Page 282 - Master Handbook of Acoustics

FIGURE 12-29 The operation of a bass trap depends on reflections of sound from the bottom of the
trap. The pressure for the frequency at which the depth is a quarter wavelength is maximal 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 maximal. Absorbent placed where the particle velocity is maximal will
absorb sound very effectively. The same action occurs at odd multiples of the quarter wavelength.

The concept of wall reflection and quarter-wavelength depths (graphically portrayed for drapes in
Fig. 12-17) also applies to bass traps. The sound pressure at the bottom of the cavity is maximal 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 velocity is maximal, which results in two phenomena. First, a
glass-fiber semirigid board across the opening offers great friction to the rapidly vibrating air
particles, resulting in maximum absorption at this frequency. Second, the zero pressure at the opening
constitutes a vacuum that acts like a sound sink. The bass trap’s effect, then, is greater than its opening
area would suggest.

The bass-trap effect, like the drape spaced from a reflective wall, occurs not only at a quarter-
wavelength depth, but also at odd multiples of a quarter wavelength. Large trap depths are required
for very low bass frequencies. For example, a quarter wavelength for 40 Hz is 7 ft. Unused spaces
above control room ceilings and between inner walls and outer shells are often used for trap space.
The famous Hidley bass trap design is an example of this type of trap.

Helmholtz (Volume) Resonators

The Helmholtz type of resonator is widely used to achieve absorption at lower audio frequencies.
The operation of resonator absorbers can be easily demonstrated. Blowing across the mouth of any