Page 186 - The Master Handbook Of Acoustics

161
REVERBERATION

Size 23.3 16 10 ft
Treatment None
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
Total sabins 339.8 119.6 63.6 53.9 88.6 111.8
Reverberation time 0.54 1.53 2.87 3.39 2.06 1.63
(seconds)
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
182.7
Example: For 125 Hz, RT60 = = 0.54 second
339.8
FIGURE 7-22
Room conditions and calculations for Example 1.

entered under Sa for 125 and 250 Hz. The absorption units (sabins) are
then figured for both materials and for each frequency. The total num-
ber of sabins at each frequency is obtained by adding that of the con-
crete floor to that of the gypsum board. The reverberation time for each
frequency is obtained by dividing 0.049 V = 182.7 by the total Sa prod-
uct for each frequency.
To visualize the variation of reverberation time with frequency, the
values are plotted in Fig. 7-23A. The peak reverberation time of 3.39
seconds at 1 kHz is excessive and would ensure quite poor sound con-
ditions. Two persons separated 10 ft would have difficulty under-
standing each other as the reverberation of one word covers up the
next word.