TRANSMISSION LOSS OF WALLS

The dB reduction of sound as it travels through a wall is termed transmission loss. When sound hits a wall, it shakes it. Then the wall shakes the air on the opposite side and the sound continues on its way modified.
At normal incidence, for a "limp" wall(when a line drawn from source to listener passes perpendicularly through the sheet), the transmission loss (the difference between the transmitted and incident sound intensities) is given by
TL = 20 logW + 20 logf - 28

where W = surface density of the sheet in lbs/ft2 and f is frequency in Hz.
A wall weighing 1 lb/ft^2 will, therefore, have a normal incidence TL of 20 dB at 250 Hz and 6 dB at 50 Hz.

Now if the wall is not normal to the sound wave, for a fixed frequency band, TL will change by 20*1og[sin(angle of incidence)].

Some materials with appreciable stiffness (not limp) may show a "coincidence notch", a region of angle for which the TL is much reduced. This angle may be estimated for steel, aluminum, or glass sheets, from a= sin-1(fc/f)1/2, where the "critical frequency", fc=500/h, and where h is the thickness of the sheet in inches.

Be sure to distinguish between the transmission loss of sound and absorption of sound.

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