Five Major Building Acoustic Design Aspects

1
PH0101 UNIT 1 LECTURE 8

• Five Major Building Acoustic Design Aspects
• Worked and Exercise Problems
• Sources of Noise

2
Five Major Building Acoustic Design Aspects

• (1) Site Selection
• A proper site with quite surroundings is to be
  selected for an auditorium. It should be away
  from the busy highway vehicular traffic, rail
  traffic, airport or any other noisy location.
  Otherwise, the vibrations produced by the traffic
  will be conveyed into the hall through the
  structures, which contribute to the noise in the
  hall. Elaborate and costly arrangements will have
  to be made to reduce the noise level.

3

• (2) Volume
• The hall should be big enough so that sound
  intensity spreads uniformly over its entire area.
  Smaller rooms lead to irregular distribution of
  sound because of formation of standing waves.
  When the length of the hall, is very large in
  comparison to the longest wavelength of sound,
  the room is considered to be large in the
  acoustical sense and the sound within such a hall
  may be regarded as spread uniformly.

4

• The floor area of the hall is computed,
  excluding the stage, based on the requirement of
  0.6 to 0.9 m2/person. The height of the hall is
  determined by the presence or absence of the
  balcony, ventilation requirement etc. An average
  height of 6 m for small halls and 7.5 m for large
  halls are usually adopted. It is desirable to
  provide slight increase in the height of ceiling
  near the center of the hall.

5

• The recommended volumes for different types of
  auditoriums are as follows
• (a) Concert halls 4.0 to 5.5 m3/person
• (b) Theatres 4.0 to 5.0 m3/person
• (c) Public lecture halls 3.5 to 4.5 m3/person

6

• For auditoriums, without air-conditioning,
  require doors and windows to be kept open during
  performance, the orientation of the hall should
  be such that the external noise is maintained at
  low level. When air-conditioning is provided care
  should be taken to reduce the plant noise and
  grill noise. Through an appropriate orientation,
  layout and structural design, the background
  noise level in the hall should be kept at around
  45 dB.

7

• (3) Shape
• The shape of the hall plays a very important
  role in determining its acoustical quality. The
  side walls and ceiling are potentially useful
  reflecting surfaces and should be carefully
  designed to maximize their usefulness. The rear
  walls and floors are potential sources of useless
  and harmful reflections which are to be avoided.
  Parallel hard walls create echo problems.

8

• The fan shaped plan provides favourable
  reflection of sound from sides.
• A concave surface within the hall is not
  desirable because it focuses sound reflections.
• Such surface must be broken up with smaller
  convex surfaces so that sound is diffused in all
  directions.

9

• .

A typical shape of an auditorium
10

• 4) Seating Arrangement
• The seats should be arranged in concentric arcs
  of the circles. Flat floor seating of more than a
  few rows is deprived of good visibility and good
  hearing.
• Sloped floor seating is essential for a large
  audience to have good visibility and good
  acoustics. The successive rows of seats have to
  be raised over the preceding ones, with the
  result that the floor level rises towards the
  rear end. The rise in level may be about 8 to 12
  cms per row.

11

• The seats in each row should be staggered
  sideways in relation to those in front so that
  the line of sight of a person in any row is not
  obstructed by the person sitting in front of him.
• The back to back distance of chairs in
  successive rows should be at least 75 cms and
  this may be increased up to 106 cms for extra
  comfort.

12

• When balcony is provided, its projection L1 into
  the hall should not be more than twice the free
  height H1 of opening of balcony (L1 ? 2H1).
• If balconies are too deep, sound shadow forms and
  the persons in the seats below the balcony do not
  receive ceiling reflections. Suitable sound
  reflectors should be positioned at appropriate
  places to get rid of this defect.

13
Formation of sound shadow by a balcony
14

• (5) Acoustic Treatment of Interior Surfaces
• The interior surface of the hall should be given
  utmost attention to make the hall acoustically
  satisfactory. If the side walls are parallel,
  they are to be covered with absorbent materials
  from a length of about 7.5 m from the proscenium
  end. As the reflections from the near walls are
  of no use, the rear wall should be covered with
  absorbents. In large halls, a false ceiling is
  usually provided.

15

• The false ceiling positioned near the proscenium
  should be constructed of reflective material and
  inclined in a proper way to help reflections of
  sound from the stage to reach the rear seats in
  the hall.
• Concave shaped ceilings in the form of dome
  should be avoided.

16

• The rear portion of the ceiling may be treated
  with sound absorbing material so that build-up of
  audience noise is prevented.
• The floor should be covered with a carpet. Carpet
  on the floor not only covers a useless reflecting
  surface but also greatly reduces audience noise.

17
Worked Example 1 A
classroom has dimensions 20 15 5 m3 . The
reverberation time is 3.5 sec. Calculate the
total absorption of its surfaces and the average
absorption coefficient.
18
Worked Example 2 For an empty assembly hall of
size 20 x 15 x 10 m3 the reverberation time is
3.5 sec. Calculate the average absorption
coefficient of the hall. What area of the wall
should be covered by the curtain so as to reduce
the reverberation time to 2.5 sec. Given the
absorption coefficient of curtain cloth is
0.5.Total absorption of the empty hall
A
owu Average absorption coefficient ?av
19

• When the walls are covered with curtain cloth
• 2.5
• The area of the wall to be covered with curtain
• S

20

• Exercise Problem
• The volume of a room is 1200m3. The wall area
  of the room is 220m3, the floor area is 120m2 and
  the ceiling area is 120m2. The average sound
  absorption coefficient (i) for walls is 0.03 (ii)
  for the ceiling is 0.80 and (iii) for the floor
  is 0.06. Calculate the average sound absorption
  coefficient and the reverberation time.
• Hint

21
Sources of Noise

• The word noise is derived from the Latin term
  nausea. Noise is defined as unwanted sound.
  Sound, which pleases the listeners, is music and
  that which causes pain and annoyance is noise.
  At times, what is music for some can be noise for
  others.
• Most leading noise sources will fall into the
  following categories roads traffic, aircraft,
  railroads, construction, industry, noise in
  building, and consumer products.

22
(1) Road Traffic

• In the city, the main sources of traffic noise
  are the motors and exhaust system of autos,
  smaller trucks, buses, and motorcycles. This type
  of noise can be augmented by narrow streets and
  tall buildings, which produce a canyon in which
  traffic noise reverberates.

23
(2) Air Craft Noise

• Now-a-days, the problem of low flying military
  aircraft has added a new dimension to community
  annoyance, as the nation seeks to improve its
  nap-of-the earth aircraft operations over
  national parks, wilderness areas, and other areas
  previously unaffected by aircraft noise has
  claimed national attention over recent years.

24
(3) Noise from railroads

• The noise from locomotive engines, horns and
  whistles, and switching and shunting operation in
  rail yards can impact neighboring communities and
  railroad workers

25
(4) Construction Noise

• The noise from the construction of highways, city
  streets, and building is a major contributor to
  the urban scene.
• Construction noise sources include pneumatic
  hammers, air compressors, bulldozers, loaders,
  and pavement breakers.

26
(5) Industrial Noise

• Although industrial noise is one of the less
  prevalent community noise problems, neighbors of
  noisy manufacturing plants can be disturbed by
  sources such as fans, motors, and compressors
  mounted on the outside of buildings.
• Interior noise can also be transmitted to the
  community through open windows and doors, and
  even through building walls. These interior
  noise sources have significant impacts on
  industrial workers, among whom noise induced
  hearing loss is unfortunately common.

27
(6) Noise in building

• Apartment dwellers are often annoyed by noise in
  their homes, especially when the building is not
  well designed and constructed. In this case,
  internal building noise from plumbing, boilers,
  generators, air conditioners, and fans, can be
  audible and annoying.
• Improperly insulated walls and ceilings can
  reveal the sound of-amplified music, voices,
  footfalls and noisy activities from neighboring
  units.
• External noise from emergency vehicles,
  traffic, refuse collection, and other city noise
  can be a problem for urban residents, especially
  when windows are open or insufficiently glazed.

28
THANK YOU
THANK YOU