Acoustic - Sound Transmission Coefficiency

7/27/2019 Acoustic - Sound Transmission Coefficiency

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5. Sound Transmission Coefficient(a) Sound transmission coefficient (T):It is the ratio which the sound energy of a given frequency transmitted through a

surface to that incident on it.(b) Sound Reduction Index (SRI):

(11)For composite partitions of n nos. of surface parts, the average transmissioncoefficient TAV can be found from the following equation:

(12)where Ti = transmission coefficients of the ith partAi = area of the ith part

A = total area of partition =

6. Sound InsulationIt is important to avoid confusion between sound absorptionand sound insulation.(a) Sound absorption is the prevention of reflection of sound or alternatively, a

reduction in the sound energy reflected by the surfaces of a room.


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(b) Sound insulation is the prevention of transmission of sound or alternatively, a

reduction of sound energy transmitted into an adjoining air space.Two types of sound insulation are to be dealt with in building construction, as

illustrated in Figure 8.(a) Ai rborne Sound Insulation: the insulation against noise originating in air, e.g.

voices, music, motor traffic, wind.(b) Impact Sound I nsulation: the insulation against noise originating directly on a

structure by blows or vibration e.g. footsteps above, furniture being moved, drilling

and hammering the structure.

6.2 Airborne Sound Insulation by PartitionAirborne sound can be transmitted in a receiving room via some or all of the paths (A)

to (D) as shown in Figure 9. Path (A) is called the direct path.All transmission paths other than path (A) are together termed the indirectorflanking

transmission. This indirect transmission becomes increasingly important when the

insulation requirement of the separating partition is about 35 dB upwards.The ideal material for good sound insulation has a very high mass and low stiffness

but some of the most convenient building material have low mass and relatively highstiffness. Details of some of these are shown in Table 2.


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(a)


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(b)Figure 8 Paths of Sound Transmission (a) Airborne (b) Impact


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Figure 9 Paths for Sound Transmission between Adjacent Rooms

Table 2 Critical Frequencies & Densities for Some Common Materials6.3 Requirements to Achieve Good Sound Insulation


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(a) MassThe average insulation from a single partition can be calculated approximately from :

(13)where, RAV = average sound reduction 2 (dB)m = mass per unit area (kg/m2)

(b) Multiple or Discontinuous Construction

If a higher degree of sound insulation is required, it is advisable to use a multiple

construction built of two or three separated leaves. But it is more expensive and

requires more careful and details of construction.Table 3 illustrates the sound insulation effectiveness of varying window glass

thickness and/or using double-glazing.

Table 3 Sound Insulation of Windows in Facades


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(c) Apparent Insulation by the Use of AbsorbentsWhile absorbent materials should not be confused with insulating ones some benefit

can often be achieved by reducing the reverberant sound in an enclosure. In practicethe placing of absorbent near to noisy machines can be more effective. However it

should be realised that a 10 dB reduction by the use of absorbers would normally be

the limit.(d) CompletenessAll sound leaks are important because sound will travel through any opening with

little loss. For example, a very small air hole in a brick wall can easily reduce

insulation from 50 dB to as low as 20 dB. Cracks, gap around pipe work through

partition, louvred doors, porous construction, etc. are to be avoided. For example, thelightweight, porous sound-absorbing tiles or panels are relatively poor isolators.It should be noted that the overall sound insulation of a construction is greatly reduced

by the presence of areas of poor insulation. When a "weak" element, such as window

or door, is used in a construction, the composite sound reduction index for the

combination is usually closer to that of the "weaker" element.Example 4A partition of total area 10 m2 consists of a brick wall plastered on both sides to atotal thickness of 250 mm and contains a window of area 2 m2. The brickwork has a

sound reduction index of 51 dB and the window 18 dB at a certain frequency.

Calculate the sound reduction of the complete partition at this frequency.Brickwork: if TB is the transmission coefficient of the brick, then


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Window : if TW is the transmission coefficient of the window, then

Now

Actual sound reduction index in dB :

It can be seen that the poor insulation of the window of small area reduces the overall

insulation very considerably. If the window had fitted badly the insulation would be

even lower.

6.4 Impact Sound InsulationInsulation against structure-borne (or impact) noise can be achieved by the use of :(a) Soft floor finish (carpet, cork, vinyl, rubber, etc.),(b) Resiliently suspended solid ceiling,


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(c) Resilient (anti vibration) mounts, and(d) Floating floor.

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Acoustic - Sound Transmission Coefficiency