Page 243 - The Master Handbook Of Acoustics

218 CHAPTER NINE

FIGURE 9-32

Development of a slot-type Helmholtz resonator from a single rectangular-bottle resonator.

spurious, unwanted modes of vibration being set up within the air
cavity.

Perforated Panel Absorbers

Perforated hardboard or plywood panels spaced from the wall con-
stitute a resonant type of sound absorber. 12–15 Each hole acts as the
neck of a Helmholtz resonator, and the share of the cavity behind
“belonging” to that hole is comparable to the cavity of the
Helmholtz resonator. In fact, we can view this structure as a host of
coupled resonators. If sound arrives perpendicular to the face of the
perforated panel, all the tiny resonators are in phase. For sound
waves striking the perforated board at an angle, the absorption effi-
ciency is somewhat decreased. This loss can be minimized by sec-
tionalizing the cavity behind the perforated face with an egg crate
type of divider of wood or corrugated paper.