Page 251 - The Master Handbook Of Acoustics

226 CHAPTER NINE

and music signals. It is true that any reso-
nant system, electronic or acoustical, has a
certain time constant associated with it.
The Q-factor (quality factor) describes the
3dB sharpness of tuning of the Helmholtz res-
onator as shown in Fig. 9-37. Once the
tuning curve has been obtained experi-
mentally, the width of the tuning curve at
the 3 dB points gives f. The Q of the
f
system is then Q f / f, where f is the
o o
frequency to which the system is tuned.
f o Measurements on a number of perforated
FIGURE 9-37 and slat Helmholtz absorbers gave Qs
around 1 or 2 but some as high as 5. Table
Once the tuning curve of a Helmholtz type resonant
absorber has been determined, its Q-factor can be 9-4 shows how the decay rate of resonant
found from the expression, f / f. The “reverberation absorbers of several Qs relates to reverber-
0
time” of such absorbers is very short for Qs normally ation time.
encountered.
With resonant absorber Qs of 100, real
problems would be encountered in a room
having a reverberation time of, say, 0.5 second as the absorbers tailed
off sound for several seconds. However, Helmholtz absorbers with
such Qs would be very special devices, made of ceramic, perhaps.
Absorbers made of wood with glass fiber to broaden the absorption
curve have Qs so low that their sound dies away much faster than the
studio or listening room itself.

Taming Room Modes
The following example of taming a troublesome room mode is based
on Acoustisoft’s ETF 5 room acoustics analyzer program. 23 The ETF

Table 9-4 Sound decay in resonant absorbers.
“Reverberation time”
Q f (seconds)
o
100 100 Hz 2.2
5 100 0.11
1 100 0.022