6. Sound Insulation

It is important to avoid confusion between sound absorption and 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.

(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) Airborne Sound Insulation : the insulation against noise originating in air, e.g. voices, music, motor traffic, wind.

(b) Impact Sound Insulation : 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 Partition

Airborne 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 indirect or flanking 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 high stiffness. Details of some of these are shown in Table 2.

 

(a)

(b)

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

Figure 9 Paths for Sound Transmission between Adjacent Rooms

 

Table 2 Critical Frequencies & Densities for Some Common Materials

6.3 Requirements to Achieve Good Sound Insulation

(a) Mass

The 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

 

(c) Apparent Insulation by the Use of Absorbents

While absorbent materials should not be confused with insulating ones some benefit can often be achieved by reducing the reverberant sound in an enclosure. In practice the 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) Completeness

All 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, the lightweight, 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 4

A partition of total area 10 m2 consists of a brick wall plastered on both sides to a total 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

 

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 Insulation

Insulation 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,

(c) Resilient (anti vibration) mounts, and

(d) Floating floor.