreasedslowly after work hours. Later versions permitted independent and smooth hourly level changesfor each day of the week. Current versions of these controllers permit numerous and highlyvariable time-vs.-level histories to be stored and used in different office zones.

The essential weakness of this concept was recognized at the time the patent was issuedand a patent disclosure was written for an adaptive level controller. Essentially, this designwould listen to the activity sounds in an office and adjust the sound masking level to maintainthe desired degree of privacy. Several hurdles had to be overcome. The social hurdle was thatthe sound sensor must not carry the entire speech spectrum to avoid being a "bugging device". Atechnical hurdle was how to separate the activity sounds from the masking sound. This washandled by calculation of percentile levels; the activity sounds being the more infrequent higherpercentiles (L10) while the masking being the more constant lower percentiles (L90 or L99). Apractical hurdle was how to distribute the detecting devices. High activity sounds in one area ofan open office might result in excessive and unacceptable masking levels in another area. At thetime, the technology was not available at reasonable cost, and the advantages of adaptive


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masking were not obvious, so no patent application was submitted. However, in 2006, a USpatent (7,460,675) was issued for an adaptive system and that system now exists (SoftdBSmartSMS-Net). If the introductory question is answered affirmatively, a second question is: Isan adaptive level controller preferred over a programmed level controller?

Much of the additional cost of an adaptive system is associated with the purchase andinstallation of sound detection equipment. In offices, acceptable masking levels create a Radiusof Distraction (RofD) of about 16 feet (see Radius of Distraction). That radius is the distancebeyond which employees have Normal Privacy from a person talking. On this basis, for anadaptive system to properly cover an office, it would seem reasonable to space the microphonesin a rectangular array of about 20 feet in every separate zone. For a 20,000 square foot openoffice, 50 microphones would provide reasonably uniform coverage. Clearly, such spacing isimpractical on a cost basis, and it is likely thatthe existing system has fewer sensors. Thedetected sound is converted to a variablevoltage, suitably averaged, and then analyzed.The generator then transmits a changedmasking level to the zone as needed. Thisprocess requires two wire systems for eachzone. An example of adaptive systemfunctioning is shown in the figure on theright. The \blue lines are the activity soundsand the red lines represent the level changes.

The original patent disclosure envisioned a hybrid system with both types of controllerssuch as shown in the figure on the right. The solid lines represent programmed level controlsthat act as limits on adaptive levels.During the workday, the adaptive levels(dashed lines) would respond to thechanging activity levels. Theprogrammed limits could be preset forvarious hours and days. Such a systemwould limit response to fire alarms at anytime or evening vacuum cleaner sounds.The author is not aware if the existingsystem has these features.

Work Hour MeasurementsAlthough speech privacy is discussed most often, the primary issue for occupants is

freedom from distractions, either speech or other sounds Distraction has two factors; themagnitude of the distraction and how often it occurs (see Distraction Potential). A means fordetermining this is to collect minute to minute time histories of sound levels and compute levelstatistics such as L10, the activity sound, and L99, the masking sound. Time histories taken at oneminute intervals in offices over work hours were analyzed. The figure below shows the timehistory of distraction exposure in dB-minutes with an adaptive masking system in operation.The table below summarizes the exposure in comparison with that resulting from a fixed sound


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masking level during work hours. The Distraction Potential is the sum of the dB-minutes overthe workday. The otherdata are the time a listener isexposed during the workdayin minutes or percent of theworkday.

In this example, it isclear that the office was arelatively quiet one with fewdistractions except for midafternoon. The resultsstrongly suggest that anappropriate fixed maskinglevel during work hoursachieved the same reductionin distraction as the adaptive system.

Another set of one-minute level data were analyzed for an office that might be considereda busy one. The results are shown the figure below as a line graph rather than a column graph.The dB-minutes for both adaptiveand fixed masking levels duringwork hours are displayed, andfollow each other closely. Therewere several times when theadaptive masking was slow torespond, resulting in more exposurethan for the fixed masking. Therewere times when the adaptivemasking responded appropriatelyresulting in less exposure than withfixed masking.The table below summarizes thecomparison. It seems clear that thisoffice had considerably moreactivity sound, and required higher levels of masking to provide freedom from distraction.However, the comparison showed that both methods resulted in a similar Distraction Potential.

AdaptiveMasking

Fixed masking45 dBA

FixedMasking46 dBA

Distraction Potential 54 87 50Minutes of Exposure 50 59 35Percent of 8 Hr Work Day 10% 12% 7%


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Recommendations Programmable level controlled systems are recommended over both

fixed level systems and adaptive systems for most offices. Adaptive systems are best applied in offices in where unexpected and

large changes of activity sound might occur during the workday. Adaptive systems are best applied to offices with suspended ceilings and

are difficult to install in open plenum ceilings or under raised floors. An adequate number of activity sound sensors are required.

Adaptive Masking Fixed masking47 dBA

Distraction Potential 861 905Minutes of Exposure 424 357Percent of 10 Hr Work Day 71% 60%


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DIRECT FIELD SOUND MASKING

Sound masking systems are being installed with loudspeakers that are face-down in thesuspended ceiling of an office. They are called direct field systems as opposed to indirect fieldsystems where intervening materials separate the loudspeakers from the listeners. The importantquestion is: Do these systems perform better than indirect masking systems? This article makesa strong case against use of direct field speakers for sound masking.

Some HistoryIt is important to note that face-down masking systems were abandoned in the 1970s for

several good reasons. When sound masking was first introduced around 1970, it was added toexisting face-down paging or music speakers on 20 foot centers. Since normal height ceilingsare on the order of 9 feet, a person walking below a speaker could not help noticing that themusic had level changes as he or she walked by and that it had annoying "static"(if masking wasused). With the advent of the GSA Public Buildings Service document for new federal offices,it was recognized that a better location for the masking speaker was needed. Lahti speakers wereplaced in the plenum above the suspended ceiling with a specified spacing between them. Thespacing was around 15 feet to provide spatial uniformity. Later, Owens-Corning entered themarket and specified that Sweeny Baffle masking speakers were to be placed in the ceilingplenum.

For most of the succeeding years, sound masking experts have specified that maskingspeakers were to be placed in the ceiling plenum. Armstrong World Industries, being a largeceiling tile manufacturer, attempted to convert certain ceiling tiles to face-down maskingspeakers, using the concepts originally developed by the Bertagni family. They recognized thedifficulty of such speakers being acceptable to occupants. To improve spatial uniformity, theyincreased the spread of the radiated sound and recommended a speaker spacing that was smallerthan that used for plenum masking. To the author's knowledge, these face-down speakers are nolonger being used for office masking. Raised floors began being used in offices. In the early1980's, the first under floor sound masking system was installed by Dynasound (see Under FloorMasking). The incredible uniformity and diffuseness of the masking sound field was surprising.When suspended ceilings were eliminated in many offices, methods for suspending maskingspeakers in the open plenum were developed. They were placed facing upward and at a heightthat resulted in acceptable spatial uniformity of the reflected sound at the occupant height.

Despite the lengthy experience with these various masking speaker locations, onemanufacturer promotes the exclusive use of face-down speakers for sound masking. Mostmanufacturers recommend indirect field masking locations. Despite some commercial successamong those unfamiliar with acoustic principles, it is clear that direct field masking violatesmany of the rules developed over the forty years of sound masking practice (See the SoundMasking Manua). These violations are discussed below.

Speaker Location ProblemsCurrent face-down speakers require the presence of a suspended ceiling, and are very

difficult to apply in spaces without them or in spaces that have discontinuous ceilings (clouds).Most suspended ceilings have been standardized on tiles having 2 by 4 foot dimensions and more


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recently to 2 by 2 foot dimensions. This implies that face-down speaker arrays should be spacedat multiples of 2 feet. Spacing examples are given in the table below. Current direct fieldspeaker spacing is recommended to be 10 feet to improve spatial uniformity. When the mostcommon plenum speaker spacing of 15 feet is compared with that recommended for face-downspeakers there is a 220% increase in the number of speakers needed. Unlike plenum speakers,each face-down speakeralso requires a ceilingpenetration.

Very few offices are purerectangles, so adjustmentsfrom a regular spacing maybe required. This is not difficult for plenum mounted speakers since any spacing is most oftenpossible; there are exceptions but these have been handled over the years. Face-down speakerspacing is further restricted by the liberal use of light fixtures or other ceiling penetrations. Face-down speakers must be installed after the ceiling material is installed. Plenum speakers can beinstalled prior to the construction of the ceiling grid; this makes wire runs considerably easier.Highly absorbing fiberglass ceiling panels are structurally weaker and deform more easily with aface-down speaker at the center. Another difficulty with small face-down speakers is theycannot be used for Mass Notification or Emergency purposes, so an additional system isnecessary. Face-down masking speakers can be used for music and paging, as can plenummounted speakers, but one is, once again, faced with the "radio has static" comment if maskingis also used. These are issues for the Architect, the Facility Manager, and the Contractor, butwhat about the office occupants?

Non-Uniformity of the Sound Field with PanelsSpatial uniformity of sound masking levels is an import factor in providing acoustical

privacy and freedom from distraction for as many people as possible in an area (see SpatialUniformity of Sound Masking). Uniformity can be a problem for face-down speakers. As withArmstrong, face-down speakers are difficult to design to provide directional uniformity in thespeech range. Face-down speakers must contend with the immutable inverse square law; theeffect of that law is strongly diminished by placing speakers behind materials. This was theprimary reason face down masking speakers were abandoned in the 1970s. As with Armstrong,it is likely the primary reason current face-downspeakers are recommended to be spaced at 10 footintervals is to improve spatial uniformity.

The directivity and spectrum of a direct fieldspeaker was measured and a computer program wascreated to evaluate uniformity. The figure on the rightshows a square array of face-down masking speakers.The array size could be varied, but the ceiling height waskept at 9 foot. The floor was carpeted but the acousticalcharacteristics of the ceiling could be varied. Thelistener was at seated height. A floor mounted furniturepanel of variable height could be inserted as shown, 36

Speaker ArrayInterval, Ft.

Area in square feet Number of speakers needed fora 10,000 square foot office

8x8 64 15610x10 100 10012x12 144 6914x14 196 5115x15 225 44


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inches from the left speakers.A three-dimensional plot of A-weighted levels is shown in the figure on the left below for

a panel 60 inches high and speakers on 10 foot centers. The floor area shown in the plot is theblue area in the upper diagram. The panel shadowing is evident resulting in a 3 dB levelvariation. If the panel system had been enclosing, the shadowing would have been in twodirections. A better appreciation of shadowing is shown in the adjacent figure on the right. Itshows a traverse between two speakers at the

upper part of the three dimensional plot. It isclear that high panels and direct fieldspeakers are not compatible due toshadowing effects. Shadowing is greatlyreduced by the improved sound diffusionoffered by other speaker positions (see nextsection). Without furniture panels, directfield speakers on 10 foot centers do createreasonably uniform masking levels as shownin the lower figure on the right. In this case,one of the negative features of face-downspeakers is avoided.

Poor Diffusion of the MaskingDiffusion of sound masking is seldom mentioned in the literature, but has a significant role

in both performance and acceptability (see Under Floor Masking). It is best appreciated byreference to lighting system design. Engineers use Equivalent Sphere Illumination and VisualComfort Probability to define the acceptability of lighting designs. The more diffuse thelighting, the less the glare, and the more acceptable the visual environment. This concept appliesalso to sound. An example of a nearly diffuse sound field is the outdoor ambient we experienceevery day. It is not normally possible to identify a direction to a specific source and most of usare not even aware of its existence. In the office environment, under floor masking is highlydiffuse so it is not possible to identify the source of sound. The figure on the next page shows asimplistic sketch of the directions masking sound impinges on a workstation occupant. Withunder floor masking, it appears that the higher sound levels in the cavity can cause undetectablyminute vibrations of the furniture panels and even the room walls which then reradiate soundfrom a number of directions. Plenum masking is less diffuse; a listener can usually determine


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that the sound is coming from above but cannot identify a specificsource if the system is well designed. Sound shadowing by panelsis minimized. Direct field sound masking has no diffusecharacteristic and is equivalent to a glare light source. A listener isperfectly capable of pointing at the source. Subjectively, diffusemasking sound in an office always appears subjectively quieter andthus is more acceptable at the same overall level as masking fromother locations. It is important to point out that diffuse sound is notonly subjectively quieter but is less attention getting than direct fieldsound.

Unusual Masking SpectrumThe masking spectrum has both an overall level and a

spectrum contour, the distribution of that level over the frequenciesin the range of human hearing. Is there a sound spectrum contourthat is preferable to people? There are a number of olderpreferred spectrum curves such as Noise Criterion (NC) and NoiseRating (NR). The Acoustical society of America has a standard that lists the Room Criterion(RC) and Noise Criterion Balanced (NCB) (ANSI S12.2-2008). Both of these criteria define afrequency spectrum thatdecreases at 5 dB per octave.This spectrum is consideredneutral, and thus acceptable.Deviations from that spectrumare considered to have "rumble"if too much low frequencyexists and "hissy" if too muchhigh frequency exists. It isimportant to point out that theNCB standard wasrecommended by Leo Beranek,an icon of acoustics.

The upper figure on theright shows a heavy solid greenline that is the result ofconverting the one octaveRoom Criterion Mark II curveto a one-third octave band withan A-weighted level of 47dBA. The numerous blacklines represent spectra specifiedby consultants and spectrarecommended bymanufacturers. They were alsoset to 47 dBA and follow the standard contour closely except for the highest frequencies where


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high panel heights reduced the need for masking. It is important to point out that one of thesespectra was recommended by Bolt, Beranek and Newman, a well known consulting firm. Thered dashed contour is the measured spectrum of a current direct field masking speaker. It is clearthat the direct field spectrum deviates markedly from that of the standards. It has two importantdeficiencies. First, the low level of the low frequency end of the spectrum is due to the smallspeaker size; most masking systems use much larger speakers and larger back boxes so theybegin to roll off only below 160 Hz. If music is added, the desired low frequency beat would belost. It is possible to make up for the low frequency deficiency if an air handling system isoperating. The next figure on the right shows the net effect of air handling sound; it modified themasking spectrum to flatten it. As a masking spectrum, it still does not meet the standards.Second, the criterion would list the spectrum as unacceptably hissy, as would any listener.Because the masking spectrum is a maximum in the frequency range where speech is bestunderstood, the level of any page would have to be set higher. Most recently, the manufacturerof this system has recognized this and added additional amplification to the page.

The most important point about the masking spectrum is that it cannot be changed. Thesound spectrum needs to be altered for two reasons. The first is for transmission through anyintervening material; this requirement is bypassed with direct field applications. The secondreason is to adjust to the acoustics of the office. Different ceiling materials and furniture panelsmake that necessary. Only generators with 1/3 octave band filters can handle theserequirements; the present direct field speakers cannot.

Radius and Area of DistractionIn open offices with very low, or no, panels, the level and directivity of a talker's voice

has significant potential for distraction of surrounding persons. There is a radius beyond whichNormal Privacy is achieved; it is called theRadius of Distraction. More accurately,there is an Area of Distraction in whichdistraction is likely to occur. A computermodel was created and calculations weremade for an office with a carpet, NRC=0.55ceiling tile, no furniture panels, and a malenormal voice level. A masking spectrumthat meets the standards was compared withthat used for the direct field speakers. Thefigure on the right shows two areas withinwhich distraction would occur; the talker isfacing to the left. The circles represent 5foot increments. The non-circularity is dueto the directivity of the human voice. TheRadius of Distraction for both was near 25feet, and the Area of Distraction was near800 square feet, suggesting that furniturepanel systems are needed to reduce that distance. For this case, the direct field maskingspectrum has no advantage over those that meet the standards. The need for furniture panelssuggests that a masking speaker location other that face-down should be chosen.


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ConclusionDirect field masking speakers do not perform better than speakers at other locations. The

masking spectrum does not meet accepted standards and is considered hissy. The lack of adiffuse sound field makes the masking more noticeable and less acceptable. The number ofspeakers required is considerably larger that for speakers located in recommended locations. Therequired regularity of the speaker array is difficult to achieve with ceilings that have numerouspenetrations such as light fixtures and air vents and more difficult to achieve when ceilings areabsent. Spatial uniformity is best achieved with face-down masking speakers in offices withsuspended ceilings, without furniture panels, and with spacing considerably closer than speakersat other locations Mass Notification or Emergency signals cannot be used with them.

Recommendations Direct field masking speakers have enough performance deficiencies

their use should be avoided whenever possible. The masking spectrum does not meet standards, cannot be altered in

the field, and is hissy. They cannot be used in open plenums, under floors, or with

discontinuous suspended ceilings. To achieve spatial uniformity, a larger number of speakers is required

than for other speaker locations. Numerous ceiling penetrations can destroy the regularity of required

close speaker spacing. They cannot be used for mass notification or emergency signals.


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RADIUS AND AREA OF DISTRACTION IN OPEN OFFICESWITHOUT FURNITURE PANELS

When people talk it is possible for those not involved to be distracted. In closed offices,the sound loss is generally sufficient so that those outside the room are not distracted. In openoffices, however, the structure of the room determines how much distraction is caused byconversations. If the office has high separating panels, the sound loss can be sufficient to avoiddistracting neighbors when a masking system is installed. Many modern offices have low or nopanels, so distraction of close neighbors is inevitable. The question here is: How far away doneighbors have to be in order to avoid distraction in an open office without furniturepanels?

The key factors related to speech in answering that question are: (1) how loud peopletalk; (2) in what direction they talk; (3) how much speech is lost enroute to a listener; (4) thelevel of the background sound; and (5) the definition of freedom from distraction. The variouslevels and directivity of the human voice are discussed below. The major factors in sound lossare: the distance, the ceiling sound absorption, and floor sound absorption. Since the acousticalproperties of these two surfaces are knowable, it is possible to use the data in calculations.Similarly, the level and spectrum of the background or masking sound can be chosen. Sincedistance is the defining factor, Thomas Koenig of Dynasound developed the concept of Radiusof Distraction (RofD). He defined the radius as the maximum distance beyond which listenershave Normal Privacy, an Articulation Index of less than 0.2 or a Privacy Index greater than 80.These are commonly accepted indices among the acoustical consultant industry and they can beused as the criterion in a calculation.

Directivity of the Human VoiceThe human voice is highly directional, so in

a completely open environment it plays asignificant role in setting the RofD. Since speechintelligibility is strongly dependent on thefrequency distribution of the voice at every angle, itis necessary to have detailed spectral informationon the human voice. Such data are available andwere used to develop the results discussed below.The figure on the right is an example of the A-weighted level of the human voice at various anglesthat show the strong directivity of the voice in anopen environment. The level when the voice isdirected at a listener is nearly 10 dBA louder than when the voice is directed away. Essentially,a talker facing a listener requires the listener to be about three times further away for privacythan for a talker facing away. For example, if the radius is 15 feet when the talker is facing way,the maximum distance (RofD) is about 45 feet when the talker is facing the listener.


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Influence of Voice Directivity on Speech IntelligibilitySince all of the acoustical factors are available, it is straightforward to model on a

computer. In all the examples below, there were no panel systems. The three free paths, direct,ceiling reflection, and floor reflection, determine the sound level at the listener. The floor wascovered with wear resistant carpet, the ceiling was 9 feet high and both talker and listener wereseated at 48 inches height. The talker was facing horizontally. Several choices of ceiling tilecould be chosen in the software as well as avariety of background/masking levels and voicelevels.

The figure on the right shows an exampleof the distances and directions at which NormalPrivacy is achieved. A common NRC=0.55ceiling tile was used with a normal male voice,and a masking spectrum at 46 dBA that meetsthe standards. Each circle in the figurerepresents an increment of 5 feet. Behind thetalker the radius was less than 10 feet, whiledirectly in front of the talker the RofD was near26 feet. At right angles to the talker the radius isabout 15 feet. The impact of voice directivity isaboundnatly clear. Tthe impact on allsurrounding persons is discussed below.

Influence of Voice LevelThere are several accepted levels of voice that are used in an office environment: casual,

normal, and raised. Casual is characterized by telephone use or a privileged conversation.Normal is when the person wishes to project his or her voice a short distance such as at ameeting. Raised is characterized by conflict resolution situations. These levels apply to bothmale and female speakers and the spectra have been measured (ASTM E1130 – 08).

The male normal voice level might be considered the worst case, since the raised voice isnormally restricted to closed rooms. The table below shows the results for five talker levels withthe talker directly facing the listener (RofD or worst case). The calculations were made with amasking spectrum at 46 dBA and a 9 foot high ceiling with a NRC-0.55 mineral tile. Althoughthe impact is worst for a talker facing a listener, there is impact on all the surrounding persons.The simplest way to account for that is to define an Area of Distraction (AofD). It is the totalarea under the contour in square feet.

Voice LevelMaleCasual

MaleNormal

MaleRaised

FemaleCasual

FemaleNormal

RofD, Feet 10 25 56 9 22AofD, Square Feet 115 873 4153 85 640


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First, it is clear that raised voices should never be used in open offices that do not havefurniture panels; the impact is enormously greater than that for normal level voices. Even atnormal voice levels and with acceptable masking levels (less than 48 dBA), the longest distanceis still quite far. For example with panels 48inches high or lower, and with workstationson six foot centers, Normal Privacy wouldbe achieved at about four workstationsaway, Sound masking is often asked to bailout a noisy office, but this situation is askingtoo much of masking. Experience hasshown that a RofD near 15 feet has beenfound to be acceptable, so male casualvoices, such as those used in call centers,can be handled with reasonable maskinglevels. To show the enormous benefit oflowered voices, the figure on the right showsa comparison of the male normal voice andthe male casual voice. The AofD reductionis significant! Female voices are even lessintrusive.

Influence of Ceiling Sound Absorbing MaterialsIn open offices with workstations having separating panels of significant height, the

ceiling materials play an important role in providing privacy. Is it worth providing highlyabsorptive and more expensive ceilings when there are no panels? The table below shows thebeneficial effect of ceiling sound absorption for a 9 foot high ceiling and with a sound maskinglevel of 46 dBA for two male voice levels.

Voice Level Ceiling NRC 0.05 0.55 0.75 0.90Male Normal RofD, Feet 29 26 26 25Male Normal AofD, Square Feet 1125 873 854 787

Male Casual RofD, Feet 10 10 9 9Male Casual AofD, Square Feet 130 115 110 106

It is clear that the direct path from talker to listener is so dominant in an open officewithout furniture panels there is little benefit in using high NRC tiles; lower cost mineral tiles(NRC=0.55) are adequate.


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Influence of the Masking Sound LevelWith only a low level of air conditioning sound, around 38 to 39 dBA, the RofD is huge

for the male normal voice. This strongly suggests that sound masking is needed in open officeswithout furniture panels. How muchsound masking is needed to reduce theRofD to an acceptable distance? Thefigure on the right shows the influenceof various levels of a masking spectrumthat meets standards and for two malevoice levels. It is clear that the upperacceptable limit of sound masking levelis needed for male normal voice levels.Using the 15 foot criterion establishedmany years ago, masking levels near 45to 46 dBA meet that criterion for malecasual voice levels.

Influence of the Masking Sound SpectrumThe normal sound masking spectrum can be found in the Sound Masking Manual and has

been used successfully for a number of years. It was based primarily on the presence ofworkstation panels of acousticallysignificant height. With the loss of thosepanels, it was important to determinewhether the spectrum contour neededrevision. A number of spectra were testedin the model. Using RofD as the criterion, itwas found that only a slight modification tothe standard spectrum was marginallybeneficial. The two spectra are shown in thefigure and in the table below. All otherspectra created larger RofD.

Frequency 160 200 250 315 400 500 630 800 1000Standard 46 45 44 43 41 40 39 37 36Modified 44 43 42 41 41 40 39 38 37Frequency 1250 1600 2000 2500 3150 4000 5000 6300 8000Standard 35 33 32 30 28 26 23 20 18Modified 36 35 34 33 31 29 25 22 18


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Estimating the RofD in an Existing OfficeThe Radius of distraction can be estimated by the use of a "walkaway test". A standing male

reads a text at casual voice level facing a standing listener, while the listener backs away slowlyuntil he or she has difficulty understanding what is said. That distance can also be used toestimate the level of the background sound with use of the figure below.

The AofD can be estimated with use of the formula:21.3*A R . For example, if the

distance is 20 feet, The AofD = 1.3*400 or 520 square feet. This can be used to estimate howmany people might be distracted.

Recommendations Casual male or female voice levels should be encouraged in open offices

with no separating panels. Ceiling tiles with NRC values near 0.55 are sufficient for most open

offices with no separating panels. The RofD in an existing office can be determined with a walkaway text.


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DISTRACTION POTENTIAL

The major emphasis on the use of sound masking in offices is to create acoustical privacyfor employees. With the use of Articulation index, Speech Intelligibility Index or PrivacyIndex it has been possible to define various degrees of speech privacy: Secret, Confidential,Normal, Transitional, and None. This metric is commonly used as a criterion for offices, but it isimportant to ask two questions. How much acoustical distraction is caused by non-speechsounds? What is the relationship between degrees of privacy and acoustical distraction?

Although speech in both open and closed offices is important, other sounds also can beimportant. Activity sounds such as coughing, sneezing, telephone ringing, copier printing, anddrawer slamming are but a few sources of distraction. Excessive use of speaker phones, paging,or radios, although speech related, are not included in the above indices. Research hasdetermined that sound processing is obligatory on the part of a listener so any intruding soundmust be part of the potential for distraction.

When the above indices are used as a privacy criterion, it is prudent to look at the worstcase where a talker is facing a listener and speaking continuously. Relying on this approach canresult in excessive use of sound masking. The Occupational Safety and Health Administrationhave addressed a similar situation associated with hearing loss. They developed the TimeWeighted Average which takes into account the level and duration of the noise exposure over theworkday. This concept can be adapted for use in evaluating distraction in an office. It may beexpressed in dB-Minutes of exposure; the highest level of any sound during a one minuteinterval over a specific threshold. The cumulative exposure is sum of the dB-minutes over anentire workday; I call it Distraction Potential (DP). What is a distraction Threshold? Sincemost occupants are not aware of level changes less than 3 dB, but are clearly aware of changesnearer to 10 dB, it seems reasonable to consider that if a transient sound level rises 5 dB, ormore, above the background or masking level, it has potential to distract. This would requirethat minute-to-minute level data be collected over an entire workday. A highly accurate means ofevaluating such data would be to track the level that is exceeded 10 percent of the time (tenthpercentile level, L10) and the ninetieth or ninety-ninth percentile level (L90 or L99) during themeasurement period. If the L10 is greater than 5 dB above the L90, or L99, distraction is likely tooccur and the excess is recorded as dB-minutes. Fortunately, some time-level histories areavailable so that the above procedure can be used.

Typical Activity SoundsAn example of activity sound

in an office is shown in the figure onthe right in the form of tenthpercentile levels in one minuteintervals. Since close-in speech canreach 70 dB, the levels appear to bethe composite of numerous voicesand other sounds. The importantpoint is that activity sounds can varyas much as 10 dB; enough level


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changes to potentially cause distractions, and the variability can occur over the entire workday.

dB-Minutes in Quiet and Noisy OfficesThe dB-minutes for the time-level history in one office were calculated and are shown in

the figure on the right for normal work hours. The reference for the exposure was not a variablebackground level, but a fixedmasking spectrum that meetsthe standards and with a levelof 46 dBA. So the measurewas (L10-46-5). Potentialdistractions occurred only 7%of the workday. Except forthe mid-afternoon rise inactivity, it must be consideredto be a "quiet" office and onein which sound maskingwould provide an adequateamount of acoustical privacy.

An example of avery active office is shownin the lower figure on theright. For this case, thesound masking level was47 dBA. Potentialdistractions occurredduring the entire workday(except for lunch hour).One must consider this a"noisy" office and one inwhich sound masking maynot provide an adequateamount of acousticalprivacy.

It is important to note that these data make no distinction between speech andother sounds.


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Influence of Sound Masking on Distraction Potential for a Quiet OfficeThe Distraction Potential for the "quiet" office is shown in the figure on the right for

several levels of sound masking. At the present time there is no body of measurements thatpermits one to state that asatisfactory office has a DPbelow a certain number.However, it is clear thatacceptable levels of soundmasking significantly reduce themagnitude of the distractions.Perhaps a more informativeapproach is look at the percentof the workday that distractionsoccur. The lower figure on theright shows that acceptablelevels of sound masking havesignificant impact on theamount of time an employee isexposed to potentiallydistracting sounds.

One conclusion for thiscase is that without soundmasking, distractions wouldhave occurred over most of theworkday. Often masking levelsare set at 47 dBA and in thiscase it is likely that 45 dBAwould be acceptable toemployees. This kind ofanalysis resulted in the development of the slow timed rise feature in products for the initiationof a masking system. The final level might have been chosen to be 47 dBA, but when 45 dBAwas reached employees might consider their privacy acceptable.


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Influence of Sound Masking on Distraction Potential for Noisy OfficesThe Distraction Potential for several "noisy" offices was calculated for normal work hours andthe result for several sound masking levels is shown in the figure on the right. Once again, thereduction in distractions causedby the addition of sound maskingis clear. Note that the magnitudeof DP is considerably higher thanthat for a "quiet" office.A more informative graph isshown in the lower figure on theright which shows the percent ofthe workday that distractionsmight occur. Here it is clear thatlow levels of sound masking donot alleviate the continuous natureof the distractions, despitereduction of the severity. Onlywhen the masking level exceeds45 dBA is there any reduction inthe percentage of the workday thatdistractions occur. Although anylevel of sound masking above thebackground can reduce theseverity of distractions, a certainlevel is required to reduce thepercent of the workday that anemployee is distracted. It is clearthat levels of sound masking nearthe upper limit of acceptability arerequired to provide any significantreduction in potential distractionsfor a noisy office.

Transition from Distraction to Annoyance to ComplaintsThe above results identify the two factors in noise disturbance: level and duration. The

results do not indicate whether one factor may be more important than the other. That remainsfor future research.

Numerous articles have pointed out that distractions reduce productivity and increasecosts for the employer. The local facility manager has the job of handling complaints resultingfrom annoyance at acoustical distractions. This becomes both a technical and administrativeproblem. Failure to meet privacy needs is often the result of unrealistic privacy expectations onthe part of the owner.


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Recommendations Consider the acoustical impact of sounds other than speech. Separate

any that can be placed outside of an open office. It is important to separate the severity of distractions from their

duration. Further research is needed to better define acceptable degrees of

Distraction Potential.

further thoughts on sound maskingccr - ccr … · further thoughts on sound masking robert chanaud,...

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