Notes for Architecture 4th Year subject Services. The topic is about Acoustic, how does it work for different places, how we can treat spaces according to acoustic and for better acoustic

1. INDEX
 Introduction
 Sound
Characteristics
Behavior in closed spaces
Reflection
Absorption
Refraction
Diffussion
Difraction
Transmission
Behavior in open spaces
Near field
Far field
Free Field
Reverberant field
 Inverse Square law
 Doppler effect

2.  Acoustic materials
Sound absorbers
Diffusers
Noise Barriers
Reflectors
Absorbers
Porous
Resonant panel Absorbents
Cavity Resonators
Composite type
• Acoustic Defect
Echo
Reverberation
Sound Focii
Dead Spots
Insufficient Loudness
• Factors Considered in acoustic design of an auditiorium

3. ACOUSTICS
 A science that deals with the production, control,
transmission, reception, and effects of sound.”
 Sound is reflected, transmitted, or absorbed by the materials
it encounters.
 Soft surfaces, such as textiles, and batt insulation, tend to
absorb sound waves, preventing them from further motion.
 Hard surfaces, such as ceramic tile, gypsum board, or wood,
tend to reflect sound waves, causing ‘echo’. Reverberation is
the term used to describe sound waves that are reflected off
of surfaces.
 Dense, massive, materials, such as concrete or brick, tend to
transmit sound waves through the material.

5. SOUND
 Sound is a vibration that propagates as a typically audible
mechanical wave of pressure and displacement, through a
medium such as air or water

6. Characteristics of sound
 Consists of alternate compressions and rarefactions that
are set up vibrating body
 Sound waves transmits or travels in all directions through
any medium whether solid liquid or gas.
 The average sound travels in air at ordinary temperatures
and pressure with a speed of 340m/sec.
 Sound cannot travel by vacuum.
 Wavelength is the distance between any two consecutive
points on a wave.Frequency is the number of cycles of
vibration per second,therefore

7. SOUND IN CLOSED SPACES
• In case of concave shaped reflecting interior surface or domed
ceiling or an enclosure, depending upon the curvature of these
surfaces, there is possibility of meeting the sound rays at
appoint called as sound foci and thus it creates the sound of
large intensity .
• This defect can be minimized by providing proper geometrical
design.
• Shape of the interior faces including ceiling and also by
providing absorbent materials on focusing areas.
• On encountering barriers posed by the enclosure, sound waves
are likely to behave in the following ways:
• Reflection
• Absorption
• Refraction
• Diffusion
• Diffraction

8.  Reflection :This occurs when the
wavelength of a sound wave is smaller
than the surface of an obstacle.
 Absorption: When sound waves hit the
surface of an obstacle, some of its energy is
reflected while some are lost through its
transfer to the molecules of the barrier.
 Refraction :This is the bending of sound
when it travels from one medium into
another medium

9.  Diffusion This is the scattering of waves
from a surface. It occurs as a result of the
texture and hardness of the obstacle
 Diffraction When the wavelength of a
sound wave is smaller or equal to the size of
the obstacle
 Transmission In this phenomenon,
sound wave is carried by molecules of the
obstacle through vibration and reemitted

10.  Near field :
The near field of a source is the region close to a source where the
sound pressure level may very significantly with a small change. In this
region the sound field does not decrease by 6 dB each time the
distance from the source is increased (as it does in the far field).
 Far field :
The far field of a source begins where the near field ends and extends
to infinity. Note that the transition from near to far field is gradual in the
transition region.It is divided into two fields:
 Free field :
The free field is a region in space where sound may propagate free from
any form of obstruction or reflecting surfaces.
 Reverberant field :
The reverberant field of a source is defined as that part of the sound
field radiated by a source which has experienced at least one reflection
from a boundary of the room or enclosure containing the source.
SOUND IN OPEN SPACES

11. Absorption Coefficient
 Sound absorbed by surface and transmitted through the
surfaces are considered together as being absorbed and are
represented by A.C
a= Sound energy absorbed
Total energy per unit area
• If absorption coefficient of material is 0.5 then it shows that 50% of
energy is absorbed by it per unit area.
• The loss of sound energy is absorbed by the material.This is because of
conversion into heat due to frictional resistance inside the pores of
material.
• The fibrous and porous nature of material contribute to their sound
absorbing capacity.
• The value of absorption coefficient depends upon the nature of material
and the frequency of sound.
• Greater the frequency ,larger is the value of the coefficient in the same
material.

12. BEHAVIOR OF SOUND .
Sound intensity level:
 The sound intensity is given as,Where, P is the sound power,
A is the area ,
To measure Sound intensity level we compare the given sound intensity
with the standard intensity.
I=P/A
 Sound Intensity Level Formula is given by,
 Where I = sound intensity and
Io = reference intensity
It is expressed in decibels (dB).
Sound Intensity Formula is used to determine the intensity of sound
waves. The S.I unit of sound intensity is Watt per meter square (W/m2
 A sound intensity level, LI , may be defined as follows:
 LI =10 log10 (sound intensity)
(ref. sound intensity)

13. INVERSE SQUARE LAW
 The Inverse Square Law teaches us that for every doubling of
the distance from the sound source in a free field situation,
the sound intensity will diminish by 6 decibels.
 As a sound wave propagates spherically, the sound energy is
distributed over the ever-increasing surface diameter of the wave
front surface.
 Under ideal conditions a free field could be represented by a
sound signal being generated from a mountain peak. In real life
situations however, rooms bounded by walls, floors and ceilings
will interrupt the inverse square law at a distance in tan average
30′ square room at approximately 10-12 feet from the sound
source. Nevertheless it is important to accept the notion that
sound will diminish in intensity with distance. For example, in a
typical classroom with a teachers voice signal of 65 decibels at a
three-foot distance from the teacher; at 6 feet away the sound
intensity will be 61 decibels and at twelve feet it will diminish down
to 54 decibels. (This is important to remember as we discuss the
Signal to Noise Ratio S/NR later on)

15. • So far, we have only considered stationary sources of sound and
stationary listeners (or observers). However, if either the source or the
observer is moving, things change. This is called the Doppler effect.
• Objects of interest may be the speed of a car on the highway, the
motion of blood flowing through an artery
• One of the most common examples is that of the pitch of a siren on an
ambulance or a fire engine. You may have noticed that as a fast moving
siren passes by you, the pitch of the siren abruptly drops in pitch. At
first, the siren is coming towards you, when the pitch is higher. After
passing you, the siren is going away from you and the pitch is lower.
This is a manifestation of the Doppler effect
DOPPLER EFFECT

16. You hear the high pitch of the siren of the approaching ambulance, and
notice that its pitch drops suddenly as the ambulance passes you. That is
called the Doppler effect

18. o study Acoustics
 To control noise at an acceptable level.
 Distinct speech or music audibility by the whole audience
inside an enclosure.
Requirement and conditions for good
acoustic
• The initial sound should of adequate intensity such that it can be
heard throughout the hall.
• The sound produced should be evenly distributed over the entire
area covered by the audience.
• In the hall used for speech, the initial sound should be clear and
distinct.
• In the hall used for music and dance the initial sound should
reach the audience with the same frequency and intensity .
• All noises whether originating from inside or outside of the hall
should be reduced to such an extent that they don’t interfere with
the normal hearing of music.

19. ACOUSTIC MATERIALS
Sound
absorbers
 Wall panels
 Ceiling clouds
 Ceiling tiles
 Ceiling baffle
 Broadband
baffle
 absorbers
Sound diffusers
• Quadra pyramid
diffusers
• Pyramid diffuser
• Quadratic diffuser
• Double duty
diffuser
Noise barriers
• Vibration control
• Composite
Sound
reflectors/
isolators

20. Sound Absorbers.
 These materials eliminate sound reflections and
are generally porous, with many pathways that
redirect sound and cause it to lose energy.
 Typical sound absorbing materials are fiberglass,
rock wool, open cell polyurethane foam, cellular
melamine foam, heavy curtain blankets and thick
fabric wall coverings.
 Absorber materials do not substantially block
sound, but absorption can enhance isolation by
stopping air movement that would otherwise allow
sound and noise to travel.
Sound Diffusers. (Alt. Diffusors.)
• These devices reduce the intensity of sound by scattering
it over an expanded area, rather than eliminating the
sound reflections as an absorber would.
• Traditional spatial diffusers, such as the polycylindrical
(barrel) shapes also double as low frequency traps.

21. Noise Barriers.
• These materials are heavy, dense and massive to prevent
sound penetration.
• A common material is drywall (gypsum, sheetrock). Thin
materials with high sound blocking characteristics are
lead foil and mass loaded vinyl.
• A sandwich of dissimilar materials such as five-eighths
inch gypsum, one- eighth inch vinyl barrier, and a half-
inch finish layer of drywall will block more effectively than
an equivalent thickness of drywall alone.
• More energy is lost as sound must change its speed for
each different material
Sound Isolators.
• These devices are resilient and prevent sound
transmission through the structural steel or concrete of
a building as well as its plumbing and air handling
systems.
• Typical devices are resilient channel for drywall,
isolation pads for floors, and special adhesives for walls
to avoid the hard connections of nails and screws that
often provide a sound path through otherwise effective
sound insulation materials.

22. Study of various absorbing materials
 All materials should absorb sound but some to a lesser extent.
 Sound wave strikes porous surface and dissipate heat channels.
 Efficiency of sound energy depends upon the porosity of
material.
 Absorption coefficient is used to express the amount of incident
sound that can be absorbed .
The need for absorbing materials
 To ensure Privacy
 Noise control
 To improve Environment for efficient working.

23. Classification of sound absorbent
Porous
absorbents
CLASSIFICATION OF SOUND ABSORBENTS
Resonant Panels
absorbent
Cavity
Resonators
Composite
type
Absorbents
• Porous absorbents: A good example
of a porous sound absorbent is stone
wool. When the sound wave penetrates
the mineral wool, the sound energy
through friction is changed into heat.
• High frequencies (above 500 Hz) are
easier to handle with 30–50 mm stone
wool thicknesses. More challenging are
the sounds in frequencies below 500 Hz.
• Here we need thicker stone wool slabs to
create better sound absorption. Material
thickness can also be compensated for
with air space behind an acoustic ceiling
or wall panel to improve low frequency
performance

24. Resonant Panels Absorbents:
 Semi hard material in the form of fibrous boards.
 In this system the absorbent material is fixed on
the sound framing with an air gap left out
between this material and the wall backing.
 Sound waves of appropriate frequency cause
vibration in the panels.
 Absorption is obtained by damping this vibration
by means of an air space.
 Resonant Panels are effective for sound
absorption at the lower frequencies.

25. Cavity Resonators: These cavity resonators
consist of a container or chamber with a small
opening in which absorption takes place by the
resonance of the air in the container which causes
loss of sound energy
 It can be designed to absorb sound of any
frequency.
 Suitable for Particular higher frequencies.
Composite type Absorbents:
 Combination of all the 3 types.
 Consists of perforated panel fixed over an air
space containing porous absorbents.
 Panel may be of metal,plywood,hard board
etc.

26. 1. Formation of echoes:
ACOUSTIC DEFECTS
Echoes mainly produced due to the reflection of sound waves (mainly from the
surface of walls , roofs , ceilings etc. ) coming from the some sources , reaches
to the ear , just when direct sound wave is already heard and thus there is a
repetitions that is nothing but echoes.
1. Normally the formation of echoes (happens when the time lag between the
two voices or sounds is about 1/17 of a second. And the reflecting surfaces
are situated at a distance more than 15 meter.
2. If the reflected surface is curved with smooth surface this problem usually
occurs. To minimizes this problem select ion of proper geometry of
auditorium and surface and also use the rough and porous material for the

27. Reverberation means the prolonged reflection of sound from wall floor or roof of a
hall.
1. When the sound is reflected back (some part of the sound is absorbed )
resulting in formation of echoes, but sometimes this reflection of sound does not
stop even the sound is died out.
2. The sound reflected back and forth against the walls, ceilings and floors for
several times This is mainly when sound in closed spaces successively
reflected by the smooth boundaries of the enclosed space.
Reverberation:

28. Sound focii
 Reflecting concave surfaces causes
concentration of reflected sound
waves at certain spot creating a
sound of large intensity.These spots
are called sound focii.
 This defect can be removed:
 Geometrical designed shapesof the
interior faces,including ceilings.
 Providing highly absorbent
materials on focusing areas

29. Dead Spots
 This defect is an outcome of the formation of sound focii.
 Because of high concentration of reflected sounds at sound
focii,there is deficiency of reflected sounds at some other
points.
 These points are dead points where sound intensity is so low
that it is insufficient for hearing.
 This defect can be removed by:
 Installation of suitable diffusers o that it can evenly distribute
sound in the hall

30.  the sound waves should be properly reflected and uniform ally spread
all over the interior part of the auditorium.
 But due to the lack of sound reflecting flat surfaces near the sound
source or stage and excessive absorption of sound in the hall resulting
the defect of insufficient loudness. This defect can be minimized by
providing hard surface near the stage and absorbent material should
be provided as per the requirements. Also the location of the loud-
speakers should be adjusted. So that there is no dead spots and
sound foci. Also use of adequate no of windows or door openings
Insufficient
loudness

37. Factors to be considered in the acoustic design
of an auditorium
 Site Selection and planning
 Volume
 Shape
 Treatment of interior spaces
 Reverberation
 Seat and Seating arrangement and audience
 Sound absorption

38. Site Selection and Planning:
 All prevailing and foreseable noises should be considered.
 Site selected should be in quietest surrounding so that the
intelligible speech and tonal quality of music is not affected.
 Keep the outdoor noise at lower level by proper orientation
 In case of AC,care should be taken to reduce the plant noise and
grill noise
Volume(size and height)
It should be in proportion to the intensity of sound to be generated.
Cinema/theatre-4.5 cu.m per person
Musical hall or concert hall-4-5 cu.m per person
Public lecture hall- 3.5-4.5 cu.m per person
Shape:
It is the governing factor in correcting the defects which are due to
reflection of sound waves

39.  The volume-depends upon the total no of
audience.
 The shape- depends upon arranging
audience for better audibility.
 Different shapes:
 Rectangular floor shape- Cross reflector
between parallel walls contribute to increase
fullness of tones.
 Concave –The concave walls are not
advisable as the rays get concentrated and
dead spots are formed.

40. Convex shaped: It is advisable the
sound waves gets dispersed in equal
directions.
Fan Shape: Accomodates more
number of people.
Lack of lateral reflections.
Side walls should be arranged to
have an angle of not more than 100
degree.
Shape is expressed in terms of ratio of
H:W:L
Ceiling height=H=1/3 Or 2/3 of W.

41. Treatment of interior surfaces:
 The ceilings and side walls should provide favourable reflections
that reach the rear parts of a large auditorium.
 Ceilings splays or spread outs and tilted portions of the ceilings
can be arranged or devised to reflect the sound.
 All reflected sound must reach within 45 seconds to avoid echo.
 Reflectors should be within 8m of the sound wave.
 The ceiling should not be parallel to the floor.
Seats,Seating arrangement of audience:
 Absorption Coefficient of seated audience =0.46 m2 Sabines
 Absorption Coefficient of plain seat =0.02m2 Sabines
 Absorption Coefficient for cushioned seat =0.2m2 Sabines
 Seating accomodation for good audibility should cover 90 degree
with horizontal,30 degree with vertical
 The distance of 1st row =3.5m for drama
4.5m for cinema

42. Ray Diagrams:
Ray diagrams are a method for analyzing whether or not reflected sounds would
cause annoying echoes. If the sound path of the reflected sound is more than
34’ longer than the direct sound path, the listener will perceive a noticeable, and
annoying, echo.
Areas distinguished by blue are “live” areas, while seats marked in red illustrate
“dead” areas. This shows that the sound reflector panels are inefficiently
designed to spread sound to all areas of the theater.

43. As evidenced by this illustration, there are no “dead” zones in the crowd where
sound will not be reflected. In addition to zone checks, the reflective path
distance was compared to the direct path distance for each sound path. The
results found that there were no differences between the two paths greater than
34’, which is acceptable for this space.

45. • The time gap between the initial direct note & the reflected note up to a
minimum audibility level is called as reverberation time.
• When the source emits sound, the waves spread out and the listener is
aware of the commencement of sound.
• If the note is continuously sounded, the intensity of sound at the listeners
ear gradually increases. After some time, a balance is reached between
the energy emitted per sound by the source and energy lost or dissipated
by walls or other materials.
• The reverberation time of a room characterizes how long acoustic energy
remains in a room. It is usually defined as the time for the acoustic
intensity (or energy density) to decrease by a factor of one million (60 dB).
Since a reasonably loud clap is about 100 dB (SPL) and a whisper is
about 40 dB, you can easily estimate the reverberation time for a room by
clapping and listening to how long you can still hear some remaining
sound from the clap. This assumes that the room is not particularly
unusual in its dimensions and that it is reasonably quiet
Reverberation Time:

46. Reverberation time = time it takes for loudness decrease by 60 dB

47. Reverberation Time
 T=0.161V
S a
V= room volume
T=reverberation Time
a= Absorption Coefficient
S=Total Absorption (Sabins)
https://www.youtube.com/watch?v=A8t
WMnr5pps

48. Criteria for Good Acoustics
Optimum reverberation time is a compromise between clarity (requiring short
reverberation time), sound intensity (requiring a high reverberant level), and
liveness (requiring a long reverberation time).
The optimum reverberation time of an auditorium is dependent on the use for
which it is designed.
Reflected sound arriving from the sides seems to be important to the overall
reverberance of the room.
Important subjective attributes of concert hall acoustics include intimacy,
liveness, warmth, loudness of direct sound, reverberant sound level,
definition or clarity, diffusion or uniformity, balance and blend, ensemble,
and freedom from noise.
In addition to the attributes above, spatial impression and early decay time are
important.
The spatial impression is dependent on contributions to the early reflections from
above and especially from the sides. The initial rate of decay of reverberation is
apparently more perceptually important than the total reverberation time.
Echoes, flutter echoes, sound focusing, sound shadows, and background noise
should be avoided in an auditorium design.
The greater the early decay time (up to two seconds), the greater the preference
for the concert hall. Above two seconds, the trend it reversed.
Narrow halls are generally preferred to wide ones.