Page 252 - The Master Handbook Of Acoustics

227
ABSORPTION OF SOUND

stands for energy, time, frequency. By means of this program the
detailed low-frequency modal structure of the room is revealed in Fig.
9-38A and B. The mode that caused an audible distortion in room
sound is the one with the pronounced reverberant “tail” at 47 Hz on
the extreme left of Fig. 9-38A. Once the culprit is identified, steps can
be taken to reduce its activity so that it will behave as the other modes
of the room.
The solution rests in the placement of a highly tuned absorber in
the room at some point of high pressure of the 47-Hz mode. The

Before
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)
A

FIGURE 9-38A
Low-frequency model structure of the sound field of a small room before introduction of the tuned Helmholtz
resonator absorber.