Building Construction IV:

Chapter 4. Insulation: Sound - Introduction

Chapter 4: Insulation

Sound - Introduction
Sound is caused by the vibrating body. Sound requires medium (solid, liquid or gas) for transmission.
Sound is transmitted through the air in the form of waves.

Inverse square law:

Sound waves from a point outdoors with no obstructions are virtually spherical and expand outward from
the source as shown below. Intensity (W/m2) varies inversely with square of the distance between the
point of observation and the source of sound.

𝐼1 𝑑2 2
= ( )
𝐼2 𝑑1

I = Sound intensity (W/m2)

d = distance from source (ft. or m)

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Building Construction IV:
Chapter 4. Insulation: ACOUSTICS:

Table: Sound source and their intensity

S.N Source Intensity level (dB) Threshold of
Range Average feeling
1. Threshold of Audibility 0 Very very faint
2. Falling leaves or whisper 0 - 20 10 Very faint
3. Quiet Conversation 20-40 30 Faint
4. Average office 40-60 50 Moderate
5. Noisy office, street noise, 60-80 70 Loud
average factory
6. Noisy traffic road, Highway, 80-100 90 Very Loud
factory area
7. Thunder, aeroplane, artillery, 100-120 110 Deafening
pneumatic hammer
8. Aeroplane at a distance of 4m 120-140 130 Pain and
deafening

ACOUSTICS:
Architectural acoustics is the science of controlling sound within buildings. It deals with the design and
construction of different building units to set an optimum condition for producing and listening to sound
(Speech, music etc). Architectural requirements should always be considered during the earliest stage of
design.

The acoustic situations can be described by three parts: source, path and receiver.
Source : Human speech, HVAC, Outside disturbances
Path : Air, earth, building materials
Receiver : Humans

Successful designers provide spatial relationships, cubic volumes, shapes, use of proper acoustic materials
so as to maintain design quality while best serving their intended purposed, whether it be for work, play or
rest.

Architectural acoustics:
Architectural acoustics plays an important role in planning buildings and deals with spaces like classrooms,
conference hall, cinema hall, theatre, FM studio, Music recording studio, amphitheatre, library, school,
hospitals, church etc.
The general requirements of a good acoustics are:
- Even distribution of sound
- Proper reverberation time
- Absence of unsuitable reflections like echos
- Acceptable background noise.

Environmental acoustics: It mainly deals with noise control and sound pollution in urban areas due to
noise of vehicle, factory, air traffic, railways and other activities. Based on the noise intensity level, urban
areas can be divided into following zones:

Group A – 80db – Industrial zone

Group B – 70 db – Public zone
Group C – 60 db – Residential zone
Group D – < 50db – Quite or noise free zone.

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Building Construction IV:
Chapter 4. Insulation: Defects due to reflected sound

PROPERTIES OF SOUND
 Transmission (T)
 Reflection (R)
 Absorption (A)

I is the intensity of the sound, If I = 1, T+R+A = 1

Defects due to reflected sound

1. Echo:
Echo is a repetition of the sound caused by its reflection, even after the source has ceased. Time interval of
repetition is 1/10th of the second or more. This is observed when the reflecting surface is more than 17 m
and when the shape of the hall/auditorium/room is curved with smooth character.

Echo can often be annoying and may cause disturbance in voice clarity. This defect can be removed by
selecting proper shape of the hall and by providing rough and porous interior surfaces to disperse energy
of echos.

2. Reverberation
It is the prolongation of the sound after the source of sound has ceased. It is due to multiple reflections in
an enclosed space. A certain amount of reverberation is desirable, specially for giving richness to music,
but too much of it can be a problem.

The time which the sound persists is called the reverberation time of the sound in the hall. In other words,
it is the time required for sound energy to decay by 60 db after the source has stopped.

Other defects: Sound Foci, Dead Spots

Reverberation time:
In an enclosed environment sound can continue to reflect for a period of time after a source has stopped
emitting sound. This prolongation of sound is called reverberation. Reverberation time (RT60) is defined as
the time required, in seconds, for the average sound in a room to decrease by 60 decibels after a source
stops generating sound.

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Building Construction IV:
Chapter 4. Insulation: Classification of noise (Sound):

Reverberation time is the primary descriptor of an acoustic environment. A space with a long
reverberation time is referred to as a "live" environment. When sound dies out quickly within a space it is
referred to as being an acoustically "dead" environment. An optimum reverberation time depends highly
on the use of the space. For example, speech is best understood within a "dead" environment. Music can
be enhanced within a "live" environment as the notes blend together. Different styles of music will also
require different reverberation times.

RT depends upon:
 The intensity of sound generated
 The absorption of the floor, ceiling, wall surface
 The absorption by the audience
 The absorption by furniture and finishing materials
 Length of the sound path

Classification of noise (Sound):

1. External (Outdoor) sound or noise - Road traffic, railways, aeroplanes, moving machinery, machines in
nearby factory or building etc.

 Major aspects of a building design

 The main barrier is the shell or the envelop of the building

Factors to be considered are:

1. Mass of the enclosing structure
2. The continuity of the structure
3. Isolation by double leaf construction

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Building Construction IV:
Chapter 4. Insulation: Classification of noise (Sound):

The main problem with insulation against external noise are windows, particularly if these are to be
opened. Any window open will give sound reduction of about 10 dB as compared to 40-50 dB reduction of
cavity walls. Closed window of single glazing gives 20 dB reduction. Thus window to wall ratio will affect
the overall sound reduction of the enclosing structure. Double glazing grossly improves the sound
insulation properties, provided the following points are observed

 Sound insulation increase with the distance between the glazed units; for a reduction of 40 dB, the
air space should be 150-200 mm wide.
 Sound insulation increases with glass thickness and .
 Double glazing designed to improve the thermal properties of a window have no real value sound
insulation.

2. Internal (indoor) sound or noise: Conversation of people, moving of people or furnitures, crying of
babies, playing of radios or other musical instruments, operation of cisterns and water closets, noise of
typewriter, banging of doors etc.

Classification of sound based on the source (Alternative classification).

1. Airborne noise or sound: Airborne sound is the one which is transmitted through air and travel directly
to our ear.
2. Impact or structural noise or sound: The sound which is transmitted through the structure is called
impact/ structural sound.

Table: Acceptable indoor noise level:

S.No Type of building Noise level (dB)
1. Radio and TV studios 25-30
2. Music room 30-35
3. Hospital and auditoria 35-40
4. Apartments, hotels, and homes 35-40
5. Conference rooms, small offices, and libraries 35-40
6. Court rooms and classrooms 40-45
7. Large public offices, banks and stores 45-50
8. Restaurants 50-55
9. Factories 55-65
Source: Building Construction, B.C. Punmia

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Building Construction IV:
Chapter 4. Insulation: Sound Absorbents:

Sound Absorbents:
Sound absorbents are the special sound absorbing materials used over the surfaces of the room to
increase sound absorption. Broadly speaking, the material that is hard, rigid and non-porous surface
provides least absorption, whereas those which are flexible, soft, and porous and can vibrate, absorb more
sound. A hard surface reflects sound. Soft and porous materials reflect less sound and permit sound
waves to penetrate into their pores.

Sound absorbents can be broadly classified into four types:

a. Porous absorbents/materials
b. Panel Resonators
c. Cavity Resonators
d. Composite type of absorbents

a. Porous absorbents:
These are soft materials with pores inside. Absorption
in porous material is mainly due to frictional losses
which occur when sound waves cause to and fro
movement of the air contained in the material. The
efficiency depends upon the porosity, the resistance to
air flow through the materials and the thickness. Eg.
Wool, Glass, Silk, Wood wool, Curtains and other soft
furnishings, drilled fibre boards and acoustic plasters.
These types of absorbents are generally used to absorb
high frequency sound waves.

b. Resonant Panels:
These panels act as sound absorbents and also as building panels. There is an air gap between the thin
panels (eg. Plywood, hard board) and the backing wall. These panels absorb the sound by damping the
sympathetic vibrations in the panel, caused by sound pressure wave of appropriate frequency, by means
of air space behind the panel. These panels absorb sound only at lower frequencies. The frequencies at
which panels vibrate depend upon their weight and depth of air spaces behind them. Damping can further
be improved by filling the air space with some porous material.

c. Cavity Resonators
Cavity resonator is virtually a container with a small opening, where the absorption takes place by the
resonance of the air. Cavity resonators can be designed to absorb sound of any frequency. These cavity

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Building Construction IV:
Chapter 4. Insulation:

resonators are adopted for sound absorption of a particular frequency such as individual machines, air
conditioning plant etc.

d. Composite type absorbents

These absorbents combine the benefits of all three qualities as described above. They can absorb wide
range of frequencies. They consist of a perforated panel fixed over an air space containing porous
absorbents as shown in the figure below:

The panels may be of metal, plywood, hard board, plaster board etc. The area of holes in the panel should
vary between 10 – 20 % of the total area of the panel. Mineral wool or glass wool is commonly used for
porous materials and it should be placed behind the perforated panel for maximum efficiency.

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Building Construction IV:
Chapter 4. Insulation: Sound Insulation/Sound Absorption

Sound Insulation/Sound Absorption

Sound Absorption: Prevention of reflection of sound waves.
Sound Insulation: Prevention of transmission of sound
The main ideas behind noise control are
 Reduce the noise emitted at the source by such devices as mufflers or mounting machinery on
resilient pads,
 Provide a reasonable degree of sound insulation reduce the amount of sound transmitted, and
 Isolate the source and the receiver.

General considerations for noise control / sound insulation.

 First of all to insulate against air borne noise is to isolate source.
 Orientation or residential buildings – no doors/windows open to noisy roads etc .
 Rooms in buildings properly planned
 Provision of furnishing material and lining of walls and ceiling by means of air filled materials (felts.
glass wool) help to reduce noise.
 Transmission of noise by vibration can be prevented by making the walls. Floors, partitions very
rigid massive/heavy.
 Control of impact sound is possible to some extent by providing resilient materials like carpet.
linoleum, cork etc to carry the whole surface Oil resilient materials like a floating floor.
 Structure borne noises/sounds can be prevented by discontinuing the path of the vibrating waves
and by using sound absorbing materials. Construction of expansion joints in large framed
structures acts in this manner. and
 Headphones/ear plugs, if used, reduced the noise to the extent of 20 dB to 30 db.

Constructional measures
The most effective barrier to the passage of sound is a material of high mass. With modern materials and
methods this form of construction is both impracticable and uneconomic. Unfortunately modern living
with its methods of transportation and entertainment generates a considerable volume of noise, and
therefore some degree of sound insulation in most buildings is not only desirable but also becomes
mandatory.

1. Walls / Partitions – barriers to transmission of sound/noise

1.1 Rigid and homogenous Partition Wall
 Sound insulation increases with the increase in thickness of the wall (Mass Law)
 Uneconomical to increase the thickness beyond certain limit.
 A solid one brick thick wall plastered on both side can act as effective insulation wall
 Hard reflecting outside increases insulation

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Building Construction IV:
Chapter 4. Insulation: Constructional measures

Table: Insulation properties of different partition walls

S.No. Type of construction Approximate wt. in Average transmission

Kg/sq.m of wall area loss (dB)
1 One brick thick wall 490 50

2 One and half brick thick wall 710 53

3 Cavity wall each of ½ brick thick 490 50-53

and 5cm cavity

4 Cavities wall each 10cm thk. 310 50

clinker block with 5cm cavity

5 ½ brick wall with 13mm plaster 170 45

on both sides

6 20mm thick hollow dense 185 45

concrete block with 13mm
plaster
7 Gypsum wall board partition wall 70 45
on timber frames

8 75mm thick hollow clay block 110 36

wall with 13mm thick plaster on
both sides

1.2 Hollow and composite partition/ double wall:

 Suitable combination of light weight materials can provide high insulation value
 A cavity wall construction with resilient materials in cavity is the best example
 Minimum connecting ties of flexible nature
 The cavity should be at least of 50mm

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Building Construction IV:
Chapter 4. Insulation: Constructional measures

2. Floors / Ceiling – horizontal barrier to noise

 Act as a horizontal barriers to noise
 Solid ceiling and floor offer adequate insulation against air-borne sounds but are poor against
impact sounds
 Structure borne sounds transmits more easily

Methods adopted to control such noise are:

2.1 Use of resilient surface material on floors

 Over the massive and rigid floor slabs a layer of resilient materials are used
 5-10 dB of insulation can be obtained over a bare concrete floor
 Softer the material, greater the insulation value
 E.g. Linoleum, insulation board, cork, mastic asphalt, carpet etc

2.2 Providing a floating floor construction

 The main principle is insulation from any other part of structure by creating discontinuity
 A floating floor is made to rest or float over the existing floor by means of a resilient material
 Resilient materials are glass wool, mineral wool, quilt, felt, cork, rubber etc
 Can be used for both concrete and wooden floors
 Also improves insulation against air-borne sound

2.2.1 Concrete floor with Floating Concrete Screed:

 This consist of concrete screed (1:1 ½: 3) not less than 50mm in thickness on a resilient layer of
mineral wool quilt, laid over the structural floor slab, turned up against structural walls at all edge.

 The quilt is covered with a waterproof paper to prevent wet concrete running over it.

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Building Construction IV:
Chapter 4. Insulation: Constructional measures

2.2.2 Concrete floor with a floating wooden raft:

 This consists of floor boarding nailed to wooden battens 50mmX50mm to form a raft, which is laid
over 20mm thick resilient quilt

2.2.3 Concrete floor with a suspended ceiling and soft floor finish or covering:
 Provides insulation against air-borne sound
 If sufficiently thick, provides insulation against impact sounds too

2.2.4 Concrete flooring with light weight concrete screed and soft floor finish:
 50mm thick lightweight concrete screed for insulation against air-borne sound but in
addition resilient floor finish for both air-borne &impact sound
 The use of dense topping is because:
- Provides a suitable base for floor finish
- Provides an impervious airtight layer sealing the top of the lightweight screed

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Building Construction IV:
Chapter 4. Insulation: Constructional measures

2.2.5 Heavy concrete floor with soft floor (resilient) finish or covering
 The heavy concrete floor gives adequate insulation against air borne sound whereas soft floor
finish of a resilient material (thick carpets, under felt, thick cork, quilt or soft rubber, linoleum etc)
gives insulation against structure borne and impact sounds.

2.3 Providing wooden floors:

 In floors constructed of wooden joists, sound insulation becomes difficult if heavy mechanical
impact sounds encounter.
 This problem does not occur in concrete floors since they are heavy, rigid and stiff enough to
sustain the vibrations of the walls due to impact sounds
 In timber floor walls, below the floor are made thicker or floor itself heavy and stiff to reduce the
vibration of walls

2.3.1 When supporting walls are 100mm or less:

 A ceiling of expanded metal and plaster loaded directly with plugging of 50mm of dry sand
 The floating floor consists of a raft rest on resilient quilt arranged over wooden joists.

2.3.2 When supporting walls are 200mm or more:

 This consist of a plaster boards with plaster finish and directly loaded with plugging of slag-wool or
mineral wool.

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Building Construction IV:
Chapter 4. Insulation: Constructional measures

2.3.3 Use of suspended ceiling with air space:

 An independent false ceiling is constructed and is connected below the solid or wooden floor by
means of hangers, acoustic clips etc. with an air space in between
 The construction offers good insulation against both air-borne as well as structure sound

3. Windows and Doors

Sound Insulation of single windows is improved by making them air-tight and by filling the air space at the
edges of such panes with sound absorbing material. Sound insulation or transmission loss further increases
with the increase in the thickness of glass. Excellent sound insulation or sound reduction is obtained by
constructing glazed windows with double panes of glass. The air space between the two glazing panes is to
be 150 - 200mm. Such windows are especially useful for sound-proofing of rooms such as broadcasting,
studios, etc..

The transmission loss or sound reduction in doors increases with the increase in weight. The sound
insulation can further be improved by packing the sound absorbing material in the space between the
jamb and frames or in shutters itself.

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Building Construction IV:
Chapter 4. Insulation: Constructional measures

4. Insulating Sanitary Fittings

For improved insulation water closets should be insulated. The pan should be made to rest upon a thin pad
of felt, linoleum, cork, rubber or other suitable resilient materials. Cisterns should be fixed direct to a bed-
room wall and should be fixed upon insulators fixed to the brackets. The pipes should be properly
wrapped, where they pass through walls or floors and be held in insulated clips.

5. Machine Mounting and Insulation of machinery

The machines or mechanical equipments such a refrigerators, lifts, fans, etc., create vibrations in an
adjoining building and are transmitted further away as structure-borne sound. To reduce this structure-
borne sound machines should be insulated by resting them on resilient supports or mountings such as
Sled springs, rubber, cork, etc ..

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