Architectural acoustics deals with controlling sound in buildings by managing how sound is transmitted, absorbed, diffracted or reflected. The goal is to reduce unwanted noise and improve listening conditions. Proper design of acoustical spaces like theaters and auditoriums is important for evenly distributing sound without defects like echoes or dead spots. The size, shape and orientation of rooms, as well as sound-absorbing and reflective materials used in construction, impact a building's acoustical qualities. Architectural acoustics provides guidance to designers on room acoustics and achieving the optimal reverberation time for different types of spaces.

1. PART I
Acoustic

2. INTRODUCTION
• ACOUSTICS is the science of sound and it deals
the origin, propagation and hearing sensation of
sound.
• The BASIC CONCEPT of the science of
ARCHITECTURAL ACOUSTICS is to reduce the un-
wanted sound generated by different sources
inside the building or outside the building and it
can improve the better listening conditions with
in the building where speech, music and lecture
need to be clear.

3. • The knowledge of this science of architectural
acoustics involves in the proper design of
some ACOUSTICAL BUILDINGS like Theaters,
Cinema halls, Auditoriums, Conference halls,
Class rooms, Lecture rooms, Hospitals etc for
equally distribution of sound, speech and
music with better quality.

4. • The Science of Architectural Acoustic deals
with the most advanced of planning and
construction for spreading of sound, speech
and music properly to the entire audience
with out effecting by any Acoustical Defects
like echo, reverberation, sound foci, sound
dead spots, insufficient loudness, sound
reflections, sound transmission etc.

5. • The Science of Architectural Acoustic can gives the
clear idea to the Architects, Engineers and
Designers about proper planning in terms of
required size, shape, volume and orientation of
each Individual rooms or Building in relation to the
surrounding areas.
• The Science of Architectural Acoustic can give the
knowledge about the construction and application
of different sound absorbent materials and their
use in certain acoustical buildings.

6. • The Science of Architectural Acoustic can give
clear idea to the architects or designer about
the use of different sound reflective materials
and their application, method of construction
etc. for distribution of sound in an enclosed
space for satisfactory hearing and
appreciation of performance of player,
speaker, audience etc.

7. Basic Theory of Acoustic
• Acoustic is the study of sound and the word
acoustic is derived from ancient Greek word
‘akouein’ meaning to hear. Hearing is the most
crucial means of survival alive in the animal
world and it is one of the most distractive
characters of human development and
culture.

8. Sound production
• Sound is the physical phenomenon (some
thing that is out of ordinary) that encourages
the sense of hearing. It is generated by
vibrated bodies in the form of waves of
compression and rarefaction in the air.

9. • These sound waves can travel from source of
origin by air in different directions and these
sound waves set the ear drum to vibrate then
the vibrating ear drum translates by the brain
in to sound sensation of hearing

10. • In humans, hearing takes place whenever
vibrations of frequencies from 15 hertz to about
20,000 hertz reach the inner ear.
• The hertz (Hz) is a unit of frequency equaling one
vibration or cycle per second. Sound has the
following characters
 When the sound waves are periodic, regular and
long continued, they produce pleasing effect of
sound and this sound is also called AUDIBLE
SOUND.

11.  When the sound waves are non-periodic,
irregular and very short duration, it produces
displeasing effect of sound and this sound is
also called AUDIBLE NOISE.

12. Sound waves are longitudinal waves and they
can move backward and forward along a line in
the direction of in which sound is traveling.
 The velocity of sound depends upon nature and
temperature of medium through which it travels.
The velocity of sound can be measured by meter
per second. See the velocity of sound through
different medium given bellow.

14. PROPAGATION OF SOUND
• When sound is generated in a place, it can move
or spread in all direction by air is called as
propagation of sound. Sound can propagate for
origin to in all directions like a person blowing up
a balloon with his mouth.
• See some of the sound propagation principles
given bellow;

15. • The propagation of sound energy through a media via
sound waves i.e. compression and rarefaction of sound
waves.
• The propagation of sound depends up on the
frequency of sound source and the capacity of listener
ear drum.
• The propagation of sound depends up on the
weather conditions like Air temperature, Presence of
Moisture, air velocity etc.
• The propagation of sound depends up on the
topographical features like ground cover, hills and
obstacles between the source and receivers, type of
space like open or closed.

17. Diffraction of Sound
• The bending of sound waves due to obstacles
called as diffraction of sound. Sound diffraction
causes decreasing of frequency, loudness, pitch
ate. Example if you brought a concert ticket
without looking seating arrangement chart. When
you enter inside the concert hall your seat will be
behind a huge pillar. After starting the film you
can not hear a clear voice due to the obstacle in
between you and the music source and because
of this the sound wave can bent around the pillar
before reaching you.

18. Hearing Sensitivity
• The ability of understanding or listening of
sound is called as sensitivity of hearing. The
hearing of sound depends up on person
eardrum, intensity of sound, type of sound
tone, distance between the source and the
listener, climatic conditions. Generally human
aural (hearing capacity) response is to be
vibrations between 15Hz to 20,000 Hz (hertz
or Hz is unit of frequency = one c/s) and the
high range of Hz is only for a young ear.

19. Physical Character of Sound
• Any type of audible sound has the following
physical characters;
1. Frequency or pitch
2. Loudness or intensity
3. Quality or timbre.

20. Frequency or Pitch
• It is the number of cycle or pressure vibrations
produced by a body in unit of time and it is
also called as pitch of sound.
• The greater the number of cycles or
vibrations, the higher will be the pitch. The
grater the frequency higher the pitch and the
lesser the frequency the lower the pitch.
• Frequency of sound is the measure of the
quality of sound.

21. • Frequency of sound can be measured in cycles
per second or hertz (Hz). The range of human
hearing is very wide extending from 20 Hz to
20000 Hz

22. Loudness or Intensity
• It is the flow of wave energy crossing per unit
time through unit area.
• Sound intensities are measured in decibels (dB).
For example, the intensity at the threshold of
hearing is 0 dB, the intensity of whispering is
typically about 10 dB.
• Sound intensities are arranged on a logarithmic
scale due to a wide range of variations of the
intensity of sound. For example two sounds of
the same character with different intensities I1 &
I2 are said to be differ by n bels.

23. • Quality or timber: It is the quality of musical
note. It is one of the important characters of the
sound that allows the ear to differentiate between
tones produced by different instruments when
the sound waves are identical in aptitude and
frequency.

24. Amplitude or Volume
• Amplitude is the characteristic of sound waves that
humans perceive as volume. The amplitude
corresponds to the distance that air molecules
move back and forth as a sound wave passes
through them. As the amount of motion in the
molecules is increased, they strike the ear drum
with progressively greater force. This causes the ear
to perceive a louder sound. This comparison of
samples at low, medium, and high amplitudes
demonstrates the change in sound caused by
altering amplitude. These three waves have the
same frequency, and so should sound the same
except for a perceptible volume difference.

26. • Measurement of sound: Generally sound can measure
in terms of pressure level or decibel (dB) and it is used as
convent unit to measure the magnitude of sound. By
measuring the sound the following objects we can get.
1. Sound measurement can helps us to improving of
building acoustics and loud speakers and thus improve
our enjoyment of music both in concert hall and home.
2. Sound measurement can indicates when sound may
causes
hearing damage and it helps in taking corrective measures
to be avoiding damage.
3. It permits evaluation of the hearing sensitivity of
individuals.
4. It permits the improvement of quality of our daily lives.

27. Behavior of sound in enclosure
• When the sound waves strike the surface of the
enclosure in the form of a room with walls, the
following three things going to be happen;
1. Some of the sound reflect back in the room
2. Some of the sound energy absorbed by the
surface of the room and the listener or persons
present in the room.
3. Some of the sound waves are transmitted outside
the room through the vibration of walls, floors
and ceiling.

28. • The amount of sound reflected or absorbed in the
enclosure depends up on type of surface of the
enclosure. While transmitted sound depends up
on the sound insulation property of the surface.

29. Reflection of Sound
• The Sound can reflect when a free sound wave
traveling through air and strike a uniform solid
plain surface in the same manner as that of a
light ray. The angle of the incident (strike wave)
makes with the plain surface is equal to the angle
which the reflected wave makes with the plane
surface.
• The study of law of reflection of sound helps in
selecting the proper shape of the room with
regarding to the distribution of sound in
enclosure.

30. The following facts in connection with the reflection of
sound from different curved surfaces to be noted;
• A concave surface leads to the concentration of
reflected sound waves at certain points and they are
considerably smaller in magnitude. This type of reflected
sound waves are most condensed(strong) and they are
amplified in nature. The concave surface may be used as
reflectors.

31. A convex surface tends to spread the reflected sound
waves which are magnified and considerably
bigger in magnitude. The convex surface may be used as
sound spreaders.

32. Defects due to reflection of sound
The following are the main defects developed due
to the reflection of sound.
1. Echoes
2. Sound foci
3. Sound dead spots
4. Insufficient loudness
5. Reverberation
6. Sound absorption
7. Sound transmission

33. 1.Echoes: An echo is a reflected sound wave.
The perceptible or long gap between the emission
(released sound) and repeat of the sound
represents the time it takes waves to travel to an
obstacle and back.
• The echoed sound is often fainter because not all
of the original waves are reflected. Generally,
echoes such as those heard in the mountains are
caused by sound waves striking large surfaces 30
or more away from their source.
• Formation of echoes normally happens when the
time lag between the two sounds is about 1/17th
of a second and the reflected surface located at a
distance greater then 15m form origin of sound.
This defect usually occurs when the shape of the
reflected surface is curved with smooth character.

34. 2. Sound Foci: In case of concave shaped reflected
interior surface or domical ceiling of enclosure
etc. causes sound waves to meet at a particular
point is called as sound foci.
• Sound foci causes concentration effect for
reflected echoes and creation of large sound
intensity.
• This can be eliminate by suitable designed
surfaces not like domical shapes surfaces
including ceiling, walls and also provided sound
absorbing materials in sound foci.

35. 3. Sound dead spots: This is the outcome of
sound foci due high consecration of reflected
sound at
sound foci, there is a limit to be deficiency or
inadequate supply of reflected sound waves at
other points.
• This point of low sound intensity causes
unsatisfactory for the audience is called as
dead spots. This defect can be rectified by
insulation of suitable sound spreader like
provision of speakers.

36. 4. Insufficient of loudness: This is formed lack
of sound reflecting surfaces near to the sound
source and excessive sound absorbing surfaces
in the enclosed space etc.
• this defect rectified by provision of hard
reflected surfaces near to the sound source,
provision of loud speakers, avoiding provision
of deep balconies inside the enclosed space.

37. 5. Reverberation: The sound persists or continues
even after the source of sound has stopped is
called as reverberation.
• Reverberation caused by multi-reflection of
sound in enclosed space.
Reverberation time: It is the period of time in
seconds which is required for sound energy to
gradually fall down. It is depend up on volume and
absorption character of reflected sound.
The time of reverberation can be calculated based
on Proof.W.C.Sabin equation.

38. Where: t = Reverberation time in seconds
V = Volume of enclosed space in m3
a1, a2, a3 …. are the absorption co-efficient of different materials
s1, s2, s3, … are the areas of absorbing surfaces in m2
Σ as or A= are some of the absorption of various units
See the quality of time for different reverberation times given
bellow.

39. Optimum reverberation time: The selection of the
correct time of reverberation is called as optimum
reverberation time.
• It will depend up on use of the room. If the
reverberation time is too long, it result in
overlapping speech and other hand if the time of
reverberation time is too short it produce the
effect of deadness.
• The bellow table shows the recommended value
of the optimum reverberation time for different
acoustical buildings for various purposes after
considering the indicated audience strength.

40. 6. Sound absorption: when a sound wave strikes a
surface (walls, floors, ceiling), a part of sound energy is
absorbed by friction is called as sound absorption. The
percentage of sound absorption depends up on type of
enclosed surface. For example a sound is generated in an
auditorium, and then the sound can be absorbed in four
ways like;

41. • Sound absorbed by air: The absorption of sound by air
mainly due to the friction between the
vibrating molecules in the air when the sound travels
through it. Generally absorption of sound by air is
extremely small.
• Sound absorbed by audience: It means the sound energy
absorbed by the clothing of audience. This is depending
up on season i.e. absorption of sound by audience in
winter is more due to heavy clothing and less in summer.
• Sound absorbed by furniture: It means the sound
absorbed by furniture, curtains, carpets etc.
• Sound absorbed by boundary surfaces: it means the
sound absorbed by walls, floors, ceilings etc in a closed
surface.

42. Absorption Coefficient: The ratio of sound
energy absorbed by the area of material in the
enclosed space is called as absorption
coefficient. The absorption coefficient depends
up on the frequency of sound and it is expressed
in per m2.

43. 7.Transmission of Sound: When a sound
generated in a room it proceeds outwards like
spherical waves until it strikes the boundary
surfaces and after strikes to the boundary it can
transmitte from one side to another side of the
boundary surface, it is called as transmission of
sound.
The amount of sound transmission depends up
on transmitte properties of the boundary,
thickness of boundary and frequency of sound
etc.

44. • Transmission loss of sound: It is the reduction
in the intensity of sound due to any obstacles
or barriers in between the source and listener.
It can be expressed by T.L and it can be
measured in decibels (dB). By mathematically
in can be calculated by the followings;
Transmission loss of sound (T.L) = Intensity of
sound at source - Intensity of sound at listener

45. For example: 100db and 60dB are the sound
levels measured on either sides of a partition
wall. What is the transmission loss or what is the
sound insulation level of the partition wall?
Intensity of sound at source = 80dB
Intensity of sound at listener = 30dB
∴Transmission loss of sound (T.L) = Intensity of
sound at source - Intensity of sound at listener
= 80dB – 30Db = 50dB

46. ARCHITECTURAL ACOUSTICS