Page 254 - The Master Handbook Of Acoustics

229
ABSORPTION OF SOUND

The length of the PVC pipe, Fig. 9-
39(A), is varied to tune the resonator to
specific frequencies. An absorbent par-
tially fills the resonator. The Q-factor of the
finished resonator was measured at Q 1.

Increasing Reverberation Time

Low-Q Helmholtz resonators are capable
of shortening reverberation time by
increasing absorption. High-Q resonators
can increase reverberation time through
storage of energy as described by Gilford. 2
To achieve the high Qs necessary, ply-
wood, particleboard, masonite, and other
such materials must be abandoned and
ceramics, plaster, concrete, etc., used in
resonator construction. By proper tuning
of the resonators, the increase in reverber-
A
ation time can be placed where needed in
regard to frequency. FIGURE 9-39A
Typical Helmholtz resonator made of readily
available materials.
Modules

The British Broadcasting Corporation has
pioneered a modular approach to the
Wood top
acoustical treatment of their numerous
small voice studios, which is very interest-
ing. 17 Because they have applied it in sev- Tube
eral hundred such studios economically
and with very satisfactory acoustical Duct
results, it deserves our critical attention. Laminated
Basically, the idea is to cover the walls with wood bottom
standard-sized modules, say 2 3 ft, having
Helmholtz Resonator Design
a maximum depth of perhaps 8″. These can
B
be framed on the walls to give a flush sur-
face appearing very much like an ordinary FIGURE 9-39B
room, or they can be made into boxes with Details of Helmholtz resonator design.