ACOUSTICS ASSIGNMENT

SUBMITTED BY-
KISHAN SIROHI
II YEAR (A)
B.ARCH
NOISE CONTROL IN BUILDINGS
INTRODUCTION
There are four major actions which can be taken to
improve noise compatibility for any type of land use or
activity. These are site planning, architectural design,
construction methods, and barrier construction.
Acoustical site design uses the arrangement of buildings
on a tract of land to minimize noise impacts by
capitalizing on the site’s natural shape and contours.
Open space, nonresidential land uses, and barrier
buildings can be arranged to shield residential areas or
other noise sensitive activities from noise, and residences
can be oriented away from noise.
Acoustical architectural design incorporates noise reducing concepts in the details of individual
buildings. The areas of architectural concern include building height, room arrangement, window
placement, and balcony and courtyard design.
Acoustical construction involves the use of building materials and techniques to reduce noise
transmission through walls, windows, doors, ceilings, and floors. This area includes many of the
new and traditional “soundproofing” concepts.
Noise barriers can be erected between noise sources and noise-sensitive areas. Barrier types
include berms made of sloping mounds of earth, walls and fences constructed of a variety of
materials, thick plantings of trees and shrubs, and combinations of these materials.
These physical techniques vary widely in their noise reduction characteristics, their costs, and
especially, in their applicability to specific locations and conditions. This section is not designed
to provide complete criteria for selecting a solution to particular noise problems and is not
intended as a substitute for acoustical design. Rather, its purpose is to illustrate the wide range
of possible alternatives which could be considered in the architectural and engineering planning
process. Knowledgeable municipal officials can provide valuable assistance to designers,
developers, and builders who may not be familiar with sound attenuation techniques that are
most applicable locally.

1.ACOUSTICAL SITE PLANNING

The arrangement of buildings on a site can be used to minimize noise impacts. If incompatible
land uses already exist, or if a noise sensitive activity is planned, acoustical site planning often
provides a successful technique for noise impact reduction.
Many site planning techniques can be employed to shield a residential development from noise.
These can include:
increasing the distance between the noise source and the receiver;
placing nonresidential land uses such as parking lots, maintenance facilities, and utility areas
between the source and the receiver;
locating barrier-type buildings parallel to the noise source or the highway; and
orienting the residences away from the noise.
The implementation of many of the above site planning techniques can be combined through
the use of cluster and planned unit development techniques.
Distance: Noise can be effectively reduced by increasing the NOISE BARRIER CAN
distance between a residential building and a highway. Distance SHIELD ONLY THE
LOWEST FLOOR OF A
itself reduces sound: doubling the distance from a noise source
BUILDING
can reduce its intensity. Distance itself reduces sound: doubling
the distance from a noise source can reduce its intensity by as
much as 6 dBA. In the case of high rise buildings, distance may
be the only means, besides acoustical design and construction,
of reducing noise impacts. This is because it is nearly impossible
to provide physical shielding for the higher stories from
adjacent noise.

Noise Compatible Land Uses as Buffers: Noise protection can be achieved by locating noise-
compatible land uses between the highway and residential units. Whenever possible,
compatible uses should be nearest the noise source.

Buildings as Noise Shields: Additional noise protection can be achieved by arranging the site
plan to use buildings as noise barriers. A long building, or a row of buildings parallel to a
highway can shield other more distant structures or open areas from noise. One study shows
that a two-story building can reduce noise levels on the side of the building away from the noise
source by about 13dBA.
If the use of the barrier building is sensitive to highway noise, the building can be
soundproofed. This technique was used in a housing project under construction in England
where a 3,900 foot long, 18 foot wide and 45-70 foot high wall (depending on the terrain)
serves as both residence and a sound shield.
The wall/building will contain 387 apartments in which the kitchens and bathrooms are placed
towards the noise, and the bedrooms and living rooms face away from the highway. The wall
facing the highway will be soundproofed and windows, when they exist, are sealed. Substantial
noise reductions are expected.

Orientation: The orientation of buildings or activities on a site affects the impact of noise, and
the building or activity area may be oriented in such a way as to reduce this impact.
Noise impacts can be severe for rooms facing the roadway since they are closest to the noise
source. The noise impact may also be great for rooms perpendicular to the roadway because a)
the noise pattern can be more annoying in perpendicular rooms and b) windows on
perpendicular walls do not reduce noise as effectively as those on parallel walls because of the
angle of the sound. Road noise can be more annoying in perpendicular rooms because it is more
extreme when it suddenly comes in and out of earshot as the traffic passes around the side of
the building, rather than rising and falling in a continuous sound, as it would if the room were
parallel to passing vehicles.
Whether the noise impact is greater on the perpendicular or the parallel wall will depend on
the specific individual conditions. Once the most severely impacted wall or walls are
determined, noise impacts may be minimized by reducing or eliminating windows from these
walls.
Buildings can also be oriented on a site in such a way as to exploit the site’s natural features.
With reference to noise, natural topography can be exploited and buildings placed in low noise
pockets if they exist. If no natural noise pockets exist, it is possible to create them by excavating
pockets for buildings and piling up earth mounds between them and the noise. Such a structure
would obstruct the sound paths and reduce the noise impacts on the residences.
Cluster and Planned Unit Development: A cluster subdivision is one in which the densities
prescribed by the zoning ordinance are adhered to but instead of applying to each individual
parcel, they are aggregated over the entire site, and the land is developed as a single entity. A
planned unit development, or P.U.D., is similar but changes in land use are included, such as
apartments and commercial facilities in what would otherwise be a single-family district.

CONVENTIONAL GRID CLUSTER SUB

SUB DIVISION DIVISION

It can be seen how the conventional

grid subdivision affords no noise
protection from the adjacent highway.
The first row of houses bears the full
impact of the noise. In contrast, the
cluster and P.U.D. techniques enable
commercial uses and open space
respectively to serve as noise buffers.
A word of caution is necessary: in a
cluster development, the required open
space can be located near the highway
to minimize noise to the residences. PLACEMENT OF NOISE COMPATIBLE LAND USES NEAR
However, many recreation uses are HIGHWAY IN PLANNED UNIT DEVELOPMENT
noise sensitive, and when one takes
advantage of the flexibility of cluster
development to minimize noise, care
must be taken not to use all of the
available open space in buffer strips,
thus depriving the development of a
significant open space area. Where high
noise levels exist, a combination of
buffer strips and other techniques (such
as berms and acoustical sound
proofing) can be employed.
The flexibility of the cluster and
planned unit development techniques
allows many of the above site planning
techniques to be realized and effective
noise reduction achieved. IN CLUSTER DEVELOPMENT, OPEN SPACE CAN BE PLACED
NEAR THE HIGHWAY TO REDUCE NOISE IMPACTS ON
RESIDENCES
2.ACOUSTICAL ARCHITECTURAL DESIGN
Noise can be controlled in a building with proper architectural design. By giving attention to
acoustical considerations in the planning of room arrangement, placement of windows, building
height, balconies, and courtyards, the architect may achieve significant noise impact reduction,
without the need for costly acoustical construction.

Solid Walls: Noise can be reduced by eliminating

windows and other openings from the walls of a
building close to noise sources. The solid wall can
then have the effect of a sound barrier for the
rest of the building. As previously discussed
,walls directly adjacent, and those perpendicular
to the noise source can be the most severely
impacted. When a solid wall is impractical,
illegal, or highly undesirable; the same effect can
be achieved by reducing window size and sealing
windows airtight. This technique is used in the
housing project described above.

One Story Houses: In cases where either the

house or the highway is slightly recessed or a
barrier has been placed in the sound path, the
noise impact may be further reduced if the
house has only one story (See Figure). Use of acoustical architectural design to reduce
noise impacts on more noise sensitive living spaces
If the single-story design is inefficient, the split-
level design may be effective. In any case the
path of the sound waves should be assessed
before the building design is drawn.

Noise impacts can be reduced by use of single story houses.

Balconies: If balconies are desired they should be given acoustical consideration. The standard
jutting balcony, facing the road, may reflect traffic noise directly into the interior of the building
in the manner illustrated in Figure 4.10. In addition to reflecting noise into the building, the
balcony may be rendered unusable due to the high noise levels. This problem is particularly
applicable to high rise apartment buildings where balconies are common. If balconies are
desired, the architect may avoid unpleasant noise impacts by placing them on the shielded side
of the buildings.

THE STANDARD JUTTING BALCONY FACING THE ROAD MAY REFLECT TRAFFIC NOISE DIRECTLY INTO THE
INTERIOR OF THE BUILDING.

Courtyards: Proper architectural design may also provide for noise reduction in an area outside
of the building. The court garden and patio houses can provide outdoor acoustical privacy. (See
Figure 4.11). Schools, rest homes, hotels, and multi-family apartment dwellings can also have
exterior spaces with reduced noise by means of court yards.

USE OF COURTYARD HOUSE TO OBTAIN QUITE OUTDOOR ENVIRONMENT

3. Acoustical Construction
Noise can be intercepted as it passes through the walls, floors, windows, ceilings, and doors of a
building. Examples of noise reducing materials and construction techniques are described in the
pages that follow.
To compare the insulation performance of alternative constructions, the Sound Transmission
Class (STC) is used as a measure of a material’s ability to reduce sound. Sound Transmission Class
is equal to the number of decibels a sound is reduced as it passes through a material. Thus, a
high STC rating indicates a good insulating material. It takes into account the influence of
different frequencies on sound transmission, but essentially it is the difference between the
sound levels on the side of the partition where the noise originates and the side where it is
received. For example, if the external noise level is 85 dB and the desired internal level is 45 dB, a
partition of 40 STC is required. The Sound Transmission Class rating is the official rating endorsed
by the American Society of Testing and Measurement. It can be used as a guide in determining
what type of construction is needed to reduce noise.
A) Walls
Walls provide building occupants with the most protection from exterior noise. Different wall
materials and designs vary greatly in their sound insulating properties.

• Increase the mass and stiffness of the wall.

In general, the denser the wall material, the more it will reduce noise. Thus, concrete walls are
better insulators than wood walls of equal thickness. Increasing the thickness of a wall is another
way to increase mass and improve sound insulation. Doubling the thickness of a partition can
result in as much as a 6 dB reduction in sound.6 However, the costs of construction tend to limit
the feasibility of large increases in wall mass.
The relative stiffness of the wall material can influence its sound attenuation value. Care must be
taken to avoid wall constructions that can vibrate at audible frequencies and transmit exterior
sounds.
• Use cavity partitions.
A cavity wall is composed of two or more layers separated by an airspace. The airspace makes a
more effective sound insulator than a single wall of equal weight, leading to cost savings.

• Increase the width of the airspace.

A three-inch airspace provides significant noise reduction, but increasing the spacing to six
inches can reduce noise levels by an additional 5 dBA. Extremely wide air spaces are difficult to
design.

• Increase the spacing between studs.

In a single stud wall, 24 inch stud spacing gives a 2-5 dB increase in STC over the common 16 inch
spacing.

• Use staggered studs.

Sound transmission can be reduced by attaching each stud to only one panel and alternating
between the two panels.

• Use resilient materials to hold the studs and panels together.

Nails severely reduce the wall’s ability to reduce noise. Resilient layers such as fiber board and
glass fiber board, resilient clips, and semi-resilient attachments are relatively inexpensive, simple
to insert, and can raise the STC rating from 2-5 dB.

• Use dissimilar leaves.

If the leaves are made of different materials and/or thickness, the sound reduction qualities of
the wall are improved.

• Add acoustical blankets.

Also known as isolation blankets, these can increase sound attenuation when placed in the
airspace. Made from sound absorbing materials such as mineral or rock wool, fiberglass, hair felt
or wood fibers, these can attenuate noise as much as 10 dB.10 They are mainly effective in
relatively lightweight construction.

• Seal cracks and edges.

If the sound insulation of a high-performance wall is ever to be realized, the wall must be well
sealed at the perimeter. Small holes and cracks can be devastating to the insulation of a wall. A
one-inch square hole or a 1/16 inch crack 16 inches long will reduce a 50 STC wall to 40.
This Figure shows a sample of
wall types ranging from the
lowest to the highest sound
insulation values. The cost of
these walls in dollars per square
foot is given for comparison of
cost effectiveness.
B) Windows
Sound enters a building through its acoustically weakest points, and windows are one of the
weakest parts of a wall. An open or weak window will severely negate the effect of a very strong
wall. Whenever windows are going to be a part of the building design, they should be given
acoustical consideration. Figure 4.14 illustrates the effects of windows on the sound transmission
of walls. For example, if a wall with an STC rating of 45 contains a window with an STC rating of
26 covering only 20% of its area, the overall STC of the composite partition will be 33, a
reduction of 12 dB.
4.14 Graph for calculating STC of composite barriers.
The following is a discussion of techniques that can be used to reduce noise in a building by
means of its windows. These techniques range from a blocking of the principal paths of noise
entry to a blocking of the most indirect paths.
Close windows: The first step in reducing unwanted sound is to close and seal the windows. The
greatest amount of sound insulation can be achieved if windows are permanently sealed.
However, openable acoustical windows have been developed which are fairly effective in
reducing sound. Whether or not the sealing is permanent, keeping windows closed necessitates
the installation of an air-conditioning system. The air conditioning system may in addition
provide some masking of noise. (Masking is discussed below). If windows must be openable,
special seals are available which allow windows to be opened.
Reduce window size: The smaller the windows, the greater the transmission loss of the total
partition of which the window is a part. Reducing the window size is a technique that is used
because (a) it precludes the cost of expensive acoustical windows, and (b) it saves money by
cutting down the use of glass. The problems with this technique are (a) it is not every effective in
reducing noise; e.g., reducing the proportion of window to wall size from 50% to 20% reduces
noise by only 3 decibels; and (b) many building codes require a minimum window to wall size
ratio.
Increase glass thickness: If ordinary windows are insufficient in reducing noise impacts in spite of
sealing techniques, then thicker glass can be installed. In addition, this glass can be laminated
with a tough transparent plastic which is both noise and shatter resistant. Glass reduces noise by
the mass principle; that is, the thicker the glass, the more noise resistant it will be. A 1/2-inch
thick glass has a maximum STC rating of 35 dB compared to a 25 dB rating for ordinary 3/16-inch
glass.
However, glass thickness are only practical up to a certain point, when STC increases become too
insignificant to justify the cost. For example, a 1/2-inch-thick glass can have an STC of 35;
increasing the thickness to 3/4 inch only raises the STC to 37. However, a double glass acoustical
window consisting of two 3/16-inch-thick panes separated by an airspace will have an STC of 51
and can cost less than either solid window.
In addition to thickness, proper sealing is crucial to the success of the window. To prevent sound
leaks, single windows can be mounted in resilient material such as rubber, cork, or felt.
Install Double-Glazed Windows: Double-glazed windows are paired panes separated by an
airspace or hung in a special frame. Generally, the performance of the double-glazed window
may be increased with:
increased airspace width
increased glass thickness
proper use of sealing
slightly dissimilar thickness of the panes
slightly non-parallel panes
In general, the airspace between the panes should not be less than 2-4 inches if an STC above 40
is desired. If this is not possible, a heavy single-glazed window can be used. The use of slightly
non-parallel panes is a technique employed when extremely high sound insulation is required,
such as in control rooms of television studios.
The thickness of double-glazed panes may vary from 1 /8 to 1 /4 inch or more per pane.
Although thickness is important, the factors which most determine the noise resistance of the
window is the use of sealant and the width of the airspace.
As in the case of all windows, proper sealing is extremely important. To achieve an STC above 43,
double-glazed windows should be sealed permanently. If the windows must be openable, there
are available special frames and sealers for openable windows which allow a maximum STC of
43.
Permanently sealed double-glazed windows often require an air pressure control system to
maintain a constant air pressure and minimal moisture in the airspace. Without this system, the
panes may deflect, and, in extremely severe cases, pop out of the frames.
To further insure isolation of noise between double-glazed panes, the panes could be of different
thicknesses, different weights, and slightly non-parallel to each other. This prevents acoustical
coupling and resonance of sound waves.
C) Doors
Acoustically, doors are even weaker than windows, and more difficult
to treat. Any door will reduce the insulation value of the surrounding
wall. The common, hollow core door has an STC rating of 17 dB.
Taking up about 20% of the wall, this door will reduce a 48 STC wall
to 24 STC. To strengthen a door against noise, the hollow core door
can be replaced by a heavier solid core door that is well sealed and is
relatively inexpensive. A solid core door with vinyl seal around the
edges and carpeting on the floor will reduce the same 48 STC wall to
only 33dB. An increased sound insulation value can be achieved if
gasketed stops or drop bar threshold closers are installed at the
bottom edge of the door. The alternative solution to doors is to
eliminate them whenever possible from the severely impacted walls
and place them in more shielded walls.

D) Ceilings
Acoustical treatment of ceilings is not usually necessary unless the
noise is extremely severe or the noise source is passing over the
building. The ordinary plaster ceiling should provide adequate sound
insulation except in extremely severe cases. An acoustically weak
ceiling which is likely to require treatment is the beamed
ceiling.Beamed ceilings may be modified by the addition of a layer of
fiberglass or some other noise resistant material. Suspended ceilings
are the most effective noise reducers but they are also the most
expensive.

E) Floors
In the case of highway noise, floors would only require acoustical treatment if the highway were
passing under the building. In this case, flooring would have to provide protection against
structural vibrations as well as airborne sound.
Two ways to insulate a floor from noise are to install a solid concrete slab at least 6 inches thick
or install a floating floor. In general, the floating floor gives the greatest amount of sound and
vibration insulation; however, it is extremely expensive. Basically, a floating floor consists of a
wood or concrete slab placed over the structural slab, but separated by a resilient material. The
resilient material isolates the surface slab from the structural slab and the surrounding walls.

F) Interior Design
Overall interior noise levels can be reduced by the extensive use of thick, heavy carpeting,
drapes, wall hangings, and acoustical ceiling tiles. These materials absorb sound. They cannot
prevent noise from coming through the walls, but they can reduce overall sound levels by
reducing sound reverberations.

G) Masking
Another way of coping with noise is to drown it out with background noise. This technique is
known as masking. It can be very effective in reducing noise fluctuations which are often the
most annoying aspects of noise. Masking can be produced by air conditioning and heating
systems, soft music, or electronic devices.
4 .Barriers
A noise barrier is an obstacle placed between a noise source and a receiver which interrupts the
path of the noise. They can be made out of many different substances:

• sloping mounds of earth, called berms

• walls and fences made of various materials including concrete, wood, metal, plastic, and
stucco
• regions of dense plantings of shrubs and trees
• combinations of the above techniques

The choice of a particular alternative depends upon considerations of space, cost, safety and
aesthetics, as well as the desired level of sound reduction. The effectiveness of the barrier is
dependent on the mass and height of the barrier, and its distance from the noise source and the
receiver. To be effective a barrier must block the “line of sight” between the highest point of a
noise source, such as a truck’s exhaust stack, and the highest part of the receiver.

TO BE EFFECTIVE, A BARRIER MUST BLOCK THE “LINE OF SIGHT” BETWEEN THE HIGHEST POINT OF A NOISE
SOURCE AND THE HIGHEST PART OF A RECEIVER

To be most effective, a barrier must be long and continuous to prevent sounds from passing
around the ends. It must also be solid, with few, if any, holes, cracks or openings. It must also be
strong and flexible enough to withstand wind pressure.
Safety is another important consideration in barrier construction. These may include such
requirements as slope, the distance from the roadway, the use of a guard rail, and
discontinuation of barriers at intersections.
Aesthetic design is also important. A barrier constructed without regard for aesthetic
considerations could easily be an eyesore. A well-designed berm or fence can aesthetically
improve an area from viewpoints of both the motorist and the users of nearby land.

A) Earth Berms
An earth berm, a long mound of earth running parallel to the highway, is one of the most
frequently used barriers.

CROSS SECTION OF A BERM

Berms can range from five to fifty feet in height. The higher the berm, the more land is required
for its construction. Because of the amount of land required, a berm is not always the most
practical solution to highway noise. Different techniques must be applied in urban as distinct
from rural settings.
A berm can provide noise attenuation of up to 15 dBA if it is several feet higher than the “line of
sight” between the noise source and the receiver. This is comparable to the noise reduction of
various walls and fences which are used as barriers. However, earth berms possess an added
advantage: instead of reflecting noise from one side of the highway to another, as walls do,and
thus increasing the noise heard on the opposite side, they deflect sound upwards.

WALL BARRIERS MAY REFLECT SOUND FROM ONE SIDE OF THE HIGHWAY TO THE OTHER

The cost of building a berm varies with the area of the country and the nature of the project. In
California, the statewide average for building a berm is about $1 per cubic yard when the earth is
at the site.
In planning a berm, one must include seeding and planting in figuring cost. Also to be included
are land costs and maintenance in relation to erosion, drainage, snowplowing, mowing, and
perhaps future seeding. It costs approximately $1,000 per acre per year to maintain a berm
which is accessible to maintenance equipment.

B) Walls and Fences as Barriers

In addition to the more usual function of keeping people, animals and vehicles from entering the
highway right of way at undesired locations, a properly designed fence or wall can also provide
visual and acoustical separation between highway noise sources and adjacent land areas.

C) Plantings
Plants absorb and scatter sound waves. However, the effectiveness of trees, shrubs, and other
plantings as noise reducers is the subject of some debate. Some conclusions can, however, be
drawn:

• Plantings in a buffer strip, high, dense, and thick enough to be visually opaque, will provide
more attenuation than that provided by the mere distance which the buffer strip represents.
A reduction of 3-5 dBA per 100 feet can be expected. Shrubs or other ground cover are
necessary in this respect to provide the required density near the ground.
• The principal effect of plantings is psychological. By removing the noise source from view,
plantings can reduce human annoyance to noise. The fact that people cannot see the
highway can reduce their awareness of it, even though the noise remains.
• Time must be allowed for trees and shrubs to attain their desired height.
• Because they lose their leaves, deciduous trees do not provide year-round noise protection.
In general, plantings by themselves do not provide much sound attenuation. It is more effective,
therefore, to use plantings in conjunction with other noise reduction techniques and for
aesthetic enhancement.

D) Combinations of Various Barrier Designs

• Often, the most economical, acoustically acceptable, and aesthetically pleasing barrier is
some combination of the barrier types previously discussed.
• For example, the Milwaukee County Expressway and Transportation Commission feels that
barriers constructed of precast concrete on top of an earth berm provide maximum benefit
for the cost.They estimate that such a combination costs $51 per linear foot.
• In addition to cost advantages, an earth berm with a barrier wall on top of it possesses
several other advantages over both a wall or a berm alone:

1) it is more visually pleasing than a wall of equivalent height;

2) the berm portion of this combination is less dangerous for a motorist leaving the roadway;
3) the non-vertical construction of the berm does not reflect noise back to the opposite side of
the highway the way a wall does
4) the combination requires less land than would be required for a berm of equivalent height
and slope; and 5) the wall provides a fencing function not provided by a berm.

• Another combination to be considered is that of plantings in combination with a barrier.

Not only do plantings and ground cover provide some additional noise attenuation, but
they also increase visual appeal.

COMMON ACOUSTIC DEFECTS IN AUDITORIUM

List of acoustical defects:

• Reverberation
• Formations of echoes
• Sound foci
• Dead spots
• Insufficient loudness
• Exterior noises
REVERBERATION

• Reverberation is the persistence of sound in the enclosed space , after the source of sound
has stopped.
• Reverberant sound is the reflected sound , as a result of improper absorption.
• Reverberation may results in confusion with the sound created next.
• However some reverberation is essential for improving quality sound.
• The time during which the sound persists is called the reverberation time of sound in the
hall.
• As per Prof. W .C. Sabins reverberation time ‘t’ is given by formula :-
t= 0.16V /A
where
V=volume of room in cubic meters
A= total absorbing power of all the surfaces of room/ hall.

Reverberation time & quality of sound

• Reverberation time should remain within
limits as per Indian Standard Code: 2526-
1963.

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.

FORMATION OF ECHOES

• Not all sound that hits matter is absorbed. Some of it is reflected. That means sound bounces
off the solid matter the way a tennis ball bounces off a wall. Sound reflected back to its
source is an echo.
• An echo is produced when the reflected sound wave reaches the ear just when the original
sound from the same source has been already heard.
• Thus there is repetition of sound.
• The sensation of sound persists for 1/10th of a second after the source has ceased.
• Thus an echo must reach after 1/10th second of the direct sound
• Multiple echoes may be heard when a sound is reflected from a number of reflecting
surfaces placed suitably.
• This defect can be removed by selecting proper shape of the hall .
• And by providing rough and porous interior surfaces to disperse the energy of 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.

SOUND FOCI

• Some times shape of the hall makes sound waves to

concentrate in some particular areas of hall creating a
sound of large quality.
• These spots are called sound foci.
• This defect can be removed by :
a. Geometrical design shapes of the interior faces.
b. Providing highly absorbent materials on critical areas
(curved spaces)

DEAD SPOTS

• This defect is the out come of formation of sound foci.

• Because of high concentration of reflected sound at
sound foci , there is deficiency of related sound at
some other points.
• These spots are known as dead spots where sound
intensity is so low that it is insufficient for hearing.
• This defect can be removed by suitably placing
diffusers and reflectors.
• Right proportions of internal spaces.
TECHNIQUES THAT CAN BE EMPLOYED TO IMPROVE ACOUSTICS IN
INTERIOR OF A RECORDING STUDIO
A recording studio is an assemblage of equipment, spaces and persons such that a performance
in sound may be created and recorded onto a medium for later reproduction.
The desired acoustic properties of a recording studio are in many ways the opposite of those of
an auditorium. Instead of enhanced reverberation, it is usually desirable for the recording studio
to be acoustically "dead", having a very short reverberation time.

RECORDING STUDIO ACOUSTICS - IMPORTANCE

• Not only does this require the enclosure itself to be very absorbent of sound, but
soundproofing becomes very important.
• In order to prevent the passage of low frequency sounds such as traffic noise, aircraft noise,
etc., the recording enclosure is often isolated from the main structure with a double wall.
• Since low frequency sounds are much more efficiently borne by solid structures than high
frequencies, the suspended "room within a room" strategy minimizes the structural linking of
the recording room to the foundation of the building.
• Careful sealing of the enclosure and careful design of the heating and air- conditioning system
are necessary.
• Sometimes additional bass traps are employed to further reduce low- frequency background.

TECHNIQUES
Recording rooms or mixing rooms should be symmetrical
to ensure reflections are identical on the left and right
sides. This is necessary to get an accurate stereo image.
Ideally, the room should also not have any parallel walls
(or parallel floor and ceiling) that can cause standing
waves.
Standing waves occur when certain
frequencies with wavelengths proportional
to the room dimensions bounce back onto
themselves, creating dips and peaks in the
amplitudes of these frequencies due to
phase cancellation and amplitude
summation. So in the worst case scenario,
you will not get an accurate representation
of the sound because the room will be
influencing the frequencies you hear, which
could vary greatly based on your physical
location in the space. Bass frequencies are
usually the biggest problem. But in addition
to the lowest standing wave or resonant
frequency, the resulting harmonics below
will exhibit standing wave behavior.
However, few of us have rooms without parallel walls or the funds to construct modifications.
More likely are rectangular situations with doors, windows or closets in troublesome locations.
These irregularities need to be minimized as much as possible, perhaps with the use of heavy
curtains, or panels that fit on top of the obstruction, etc.
Mixing And Recording Desk Position

Rather than addressing the concerns of window and door locations,

etc. (which will vary from room to room), let’s talk about how to best
mediate acoustical problems in common rectangular shaped rooms.
In any rectangular situation you have a few position choices for your
mixing desk as seen below.
Choice 1 is problematic because although it is centered in the room,
the early reflections will arrive with more delay than is necessary
due to an increased distance from the side walls.
Choice 2 is problematic because placement is not symmetrical in the
room and reflections will be unbalanced.
Choice 3 is also problematic because its placement is not
symmetrical in the room and reflections will be unbalanced.
Choice 4 is optimal placement.
So in a rectangular room, it is advisable to position your mixing desk
centered on one of the short walls.

Speaker Position
Bass Traps and Absorption
Panels

Add bass traps to alleviate bass

buildup in the corners.
Positioning absorption panels
spaced out away from the wall
will increase their effectiveness in
absorbing low end frequencies.
Position side wall absorption
panels using the mirror
technique:
Have a friend take a mirror and
move it flat along the wall starting
at the front. Rotate your head to
follow the mirror without moving
from the listening position. If you
can see a speaker in the mirror,
that is a point of reflection and a
perfect spot for an absorption
panel.
Panels should be placed in
identical locations on the left and
right to ensure balanced
reflections.

Diffusion Panel and Cloud Positioning

Add a diffusion panel to
the back wall to spread
out reflections evenly
throughout the room.
Finally, add a cloud
diffusion or absorption
panel at an angle on the
ceiling to alleviate
possible standing waves
between the floor and
ceiling.

DIY Diffusion Panels

Having a good balance of diffusion and absorption is essential. Too much absorption will give the
room an unnatural coffin-like sound.
Professionally made diffusers are often designed using a mathematically derived series of bumps
and wells intended to distribute and reflect incident waves equally throughout the room. The
idea is to prevent the majority of the wave’s energy from bouncing back onto itself creating the
standing wave problem.
There are many DIY diffuser types and patterns freely available online and if you have access to
the right tools you can make your own. Cost will be largely dependent on the type of wood and
overall size.
Poor man’s diffuser: Position a large bookshelf centered on the back wall of your studio and
slightly pull out every other book an inch or two. While this is by no means scientific, it will
certainly do some diffusion and will definitely be better than a flat open wall
.
Speaker Isolation

Speakers stands are always a good idea and there are many available that will help to isolate
vibration and transference (mostly a problem with low end frequencies).
But if you don’t have stands and are forced to place speakers on the desk itself, be sure to place
them on acoustic foam. The Auralex Mopads can be purchased for about $40 a set. Even a piece
of styrofoam is better than nothing in a pinch.
Failure to isolate speakers will create low end resonance not present in the original sound.
CONSTRUCTION TECHNIQUES
Walls
• Building a home recording studio requires good walls.
• The most effective soundproofing must be designed into a house when it is first built.
• A typical residential wall is made of a frame of 2x4 wood studs covered with 5/8" thick gypsum
board. Doubling the thickness of gypsum gives another 3 to 6 dB of overall isolation, but its most
important effect is lowering the resonant frequency, hopefully below the audio range.
• There are two common strategies for reducing coupling between the two sides of the wall.
One is to make the gypsum to stud connection springy, either by using metal studs or by hanging
the gypsumboard on resiliant metal bars.
• The most effective trick is to use separate studs for each face of the wall so there is no direct
connection.
• This eats up a lot of space, but can give a transmission loss of over 60 dB. This is actually better
performance than simple cinder block or poured concrete construction

The picture shows the blue wall studs attached to one side and the green ones to the other
(diagram is looking down at the top of the wall).
INSULATION

Key idea to consider: fill the walls with cellulose insulation . The
result is much more sound blocking. Showing cellulose in the
walls and fiberglass in the floor.ALL the joints where studs meet
the outside wall. This makes the wall airtight, and increases the
sound proofing.

FLOORS

This is an effective way to decouple the floor from the structure,

helping to eliminate noises traveling in the frame of a building.

WINDOWS
SOUND ABSORPTION MATERIALS
When sound waves strike a surface some of the sound is absorbed by friction. The materials that
absorb are known as absorbants. Absorption in auditoriums or theatre the takes place in three
ways –

In Air – This absorption happens due to friction between two molecules though this is very small
By Audience — Cloth is an absorbant. So in theatres and auditoriums the sound is absorbed by
the clothes o f the audience. The more the audience the more the absorption takes place.
By furniture and furnishing – Materials such as curtains, carpets absorb sound.

Requirements of a good acoustic material-

Should have high absorption power

It should be able to absorb a wide range of frequencies
Should be cheap and easily available
Should look attractive after fixing it
It should be fire resistant
Should have adequate structural strength
Should be non- hygroscopic.
It should be insects and termites free

Types of sound absorbing materials according to their physical composition,

Porous materials
Non- perforated or membrane absorbers.
Cavity or helmholtz resonaters

Porous materials

• Basic acoustical characteristic of all porous materials is a cellular network of minute

interlocking pores. They convert the incident sound energy into heat energy by the frictional
and viscous resistance within these pores and by vibration of their small fibres. Good for high
frequency range.
• Fibreboards, mineral wools, insulation blankets, etc. are some of the examples .

ROCK WHOOL GLASS WHOOL

Non perforated panels & membrane absorbers

• Any impervious material, installed on a solid

backing but separated from it by an air space, will
be set to vibration when struck by sound waves.
good for low frequency range.
• wood and hardboard panelling, gypsum boards,
suspended plaster ceilings, furred out plasters, rigid
plastic boards, windows, glazing, doors, wood floors
and plat-forms, etc
GYPSUM BOARD
Cavity or helmholtz resonaters

• They consist of an enclosed body of air confined

within rigid walls and connected by a narrow
opening(called the neck) with the surrounding
space in which the sound waves travel.
• Cavity resonators can be applied

1.as individual units

2. as perforated panel resonators
3. as slit resonator panels.
Types of sound absorbing materials according to their trade name and availability in market,

hairfelt,
acoustic plaster,
acoustical tiles,
strawboard,
pulp boards,
compressed fibreboards,
compressed wood particle boards,
perforated plywood,
wood wool board,
quilts and mats.

HAIRFELT:
The material was used by prof. Sabin in his experimental works, made up of wool and fur.
The average value of coefficient of absorption of 25mm thick hairfelt is 0.60.
It is used at rooms and halls.
ACOUSTIC PLASTER
Also known as the fibrous plaster and it includes granulated
insulation insulation material mixed with cement.
The average value of coefficient of absorption of 20mm thick
and density of 1kN/m3 acoustic plaster is 0.30 at 500 cps.
For acoustical plaster boards- 0.15-0.30

ACOUSTICAL TILES
They are factory made, uniform sound absorbers, easily
fixable but expensive.
Most suitable for rooms in which small area is available for
the acoustical treatment.

STRAWBOARD
Prepared from compressed straw and covered with thick
paper or hardboard are used.
The average value of coefficient of absorption of 13mm thick
and density of 12.4kN/m3 is 0.3 at 500 cps.
It is used at rooms and medium capacity halls.

PULPBOARDS
These are the softboards prepared from compressed pulps.
The average value of coefficient of absorption of 13mm thick,
is 0.17 at 500 cps.
They are cheap and can be fixed by ordinary panelling.

COMPRESSED FIBREBOARD
Made up of compressed fibre, may be perforated or
unperforated.
The average value of coefficient of absorption of perforated
one with the density of 3kN/m3 is 0.30 and of other one is
0.52.

COMPRESSED WOOD PARTICLE BOARD

Prepared from compressed wood and provided with
perforations and it can be painted also.
The average value of coefficient of absorption of 13mm thick
is 0.40 at 500 cps.
PERFORATED PLYWOOD
Can be used by forming composite panels with mineral wool
and cement asbestos or with mineral wool and hardboard.
Generally suspended from trusses.
The average value of coefficient of absorption for the former
one is as high as 0.95 and for the latter one, it is about 0.20.

WOOD WOOL BOARD

Prepared from a mixture of portland cement and wood wool or
wood shavings. A small quantity of Gypsum is sometimes added.
The average value of coefficient of absorption of 25mm thick
and density of 4kN/m3 is 0.20 at 500 cps.

QUILTS AND MATS

Prepared from mineral wool or glass wool and are fixed in the
form of acoustic blankets
The absorption coefficients of such quilts and mats depend on
the tickness, density, perforations, mode of fixing, nature of
backing and frequency of sound.

SOUND INSULATION MATERIALS

Sound insulation/sound proofing is a method used to subdue the level of sound passing through
the insulating building component. People generally confuse sound absorption and sound
insulation to be the same. The two are very different from each other. Sound absorbents which
are mostly pours materials absorb thus reduce the sound which is reflected from surfaces. On
the contrary sound insulating construction reduces sound passing through it. Sound absorbers,
are poor sound insulators. While hard material used for sound insulation are inferior sound
absorbers.

Sound Insulating Materials –

Non porous rigid partitions- The sound insulation of non-porous rigid constructions such as
plastered solid brick masonry walls varies. It depends upon the weight per unit area. There is a
point when it requires major increase in thickness to provide small increase in sound insulation.

Porous Rigid Materials — Porous concrete masonry and cinder concrete are some examples of
porous rigid materials. They provide 10 percent higher insulation as compared to non-porous
rigid partitions due to their sound absorptive quality. To enhance the results of insulation it is
recommended that porous partitions should be plastered on at least one side and if possible
then on both sides.

Flexible Porous Material — These materials provide low insulation. Even lower than rigid
materials. Flexible porous materials consists of mineral wool, quilt etc. To increase insulation
rigid materials and porous absorbers can be combined together and then applied. This will
produce better insulation per unit area.
WALL INSULATION

Walls are a vertical barrier of sound. Proper construction of walls can increase the level of sound
insulation. Construction of walls for sound insulation can be of four types –

Rigid Homogeneous Walls- Stone, brick or concrete masonry constructions come under this
section. The sound insulation in these walls depends upon their weight per unit area. Sound
insulation in these increases if the thickness of wall increases. Due to this these walls become
uneconomical and bulky after a certain limit.
Partition Walls of porous materials- These can be rigid or non-rigid. Rigid porous materials such
as porous concrete masonry, cinder concrete etc. increase insulation about 10.%. While partition
walls of non rigid porous materials provide very low sound insulation. However they can be used
if combined with rigid materials.
Double wall partition – A double wall partition comprises of Plaster boards Or fibre boards or
plaster on laths on both sides. With sound absorbing cushion in between. Rough wooden blocks
are provided to support the cushion. Double wall partition is a wall of rigid as well as non–rigid
porous materials.
Cavity Wall Construction- This is said to be the most adequate wall construction from the sound
proofing point of view. In this two walls are made with a gap of minimum 5cm between them.
This gab can be left air filled as we know sound travels the slowest in air or can be filled with
some flexible material, like quilt etc. On the surface of the wall celotex or other insulating board
may be fixed.

An example of wall insulation

FLOOR AND CEILING INSULATION

Like walls are vertical sound barriers similarly floor and ceiling are horizontal sound barriers. The
materials used for the construction of floors and ceiling i.e. R.C.C, stone etc. provide great
insulation again air borne noise but do not function well for structure and impact borne noise.
The target of sound proofed floors is to provide insulation against impact and structure borne
noise.
This can be achieved by the following ways-
APPLING RESILIANT SURFACE MATERIALS ON FLOOR – In this a thin concrete layer is provided as
R.C.C floor slab. On top of this then a soft floor finish or covering is applied. This finish or
covering can be of linoleum, insulation board, cork, carpet, etc. this helps in reducing impact
noises.
CONCRETE FLOOR FLOATING CONSTRUCTION – This is similar to Cavity wall construction as
discussed earlier. In this method we construct an isolated floor from the existing concrete floor.
Then a resilient or porous material like glass wool is laid on the R.C.C flooring. On top of this a
water proof sheet is put and then a 5cm thick layer of concrete is given. This kind of construction
provides complete insulation against impact sounds.
TIMBER FLOOR FLOATING CONSTRUCTION – In case of wooden flooring the problem of sound
insulation is even more. This process is similar to concrete floor floating construction. The only
difference is that mineral or glass wool quilts are used for isolation purpose. At times sand or
ashes are also used for isolation.

An example of ceiling insulation

WAVE THEORY OF SOUND
Sound is produced when something vibrates. The vibrating body causes the medium (water, air,
etc.) around it to vibrate. Vibrations in air are called traveling longitudinal waves, which we can
hear. Sound waves consist of areas of high and low pressure called compressions and
rarefactions, respectively. Shown in the diagram below is a traveling wave. The shaded bar above
it represents the varying pressure of the wave. Lighter areas are low pressure (rarefactions) and
darker areas are high pressure (compressions). One wavelength of the wave is highlighted in red.
This pattern repeats indefinitely. The wavelength of voice is about one meter long. The
wavelength and the speed of the wave determine the pitch, or frequency of the sound.
Wavelength, frequency, and speed are related by the equation speed = frequency * wavelength.
Since sound travels at 343 meters per second at standard temperature and pressure (STP), speed
is a constant. Thus, frequency is determined by speed / wavelength. The longer the wavelength,
the lower the pitch. The 'height' of the wave is its amplitude. The amplitude determines how
loud a sound will be. Greater amplitude means the sound will be louder.

Interference
When two waves meet, there can be two kinds of interference patterns; constructive and
destructive. Constructive inteference is when two waveforms are added together. The peaks add
with the peaks, and the troughs add with the troughs, creating a louder sound. Destructive
interference occurs when two waves are out of phase (the peaks on one line up with troughs on
the other). In this, the peaks cancel out the troughs, creating a diminished waveform. For
example, if two waveforms that are exactly the same are added, the amplitude doubles, but
when two opposite waveforms are added, they cancel out, leaving silence.
Standing Waves
Vibration inside a tube forms a standing wave. A standing wave is the result of the wave
reflecting off the end of the tube (whether closed or open) and interfering with itself. When
sound is produced in an instrument by blowing it, only the waves that will fit in the tube
resonate, while other frequencies are lost. The longest wave that can fit in the tube is
the fundamental, while other waves that fit are overtones. Overtones are multiples of the
fundamental. The areas of highest vibration are called antinodes (labeled 'A' on the diagram),
while the areas of least vibration are called nodes (labeled 'N' in the diagram). In an open pipe,
the ends are antinodes. However, in a pipe closed at one end, the closed end is a node, while the
blown end is an antinode. Thus, closed pipes yield only half the harmonics.

Transverse Waves
If a string that is fixed on both ends is bowed or plucked, such as in a violin, vibrations are formed
that are in a standing wave pattern, having nodes at the fixed ends, and an antinode in the
center. Several harmonics are also produced, in a similar way to the standing wave.

Overtones
Overtones are the other frequencies besides the fundamental that exist in musical instruments.
Instruments of different shapes and actions produce different overtones. The overtones combine
to form the characteristic sound of the instrument. For example, both the waves below are the
same frequency, and therefore the same note. But their overtones are different, and therefore
their sounds are different. Note that the violin's jagged waveform produces a sharper sound,
while the smooth waveform of the piano produces a purer sound, closer to a sine wave.
DEAD SPOTS
• This defect is the out come of formation of sound foci.
• Because of high concentration of reflected sound at sound foci , there is deficiency of related
sound at some other points.
• These spots are known as dead spots where sound intensity is so low that it is insufficient for
hearing.
• This defect can be removed by suitably placing diffusers and reflectors.
• Right proportions of internal spaces.

REVERBERATION

• Reverberation is the persistence of sound in the enclosed space , after the source of sound
has stopped.
• Reverberant sound is the reflected sound , as a result of improper absorption.
• Reverberation may results in confusion with the sound created next.
• However some reverberation is essential for improving quality sound.
• The time during which the sound persists is called the reverberation time of sound in the
hall.
• As per Prof. W .C. Sabins reverberation time ‘t’ is given by formula :-
t= 0.16V /A
where
V=volume of room in cubic meters
A= total absorbing power of all the surfaces of room/ hall.
Reverberation time & quality of sound

• Reverberation time should remain within

limits as per Indian Standard Code: 2526-
1963.

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.

FLUTTER ECHO
Flutter Echoes are produced by sound traveling quickly between two parallel reflective surfaces.
Also referred to as "zing" or "chatter," Flutter Echo can result in tone smearing through comb
filtering.

A partially treated room is normally to blame for Flutter Echo, wherein two portions of walls,
ceiling or floor are non-absorptive and face directly at one another. Depending on room size, the
nature of this echo can vary from a fast procession of separate, distinct sound events, to a
seemingly unbroken series of echoes. Except for rare use as an intentional sound effect, Flutter
Echo is generally not desirable in any space.
Solutions include Diffusion and Absorption.
ECHOES

• Not all sound that hits matter is absorbed. Some of it is reflected. That means sound bounces
off the solid matter the way a tennis ball bounces off a wall. Sound reflected back to its
source is an echo.
• An echo is produced when the reflected sound wave reaches the ear just when the original
sound from the same source has been already heard.
• Thus there is repetition of sound.
• The sensation of sound persists for 1/10th of a second after the source has ceased.
• Thus an echo must reach after 1/10th second of the direct sound
• Multiple echoes may be heard when a sound is reflected from a number of reflecting
surfaces placed suitably.
• This defect can be removed by selecting proper shape of the hall .
• And by providing rough and porous interior surfaces to disperse the energy of 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.

NOISE REDUCTION COEFFICIENT

The Noise Reduction Coefficient (commonly abbreviated NRC) is a scalar representation of the
amount of sound energy absorbed upon striking a particular surface.

An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption.

It is the arithmetic average, rounded to the nearest multiple of 0.05, of the absorption
coefficients for a specific material and mounting condition determined at the one octave band
centre frequencies of 250, 500, 1000 and 2000 Hz.
The absorption coefficients of materials are commonly
determined through use of standardised testing procedures,
such as ASTM C423[2] that is used to evaluate the absorption
of materials in eighteen one-third octave frequency bands
with centre frequencies ranging from 100 Hz to 5000 Hz.
Absorption coefficients used to calculate NRC are commonly
determined in reverberation rooms of qualified acoustical
laboratory test facilities.
NRC is most commonly used to rate general acoustical
properties of acoustic ceiling tiles, baffles, and banners, office
screens, and acoustic wall panels.