ARCHITECTURAL SERVICES – V (ACOUSTICS) (RAR – 806) MODULE-1 – BUILDING ACOUSTICS (COMMON ACOUSTICAL DEFECTS AND RECOMMENDED REMEDIES) Presented by Ar. Manish Kumar, Assistant Professor in Architecture Department at Axis Institute of Architecture

1. ARCHITECTURAL SERVICES – V (ACOUSTICS)(RAR – 806)
MODULE-1 – BUILDING ACOUSTICS
(COMMON ACOUSTICAL DEFECTS AND
RECOMMENDED REMEDIES)
AR. MANISH KUMAR AXIS SCHOOL OF ARCHITECTURE 1

2. CONTENTS
▪ ECHO,
▪ SOUND FOCI,
▪ DEAD SPOTS,
▪ SOUND SHADOWS,
▪ RESONANCE,
▪ INSUFFICIENT LOUDNESS,
▪ EXTERNAL NOISE
▪ REVERBERATION
▪ SABINE’S EXPRESSION FOR CALCULATION OF
REVERBERATION TIME
▪ ABSORBENTS AND ABSORPTION COEFFICIENT
AR. MANISH KUMAR AXIS SCHOOL OF ARCHITECTURE 2

3. COMMON ACOUSTICAL DEFECTS
▪ Acoustical conditions in a big room, ball or auditorium etc`. are achieved
when there is clarity of sound in every part of me occupied space.
▪ For this, the sound should rise to suitable intensity everywhere with no
echoes or near echoes or distortion of the original sound ; with correct
reverberation time.
COMMON DEFECTS :
(1) Echo
(2) Sound foci
(3) Dead spots
(4) Sound shadows,
(5) Resonance,
(6) Insufficient loudness
(7) External noise
(8) Reverberation
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ECHO:
• A SOUND REFLECTED OFF A
SURFACE THAT ARRIVES AT THE
LISTENER AFTER THE DIRECT
SOUND. SOMETIMES THOUGHT OF
AS REVERBERATION, BUT AN ECHO
IS VERY DISTINCT WHILE
REVERBERATION IS A MIXED
TOGETHER SOUND WHICH DECAYS
GRADUALLY.
• ECHO IS HEARD AS DISTINCT
REPEAT, 100 MILLI-SECONDS (1/10
SEC) OR GREATER, FROM WALLS
AND CEILING WITH PATH-LENGTHS
GREATER THAN 15 METERS (45FT)
APART.
REMEDIES: DEFECTS CAN BE
REMOVED BY - BY SELECTING PROPER
SHAPE OF THE HALL. - PROVIDING
ROUGH AND POROUS INTERIOR
SURFACES.
COMMON ACOUSTICAL DEFECTS

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• With practice this test can be
approximated with a single handclap, in a
quite room, as in the above graph. but with
continuous sound (music) reverberation
builds up and remains at a constant level.
SOUND FOCI:
• Reflecting concave surfaces cause
concentration of reflected sound waves at
certain spot, creating a sound of large
intensity.
These spots are called sound foci.
REMEDIES: This defect can be removed by
(a) geometrical designed shapes of the
interior faces, including ceilings
(b) providing highly absorbent materials on
focusing areas.
COMMON ACOUSTICAL DEFECTS

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DEAD SPOTS:
• This defect is an outcome of the formation of sound
foci.
• Because of high concentration of reflected sound at
sound foci, there is deficiency of reflected sound at
some other points.
• These points are known as dead spots.
• where sound intensity is so low that it is insufficient
for hearing.
COMMON ACOUSTICAL DEFECTS
REMEDIES: This defect can be
removed by
(a) installation of suitable diffuser so
that there is even distribution of
sound in the hall.
(b) Right proportions of internal
spaces.
(c) So that sound gets reflected in all
direction.
USE OF REFLECTORS AT
AUDITORIUM CEILING
PLACING DIFFUSERS
INSIDE A ROOM

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SOUND SHADOWS
• An acoustic shadow or sound shadow is an area through
which sound waves fail to propagate, due to topographical
obstructions or disruption of the waves via phenomena such
as wind currents, buildings, or sound barriers.
• Due to diffraction around the object, it will not be completely
silent in the sound shadow.
• Sound shadow are the area of poor audibility.
REMEDIES: This defect can be removed by
COMMON ACOUSTICAL DEFECTS
MINERAL WOOL FIBER TILES
RESONANCE
Resonance occurs due to the matching of frequency. If the
window panels and sections of wooden portions have not been
tightly fitted, they may start vibrating, thereby creating an extra
sound in addition to the sound produced in the hall or room.
REMEDY
The resonance may be avoided by fixing the window panels
properly. Any other vibrating object which may produce
resonance ban be placed over a suitable sound absorbing
material.

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INSUFFICIENT LOUDNESS
• This defect is caused due to
(a) Iack of sound reflecting flat surface near the sound source
(b) excessive sound absorption treatment in the hall.
REMEDIES: The defect can be removed by
(a) providing hard reflecting surface near the source,
(b) by adjusting the absorption of the hall so as to get optimum time of reverberation.
(c) When the length of the hall is more, it may be desirable to install loud speakers at
proper places.
EXTERNAL NOISE
External noise from vehicles, traffic engines , factories, cooling plants etc. may enter the
hall either through the openings (such as doors, windows, ventilators etc.) or through
walls and other structural elements having improper sound insulation. This defect can be
removed by a) proper planning of the hall with respect of its surroundings b) proper
sound insulation of exterior walls.
REMEDIES: The defect can be removed by proper planning of the hall with respect to its
surroundings and by proper sound insulation of external walls.
COMMON ACOUSTICAL DEFECTS

9. COMMON ACOUSTICAL DEFECTS
AR. MANISH KUMAR AXIS SCHOOL OF ARCHITECTURE 9
REVERBERATION
• It is 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.
• Excessive reverberation is one of the most common defect,
with the result that sound once created longs for a longer
duration resulting in confusion with the sound created next.
• However, some reverberation is essential for improving
quality of sound.

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COMMON ACOUSTICAL DEFECTS

11. COMMON ACOUSTICAL DEFECTS
AR. MANISH KUMAR AXIS SCHOOL OF ARCHITECTURE 11
REVERBERATION TIME: Is the time interval with in which ,the intensity of sound
produced, diminishes to one millionth of its original intensity.
• Depends on the size of room as if room is small reflections will take place quickly as
waves have to travel less distance, so time will be less.
• Plays and important role in achieving desirable acoustical conditions.
• In case of cinema halls r.t should be short.
RT60 IS TIME IN SECONDS FOR
REVERBERATION TO DIMINISH
TO - 60DB (1/1,000,000).
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.

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COMMON ACOUSTICAL DEFECTS

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COMMON ACOUSTICAL DEFECTS
SABINE’S EXPRESSION FOR CALCULATION OF REVERBERATION
TIME
Measuring the reverberation time of a space is a good way to identify a noise control
problem. If your large open space is plagued by echo and difficulty understanding speech,
you may have a reverberation problem. Here we will discuss how to calculate the
reverberation time for your multi-purpose use space.
Acoustic Calculation
The first step to calculate the reverberation time is to calculate the Sabins (a) with the below
equation.
Formula for Sabins:
a = Σ S α
Where:
Σ = sabins (total room absorption at given frequency)
S = surface area of material (feet squared)
α = sound absorption coefficient at given frequency or the NRC
After we calculate a, we can then use the Sabine Formula to calculate the reverberation
time.

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COMMON ACOUSTICAL DEFECTS
After we calculate a, we can then use the Sabine Formula to calculate the
reverberation time.
Sabine Formula:
RT60 = 0.049 V/a
Where:
RT60 = Reverberation Time
V = volume of the space (feet cubed)
a = sabins (total room absorption at given frequency)
Remember:
Units used in the formula id feet. For unit of meter the formula is modified as
follows….
RT60 = 0.161 V/a

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COMMON ACOUSTICAL DEFECTS
Location Material
Absorption Coefficient (a) or Sabins
Octave Band Center Frequencies
125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz
All Walls Gypsum
Board
0.29 0.10 0.06 0.05 0.04 0.04
Ceiling Metal
Deck
0.05 0.04 0.03 0.03 0.02 0.01
Floors Concrete
Painted
0.01 0.01 0.02 0.02 0.02 0.02
Ceiling Acoustic
Baffles
0.27 0.65 1.06 1.16 1.14 1.12

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COMMON ACOUSTICAL DEFECTS
Sr. No. Room Type RT60(sec)
1 Class rooms having volume @ 200m3 0.6
2 Class rooms having volume @ 300m3 0.7
3 Rooms like audio-visual room (Shorter
reverberation time is suitable)
0.4
4 Rooms like music practice room (Longer
reverberation time is suitable)
0.8
5 Spaces like gymnasiums having volumes @
5000m3
1.5

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COMMON ACOUSTICAL DEFECTS
➢ Example:
Calculate the reverberation time for a classroom having following details.
Dimensions.... Length=10m, Width=6m and height =4m Specifications....
Flooring... Terrazo, walls.... Brickwork and painted, ceiling... slab Wooden Doors.... 2 of
size 1.2m* 1.8m, Glass windows....4 of size 1.2m*1.8m No. of benches in class...70,
surface area of each bench....1m²
Calculate the reverberation time and comment.
➢ Solution:
➢ Floor area = 10m X 6m = 60 m²
➢ Ceiling area = 10m X 6m = 60 m²
➢ Door area = 2 X1.2mX1.8m=4.32 m²
➢ Window area =4X1.8mX1.2m= 8.64 m²
➢ Wall area = (32mX4m)- Door area -Window area = 115.04 m²
➢ Bench area = 70 X 1m2 = 70 m²
➢ Volume of the room = 10mX6mX4m= 240 m³

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COMMON ACOUSTICAL DEFECTS
Sr. No Item Area(m2) S α a
1 Floor area 60 m2 0.02 1.20
2 Ceiling area 60 m2 0.02 1.20
3 Door area 4.32 m2 0.3 1.30
4 Window area 8.64 m2 0.2 1.73
5 Wall area 115.04 m2 0.04 4.60
6 Bench area 70 m2 0.3 21.0
Σ…… 31.03
❖ RT60 = 0.161 V/a = 0.161X240/31.03 = 1.24 sec.
Comment.....
• Expected reverberation time is around 0.7 sec.
• So the efforts are to be taken to reduce reverberation time. Possible
measures may include use of curtains, use of carpets, Open-cell foams,
Acoustic Partitions.

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COMMON ACOUSTICAL DEFECTS
ABSORBENT:
When one substance enters the volume or bulk of another substance, this
process is Absorption. Solid soaks the liquid or gas rather than any forces
applied on molecules. The substance which gets absorbed is called
absorbate and the substance which absorbs is called the absorbent.
ABSORPTION COEFFICIENT
• The coefficient of absorption (a) of a material is defined as the ratio of
sound energy absorbed by the surface to that of total sound energy
incident on the surface.
• Since the open window is fully transmitting the sound incident on it, it is
considered as an ideal sound absorber.
• Thus the unit of absorption is the open window unit (OWU) and is named
as ‘sabine’ after the scientist who established the unit.
Absorption coefficient.pdf

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THANK YOU
Presented by Ar. Manish Kumar, Assistant Professor in
Architecture Department at Axis Institute of
Architecture