acoustical considerations in hvac systems (1)

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Acoustical Considerations in HVACSystems

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Center Frequency 63 125 250 500 1000 2000 4000 8000Band Designation 1 2 3 4 5 6 7 8

Acoustical Applications and Factors NC - Noise Criteria

Table 11. Octave Band Designations

Acoustical Applications and FactorsAir terminals are the most noise sensitive of all HVACproducts since they are almost always mounted in ordirectly over occupied spaces. They usually determine theresidual background noise level from 125 Hz to 2,000 Hz.The term Air Terminals has historically been used todescribe a number of devices which control airflows intooccupied spaces at the zone (or individual temperaturecontrol area) level. There are two types: those that controlthe amount of airflow to a temperature zone (Air ControlUnits, ACUs, or more commonly Boxes), and those thatdistribute or collect the flow of air (Grilles & Diffusers,GRDs). On some occasions, the two functions arecombined. As these two elements are the finalcomponents in many built-up air delivery systems andthose closest to the building occupants, both are criticalcomponents in the acoustical design of a space. There isalso a critical interplay between acoustics and the primaryfunction of these devices; providing a proper quantity ofwell mixed air to the building occupants. Before discussingtypes of devices, we must have an understanding of someissues regarding sound levels in occupied spaces.

The sound level in an occupied space can be measureddirectly with a sound level meter, or estimated frompublished sound power after accounting for room volumeand other acoustical factors. Sound level meters measurethe sound pressure level at the microphone location.Estimation techniques calculate sound pressure level at aspecified point in an occupied space. Measured soundpressure levels in frequency bands can then be plotted andanalyzed, and compared with established criteria for roomsound levels.

Sound power cannot be measured directly (except usingspecial Acoustic Intensity techniques) and is a measure of the acoustical energy created by a source. It is normallydetermined in special facilities and reported for devicesunder stated conditions. Sound Power Level (Lw) valuesfor Air Terminal devices are usually reported as the soundpower level in each of several octave bands with centerfrequencies as shown in Table 11. Sound Power Levels are given in decibels (dB) referenced to a base power in watts, typically 10-12 watts. Sound power levels can also be reported for full or 1/3 octave bands, but usually as full octave bands, unless pure tones (narrow bandssignificantly louder than adjacent adjacent bands) are present.

NC - Noise CriteriaSound Pressure Levels (Lp) are measured directly bysound level meters at one or more points in a room. They reference a pressure rather than a power. A productsestimated Sound Pressure Level (Lp) performance curve isobtained by subtracting space (or other appropriate) soundattenuation effects from the unit sound power (Lw).Currently, most Air Outlet and Inlets (GRDs) soundperformance is reported by subtracting a 10 dB attenuationfrom all octave band sound power levels, and determiningthe NC rating. (This room effect approximates a 3,000 cu. ft. room, 10 ft. from the source for VAV Boxes,which peak in lower frequencies, and a 2,500 cu. ft. room 7 ft. from a diffuser, which typically peaks @ 1000 Hz. (This is defined in the ASHRAE\ARI Standard 885 spaceeffect calculation described later in this section). NC curveswere developed to represent lines of equal hearingperception in all bands and at varying sound levels. Mostair terminal products are currently specified and reportedas single number NC ratings.

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Acoustical Applications and Factors NC - Noise Criteria

NC CURVE EXAMPLE

10

20

30

40

50

60

70

80

63 125 250 500 1K 2K 4K 8K

MID - FREQUENCY, Hz

dB

APPROXIMATETHRESHOLDOF HUMAN HEARING

NC RATING GIVEN IS NC-30 SINCE THIS IS HIGHESTPOINT TANGENT TO AN NC CURVE

SOUND POWER

NC-70

NC-20

NC-60

NC-50

NC-30

NC-40

SOUND POWER LESS 10 dB INEACH BAND

Figure 110. Typical NC Graph for a Diffuser

In this example, the outletLp spectrum does notexceed the NC curve of 30in any of the eight octavebands and is thus referredto as meeting an NC30criteria (and specification).It should be noted that whilethis spectra meets NC30,if the critical band resultedin an NC33, most buildingoccupants would not beable to discern thedifference.

NC ratings have been common in specifications for anumber of years, with an NC-35 being the most commonrequirement. While NC is a great improvement overprevious single number ratings, including Sones, Bels,and dBA requirements, it gives little indication of thequality of the sound. A more comprehensive method, RC,has been proposed; while a good analysis tool, RC is avery poor design tool.

This yields a range of deductions which differ in eachoctave band, as shown in Table 12.

The 10 dB room effect which has been traditionally used fordiffuser sound ratings, which typically peak in the 5th band,can be considered to be equivalent to a room about 2,500 cu. ft. in size, with the observer located about 7 ft. from the source.

With VAV terminals, which peak in lower bands, the 10 dBroom size is larger, or the distance is greater.

The use of a 10 dB room effect as usedin this example, while in common practiceand accepted for many years, is not asaccurate a prediction as is possible usingnewer techniques. The ASHRAEHandbook and ARI Standard 88598present an equation for determining thespace effect based on both room volumeand the distance from the observer to apoint sound source.

Space Effect = (25) - 10 Log (ft.) - 5 Log (cu. ft.)

- 3 Log (Hz)Where ft. = Distance from

observer to sourcecu. ft. = Room volume

Hz = Octave band center frequency

Acoustical Applications and Factors (continued)

Table 12. Space Effect (ARI 885 and ASHRAE)Room Band 63 125 250 500 1000 2000 4000 8000

Volume Hz 1 2 3 4 5 6 7 8@ 5ft -4 -5 -6 -7 -7 -8 -9 -10

2000 CuFt @ 10ft -7 -8 -9 -10 -11 -11 -12 -13@ 15ft -9 -10 -10 -11 -12 -13 -14 -15@ 5ft -4 -5 -6 -7 -8 -9 -10 -11

2500 CuFt @ 10ft -7 -8 -9 -10 -11 -12 -13 -14@ 15ft -9 -10 -11 -12 -13 -14 -15 -15@ 5ft -5 -6 -7 -7 -8 -9 -10 -11

3000 CuFt @ 10ft -8 -9 -10 -10 -11 -12 -13 -14@ 15ft -10 -10 -11 -12 -13 -14 -15 -16@ 5ft -6 -7 -8 -9 -9 -10 -11 -12

5000 CuFt @ 10ft -9 -10 -11 -12 -12 -13 -14 -15@ 15ft -11 -12 -12 -13 -14 -15 -16 -17

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Acoustical Applications and Factors RC - Room Criteria

RCRoom CriteriaRoom Criteria (RC) is based on ASHRAE sponsoredstudies of preference and requirements for speechprivacy, along with ratings for Acoustical Quality. RCratings contain both a numerical value and a letterQuality rating. The RC numerical rating is simply thearithmetic average of the sound pressure level in the 500,1,000, and 2,000 Hz octave bands, which is the speechinterference level (SIL). These are the frequencies thataffect speech communication privacy and impairment.Studies show that an RC between 35 and 45 will usuallyprovide speech privacy in open-plan offices, while a valuebelow 35 does not. Above RC45, the sound is likely tointerfere with speech communication.

Recommended NC and RC goals forvarious space applications, given inthe current ASHRAE Handbook, areshown in the table to the right.

Occupancy RC NC

Private residence RC 25-30(N) NC 25-30Apartments RC 30-35(N) NC 30-35Hotels/motelsIndividual rooms or suites RC 30-35(N) NC 30-35Meeting/banquet rooms RC 30-35(N) NC 30-35Halls, corridors, lobbies RC 35-40(N) NC 35-40Service/support areas RC 40-45(N) NC 40-45Offices Executive RC 25-30(N) NC 25-30 Conference rooms RC 25-30(N) NC 25-30 Private RC 30-35(N) NC 30-35 Open-plan areas RC 35-40(N) NC 35-40 Business mach Computers

RC 40-45(N) NC 40-45

Public circulation RC 40-45(N) NC 40-45Hospitals and clinics Private rooms RC 25-30(N) NC 25-30 Wards RC 30-35(N) NC 30-35 Operating rooms RC 25-30(N) NC 25-30 Laboratories RC 35-40(N) NC 35-40 Corridors RC 30-35(N) NC 30-35 Public areas RC 35-40(N) NC 35-40Churches RC 30-35(N) NC 30-35Schools Lecture and classrooms RC 25-30(N) NC 25-30 Open-plan classrooms RC 35-40(N) NC 35-40

Table 14. NC/RC Guidelines

In addition to the numerical SIL portion of the RC method,there is a Quality portion of the RC rating which involvesan analysis of potential low and high frequency annoyance.The goals of acoustical quality are described in thefollowing table.

Table 13. ASHRAE Defined Acoustic Quality

NOT TOO QUIET

NOT TOO LOUD

NOT TOO ANNOYING

NOT TO BE FELT

DONT DESTROY ACOUSTICPRIVACY

AVOID HEARING DAMAGEDONT INTERFERE WITH

SPEECH

NO RUMBLE, NO HISS NO IDENTIFIABLE MACHINERY

SOUNDS NO TIME MODULATION

NO FEELABLE WALL VIBRATION

Acoustical Applications and Factors (continued)

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Acoustical Applications and Factors RC - Room Criteria

Acoustical Applications and Factors (Continued)

Figure 111. RC Curves

Figure 112. Hissy Spectrum

The Four Quality letterdesignations currently in use are:

R RumbleH HissV Vibration (acoustically

induced)N Neutral

These letters are determined byanalyzing the low and highfrequency spectra compared to a line drawn with a -5 dB slopeper band through the numericalRC point @ 1,000 Hz. Thisestablishes the SIL (SpeechInterference Level) line. Lines of -5 dB slope create the RCchart, shown in Figure 111.

Also shown in Figure 111 areareas of rumble (B) and vibration(A), as well as a threshold ofaudibility. Note that the RC chartgoes well below the 63 Hz lowercutoff of the NC chart, as theselow frequency sound levels havebeen discovered to be a majorsource of discomfort tooccupants.

If the sound spectrum beinganalyzed exceeds a line drawnparallel to the SIL line plus 3dB in the higher frequencies (> 2,000 Hz), the hiss roof,then it is declared to be Hissyand gets an H designation.

RC50454035

3025

A

B

C

ROOM CRITERIA (RC) CURVES

ADAPTED FROM 1989 ASHRAE FUNDAMENTALS HANDBOOK - ATLANTA, GA

10

30

40

50

60

70

20

80

90

16 63 250 1K 4K

OCTAVE BAND CENTER FREQUENCY, Hz

MIC

RO

PASC

ALS

THRESHOLD OFAUDIBILITY

OCTAVEBAND

SOUND PRESSURE

LEVEL,dB re 20

REGION A: High probabilitythat noise induced vibrationlevels in light wall andceiling structures will benoticeable. Rattling of lightweight light fixtures, doors, and windows should be anticipated.

REGION B: moderate probability that noise induced vibration will be noticeable in lightweight fixtures, doors, and windows.

10

20

30

40

50

60

70

80

90

16 63 250 1K 4KOCTAVE BAND CENTER FREQUENCY, Hz

OCTAVE BAND

SOUND PRESSURE

LEVEL

ADAPTED FROM 1989 ASHRAE FUNDAMENTALS HANDBOOK - ATLANTA, GA

C

Measured data is outside the referenceregion by >3 dB, above the 1000 Hz octave band,therefore the noise is likely to beinterpreted as "hissy."

PSIL = (35+36+34) / 3 = 35

RC-35(H)

HISSY SPECTRUM (H)

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Acoustical Applications and Factors Air Terminal Sound Issues


Figure 114. Typical Sound Sources for Fan Terminal System

C

O

C

D

O

= Casing Radiated and Induction Inlet= Discharge Sound

= Outlet Generated Sound

SOUND POWER Lw

D

Air Terminal Sound IssuesSound is an important design criterion in theapplication of air terminals. In the context ofthe total building environment, comfortcannot be achieved with excessive sound ornoise levels. By definition, sound is a changein pressure for a medium, such as air. Thischange in pressure involves a radiation ofenergy. Energy is used in the generation ofsound and this energy is radiated from asource.

All sound has a source and travels down apath to a receiver. Air terminals are onesource of sound in a mechanical system.The path for sound emanating from the airterminal is through the plenum or down theduct into the conditioned space where itreaches the occupant or receiver.

Mechanical system designers should not beconcerned so much with sound, but ratherwith noise. Noise can be thought of asunwanted or excessive sound. Good designpractice dictates that a designer establishthe acceptable noise for the occupied spaceand then determine the selection criteria forthe mechanical system components.

In any application, both radiated anddischarge sound should be considered.Radiated sound "breaks out" from theterminal casing or induction port and travelsthrough the plenum and ceiling to enter theoccupied space. Discharge sound travels out the discharge of the terminal through the duct work and outlet to enter theoccupied space.

10

20

30

40

60

50

70

8080

8090

16 63 250 1K 4K

OCTAVE BAND CENTER FREQUENCY, Hz

OCTAVE BAND

SOUND PRESSURE

LEVEL

A

B

C

RUMBLE AND INDUCED VIBRATION (RV)

Even though the PSILis only 33 dB, thenoise spectrum falls within regionsA & B indicating ahigh probability ofmoise-induced vibration in lights,ceilings, air diffusers,and return air grilles.

PSIL= (38+32+29) / 3 = 33 RC-33(RV)

Figure 113. Rumble Spectrum

If the plotted spectra exceed a +5 dBRumble roof in the lower frequencies, itgets an R (Rumble) Rating. Finally, if thesound spectrum enters the A or Bzones in the very low frequencies shownon the RC graphs, it warrants a V forpossible wall or furniture vibration inducedby acoustical energy in low frequencies.The B region may be characterized as BeCareful where one may have complaints,but the A region is Awful, andcomplaints should be expected. This is anexample of an RV spectrum.

When room sound levels are analyzedusing RC curves, air diffusers tend to givethe same ratings as they do in an NCanalysis. Boxes, on the other hand, whichare typically predominant in lowerfrequency sound and often characterizedas Roar (250-500 Hz), often yield RCvalues lower than NC, but with an Rclassification.

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Acoustical Applications and Factors ARI 885

ARI Standard 885-98ARI Standard 885-98, Procedure for Estimating OccupiedSpace Sound Levels in the Application of Air Terminal andAir Outlets, provides the most current application factors forconverting rated sound power to a predicted room soundpressure level. This standard is the basis by which most airterminal manufacturers convert sound power, as measuredin reverberant rooms per ARI Standard 880-98, to apredicted room sound pressure level. The standardprovides a number of equations and tables availableelsewhere, but puts them all in one document, and includessome unique tables as well. It also includes examples anddiagrams to make the process easier to use. The mostimportant of those are included here.

Environmental Adjustment FactorIn order to use the 885 Standard, sound power must becorrected for differences between reverberant room andfree field calibrations when ARI 880 sound power is thebase. This Environmental Adjustment factor is listed in theARI Standard 885.

According to ARI Standard 885-98, an environmentaladjustment factor must be applied to manufacturers data ifthe sound power data has been taken under a free fieldRSS (reference sound source). According to ARI, this isnecessary because at low frequencies, all real occupiedspaces behave acoustically more like reverberant roomsthan open spaces (free field). In other words,manufacturers sound power data which is based onILG/RSS with a free field calibration must be adjusted tomatch actual operating conditions found in the field. Thisapplies to TITUS and other participants in the ARI 880Certification program. For data gathered using ARI 880, theenvironmental adjustment factor must be subtracted fromthe manufacturers sound power level data in order to usethe adjustments provided in ARI Standard 885.

Determining Acceptable Radiated SoundPower LevelsTo determine the maximum allowable radiated soundpower levels for a project, the attenuation from theceiling/space effect must be added to the desired roomsound pressure for each octave band.

Ceiling/Space EffectARI 885 combines the effect of the absorption of the ceilingtile, plenum absorption, and room absorption into theCeiling/Space Effect. Experience has shown that the STCrating (Sound Transmission Class) for ceiling tiles, which isbased on a two room pair test, is not well correlated withobserved data for a noise source located above a ceiling.The ARI 885 Ceiling/Space Effect table D14 (Table 16) isderived from a number of manufacturers observations andis only found in the ARI Standard. This table assumes thatthe plenum space is at least 3 ft. deep, is over 30 ft. wide orlined with insulation, and that there are no penetrationsdirectly under the unit.

From the ARI Standard, the following attenuation values, or transfer functions, should be used for the ceiling/spaceeffect:

Once the ceiling/space effect has been determined, theyare added to the sound pressure level to determine themaximum acceptable sound power levels. This must bedone for each octave band.


Table 15. Environmental Adjustment Factor

LW RAD = LP + S + P/C + Env

where: LW RAD = Radiated Sound Power LevelLP = Sound Pressure LevelS = Space EffectP/C = Plenum/Ceiling EffectEnv = Environmental Effect

Table 16. Ceiling / Space Effect (Table D14, ARI885-98)

Frequency 125 250 500 1K 2K 4K

Octave Band 2 3 4 5 6 7

Mineral Fiber 16 18 20 26 31 36Tile Ceiling

Glass Fiber Tile 16 15 17 17 18 19Ceiling

Solid Gypsum 23 27 27 29 29 30Board 58

Octave Band 2 3 4 5 6 7 8Env Factor 2 1 0 0 0 0 0

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Duct Octave BandDimension 2 3 4 5 6 7

6 x 6 0.6 1.5 2.7 5.8 7.4 4.312 x 12 0.4 0.8 1.9 4.0 4.1 2.824 x 24 0.2 0.5 1.4 2.8 2.2 1.848 x 48 0.1 0.3 1.0 2.0 1.2 1.2

Determining Acceptable Discharge SoundPower LevelsDischarge sound (sometimes called airborne sound) is the sound that travels down the duct and discharges intothe room along with the conditioned air. The procedure fordetermining the maximum acceptable discharge soundpower levels requires the addition of the space effect, endreflection, duct insertion, flow division (or branch powerdivision), and elbow and tees to the maximum acceptableroom sound pressure levels. If more than one outletsupplies air to a room, separate evaluations should occurfor each discharge path. This is done for each octave band.

Space Effect. The discharge sound space effect isdetermined in the same manner as the radiated soundspace effect. The sound source in this case might be theoutlet (i.e., grille, register, or diffuser) supplying air to thespace, or may be sound from an upstream noise source(damper or fan) which passes through the outlets, or a sumof both. (Table 12)End Reflection. When the area across the airstreamexpands suddenly as the duct work terminates or ends atthe outlet to the occupied space, a significant amount oflow frequency sound is reflected back into the duct work.This is called end reflection. The amount of end reflectionvaries based on the inlet size and type of duct.

Duct Insertion Loss. The addition of lined duct workresults in significant attenuation of higher frequency sound.The amount of attenuation varies with duct size and liningthickness. ARI 885 contains several tables helpful indetermining the appropriate attenuation values. Eachsection of duct work inserted downstream of the terminalmust be evaluated. For example, one might have separateduct insertion attenuation values for straight lined dischargeduct, branch duct (if lined), and flex duct from the branch tothe outlet. The ARI Standard, as well as the ASHRAEHandbook, provide tables of insertion loss per foot of ductbased on inside duct dimensions.

While lined duct factors are available in both ASHRAE andARI documents, flexible duct insertion loss data is availableonly from manufacturers or as found in the ARI 885Standard. Table 20 is the flexible duct insertion loss datafrom ARI 885.

Acoustical Applications and Factors Discharge Sound


Table 19. Rectangular, 1-inch Lined Duct, dB / ft. (ARI 885-98)

Table 18. Round 1-inch Lined Spiral Duct, dB / ft. (ARI 885-98)

Table 20. Flexible Duct Insertion Loss, dB

Two tables are provided here for rectangular and roundlined duct from the ARI Standard.

Duct Octave BandDiameter 2 3 4 5 6 7

6 0.59 0.93 1.53 2.17 2.31 2.0412 0.46 0.81 1.45 2.18 1.91 1.4824 0.25 0.57 1.28 1.71 1.24 0.8548 0 0.18 0.63 0.26 0.34 0.45

Eq. Dia. or Duct Octave BandWidth 1 2 3 4 5 6 7 8

6 17 12 6 3 1 0 0 08 15 9 5 2 0 0 0 010 13 8 3 1 0 0 0 012 12 6 3 1 0 0 0 016 9 5 2 0 0 0 0 024 6 3 1 0 0 0 0 0

Table 17. End Reflection (Table D13, ARI 885-98, dB)Duct Duct Insertion Loss, dB

Diameter Length Octave BandsInches Feet 2 3 4 5 6 7

10 10 9 27 33 38 244 5 7 5 16 24 27 18

3 5 4 12 20 23 1510 10 12 28 33 37 23

5 5 6 7 17 23 25 163 5 5 13 19 21 1310 10 15 28 33 35 22

6 5 6 9 18 22 24 153 5 6 13 17 19 1110 10 18 29 32 32 20

8 5 6 10 18 20 21 123 4 7 14 15 16 810 9 19 28 31 29 18

10 5 5 11 18 18 18 93 4 7 14 12 13 610 8 17 26 29 26 15

12 5 4 9 16 16 15 73 3 6 12 10 11 410 6 13 23 26 23 12

14 5 3 7 14 14 13 63 2 4 10 9 9 410 4 7 19 24 20 8

16 5 2 2 11 12 11 53 1 0 8 8 8 4

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Acoustical Applications and Factors Total Sound


Flow Division. When the airstream is divided, the soundcarried in each downstream branch is less than the soundupstream of the branch take-off. This shows the percent oftotal airflow carried by the branch. The appropriate level ofattenuation can then be determined from Table 21.

Elbows and Tees. A certain amount of attenuation ofhigher frequency sound is gained when an airstream entersan elbow or tee duct connection. If the elbow is round andunlined, the attenuation is considered by ARI Standard88598 to be negligible. Attenuation of rectangular tees isdetermined by treating the tee as two elbows placed sideby side.

Table 21. Duct Splits, dB

Once all of the attenuation factorshave been determined, they areadded to the sound pressure level todetermine the maximum acceptablesound power levels. This must bedone for each octave band, andagain the Environmental Adjustmentfactor must be added.

LW DIS = LP + S + ER + I + D + T/E + Env

where: LW DIS= Discharge Sound Power Level

LP = Sound Pressure Level

S = Space EffectER = End Reflection

I = Duct InsertionD = Flow Division

T/E = Tee/ElbowEnv = Environmental

Factor

Approximate Attenuation of 90 Elbows without Turning Vanes

Determining Acceptable Total Sound in aSpaceOnce the Radiated and Discharge sound pressure pathsand effects are known, the resulting room sound level canbe evaluated. Other factors may play a part in determiningthe final room sound levels. All these factors must beincluded to achieve an accurate prediction or analysis. Theresults may be very complex.

In the example here, many paths are shown. In practice,only a couple are significant, but changes in designs maymake a one time insignificant path become predominant.For example, should duct lining be eliminated, and no flexduct employed, discharge sound may be much moreimportant than radiated, the usual acoustical problem. Poorduct design may cause duct breakout to be the highestsound heard in the space.

The ARI 885 Standard provides guidance on all thepossible paths. Not shown here is background sound,which is often at an RC35 or greater in occupied spaces.

Table 22. Lined and Unlined Rectangular, dB (Table D12,ARI 885-98)

Table 23. Circular with Lining Ahead or Behind Elbow, dB(Table D10, ARI 885-98)

= Flex Duct Breakout

= Duct Breakout

2 3 45

6

= Casing Radiated & Inlet

= Distribution Duct Breakout

= Discharge1

= Outlet Generated Sound

1

2

3

4

5

6

SOUND PRESSURE Lp

Figure 115. Fan Powered Terminal or Induction Terminal - SummaryCalculation, Sound Sources and Paths

% of Total Air 5 10 15 20 30 40 50 60FlowAttenuation 31 10 8 7 5 4 3 1

Duct Octave BandWidth 2 3 4 5 6 75-10 0 0 1 6 11 1011-20 1 6 6 11 10 1021-40 6 6 11 10 10 1041-80 6 11 10 10 10 10

Duct Octave BandWidth 2 3 4 5 6 75-10 0 0 1 2 3 311-20 1 2 2 3 3 321-40 2 2 3 3 3 341-80 2 3 3 3 3 3

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Acoustical Applications and Factors Total Sound


All the sound paths must be combinedto predict the room sound level.

When combining path elements, themath is done using log addition, notalgebraically. Logarithmic (log)addition requires taking the antilog ofthe dB in each band, adding themtogether, then taking the log of theanswer. While this soundscomplicated, Figure 116 here showsan easier way of estimating the result.

To Add Two Decibel Values:

80 dB+ 74 dB

154 dB (Incorrect)Difference in Values: 6 dB

From Chart: Add 1.0 dB to higher Value

80 dB+ 1 dB

81 dBDifference In Decibels

Between Two Values BeingAdded (dB)

Corre

ctio

n To

Be

Adde

d To

High

er V

alue

(dB)

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10

(Correct)

Figure 116. Decibel Addition Example (Incoherent Sound)

Figure 117. Quiet VAV and Pan Terminal Recommended Installation

VAVUNIT

Lined Sheet Metal Plenum(Max velocity 1,000 fpm)

Flexible DuctsTo Diffusers

Flexible ConnectorsFor Fan Powered Units

D

> 3 D

Ceiling

Maximize HeightAbove Ceiling

4' Min.

More importantly, most people cannotdifferentiate between two sourceswhich differ by less than 3 dB. If thebackground sound is an NC35, andthe device in question is predicted atan NC35, it is likely that the spacewill be at an NC38 (although this isdependent on which octave bands arecritical), but most people cannot hearthe difference.

In a similar manner, sound from VAVboxes and diffusers combine to createthe room sound pressure level. Sincethey peak in different bands, however,they often complement each other. Inmany cases, a Series Fan Terminalwill be predicted to have an NC30 ina space, but when combined with anNC40 diffuser, will result in a roomsound pressure level of NC40, whichis optimum for providing speechprivacy in open plan spaces.

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Acoustical Applications and Factors Specifying Maximum Sound Power


Engineers can minimize the sound contributionof air terminals to an occupied space throughgood design practice.

Whenever possible, terminals should belocated over areas less sensitive to noise.This includes corridors, copy rooms,storage rooms, etc. Quiet air terminalsfacilitate the location of terminals overunoccupied space as with these unitslarger zones are possible resulting infewer terminals. This also reduces firstcost and improves energy efficiency.

The use of lined duct work ormanufacturers attenuators downstream ofair terminals can help attenuate higherfrequency discharge sound. Flexible duct(used with moderation) is also anexcellent attenuation element.

Sound will be reduced when appropriatefan speed controllers are used to reducefan rpm rather than using mechanicaldevices to restrict airflow. This form ofmotor control is often more energyefficient.

The air terminal and the return air grillelocation should be separated as far aspossible. Radiated sound can traveldirectly from the terminal through thereturn air grille without the benefit ofceiling attenuation.

Designing systems to operate at lowsupply air static pressure will reduce thegenerated sound level. This will alsoprovide more energy efficient operationand allow the central fan to be downsized.

Sharp edges and transitions in the ductdesign should be minimized to reduceturbulent airflow and its resulting soundcontribution.

Octave 2 3 4 5 6 7Band

LP (RC 40N) 55 50 45 40 35 30Rumble 5 5 0 0 0 0Roof

Hiss Roof 0 0 0 0 0 3

Max Room60 55 45 40 35 33Sound

Pressure

Specifying Maximum Sound Power Levels for Manufacturers DataA proper specification for acoustical performance in aspace will limit the maximum sound generation for aproduct. This should be based on the desired resultantsound in the space and accepted and clearly stated soundpath attenuations/reductions. ARI 885 provides a consistentmethod for accomplishing this task.

Example:A designer wants to achieve RC 40N for an open office toachieve an acceptable level of speech privacy. The officehas a 9 ft. ceiling and a volume of 3,000 ft.3. The terminalwill be located over the occupied space with 5 ft. of lined duct on the discharge.The lined discharge ductis 14" x 14" outside with one inch of insulation (12" x 12"internal cross section). This duct branches into an unlinedtrunk duct with two runs of lined flex duct taking off to theoutlet. The flex duct is 6 ft. long and 8" in diameter. Theterminal will supply 600 CFM with each diffuser taking 300 CFM. The ceiling is made of 5/8 inch mineral fiber tilewith a 35 lb. ft.3 density. The diffuser is selected toprovide an NC 40(N) at design flow.Desired Room Sound Pressure Levels

From ARI 885 (or ASHRAE) the appropriate soundpressure levels for an RC 40 can be determined. Since aNeutral Spectrum is desired, the Rumble Roof and HissRoof can be added to the spectra and still result in a neutraldesignation. The resultant maximum room sound pressurespectra are:

Table 24. Maximum LP for (RC 40N), dB

Figure 118. Sound Design Guidelines

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Acoustical Applications and Factors Specifying Maximum Sound Power

Octave Band 2 3 4 5 6 7Mineral FiberTile 5/8 16 18 20 26 31 3635# / Ft3

Radiated Sound Power LevelSpecificationsThe ceiling/space effect may be determined from Table 16,page B69.

The maximum acceptable radiated sound power levels aredetermined by adding all the factors, including theenvironmental factor. For example, in the 2nd band:

LW RAD 2 = 55 (LP) + 16 (C/S) + 2 (Env) = 73(Octave band 2. LW in bands 37 is calculated in a

similar manner.)For all bands, the following table results in a maximumallowed sound power, per ARI 88098, to achieve an RC 40N:

Discharge Sound Power LevelSpecificationsThe room absorption is determined by the space effectequation. With a 10 ft. ceiling and the terminal located a few feet away from the receiver, r will equal 10 ft. The volume of the room is 3,000 ft.3. The effect varies withthe octave band. The space effect will be obtained fromTable 12, page B65.

End reflection is based on Table 17, page B70 and an 8 in.duct connection.

Duct insertion for 5 ft. of 8 in. lined flex duct can be takenfrom Table 20, page B70.

No insertion value will be gained from the unlined trunk duct.

Flow division based on a 50 percent split (300 CFM / 600CFM) can be taken from Table 21, page B71.

The rectangular tee attenuation can be taken from Table 22, page B71.

Duct insertion for the 5 ft. of rectangular discharge duct canbe taken from the ARI 885 Standard, or from Table 19,page B70 in this case.

For this specification to be compared evenly against allmanufacturers, the environmental adjustment factor formanufacturers using a free field calibration ReferenceSound Source (RSS), as required in ARI 88098, has been subtracted from the appropriate manufacturers dataor added to the maximum acceptable sound power levels. If data is tested in another method, the appropriateness ofthe environmental factor must be understood and properlyapplied.

As with radiated sound, the environmental adjustmentfactor for manufacturers using a free field calibration RSS,as required in ARI 88094, has been subtracted from theappropriate manufacturers data or added to the maximumacceptable sound power levels. If data is tested in anothermethod, the appropriateness of the environmental factormust be understood and properly applied.


Octave Band 2 3 4 5 6 7Space Effect 9 10 10 11 12 13

Octave Band 2 3 4 5 6 7Flex Insertion Loss 6 10 18 20 21 12

Octave Band 2 3 4 5 6 7Flow Division 3 3 3 3 3 3

Octave Band 2 3 4 5 6 7Tee Attenuation 0 0 1 2 3 3

Octave Band 2 3 4 5 6 7Duct Ins. Loss 2 4 10 20 21 14

Octave Band 2 3 4 5 6 7LP (RC 40N) 60 55 45 40 35 33Env Effect (Table 15) 2 1 0 0 0 0Space (Table 12) 9 10 10 11 12 13End Ref (Table 17) 9 5 2 0 0 0FLEX (Table 20) 6 10 18 20 21 12Flow Div (Table 21) 3 3 3 3 3 3Elbow & Tee (Table 22) 0 0 1 2 3 3Rect Duct (Table 19) 2 4 10 20 21 14Lw 91 88 89 96 95 78

Octave Band 2 3 4 5 6 7End Reflection 9 5 2 0 0 0

Octave Band 2 3 4 5 6 7Lp (RC 40N) 60 55 45 40 35 33C/S (Table 5) 16 18 20 26 31 36Env Effect 2 1 0 0 0 0(Table 4)Lw 78 74 65 66 66 69

Table 25. Allowed Sound Power Maximums (ARI880-98)

B75Engineering G

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Acoustical Applications and Factors Diffuser Sound

It can be seen that discharge sound is not likely to be aproblem, especially in the mid-frequencies. If duct liningis eliminated, however, the maximum allowable poweris reduced by the duct insertion loss data on theprevious page. If flex duct is disallowed, the maximumsound power allowed is further decreased by flexinsertion loss. This can result in the followingrequirement for the unit:

In many cases, this is a borderline acceptable case formany unit sizes and flow rates, especially in smallerrooms with RC 35N requirements. Flexible duct can be included as a solution to discharge noise in thesecases.

diffusers, and so the NC rating must be used. For a closeapproximation of diffuser sound power when only NC isknown, one can assume that the sound power for thediffuser in the 5th octave band (1,000 Hz) is equal to thereported NC plus 10 dB, the 4th band (500 Hz) is 3 greaterthan this, and the 6th band (2000 Hz) is 5 less. This will besuitable for most applications.

The room sound pressure level requirements should bebased on the resultant desired acoustical environment. As the only attenuation element for diffusers is the roomeffect, this should be the primary attenuation path.Diffusers, moreover, have typically been tested in the samefacilities as VAV terminals, with the same reference soundsource, and therefore the ARI 885 Environmental Effectmust be included as well. The following is a proposedprocedure for determining the Diffuser NC requirementbased on an RC analysis:

Step 1: Determine the desired RC level for the space.This is the sound pressure level requirement inthe 5th band.

Step 2: Determine the room effect in the 5th (1,000 Hz)band, based on room volume and distance tothe diffuser from the observer. Add this to theRC number.

Step 3: Subtract 10 dB from the result in Step 2. This isthe required diffuser NC.

Diffuser SpecificationsDiffusers are commonly specified and reported in NC,rather than RC. In most cases, there is no differencebetween NC and RC for diffusers as they usually peak inthe 5002,000 Hz region, and the resultant numericalspecification is the same for both NC and RC. Diffuser NCratings commonly subtract 10 dB from measured soundpower levels in all bands to account for room attenuation.As described earlier, this will be a valid assumption for anumber of combinations of room volume and distance tothe source. While an ideal specification will be based onoctave band sound levels, these are seldom available for

Figure 119. Discharge Path Elements


Octave Band 2 3 4 5 6 7No Lining/Flex 83 74 61 56 53 52

Discharge Attenuation Elements

20

30

40

50

60

70

80

90

100

Octave Band Center Frequency, Hz

dB

RC 40

Lp (RC 40N)Rumble Roof

Hiss Roof

Env Effect

Space

End Ref

Flex

Flow Div

Elbow & Tee

Rect Duct

125 250 500 1K 2K 4K

Discharge Attenuation Elements

B76En

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erin

g G

uide

lines

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Acoustical Applications and Factors Specification Compliance

As the estimated room sound pressure level exceeds therumble roof in the 2nd band, this unit must be classified RC 29 (R). If an RC 35 (N) diffuser is also supplied,however, the sound from it must be added to the terminalsto get the room total sound level. Using the proceduredescribed under Diffuser Specification, we can estimatethe sound power level of an NC35 diffuser:

When these are added, the resulting spectra is:

The RC = (41 + 36 + 29)/3 = 35. The rumble roof for this spectra is therefore:

Therefore the sound pressure level in the space is an RC 35 (N).This works because diffusers and VAV terminalsseldom peak in the same frequencies, with diffusers beingcritical in the speech bands (5002,000 Hz) and boxesproducing the most sound in the 125250 Hz region. Whenthese two sounds combine in the space, they oftencomplement each other, producing a full spectrum of soundand resulting in an N rating.


Example:

The open office from the previous example is used here.The room is large and speech privacy is desired,requiring an RC 40N specification.

Step 1: From ARI 885 (or ASHRAE) the appropriatesound pressure levels for an RC 40 can bedetermined. In the 5th band the RC is = to thesound pressure level requirement, 40 dB.

Step 2: The room absorption is determined from thespace effect equation. With a 10 ft. ceiling andthe terminal located a few feet away from thereceiver, the distance variable will equal 10 ft.The volume of the room is 3,000 ft.3. Thefrequency varies with the octave band. Thespace effect in the 5th band will be obtainedfrom Table 12, page B65, and is = 11 dB. 40 + 11 = 51 dB.

Step 3: Subtract 10 dB = the diffusers NC requirement,or NC 41.

Determining Compliance to a SpecificationWhen determining if a unit will meet a specification, it maybe necessary to conduct a total room sound evaluation withmultiple sound sources and multiple paths. These areadded using logarithmic addition to determine total soundlevel. Once all path elements are identified, the noncriticalpaths can be determined using Figure 116, page B72.Paths which are 10 dB or more below the loudest, in anygiven band, can usually be ignored.

VAV terminals, if evaluated by themselves, often result in anR classification because of the high mid-frequencyabsorption provided by lined and flexible duct. The diffuser,however, can overcome this apparent Rumble spectra byfilling in the resultant sound with its high frequency soundgeneration. This results in an N rating, as required. Usingthe estimated sound power procedure from the DiffuserSpecification section above, the diffusers contribution canbe added (using log addition) to the VAV boxes soundpressure level, and a resultant sound pressure levelclassification developed.

Example:

A project engineer desires a space sound pressure levelof RC 35N for a private office. He has selected a TITUSTQS Fan Terminal, size 512, at 1500 CFM, with adesign inlet pressure of 1 in. static pressure. From thesound tables for the product, the sound power levels forthis unit and the reduction factors as in the previousexample are shown above:

Octave Band 2 3 4 5 6 7Diffuser Pwl 48 45 40Space Effect (Table 12) 9 10 10 11 12 13Env Effect (Table 15) 2 1 0 0 0 0Estimated Diffuser Spl 38 34 28

Octave Band 2 3 4 5 6 7Diffuser Pwl 38 34 28Terminal Spl 53 45 39 31 19 9Log Sum 53 45 41 36 29 9

Octave Band 2 3 4 5 6 7Unit Pwl 71 64 59 57 50 45C/S (Table 16) 16 18 20 26 31 36Env Effect (Table 15) 2 1 0 0 0 0Estimated Room Spl 53 45 39 31 19 9

Octave Band 2 3 4Rumble Roof 55 50 40

acoustical considerations in hvac systems (1)

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