Building acoustics

1. Accoustics of a Building
By: Mohibullah

2. Acoustics
Acoustics: is the science of sound which
assures the optimum conditions for producing
and listening to speech, music etc.
Acoustical design and construction is done to
for dissipation of the noises, external and
internal and insulation against sound.

3. Acoustics and Sound Insulation
Acoustics can be achieved by considering two
aspects:
 proper control and remedy of the acoustical
defects in buildings
 sound insulation

4. Acoustics and Sound
Insulation
 Medium: Solid, liquid & Gas.
 velocity of sound depends on nature and
temperature of medium.
Frequency or pitch: Number of cycle or
vibrations / sec. Greater is no. of cycle higher
will be the pitch.
 Pitch is the measure of quality of sound.

5. Cont...
Intensity or loudness of sound: flow of sound
energy /sec through unit area.
 intensity is a measure of quantity of sound energy
Measurement of Sound: unit - Bel = 10 dB
Ear doen't respond in proportion to the intensity of
sound, and that its response in proportional to the
logarithm of the intensity of sound.

6. Cont...
Sounds or Noises Intensity level (dB) Feeling of
sound
Range Average
Pneumatic drill or aeroplane
at a distance of 4 m.
120-140 130 Pain &
Discomfort
Thunder, Air plane motors 100-120 110 Deafing
Noisy factory areas, loud
street noise
80-100 90 very loud
Noisy home, average
conversation
40-60 50 moderate
Quiet living room, quiet
conversation and avg
auditorium
20-40 30 faint

7. Acceptable Indoor Noise Level
Location Noise Sources in Different
Buildings
Noise Levels
(dB) range
Radio and TV studios 25-30
Music Room 30-35
Hospital Auditoria 35-50
Apartments, hotels, & homes 35-40
Conference rooms , small offices, and
libraries
35-40
Court rooms and class rooms 40-45
Large public offices, banks and stores 45-50

8. Behaviour of Sound
 Sound originated from any source either as a
speech or music, it is transmitted from the
source in all directions.
 Sound travels till it strikes on some surface
from where, a part of it is reflected back, a
part being absorbed, or transmitted.

9. Cont....
Refelction Coefficient: ratio of the reflected intensity
over the incident intensity
Absorption Coefficient: ratio of energy absorbed by
the area to the energy striking.
Transmission loss: reduction in intensity of sound in
passing through the barrier or measure of the
effectiveness of a barrier or surface in insulating
against the transmission of sound.




10. Acoustical Defect
 Echoes: Echo is an indirect or a reflected voice which
is heard just after the direct hearing of the voice
coming from the same sound source.
 Echoe forms when time lag between the two voices or
sound is about 1/17th of a sound and the reflecting
surfaces are situated at a distance greater than 15m.
 Remedy of this defect is to select the proper shape of
the auditorium and surfaces and to use the rough and
porous material for interior surface.

11. Acoustical Defect
Reverberation: Prolongation of sound after
the source producing it has died out.
Acoustical correction: Addition of absorption
units, to make up the deficiency of actual
absorption available for a hall or room in order
to obtain optimum reverberation time or
conditions.

12. Reverberation
 Reverberation time:
 t >5 sec - very bad
 5>t >3 sec - bad
 3>t >2 sec - fairly good
 2>t >1.5 sec - good
 1.5>t >0.5 sec - very good
 Reverberation should be short for
auditorium as a cinema theatre or
public address.

13. Acoustical Defect
Sound Foci: when reflected sound rays meets at a point
which causes concentration effect for the reflected echoes
and creates a sound of large intensity.
can be eliminated by providing geometrical designed shape of
interior face including ceiling/providing absobing material.
Dead Spot: Side effect of sound foci. deficiency of
reflected sound rays causes low sound intensity at some
points.
Insufficient loudness: In large auditorium, due to lack of
sound reflecting flat surfaces near the source or stage and
excessive absorption of sound in the hall, results in this
defect of insufficient loudness.
Provide loud speaker

14. Acoustics defect
Exterior Noise or Outdoor Nuisance:
cause mainly due to poor sound insulation an partly
due to poor planning.

15. Defects and Recommended Remedies
Defect Causes
Recommendation
New Design Existing Design
Excessive
reverberation
Insufficient
Absorption
Add absorbents Add absorbents
Echoes unsuitable shapes
& proper reflecting
surface
avoid unsuitable
shapes & make
offending surface
highly absorbent
-
Sound Foci Concave reflecting
interior surfaces
avoid curvilinear
interior
use absorbents on
focusing surface
Deat spots Irregular
distribution of
sound
provide even
diffusion of sound
introduce suitable
diffusers
Insufficient sound
volume
lack of reflections
close to source of
sound or excessive
absorption
disposition of hard reflecting surfaces
near the source of sound.
adjust absorption to give optimum
reverberation

16. Acoustics -Requirements and conditions
 Adequate sound intensity.
 Evenly distributed
 Clear and distinct
 Reach the audience with the same
frequency and intensity
 Noise protection to be done

17. Factors in Acoustical Design
 Site Selection
 Volume
 Shape
 Interior Surface
 Reverberation
 Seating arrangements and audience
 Sound Absorption

18. Factors in Acoustical Design
Site selection:
 if possible, site should be away from busy
street, railways, airports, etc
 if not, then orientation, layout, structural
design should be done accordingly.
 Noise reduction

19. Factors in Acoustical Design
Volume:
 Puropose of hall.
 Height is of greater importance.
 Public lecture hall - 2.8 to 3.7 m3/person
 Musical Concert Hall - 4.2 to 5.6 m3/person
 Cinema Theatres - 3.7 to 4.2 m3/person

20. Factors in Acoustical Design
Shape:
 For correcting the defects.
 For better distribution of sound.
 Ceiling height for a room or hall to be
used for speech and music is kept
varying from 1/3rd to 2/3rd of the
width of the room.

21. Relationships between ratio & Vol per seat
Type of
Auditorium
Vol./per
seat in m3
Ratio of
H:W:L
Approx
Capacity in no.
of person
Lecture Hall
(Small)
2 - 3 1:1.5:3 150 to 300
Lecture Hall
(Large)
3.5 1:2:4 400 to 500
Lecture halls
with loud
speakers
4.5 1:2.5:5 500 to 1000
Cinema 3.5 to 4.5 1:2:3 800 to 1000
(normally)
Theatre
(Drama)
3.5 to 4.5 1:2:3 upto 750 (small
theatre)

22. Factors in Acoustical Design
Treatment of interior surfaces:
 Provide favourable reflections
 Tilted portions of the ceiling can be
arranged
 Max time gap between sound
source and listener should be 45
milli-sec
 ceiling and wall reflector should be
within 8m of the sound source.

23. Cont...
 Path difference between direct and
reflected sound at no listening point
should exceed 12 m.
 Plain side wall are normally found
suitable.
 Concave ceiling surfaces such and
domes should be avoided.
 smooth ceiling should not be
parallel to floor

24. Cont...
 Convex shaped wall considered best
 Rear wall should not be provided as
concave walls unless treated with
sound absorbent

25. Factors in Acoustical Design
Reverberation:
 Optimum reverberation time
 For orchestral music. t is about 15%
longer than that for unaided
speech.
 For music, it is 40% longer.

26. Factors in Acoustical Design
Seating Arrangements:
 largest contributer to the absorption
of sound
 distance of front row should be
about 3.5 m for drama and 4.5m for
cinema.
 Width of seat 45 to 55cm.
 Back to back distance - 85 - 105 cm

27. Factors in Acoustical Design
Sound absorption:
 Usefull positioning of the sound
absorbing material.
 coefficient absorption for distemper
is higher than paint.
 wood panel has higher coefficeint at
lower frequency.

28. Acoustical Treatment
Open Air Theatres:
 Selection of site is most important.
 Avg noise level should not exceed
40 dB.
 Slope of seating area should not be
less than 12 degree.
 Sound amplification system

29. Acoustical Treatment
CinemaTheatres for Sound Films:
 Fan shaped floor plan with diverging
side wall.
 Splayed type ceiling with slight
upward slope towards the rear end.
 Ratio (H:W:L) should be approx
1:2:3.
 Surface near sound source should
be hard polished.
 Vol - 3.5-4.5 m3.

30. Acoustical Treatment
Radio Broad Casting Studios:
 Perfect Sound Proofing/Noise
insulation:
 Floor, walls, & ceiling must be rigid
material.
 Variable reverberation time:
 Different absorption is required in the
same room as suitable for type of
studio.
 Noise level - 25-30 dB.

31. Acoustical Treatment
Radio Broad Casting Studios:
 Ratio of H:W:L - 2:3:5.
 Shape should be rectangular with flat
ceiling.
 Provision of window should be min.
 Air tight.
 Heavy Curtains can be used

32. Acoustical Treatment
Class Lecture Rooms:
 Room dimen - 7:8.5:4 considered
satisfactory for 40 students.
 Length to width - 1.2 to 1(wide rooms
are more satisfactory.
 Noise level- 40dB.
 t - 0.75 sec at freq.(500-2000) & 9 sec
at freq.(125CPS)
 Vol per can be kept as small as possible
- 12m2 or less.

33. Classification of Sound Absorbents
 Porous Absorbents:
 Sound waves strike surface of a porous
of a material, a part of sound waves gets
reflected while the other part enter the
porous material.
 The part of reflected waves energy is
reduced while , the part of waves that
entered into the porous material is
converted into heat
 rock wool, glass wool, foamed plastics,
asbestos fibre, curtains, spray felt etc.

34. Classification of Sound Absorbents
Resonent Panel's
Absorbent:
 Semi hard material
in the form of porous
fibre boards.

35. Classification of Sound Absorbents
Cavity Resonators:
 When sound waves enter the resonator;
due to multiple reflections inside , the
sound waves are absorbed .
 The cavity resonator may be designed
to absorb sound of any particular
frequency generated by machines such
as air conditioner plant, machines etc.

36. Classification of Sound Absorbents
Composite Type-Absorbents:
 A composite absorber is a single unit
which does the function of the other three
absorbers mentioned before.
 The composite absorber of a perforated
panel fixed over an air space containing
porous absorbent.
 When sound waves strike the panel, they
pass through it and damped by resonance
of the air in the cavity.

37. Noise
Unwanted sound
Noise created from high frequency is
more unpleasant and harmful.
 Two types:
 Outdoor Noise
 Indoor Noise

38. Noise
Classification based on their origin:
Oudoor Noise: source of origin outside
from the room or building.
 Road traffic, railways, aeroplanes,
loudspeaker, moving machineries
Indoor Noise: source of origin inside the
room.
 conversation of occupants,
banging of doors, footsteps, etc.

39. Noise
Classification based on analysis of sound
insulation problem:
Airborne Noise: generated in the air & is
transmmitted through air.
Structure Borne or Impact Noise:
originates and progress in the building
structure.
 caused due to structural vibration

40. Sound Terminology
Sound Absorption: prevention of reflection of
sound waves
Sound Insulation: prevention of transmission
of sound
Sound Structure: indicate loss of sound energy
on reflection at a surface.
Transmission loss: reduction in sound
intensity during the transmission of air-borne
sound from the source to the recipient.

41. Max Acceptable Nosie
SNo. Type of building /structure Noise level (dB)
1 Studios for radio broadcasting 25-30
2 Auditoriums 35-40
3 Small offices, court room, libraries
40-45
4 Hospitals 40-50
5 Lecture Rooms 45-50
6 Residential Building 45-55
7 Large Offices 50-60
8 Factories 60-65

42. Noise Control & Sound Insulation
General Consideration:
 Isolate it at source
 Selection of location and orientation
 Lining of wall and ceiling by means of airfilled
materials, like felts, straw boards etc
 Rigid wall, floor, partitions can prevent
transmission of noise
 struture borne noise can be prevented by
introducing discontinuities in the path of vibrating
waves or expansion joint.
 personal protective devices - ear plugs, head
phones etc.

43. Noise Control & Sound Insulation
Constsructional Measures:
 Wall and Partitions
 Floor and Ceilings
 Windows and Doors
 Insulating Sanitory Fittings
 Machine mounting or Insulations of Machinery

44. Noise Control & Sound Insulation
Wall and Partitions(vertical barriers):
 Rigid and Homogenous Patitions: Thickness of wall
affects the sound insulation property.
 Partitions of Porous Material:Porous Concrete
Masonry, cinder concrete etc. 10% more
absorbent than non porous.
 Hollow & Composite Partition walls or Double wall
Construction:Composite partition of cavity wall
type construction by filling the cavity or leaving
the air space with some resilient material

45. Noise Control & Sound Insulation
Floor &
Ceilings(Horizontal
barrier to noise):
 Use of resilient material
on floors.

46. Noise Control & Sound Insulation
Floor & Ceilings(Horizontal barrier to
noise):
 Providing a floating floor construction:
Additional floor float over the existing
surface by means of resilient material.