Page 229 - The Master Handbook Of Acoustics

204 CHAPTER NINE

kinds of low-frequency sound absorbers, such as panel absorbers, but
perhaps it should really be reserved for a special type of reactive cav-
8
ity absorber that has been widely used in sound recording circles. A
true bass trap is shown in Fig. 9-20. It is simply a box or cavity of crit-
ical depth but with a mouth opening of size to suit particular pur-
poses. This is a tuned cavity with a depth of a quarter wavelength at
the design frequency at which maximum absorption is desired. Sound
absorption at the lowest octave or two of the audible spectrum is often
difficult to achieve. The bass trap is commonly used in recording stu-
dio control rooms to reduce standing waves at these bass frequencies.
The sound pressure at the bottom of the cavity is maximum at the
quarter-wavelength design frequency. The air particle velocity is zero
at the bottom. At the mouth the pressure is zero and the particle veloc-
ity is maximum, which results in two interesting phenomena. First, a
glass fiber semi-rigid board across the opening offers great friction to
the rapidly vibrating air particles resulting in maximum absorption at
this frequency. In addition, the zero pressure at the opening consti-

Pressure velocity

Particle

Glass fiber
FIGURE 9-20
The bass trap depends upon reflections of sound from the bottom for its action. The
pressure for the frequency at which the depth is a quarter wavelength is maximum at
the bottom and the particle velocity is zero at the bottom. At the mouth, the pressure
is zero (or very low) and the particle velocity is maximum. Absorbent placed where the
particle velocity is maximum will absorb sound very effectively. The same action occurs
at odd multiples of the quarter wavelength.