(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.
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.
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
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.
(a) Soft floor finish (carpet, cork,
vinyl, rubber, etc.),
(b) Resiliently suspended solid
(c) Resilient (anti vibration) mounts,
(d) Floating floor.