i

INDOOR ACOUSTIC STANDARD

WRITTEN

BY

SANYAOLU K.A ARC/03/1956

ALAO O.O ARC/03/1907

SODIYA L.O ARC/00/4992

SUBMITTED

TO

PROFESSOR OGUNSOTE O.O

IN PARTIAL FULFILLMENT OF

ENVIRONMENT CONTROL III (ACOUSTIC AND

NOISE CONTROL)

ARC 507

MARCH 2008.

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Contents

1.0 INTRODUCTION........................................................................................................1

2.0 AUDITORIUM ACOUSTICS ..........................................................................................2

2.1 Auditorium Acoustics ..........................................................................................4

2.2 Reviewing the Basics of Auditorium Design ....................................................5

2.3 Auditorium Acoustic Design ..............................................................................9

2.4 The Classic Auditorium......................................................................................11

2.5 Oldies but Goodies ...........................................................................................15

2.6 Sound Propagation in an Auditorium ............................................................15

2.7 Direct Sound and Early Reflections ..............................................................16

2.8 Late Reflections ...............................................................................................17

2.9 Calculating Reverberation Time..................................................................17

2.9.1 Air Absorption............................................................................................18

3.0 CONCLUSION ........................................................................................................19

4.0 REFERENCES ...........................................................................................................20

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1.0 INTRODUCTION

This write-up shall critically examine auditorium indoor acoustic standard, indoors

acoustic standard varies as the spaces involve also varies. In this write-up various

concept will be considered in the study of auditorium indoor acoustic standard and

these include absorption, diffraction, echo, reverberation, transmission of sound

among others.

Also, the behavior of sound within auditorium space is also considered, and how

dimensions, shapes, height and indoor content affect the auditorium acoustic

standard of the buildings.

Furthermore, the way building noise is reduced through reflection and absorption are

also considered also how building materials can be used to enhanced indoor acoustic

standard. All of these and many more are discussed in this write-up about auditorium

indoor acoustic standard.

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2.0 AUDITORIUM ACOUSTICS

Multi-purpose use demands that auditorium acoustics satisfy acoustics

needs for theatrical performances, concerts, speeches, assemblies, and

public gatherings equally well.

Acoustics by Design’s acoustical engineers consult with auditorium

architects, facilities managers, and technical directors to shape and

design spaces for optimal performance of auditorium acoustics. In the

design phase, we use sophisticated acoustical modeling software to

predict and analyze projection, reverberation, reflection, vibration, and

other acoustical issues. When an auditorium is already built, we use

scientific acoustic measurement technologies and predictive modeling

to analyze existing acoustics and evaluate alternative remedies for

acoustical problems.

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Our experience includes the design of auditorium acoustics and

technical systems for a wide range of schools and universities,

convention and meeting centers, municipal and community buildings,

and corporate office and conference centers.

Acoustics By Design’s auditorium acoustics professionals also include

audio-visual and lighting consultants who provide independent analysis

and recommendations regarding auditorium technical systems design.

These professionals ensure desired sound, visual reproduction, and

lighting through the integration of technical systems design, delivering

the flexibility to host diverse events and support them with optimum

audio and visual clarity.

As independent acoustical and technical systems consultants, we don’t

represent any product manufacturers, suppliers, or other service

providers. Accordingly, our clients are assured our recommendations

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are free from bias and focused only on the best solutions for the issues

at hand.

Acoustics by Design’s auditorium acoustics consulting services include:

• Acoustical design

• Acoustical modeling

• Acoustical measurement

• Noise isolation

• Mechanical systems noise control and vibration isolation

• Audio-visual, lighting, and technical systems design

Acoustics by Design, Inc.

Consult as in Acoustics, Noise, Vibration, & Technical Systems

M Michigan, Indiana, Ohio, and Illinois, and in the major metropolitan areas of

Chicago, Fort Wayne, Indianapolis, Cleveland, Dayton, Columbus, Toledo,

Detroit, and West Michigan.

2.1 Auditorium Acoustics The room in which we listen to sounds has an important influence on what we

hear. This section will identify some of the principal means currently available for

judging the quality of an auditorium. However, the design of such spaces is still

considered an inexact science. When working with an acoustician in the design

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or renovation of a hall it is helpful for all to have an understanding of the basic

concepts in auditorium acoustic design. You can’t really design a hall just by

knowing the basics of auditorium design. The acoustician maintains an arsenal

of trade secrets and insider techniques, reserved to managing sound once it’s

been launched from the loudspeaker. But by understanding the basics, you can

at least keep track of what and why various things are being done. You can

explain things to fellow parishioners. Also, long before you even start, you have

to find someone to help with the design work. Understanding auditorium design

helps you interview candidates and recognize who is not able to answer straight

forward questions. Finally understanding auditorium acoustics will help give you

second thoughts when you find there is a problem with understanding sound

from a loudspeaker. Your speaker was probably installed by perfectly

competent sound contractor and is still working just fine. If it’s loud enough to

hear and you still can’t understand it, then your problem isn’t loud speakers, its

acoustics.

Besides keeping the rain out, the next most important thing an auditorium must

do is to provide a place where speech can be clearly understood. This means a

good auditorium will have a good intelligibility rating. The set of minimum

acoustic requirements that are met by a working auditorium starts with the

direct sound from the speaker being loud enough, that means it replicates

conversational sound levels. The background noise in the hall has to be fairly

quiet. The hall acoustics should be fairly free from echoes and other types of late

reflections. And finally, the hall acoustic is not very reverberant at all. Here we

take a look at each of these factors as they apply to the three basic types of

auditoriums that have evolved over the last century.

2.2 Reviewing the Basics of Auditorium Design Auditorium design begins with the loudspeaker and how it plays sound

into the hall. It ends with how the hall returns reflections of the sound back to the

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audience. The speakers should produce a sound level at about 65 dB,A

everywhere in the seating area.

It should have at least 20 dB of “head room” so that short lived bursts of sound

up to 85 dB, A can be replicated without any hint of speaker or electronic

distortion. Electronic distortion must be avoided. Distortion of the signal is one of

the fastest ways to cause people to lose their understanding of the sound. In

addition, the loudspeaker system should sound similar no matter where a person

is seated. This is achieved when the speaker system is tested and confirmed to

provide a fairly flat frequency response curve for every seat in the house.

The natural noise floor of the hall itself should be at least 20 to 30 dB quieter than

the speaker level heard at the seats throughout the entire frequency range for

speech. Generally the ambient noise levels for an empty good auditorium

would be about 25 to 30 dB,A. This noise is the sound of the hall when everything

is turned on, lights, air conditioning and even the sound system. The only thing

that isn’t happening is that someone isn’t talking into the mic. Only a fairly good

sound meter can measure sound levels this low. Then we open the doors and

the audience moves into the hall. The background noise levels rise up to about

35 dB,A because of the breathing and other rustling that people naturally do. It

is a well-established fact of human behavior that in a group, we collectively

manage to make just a little more noise than the background noise level. That’s

why we act quietly in a library and noisily in a packed diner. (See Figure D)

Early reflections are very helpful but not necessary for achieving good

intelligibility In a quiet hall. But many halls are quite perfect and that’s when

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early reflections become a good little helper, especially for those of us who

have begun to lose some hearing efficiency. A bright and clear sounding

auditorium will be providing something like 30 separate early reflections, each

arriving well within the first 1/40th second following the initial impact of the direct

signal. Some of these will be strong and some will be weak but the overall

summed power of all the early reflections should be in the range of 60 dB,A to

provide good speech reinforcement. (See Figure C & E)

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The next group of reflections is to be avoided. They are those that arrive at the

listener’s ear within a time delay following the direct signal between 1/40th

second and ¼ second. These reflections should be relatively indistinct and quiet,

about 15 dB below the speech signal range. The late reflections and echoes are

the most problematic part on auditorium acoustics. Some designers simple

eliminate all late reflections by absorbing them. Other more tricky designers like

to cover the surfaces of the hall with sound scattering devices to disburse late

reflections, breaking them up into a plethora of fairly quiet reflections that do

not obscure the perception and understanding of speech.

Following this should be a low level rise and fall of reverberation, again in the

range of about 15 dB below the direct and early reflected signals. The overall

reverberation should fall away and become inaudible within 1 second following

the initial direct signal. (See Figure F & G)

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Starting in about 1985 a new concept was being introduced (by SynAudCon) to

a few of the top sound contractors in the country. Sound installations were soon

going to be judged by a new set of performance criteria, the intelligibility spec

and contractors had to bone up to be able to do jobs that would meet this

new, higher level of performance. . Prior to this, sound systems only had to meet

loudness specifications. Now, loudness was not enough, understandability was

the final judge of a system.

This was a major step forward in the evolution towards good sound.

The only problem was that providing good loudspeakers was no longer good

enough. Bad hall acoustics could easily ruin any good sound system.

The top sound contractors in the country had to learn about acoustics.

Intelligibility meters soon become available and have become more affordable

over time. Nearly any good sound system was required to achieve a score of 80

to 85% in every seat in the house when measured with intelligibility test

equipment.

2.3 Auditorium Acoustic Design Today’s auditorium is generally quite different than those built early in the last

century.

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Both types use loudspeakers but that’s where the similarity ends. We will consider

both types here and the transition auditoriums built within the last few decades.

We start with the traditional auditorium, made out of heavy rock blocks or pour

in place concrete walls with ceilings made out of wood or concrete beams. We

end up simplifying construction to reduce costs. Today, concrete block or tilt-up

concrete walls are used to outline the space and roofs are made out of

corrugated metal supported by exposed metal trusses. The shell of today’s

auditorium is built not much different from an industrial space.

Fundamentally the old world auditorium is a “what you see is what you get”

type of building. The interior surfaces of the building are what manage the

sound. The seating, the height and the interior architecture all work in unison to

produce the required intelligible acoustic condition for reasonable listening. The

reflecting surfaces of the hall provides for some early reflections but not much.

The sheer volume of the hall helps to avoid generating late reflections and the

multifaceted ceiling and upper wall surfaces further act to diffuse the late

reflections. The audience provides the acoustic materials that act to control the

reverb time.

Architects are flocking to the new design trend in auditorium design and it’s very

different from the classical auditorium. These new spaces are large concrete

boxes that have been decked out with sculpted wooden, plastic, metal,

sheetrock and sometimes even glass panels. The hall is full of big, curved panels

that are suspended off the walls and again high overhead. The new look and

sound in auditorium, church and music hall design is one of acoustic clouds, lots

of acoustic clouds hanging in midair, below a completely blacked out high bay

ceiling. Although efficient to build and outfit, to the traditionalist, these halls,

sporting their marching arrays of flying sound panels seem a little strained,

possibly too technical, if not somewhat contrived. The acoustic clouds however

are intended to adjust the signal to noise ratio in a direct and effective way.

They provide for early reflections, diffuse and weaken the late reflections and

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regulate the reverb level and decay rate. The audience provides some acoustic

absorption and the rest is located way up out of sight, behind the acoustic

clouds.

In between these two styles we find built in the recent past, large sweeping

rooms with padded seats and carpet, topped off with the largest expanse of an

acoustic tile ceiling one could ever imagine. This type of hall is also a large

concrete box but its interior surface has been built out to create a very dead

hall. Its design seems directly opposite to that of the classic concrete and

marble auditorium. There are no early reflections designed into the space,

certainly no late reflections and as well, no reverberation. The only heard sound

in the hall is the direct sound from the loudspeakers. Built on the supposition that

if reverberation is bad for speech, then an acoustically dead space must be

good for speech. These spaces are so big and so dead that the audience

suffers from sensory deprivation. Distributed sound systems have to be installed in

the acoustic ceiling in an effort to help inject life back into the space. One thing

is for sure about these spaces. Because they are so acoustically dead, they work

great for making TV shows.

2.4 The Classic Auditorium This is the type of auditorium that was widely built during the WPA years to help

bring relief to the grips of the great depression in the early 1930’s. The features of

this hall are well documented in older architectural and acoustic design books.

It is tall, a little longer than wide and has balcony seats running on the two

sidewalls and the back wall. The ceiling has a deeply coffered design and the

sidewalls are lined with pillars or rounded pilasters. This hall uses a central

speaker cluster elevated high over the proscenium of the raised stage.

This hall is a classic example of minimalist design. It employs an elevated, central

speaker position to provide fairly uniform sound levels to every seat in the

audience. The room is a high volume hall, with ceilings 40 to 50 feet high over

the main floor.

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From the viewpoint of the speaker cluster, the main floor, two sidewalls and rear

wall is nearly completely absorptive when the hall is fully occupied. The only

remaining surface that the speakers can see is the ceiling and it appears to be

a very diffusive surface acting to scatter sound in all directions. (See Figure H)

It is instructive to run through the basic acoustic calculations for the large classic

auditorium design. Here are some basic ratios. Each seated person occupies

about 7 square feet of floor space and provides about 3 square feet of sound

absorptive surface. A hall that is 200 feet wide and 300 feet long will provide

about 7,000 seats on the main floor. The rear balcony section will be about 50

feet deep and provide about 1250 seats. The sidewall balconies will be about

30’ deep and provide 1000 seats each. The total seating of the hall is 9250 seats.

Occupied hall calculations are based on the hall being 2/3rds full, just over 6000

people.

The audience provides about 18,500 square feet of absorption distributed over

the floor, side and back walls. The hall has a volume of 3 million cubic feet.

Empty it will have a reverb time of about 7 seconds. This means in all its

complexity, it has a physical acoustic surface equivalent of about 21,400 square

feet. The hall has a floor and ceiling surface area of 60,000 sqft, surface area

each. The walls have a surface area of 50,000 sqft. The average absorption

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coefficient of the surface of the hall is already about 12.5%, including

atmospheric absorption effects.

When the audience arrives, they bring into the hall their additional component

of sound absorption, bringing the total absorption up to about 40,000 square

feet. The reverb time for the hall will now be about 3 ¾ seconds, far from the

“required 1 second”. To be able to meet the desirable 1 second reverb time it

would take a total of 150,000 square feet of absorption in the hall. This means

that nearly 100% of the 170,000 square feet of total surface area of the hall

would have to be covered with sound absorption. This was not possible in the

early days of marble surfaced halls. But auditorium designers didn’t stop trying.

They invented “acoustical plaster”, a surface that looked hard but wasn’t. There

were numerous formulas for this material but over time the art of acoustic

plaster, the formulations and the skilled people who applied it by hand have all

disappeared. Still, the hall acoustics could not come close to accommodating

a 1 second reverb time.

The floor, sidewalls and rear wall present 100,000 sqft of surface area that

intercept the sound from the speakers. With the sound absorption of the people

added to the natural absorption of the hall surfaces, these 3 surfaces present

about 31,000 sqft of absorption facing the loudspeaker. This means that 31% of

the incident sound is absorbed during the first reflection. Sound reflecting off

these 3 surfaces is reduced in strength by about 2 dB. If we assume that the

sound reflects back across the room, traveling an average of 225 feet and

taking about 1/5 second. Sound is further attenuated by the natural surface

absorption of about 10% upon this reflection. As a result, by the time sound

makes one round trip in the hall from the proceneum wall into the audience

and back, it has been attenuated down to 62% of its original strength, about -2.5

dB all in one round trip time period of 1/10th second. After one second, there will

be 20 such round trips and the overall sound will have dropped by about 25 dB

in strength. The hall will have a reverb time of about 2.4 seconds. Adding stage

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curtains that show even when pulled back will drop the reverb time to about 1.8

seconds. This is a reasonable reverb time for a large hall.

But there is more than “reverb time” to hall acoustics. There are the good early

reflections and the bad late reflections. The early reflections need to be

cultivated. The late reflections need to be weeded out. The balcony facing is

sculpted to provide early reflections back down to the main floor.

The back wall of the balcony and ceiling is sculpted to provide early reflections

into the balcony seats. (See Figure A)

The late reflections are mainly dealt with by combining two features. It begins

with having a heavily coffered ceiling. Any sound that is heading upwards

eventually hits the coffered ceiling, only to be splintered into a cascade of tiny

and off angled reflections. The second factor is the height of the ceiling. A 70’

ceiling with a loudspeaker mounted some 35’ off the floor starts splashing sound

back onto the main floor at about 40 ms after the direct signal has passed

through the audience. The coffering of the ceiling breaks this reflection up into

dozens of low-level reflections with a variety of additional time delays. The high

coffered ceiling acts to diffuse and randomize the only possible late reflections

in the hall. Other sounds that are traveling upward that went over the heads of

the balcony seating continue upwards after the wall bounce and are also

intercepted by the deeply coffered ceiling scattering grid ceiling. The low level

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of time-delayed backfill continues until it is overwhelmed with the rise and

decay of the reverberant part of the hall sound. (See Figure B)

The classic auditorium of the early 1900’s was almost, nearly a perfectly

balanced system of people, space, speakers and surfaces. It was a symphony in

sound and architecture.

2.5 Oldies but Goodies The old auditoriums, built in the early part of the last century could be designed

and built to sound pretty good. They were giant, expensive hand built civic halls.

But over the wear and tear of time, they began to look run down, worn out and

shabby. After WWII, a new product took the architectural and building world by

storm, acoustic ceiling tiles. During the same time a new form of civic pride

dictated “off with the old and on with the new” and this kind of new meant

concrete block walls and acoustic tile ceilings, and a whole new way to build

auditoriums. The era of fine sounding old traditional civic auditoriums ended in a

bang, sounded by the wrecking ball.

2.6 Sound Propagation in an Auditorium

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• As sound waves travel at about 345 meters/second, the sound coming

directly from a source within an auditorium will generally reach a listener

after a time of anywhere from 0.01 to 0.2 seconds.

• sound, a series of semi-distinct reflections from various reflecting surfaces

(walls and ceiling) will reach the listener. These early reflections typically

will occur within about 50 milliseconds.

• The reflections which reach the listener after the early reflections are

typically of lower amplitude and very closely spaced in time. These

reflections merge into what is called the reverberant sound or late

reflections.

• If the source emits a continuous sound, the reverberant sound builds up

until it reaches an equilibrium level. When the sound stops, the sound level

decreases at amore or less constant rate until it reaches inaudibility.

• For impulsive sounds, the reverberant sound begins to decay immediately.

2.7 Direct Sound and Early Reflections

• Direct sound will decrease by 6 dB for each doubling of distance

propagated.

• Our auditory system will determine the direction of a sound source from

the direct sounds reaching the ear.

• Early reflections which arrive within about 35 milliseconds are not heard as

separate from the direct sounds. Rather, they tend to reinforce the direct

sound.

• The source is perceived to be in the direction from which the first sound

arrives provided that (1) successive sounds arrive within about 35

milliseconds, (2) the successive sounds have spectra and time envelopes

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reasonably similar to the first sound, and (3) the successive sounds are not

too much louder than the first. This is referred to as the precedence effect.

• From a study by Leo Beranek (1962), a concert hall is considered

``intimate'' if the delay time between the direct and first reflected sound is

less than 20 milliseconds.

• First reflections usually arrive from the nearest side wall or from the ceiling

for those seated in the center.

• Reflections from the ceiling or overhead reflectors are not as perceptually

desireable as those from side walls.

2.8 Late Reflections

• During a continuous sound, the reverberant sound level is reached when

the rate at which energy is supplied by the source is equal to the rate at

which sound is absorbed by the room and its contents.

• Too much reverberant sound will result in loss of clarity.

• In a bare room, where all surfaces absorb the same fraction of the sound

that reaches them, the theoretical reverberation time is proportional to

the ratio of volume to surface area.

• Reverberation time is typically defined as the time required for the sound

level to decrease by 60 dB (or ).

2.9 Calculating Reverberation Time

• When expressed in units of cubic and square meters, the reverberation

time is given by RT =, where is the volume of the room and is the

effective ``total absorption'' area.

• The ``total absorption'' area is calculated as the sum of all surface areas in

the room, each multiplied by its respective absorption coefficient.

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2.9.1 Air Absorption

• Air contributes a substantial amount to the absorption of high frequency

sound.

• Taking account of air absorption, RT = 0.161 , where is a

constant which varies with air temperature, humidity, and frequency.

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3.0 CONCLUSION

Since acoustic is the study of sound behavior in buildings, it is evident in this write up

that when any building for any purpose is being design there should be a critical

consideration of sound behavior so as to ensure that the building is functional regards

to sound behavior (acoustic of the building) so that sound do not turn to be noise in

the designed space or building.

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4.0 REFERENCES

[1] H. Kuttruff, Room Acoustics (Elsevier, New York, 1991).

[2] M. Barron, Auditorium Acoustics and Architectural Design (E & FN Spon, London,

1993).

[3] L. L. Beranek, How They Sound, Concert and Opera Halls (Acoustical Society of

America, Woodbury, 1996).

[4] P. Lord and D. Templeton, the Architecture of Sound; Designing Places of

Assembly (Architectural Press, London, 1986).

[5] L. Nijs, D. de Vries and D. Petri, ‘‘The young architect’s guide to room acoustics,’’

Int. Symp. Room Acoustics: Design and Science (2004).

[6] L. Nijs, P. Versteeg and M. van der Voorden, ‘‘The combination of absorbing

Materials and room shapes to reduce noise levels,’’ Int. Congr. Acoustics, Kyoto,

Japan (2004).

[7] L. Cremer and H. A. Muller, Principles and Applications of Room Acoustics (Applied

Science, London, 1982).

[8] C. W. Kosten, ‘‘A new method for the calculation of the reverberation time of

halls for public assembly,’’ Acoustic, 16, 325–330 (1966).

• Acoustic Quality in Office Workstations, as Assessed by Occupant Surveys by

Jensen, K., and E. Arens. Proceedings, Indoor Air 2005, Beijing, China, Sept. 4-

9, 2005.

• Architectural Graphic Standards (AGS), 10th Edition by Ramsey/Sleeper.

Washington, DC: The American Institute of Architects, April 2000 (pages 63-70).

• ASID Sound Solutions: Increasing Office Productivity through Integrated

Acoustic Planning and Noise Reduction Strategies by Bullock Associates. A

Professional Paper from: American Society of Interior Designers, Armstrong

World Industries, Inc., Dynasound, Inc., Milliken & Co., and Steel case Inc.

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• Classroom Acoustics by Acoustical Society of America

• GSA Facilities Standards for the Public Building Service, P100 2005. (Section

3.4, Special Design Considerations - Acoustics)

• High Performance School Buildings by the Sustainable Buildings Industry

Council. Washington, DC: SBIC.

• National Research Council of Canada Acoustics Research

• NIH Design Policy and Guidelines (p. 15)

• Sustainable Building Technical Manual (Chapter 14, Acoustics)

• U.S. Courts Design Guide, 4th edition by the Administrative Office of the U.S.

Courts. Washington, DC: 1997.