Page 253 - The Master Handbook Of Acoustics

228 CHAPTER NINE

After

20 40 60 80 100 120 140 160 180 200
0
-4
-8
-12
-16
Level (dB)
-20
-24
-28
-32
-36
-40
0
300
600
900
1200 Time (ms)
1500
1800
20 40 60 80 100 120 140 160 180 200
Frequency (Hz)
B
FIGURE 9-38B

Low-frequency model structure of the sound field of the same small room after the introduction of the tuned
Helmholtz resonator absorber.

locating of high-pressure points of the 47-Hz mode is found simply
by energizing the room with a 47-Hz sine wave on the loudspeaker
and exploring with a sound-level meter. A spot both convenient and
effective will probably be found in a corner.
The finished Helmholtz resonator is shown in Fig. 9-39A. Con-
structional details are obtained from resonator design instruction
included in the ETF 5 program. The resonator is made from a concrete-
forming tube available in hardware stores. Laminated wood covers are
tightly fitted into both ends of the tube.