acoustics in different buildings

1. SUBJECT : ACOUSTICS
TOPIC : ACOUSTIC DESIGN PROCESS
AND
ACOUSTICS OF DIFFERENT BUILDINGS

2. FACTORS AFFECTING ACCOUSTICS OF A BUILDING
REVERBERATION TIME
• If a hall is to be acoustically satisfactory , it is essential that it should have the right reverberation time.
• The reverberation time should be neither too long nor too short .
• A very short reverberation time makes a room ‘dead’ .On the other hand , a long reverberation time
renders speech unintelligible.
• The optimum value for reverberation time depends on the purpose for which a hall is designed.
REMEDIES
• The reverberation time can be controlled by the suitable choice of building materials and furnishing
materials.
• Since open windows allow the sound energy to flow out of the hall , there should be a limited number of
windows . They may be opened or closed to obtain optimum reverberation time.
• Cardboard sheets, perforated sheets, felt , heavy curtains , thick carpets, etc. are used to increase wall and
floor surface absorption . Therefore, the walls are to be provided with absorptive materials to the required
extent and at suitable places.
• Heavy fold curtains may be used to increase the absorption.
• Covering the floor with carpet also increase the absorption.
• Audience also contribute to absorption of sound . The absorption coefficient of an individual is about 0.45
sabines.
• In order to compensate for an increase in the reverberation time due to an unexpected decrease in
audience strength , upholstered seats are to be provided in the hall.
• Absorption due to an upholstered chair is equivalent to that of an individual.

3. REVERBERATION TIME

4. • LOUDNESS
• Sufficient loudness at every point on the hall is important factor for satisfactory hearing.
• Excessive absorption in the hall or lack of reflecting surfaces near the sound source may lead to decrease
in the loudness of the sound.
REMEDIES
• A hard reflecting surface positioned near the sound source improve the loudness.
• Low ceilings are also of help in reflecting the sound energy towards the audience.
• Adjusting the absorptive material in the hall will improve the situation.
• When the hall is large and audience are more , loud speakers are to be installed to obtain the desired level
of loudness.
FACTORS AFFECTING ACOUSTICS OF A BUILDING

5. • FOCUSSING
• Reflecting concave surfaces cause concentration of reflected sound , creating a sound of larger intensity at
the focal point . These spots are known as sound foci.
• Such concentrations of sound intensity at some points lead to deficiency of reflected sound at other points.
• The spots of sound deficiency are known as dead spots . The sound intensity will be low at dead spots and
inadequate hearing.
• Further , if they are highly reflecting parallel surfaces in the hall , the reflected and direct sound waves
may form standing waves which leads to uneven distribution of sound in the hall.
REMEDIES
• The sound foci and dead spots may be eliminated if curvilinear interiors are avoided . I f such surfaces are
present , they should be covered by highly absorptive materials.
• Suitable sound diffusers are to be installed in the hall to cause even distribution of sound in the hall.
• A paraboloidal reflecting surface arranged with the speaker at its focus is helpful in directing a uniform
reflected beam of sound in the hall.
FACTORS AFFECTING ACOUSTICS OF A BUILDING

7. • ECHOES
• When the walls of the hall are parallel , hard and separated by about 34m distance , echoes are formed.
• Curved smooth surfaces of walls also produce echoes.
REMEDIES
• This defect is avoided by selecting proper shape for the auditorium.
• Use of splayed side walls instead of parallel walls greatly reduces the problem and enhance the acoustical
quality of the hall
• Echoes may be avoided by covering the opposite walls and high ceiling with absorptive material.
FACTORS AFFECTING ACOUSTICS OF A BUILDING

8. • ECHELON EFFECT
• If a hall has a flight of steps , with equal width, the sound waves reflected from them will consist of echoes
with regular phase difference .
• These echoes combine to produce a musical note which will be heard along with the direct sound . This is
called echelon effect.
• It makes the original sound unintelligible or confusing.
• REMEDIES
• It may be remedied by having steps of unequal width.
• The steps may be covered with proper sound absorbing materials , for example with a carpet.
FACTORS AFFECTING ACOUSTICS OF A BUILDING

9. • RESONANCE
• Sound waves are capable of setting physical vibration in surrounding objects , such as window panes ,
walls , enclosed air , etc.
• The vibrating objects in turn produce sound waves .
• The frequency of the forced vibration may match some frequency of the sound produced and hence result
in resonance phenomenon.
• Due to the resonance certain tones of the original music may get reinforced that may result in distortion of
the original sound.
• REMEDIES
• The vibrations of bodies may be
suitably damped to eliminate
resonance due to them by proper
maintenance and selection .
FACTORS AFFECTING ACOUSTICS OF A BUILDING

10. ACOUSTICAL DESIGN CHARACTERISTICS

11. TEXTURE
BLEND AND ENSEMBLE
“Intimacy” refers to the feeling that listeners have
of being physically close to the performing group.
Intimacy is achieved whenever the first reflected
sound reaches the listener less than about 20 ms
after the direct sound.
INTIMACY

12. Acoustical Design Steps
• An acoustical consultant should take the following steps in designing a room for speech communication:
Assess the acoustical requirements.
• Will the room be configured in a lecture style with a fixed speaker location or will it be a more interactive
type of use as in many meeting rooms and classrooms? Will users of the room regularly include those with
special acoustical needs such as very young, very old or hearing-impaired listeners who would require
superior listening conditions? (Listeners with more severe hearing impairment may require the
installation of systems that transmit speech to personal receiving devices.)
Choose a maximum noise level goal.
• Determine the maximum ambient noise level criterion from Figure 2 and modify it to meet the needs of
particular users.
Design to meet noise criteria.
• Set the maximum noise level criteria for mechanical systems. Room boundaries must be adequate barriers
to intruding noise from outdoors and from adjacent spaces.
Choose the reverberation time criterion.
• Select the RT design goal from Figure 3 and use the lower (special needs) curve if more critical uses are
expected.
Determine the required total sound absorption and amount of material to be added.
• Figure 4 gives a simple estimate of the total required sound absorption. Calculating the amount of added
sound-absorbing material requires knowledge of the sound absorption of various products and an
estimate of the absorption of the expected room occupants.
Consider the room shape and the location of the sound-absorbing material.
• In smaller rooms, sound-absorbing material should usually be distributed evenly over the room surfaces
but avoiding the centre portion of the ceiling and surfaces close to talker locations such as fixed podiums.
In larger rooms the shape of the room and the location of absorption will also be important.

13. ACOUSTICS FOR DIFFERENT TYPES OF
BUILDINGS

14. Lecture Halls
Acoustic considerations
• Speech intelligibility and rhetoric must be in focus in lecture halls.
• Ideal acoustic conditions are ensured by creating a sonorous room – a rich sound environment with a hint
of resonance.
• Furthermore, good sound diffusion is necessary in order to spread the consonants to the audience.
Acoustic design
• Recommended reverberation time is 1 second.
• the NC level should not exceed 25 to 30.
• The front wall and ceilings can be reflective, enabling sound to reach everyone.
• Absorptive material on the back and side walls will help reduce the reverberation time and unwanted
reflections.
• If possible, try to avoid parallel surfaces, which can cause flutter echoes.
• Consider splaying or canting the sidewalls.
• Lecture halls must be fitted with sound reflecting elements above the speaker.
• In addition, it is important to enable the sound to be angled so that it can reach the audience.
• The ceiling can be created as a separate shape that prevents the sound from being reflected back to the
speaker.
• Walls should have a Sound Transmission Coefficient (STC) rating of not less than 50.
• Walls must extend to the floor above or to the roof construction, and not stop at the ceiling.

15. • The mix of sound-reflectant and sound-absorbent materials must be carefully calculated to control
reverberation without creating a sound-deadened room .
• Ceilings should be sloped or stepped and primarily of a hard surface.
• Acoustical treatment for the ceiling, if required, should be installed around the perimeter of the sides and
rear in the form of a ‘U’ with the front and middle sections of hard surfaced , sound reflectant materials.
• Acoustical treatment normally will not exceed 40-50% of the ceiling surface.
• Partial wall-surface treatments should be considered as an alternative to ceiling treatment.
• The back wall may need to be 50-100% covered with acoustical absorption materials.
• Care must be exercised in isolating the facility from exterior noise as well as controlling the background
noise level in the room, especially that generated by the mechanical systems.
• Ambient sound levels measured at 4 feet above the floor at all points throughout the room must have a
Noise Criterion (NC) rating of more than 35.
• The mechanical system should generate a background noise of not more than NC 15-20.
• Factors that have been identified in the design of a quiet operating system include :
 Air handlers or fans located away from the lecture hall
 Low velocity of air within the lecture hall
 Proper sizing and acoustical treatment of ducts , returns and
diffusers.
• For lecture halls noise limits of 35 dB(A) or 30 dB(A) are
recommended.
• Requirements Maximum Noise Level
low 40 dB(A)
middle 35 dB(A)
high 30 dB(A)
• Maximum allowable background noise levels

16. CLASSROOMS
Acoustic considerations
• in a class room, it is important to ensure good speech and listening comfort.
• the most important aspect in achieving listening comfort is the audibility and clarity of consonants.
• this is due to the fact that comprehension of speech is dependent on consonants.
• in addition, interfering noise must be reduced so that it does not drown or ”mask” the relevant sound.
• speech comfort is ensured by creating a sense of reverb in order for the rhetoric effects to be used.
• if too much sound is absorbed, the rhetoric effects of the speech will disappear.
• if too little sound is absorbed, the noise will impede the listening comfort.
Acoustic design
• rectangular room, 50 to 70 m². ceiling height max. 3 m.
• room dimension may not be close to or exceed 1:2.
• Ceiling:
 Acoustic materials with sound absorbing and diffusing properties, as well as a small amount of reflection.
 The area of the ceiling to be acoustical tile is a function of ceiling height.
 ceiling height %of acoustical tile
 10feet 40-50
 12feet 50-60
 These numbers presume the use of Noise Reduction Coefficient (NRC) 0.55-0.65 tile in a ceiling
suspension system.
 The acoustical tile should be arranged in the form of a ‘U’ around the perimeter of the room , with the
opening at the front and rest of the ceiling a hard material such as gypsum board or plaster.

17. • Walls:
 Sound absorbing materials with diffusion characteristics.
 Sound Transmission Coefficient (STC) not less than 50.
 Walls must extend to the floor above or to the roof construction, and not stop at the ceiling. This will reduce noise
transmission.
 Higher STC ratings and special wall construction details must be included whenever classrooms are located to
,adjacent, or below restrooms, mechanical rooms, elevator shafts, athletic facilities , or other sources of high noise
levels or where the classroom function generates a significant amount of noise.
 Concrete masonry units may be used, but may have to be covered with another finish in order to provide proper
acoustical treatment.
 Folded walls are extremely undesirable and should not be used except under very extraordinary circumstances. It
is difficult to develop a folding-wall design that is able to maintain adequate sound separation between classrooms.
 Sound levels as generated by mechanical systems or other ambient noise measured at all points in a classroom at 4
feet above the floor must have a Noise Criterion (NC) rating of not more than 35.
• Mechanical systems:
 The mechanical systems supporting general purpose classrooms should generate a background noise of not more
than NC 35.
 The air changes and circulation of air per ASHRA standards is a critical factor instructional spaces . This must be
achieved with effective control of HVAC system-generated background noise.
• Utility Boxes:
 When classrooms share a common wall, electrical receptacles
or other utility boxes should not be installed back-to-back with
similar receptacles in the next room.
 Off setting the boxes will reduce sound transmission between
rooms.

19. OPEN OFFICES
Acoustic considerations
• Open plan offices are best suitable for employees with related work functions and the
occupations requiring visual and verbal contact.
• The most important acoustic measure is sound separation.
• The interior fit-out plays therefore an important acoustic role.
• In addition, personal relations have a decisive influence of the perception of sound versus
noise.
• It is thus in some cases required to plan the fit-out on a more individual level.
Acoustic design
• It is important to create discontinuous ceiling and wall surfaces, both as structures, inclinations or curves.
• Hard surfaced elements that reflect sound should be avoided or equipped with sound absorbers.
• Alternatively, the groups should be separated by room dividing elements, stretching from the ceiling to the
floor.

20. • BLOCK SOUND
• To stop the direct path of sound, we erect barriers (system wall panels) which stop sound from passing
through. The STC, or Sound Transmission Class, of a partition measures the ability of a barrier to stop
sound from passing through it. A material of an STC of 21 will prevent 21 decibels from passing through.
• The most sound reduction that can be expected between workstations is 21 decibels, because sound will
diffract, that is, bend over the top and around the side of partial height partitions. An STC of more than 21 is
not generally an advantage.
• ABSORBING SOUND
• Acoustically absorbent panels absorb rather than reflect sound. The reflection of sound off hard surfaces is
called reverberation. The absorption of sound, on the other hand, actually refers to energy conversion.
• Sound is created when something resonates i.e. pushes against the air and retracts creating waves of
dense and rarefied air – fluctuations in air pressure.
• Sound is a form of energy, and energy cannot be destroyed. It can be dissipated as it spreads out over
distance or converted into another form of energy. Acoustical panels convert sound energy into mechanical
energy. As sound waves impact the material, the material responds by vibrating. Those vibrations
(“wiggles’) are then dissipated as a minute amount of heat. The ability of a material to convert sound energy
to mechanical energy is measured in a test that provides the Noise Reduction Coefficient (NRC). An NRC of
70 means that a material absorbs 70 percent of the sound that hits it. The reciprocal, 30 percent, is
returned.
• The ability of a material to absorb sound determines its acoustical capabilities. The most effective sound
reduction in a office environment is achieved when the higher frequencies of human speech, those that lend
intelligibility, are those which are absorbed. These are often referred to as the articulate speech frequencies.
• The NRC is a simple average of the material’s absorption of sound at frequencies of 250, 500, 1,000 and
2,000 Hertz (Hz). Hertz cycles per second (CPS), and frequency all refer to the number of fluctuations per
second which determines the pitch of a sound.
• In evaluating an NRC rating for open plan acoustics, it is important that the higher absorption coefficients
are at the higher frequencies (the articulate frequencies of human speech).
• Two different materials may have the same NRC but, the one which absorbs more of the intelligible (higher)
speech frequencies is a more effective material for controlling sound in an office environment. In fact, the
reflection of low frequencies may be an advantage in that it permits an ambient sound level that can
reinforce the background masking sound.

22. CONCERT HALLS
• The reverberation time will depend on what type of concert is performed.
• For classical or orchestral music, a higher reverberation time would be
appropriate (approximately 2 sec), for a rock concert, a lower reverberation time
would be appropriate (approximately 1 sec). Find a happy medium, perhaps 1.5
sec. This only applies to indoor venues.
• It is vital to control the reflections from the back wall. If not, the presentation
could reflect off the back wall and "slap back" to the presenter(s). This won't
necessarily impact the audience, but could be disastrous and distracting for the
people on stage. Because of this, it's usually necessary to splay or tilt the back
wall to avoid slap back. A concave back wall could compound this problem. If
not, it's imperative that it be treated with absorptive material.
• Control the reverberation time on the stage. Ideally, the reverberation time in
the stage area should be the same as in the house. Since the stage area might
have a higher ceiling than the rest of the auditorium, more absorptive materials
might be required in this area. Frequently, the back wall of the stage, and
possibly one or two of the side walls, is treated with an acoustically absorptive
material, typically black in color.
• Beware of potential noise impact to your space from exterior sources and/or
excessive HVAC noise. In the design, the NC level should not exceed 25 to 35.
When specifying NC, specify an actual rating, such as NC 30, rather than a
range, such as NC 30-35. Although specifying a lower number will ensure
minimal background noise, it might be cost prohibitive to achieve.

23. Concert halls

24. CINEMA HALL
• Recommended reverberation time: 0.8-1.2 seconds.
• Background noise levels should be kept to a minimum. Three primary potential
noise sources are mechanical equipment (HVAC), noise from adjacent theaters
and lobby, and outdoor noise.
• HVAC noise, which is often overlooked, can negatively impact the usability of a
space. To help protect your design, the NC level should not exceed 30 to 35.
When specifying NC, specify an actual rating, such as NC 30, rather than a
range, such as NC 30-35. Although specifying a lower number will ensure
minimal background noise, it might be cost prohibitive to achieve
• Noise from the lobby area can be disruptive. Be sure openings such as doorways
are properly sealed. Consider a vestibule door system.
• Excessive room length should be avoided.
• Beware of potential outdoor noise impacting your space.

25. • A common, and often ineffective, practice is to use absorptive materials
only on the back wall of a cinema. This is meant to reduce slap back; but
slap back impacts the performers, which, in this case, are actors on a movie
screen. Primary concern should be given to the audience. Although
absorption on the back wall might be necessary, the critical areas are the
side walls. Walls, except possibly those closest to the screen, should be
absorptive.
• It is vital to control the noise transfer between theaters. The STC rating
should be at least 60-65, but keep in mind that STC ratings only address
noise isolation from 125 Hz to 4000 Hz. This range is fine when considering
speech frequencies, but gives no information for sounds that register at
lower frequencies, such as the base notes of the music or explosion sound
effects, which often register below 125 Hz. In such cases, be aware that the
STC rating might not accurately describe how well that particular wall will
work at that noise level.

26. AUDITORIUMS
• Recommended reverberation time is 1.0-1.5 seconds (might be higher for some
auditoriums).
• Although the seating area will provide absorption, thereby reducing the reverberation
time, absorptive materials are to be added to the other surfaces within the space.
• It is vital to control the reflections from the back wall. If not, the presentation could
reflect off the back wall and "slap back" to the presenter(s). This won't necessarily
impact the audience, but could be disastrous and distracting for the people on stage.
Because of this, it's usually necessary to treat the back wall with an absorptive
material. A concave back wall could compound this problem. If not, it's imperative
that it be treated with absorptive material.
• Splay or use irregular surfaces on the walls to avoid flutter echoes. Parallel reflective
surfaces can allow sound to "ricochet" back and forth between the surfaces. This
potentially annoying condition is referred to as standing wave or flutter echo. It is
avoided by constructing non-parallel surfaces or by adding absorptive materials to the
surface(s).

27. • Consider faceting the ceiling to help with sound dispersion.
• Control the reverberation time on the stage. Ideally, the reverberation time in the stage
area should be the same as in the house. Since the stage area might have a higher ceiling
than the rest of the auditorium, more absorptive materials might be required in this area.
Frequently, the back wall of the stage, and possibly one or two of the side walls, is treated
with an acoustically absorptive material, typically black in color.
• Remember the space will be less absorptive when only half full, since the audience itself is
absorptive. By using absorptive seating areas, the reverberation time will remain more
consistent regardless of the audience size.

28. • Noise from the lobby area can be disruptive. Be sure openings such as doorways are
properly sealed. Consider a vestibule door system.
• Persons seated deep under a balcony might experience auditory distortion. To avoid this, the
balcony should be no deeper than twice its height. Ideally, the balcony should not be any
deeper than its height.
• Even if everything else is controlled perfectly, the space might not be usable if the
background noise (e.g. HVAC system) is too loud. For good design, the NC level should not
exceed 20 to 35. When specifying NC, specify an actual rating, such as NC 20, rather than a
range, such as NC 20-30. Although specifying a lower number will ensure minimal
background noise, it might be cost prohibitive to achieve.

30. SEMINAR ROOMS
For Acoustic Treatment in conference halls and Seminar halls three basic steps kept in mind.
• 1. Sound isolation to avoid noise disturbance and to achieve good speech privacy.
• 2. Room finishes to create an acoustical environment conducive to discussion over the length of long
boardroom tables.
• 3. HVAC system noise control to reduce background noise and further provide for discussions over those
long tables.
A conference room is designed for two acoustic purposes:
• Confine sound within the walls so people on the outside cannot hear what is said inside.
• Create a good environment for conversations, both those made within and those made
through a speakerphone or similar.
• This means that the walls are thick and rigid – not letting much sound through and thus the sound is
trapped inside in the form of echoes or reverberations.
• These echoes pollute the acoustic environment on the inside and must therefore be eliminated –
something which is best done by sound absorption.
• Since the conversation that created these echoes consists of both high and low frequencies then the
echoes also consists of both high and low frequencies.
• Because of this both high and low frequencies alike need to be removed, or absorbed, and preferably
equally much of each.
• Aim for having at least 15% of the wall surface covered with panels for the best result (if there are no other
acoustic improvements).
• The best way is to either have them on all walls or on two walls out of four - this way there is no room for
echoes to bounce back and forth.