PH0101 UNIT 1 LECTURE 81 Five Major Building Acoustic Design Aspects Worked and Exercise Problems...

PH0101 UNIT 1 LECTURE 8 1

PH0101 UNIT 1 LECTURE 8

• Five Major Building Acoustic Design Aspects

• Worked and Exercise Problems

• Sources of Noise


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.


(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.


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.


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


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.


(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.


• 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.


.

A typical shape of an auditorium


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.


• 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.


• 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.


Formation of sound shadow by a balcony


(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.


• 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.


• 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.


S

VT

161.0

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.

695.3

)51520(161.0 3

s

mS

average

69 69

2(20 15 15 5 20 5) 950

0.07


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

1385.3

101520(161.0

A = owu

Average absorption coefficient

av = 106.0)102010151520(2

138


• When the walls are covered with curtain cloth

2.5 =

• The area of the wall to be covered with

curtain

S =

S5.0138

)101520(161.0

483 2.5 138

2.5 0.5

2110.4m


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 : 24.0

321

332211

sss

sasasaa

0.16VT

A 1.8s


• 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.

Sources of Noise


• 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.

(1) Road Traffic


• 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.

(2) Air Craft Noise


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

(3) Noise from railroads


• 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.

(4) Construction 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.

(5) Industrial Noise


• 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.

(6) Noise in building


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PH0101 UNIT 1 LECTURE 81 Five Major Building Acoustic Design Aspects Worked and Exercise Problems...

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