Page 188 - The Master Handbook Of Acoustics

163
REVERBERATION

Size 23.3 16 10 ft
Treatment Acoustical tile
Floor Concrete
Walls Gypsum board, 1 /2", on frame construction
Ceiling Ditto
Volume (23.3) (16) (10) 3,728 cu ft

125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz
Material S
sq ft
a Sa a Sa a Sa a Sa a Sa a Sa
Concrete 373 0.01 3.7 0.01 3.7 0.015 5.6 0.02 7.5 0.02 7.5 0.02 7.5
Gypsum board 1,159 0.29 336.1 0.10 115.9 0.05 58.0 0.04 46.4 0.07 81.1 0.09 104.3
Acoustical tile 340 0.09 30.6 0.28 95.2 0.78 265.2 0.84 285.6 0.73 248.2 0.64 217.6
Total sabins 370.4 214.8 328.8 339.5 336.8 329.4
Reverberation time 0.49 0.85 0.56 0.54 0.54 0.55
(seconds)

S area of material
a absorption coefficient for that material
and for that frequency (See Appendix)
Sa S times a, absorption units, sabins
(0.049)(3728) 182.7
RT60
Sa Sa
FIGURE 7-24

Room conditions and calculations for Example 2.

on which the Sabine equation is based) the mean free path (the
average distance sound travels between reflections) is 4V/S or (4)
(3,728)/1,533 = 9.7 ft. If the reverberation time is 0.3 second, there
would be at least 35 reflections during the 60 dB decay. This would
appear to be a fair involvement of all room surfaces.
In a small, relatively dead room such as the average studio, control
room, and listening room, one never gets very far away from the direct
influence of the source. A true reverberant field is often below the
ambient noise level. The reverberation time equations have been
derived for conditions that exist only in the reverberant field. In this
sense, then, the concept of reverberation time is inapplicable to small,
relatively dead rooms. And yet we measure something that looks very
much like what is measured in large, more live spaces. What is it?
What we measure is the decay rate of the normal modes of the room.
Each axial mode decays at its own rate determined by the absorbance
of a pair of walls and their spacing. Each tangential and oblique mode