The sound which causes annoyance, interference with speech, damage to hearing and results in reduction in efficiency of work performance is called noise. So in modern building construction sound insulation is very much important topic. Noise may also defined as a disturbance in an elastic medium which includes solid, liquid and gases. It is a form of wave motion created by a vibrating body and is transmitted in all directions in the form of spherical wave consisting of alternate compressions and rarefactions. The procedure of obtaining an acceptable noise environment at a particular point is called noise control.
CLASSIFICATION OF SOUND
Depending upon the position of source, sound can be broadly divided in
two categories.
1.Airborne sound.
2. Impact sound.
Airborne sound:
An airborne sound is one which is transmitted through air and travels
direct to the ear of the person. This type of sound travels from one part of
the building to another or from outside of the building to the inside through
open doors, windows or other openings or through small gaps around doors and
windows.
Impact sound:
The sound which is transmitted first through the structure is called
impact sound. The noise of the footsteps, furniture, movement, dropping of
utensils on the floor fall under this category. Impact sounds are troublesome
and are often very sharp.
MEASUREMENTS OF SOUND
Human ear is incredibly sensitive. It can react to the rustle of the wind
in a leaf in a night as well as to the noise of a rock drill, latter being
10,000,000,000 times stronger than the former. Sound is usually measured and
expressed in terms of sound pressure levels or decibles.
Decible is therefore used as a convenient unit to measure the magnitude
of sound, sound levels or sound insulation. 0.002 Dynes/sq.cm. is the lowest
value of sound pressure that an average human ear can perceive and hence this
value is taken zero on the decible scale.
TRANSMISSION OF SOUND
When sound is produced in a room it proceeds outward in spherical waves
until it strikes the boundaries of the room. Thereafter, the sound waves are
reflected, transmitted and absorbed in varying amounts depending upon the
characteristics of the walls of the room and the frequency of the sound.
TRANSMISSION LOSS
The reduction in the intensity of airborne sound that takes place during
its transmission from the source to the recipient is called transmission loss.
Transmission loss is numerically equivalent to the loss in the intensity of
sound expressed is decibles.
DEFECTS DUE TO REFLECTED SOUND
The behaviour of the reflected sound plays an important role in sound
insulation or acoustical design of a room/hall. The main defects which
reflected sound waves may cause in arrom are:
1.Echoes
When a reflecting surface is so far away from the source that the sound
is reflected back as a distinct repetition of the direct sound, the reflected
sound is called an echo. Echoes are produced, when the time interval between
the direct and reflected sound waves is about 1/15th of a second.
This defect is particularly common when the reflecting surface is curved in
shape. To minimise this defect in curved walls, the walls are covered with
highly absorbent material or grills on the face work.
2. Reverberation:
When the sound waves get reflected, a part of the sound energy is
converted into heat energy by friction and is absorbed by the walls.
Subsequently the reflected waves get inter reflected from one surface to
another until they gradually fade and become inaudible. This phenomenon of
undue prolongation of sound by successive reflections from surroundings
surfaces, after the source sound has ceased is termed as reverberation. A
certain amount of reverberation is necessary to enhance the sound. However
excessive reverberation is damaging to clarity.
Reverberation time:
The reverberation period is the time taken for the sound energy to decay
by 60 decible, after the sound source has stopped. The reverberation time
depends principally on the volume and absorption of the room.
The time of reverberation plays a significant role in achieving desired
acoustical condition. If the time of reverberation is too long, it results in
overlapping of speech and loss of intelligibility, and if it is too short, it
produces the effect of deadness and loss of brilliance of sound. If the time of
reverberation works out to a value greater than 3 seconds, it is considered
bad, between 3 to 2 seconds as fairly good and between 2 to ½ second as very
good. The time of reverberation to allowed varies with the purpose for which
the enclosure is to be used. If the enclosure or auditorium is to be used as a
sound film theatre or for public address system, the time of reverberation
selected should be short, whereas for concert halls and churches, it should be
longer. For enclosure to be used for both speech and music, a value midway
between the two should be adopted.
The selection of the correct time of reverberation is called optimum time
of reverberation.
It is noted that presence of audience in a room reduces the time of
reverberation. This is on account of the absorption provided by the audience.
This shows that a theatre will have a greater time of reverberation, when it is
empty, then when it is full with audience.
SOUND ABSORBENT
Broadly speaking, the material having hard, rigid and non-porous surface,
provide the least absorption whereas those which are flexible, soil porous and
can vibrate, absorb more sound. The efficiency of the sound absorption, however
depends more upon the porosity of the material used as sound absorbent. The
term used to express the percentage of the incident sound that can be absorbed
by a material is known as absorption co efficient of the material. Thus if the
absorption co efficient of a material is 0.75, this would mean that the
material is capable of absorbing 75% of the incident sound. The absorption co
efficient differs with the frequency of the incident sound. In general, low
density materials have higher absorption co efficient at the higher frequencies
than at low frequencies.
CLASSIFICATION OF ABSORBENTS
Sound absorbents can be broadly classified into following four
categories:
1.Porous absorbents
2. Resonant absorbents
3. Cavity resonators
4. Composite type of absorbents
1.Porous absorbents:
When sound waves strike the surface of porous material, a part of the
waves get reflected while a part enters the pores of the material and is
thought to be dissipated into heat energy. The efficiency of this type of
absorbent increases with the increase in the resistance offered by the material
to air flow, its thickness and the porosity. Slag wool, glass wool, wood wool,
asbestos fibre spray, foamed plastic and perforated fibreboards are some of the
categories of porous absorbents. In general, porous materials are selected
mainly to absorb sound having high frequency.
2. Resonant absorbents:
In this system, the absorbent material is fixed on sound framing with an
air space left out between the framing and the wall at the back. Such an
arrangement works most efficient for absorbing sound waves at low frequency. The
principle of sound absorption in this method is that sound waves of the
appropriate frequency cause sympathetic vibrations in the panel which acts as a
diaphragm. The absorption of sound takes place by virtue of the dampening of
the sympathetic vibration in the panel by means of the air space behind the
panel. Dampening effect of this system can be improved appreciably by placing a
porous material in the air space.
3. Cavity resonators:
Cavity resonators essentially consists of a chamber with a narrow opening
in which absorption takes place by resonance of the air in the chamber which
gives loss of sound energy. Such an arrangement can act effectively over a
single selected frequency. Application of cavity resonator is normally
restricted to absorption from individual machine or in similar cases.
4. Composite type of absorbents:
They consist of perforated panels mounted on battens so as to leave a
cavity between panels and wall at the back. The panels may be of metal, wood,
plaster board etc. The area of holes in the panel should vary between 10 to 20%
of the total area of the panel. When the sound strikes the panel, the sound
waves pass through the holes and get damped by the resonance of the air in the
cavity. The effectiveness of this system can be increased by placing a porous
material like mineral wood etc. in the cavity. This type of absorbent is
commonly used, as it is easy to install, economical and it can accommodate wide
range of frequencies.
SOUND INSULATION OF BUILDINGS
Control of noise transmission is essential to minimise the disturbing
effect of sound passing from one room to another, through walls, partitions,
and floors or ceilings. Good planning in respect of the location of building as
well as the placement of quiet and noisy areas in the building itself plays an
important role in controlling noise transmission.
WALL CONSTRUCTION:
The sound insulation rating of a wall is generally governed by the net
sound transmission loss it provides and also the efficiency with which it
serves as a barrier for speed sound. Weight of the wall is the governing factor
in wall insulation. It is seen that a solid one brick thick wall plastered on
both sides, proves quite effective as a sound insulation partition wall. It has
an average reduction of 50 decible. It is now, however, possible to have wall
made from a suitable combination of materials which are light in weight and yet
have high insulation value.
A cavity wall type of construction can be made to have increased
insulation value by filling the cavity with some resistant material. In this
type of construction, the cavity should be at least 5 cm. in width and the two
wall leaves should be tied by use of only light butterfly wall ties. Partition
walls having the value of transmission loss of 45 decible or less are
considered adequate for separating critical areas of adjacent dwellings.
FLOORS:
Transmission of sound takes place more easily through floors. This is on
account on the fact, that invariably the sound producing source has actual
contact with the floor. Hence the floor serves as the most common path for the
transmission of impact noise. The ordinary R.C.C. floor weighing less than 220
kg/sq.m. has a sound reduction of only 45 decible. Thus bare concrete and
timber floors do not function effectively as barrier against impact sound. A
floating floor resting on a resilient material like glass wool, mineral wool,
quilt, hair felt, crock, rubber etc. has an increased rating for impact sound
insulation. The principle underlying the design of the floating floor is its
insulation from any other part of structure. To achieve this, the resilient
layer on which the floor rests is turned up at all edges which abut the walls,
partitions or other parts of the structure. The partition should be built off
the structural floor so that the floating screed is self-contained within each
room.
The wood raft floating floor consists 50 mm deep X 50 mm. wide wooden
battens, on which 20 mm. thick resilient quilt is laid over the structural
floor slab.
The concrete screed floating floor consists of a 70 mm. thick layer of
1:1 ½: 3 concrete screed laid on a 25 mm. thick resilient layer of mineral wool
quilt. The quilt is covered with water proof paper to prevent the moisture from
concrete screed travelling below on the structural floor slab.
Floors with suspended ceilings have an added advantage of insulation
against air borne sound, provided a soft floor finish is provided on top to
give necessary insulation against impact sound refer.
Wooden joist floors:
The performance of wooden joist floors is greatly influenced by the
amount of indirect or flanking sound, transmitted through the walls. This
factor is not important in concrete floors, since the concrete floors are
considered heavy, rigid and stiff enough to restrain the vibration from the
walls. In timber floor, this factor can be taken care of by increasing the
stiffness and thickness of the wall below the floor. Alternatively, the floor
should be made heavy and stiff enough to reduce vibrations of the walls. A
noteworthy feature in the insulation of these floors are the floating floor
effect achieved by using a 25 mm. thick mineral wool or glass wool quilt which
is dropped over the joist and turned up at the edges of the boards. The
efficiency of the flooring system is improved by increasing its weight with
pugging between the joints.
ACOUSTICAL DESIGN OF AUDITORIUM
The important factors which influence the acoustical design of an
auditorium are the volume, the shape and the sound absorption.
The volume of the auditorium should be in proportion to the intensity of
the sound that is expected to be generated in the hall. In deciding the volume
of the hall, its height plays a significant role than its length or breadth.
This is on account of the fact that a small increase in height increases the
volume considerably. The volume required for musical concert halls is larger
than that required for halls to be used for speech alone. This is necessary
from the point of view of proper distribution of musical sound. In case,
however, the auditorium is to be used for both musical concerts as well as
speech, the volume of the auditorium should be so chosen as to have a
intermediate between the two. The following data may be used as a rough guide
for deciding the volume of an auditorium.
For cinema theatres
3.7 to 4.2 cu .m. per person
For public lectures halls
2.8 to 3.7 cu. m. per person
For concert halls 4.2 to 5.6 cu.
m. per person.
The shape of auditorium is the governing factor in avoiding the defects
like echoes or other types of reflections of sound waves. On account of the
introduction of sound amplifiers, this aspect of planning has become still more
important. Since the behaviour of sound in a hall is different from that in the
open, it is rather easier to create desirable acoustical conditions in an
auditorium rather than in an open air theatre. Rectangular, fan, horse shoe,
circular or oval are the typical possible shapes of the floor plan of an
auditorium.
The side walls and ceiling are advantageously used to provide favourable
reflections. The walls of the hall are so shaped and placed as to minimise the
possibilities of echoes. Plain walls are normally found suitable. The convex shaped
walls are however, considered best to reduce echoes to a great extent. In
general, the ceiling height of the hall should be about ½ to 2/3 rd of the
width. For reducing the focussing effect of a curved ceiling, the radius of
curvature of the ceiling should be made at least twice the height or less than
half ceiling height. Ceiling plays a significant role in reflecting the sound
to the rear areas of the auditorium. A noteworthy point in the selection and
installation of the ceiling is that it should be ensured that the sound waves
get reflected either directly or via the walls to the audience in such a manner
that the waves do not concentrate at certain spots.
Sound absorbing materials are also used to minimise objectionable
reflection of sound. However, to ensure effectiveness of the sound absorbing
material, the zones of installations have to be decided very carefully. A
variety of acoustical materials are manufactured these days. While making a
selection, due consideration should be given to their appearance, light
reflection, flame resistance, workability, durability and cost.
In addition, the furnishings and the audience contribute to a great
extent to the absorption in the room. In fact the audience may be largest
contributors to the absorption in any auditorium. With a view to ensure optimum
absorption from the audience, the seats in the hall are raked so that the heads
in one row do not intercept the passage of direct sound to the persons in the
row immediately behind.
0 Comments