Page 250 - The Master Handbook Of Acoustics

225
ABSORPTION OF SOUND

in which
f frequency of resonance, Hz
o
p percent perforation (see Fig. 9-33)
D airspace depth, inches
d thickness of slat, inches
Suggestion: If slats are mounted vertically, it is recommended that
they be finished in a dark color conforming to the shadows of the slots
to avoid some very disturbing “picket fence” optical effects!

Placement of Materials

The application of sound-absorbing materials in random patches has
already been mentioned as an important contribution to diffusion.
Other factors than diffusion might influence placement. If several
types of absorbers are used, it is desirable to place some of each type
on ends, sides, and ceiling so that all three axial modes (longitudinal,
transverse, and vertical) will come under their influence. In rectangu-
lar rooms it has been demonstrated that absorbing material placed near
corners and along edges of room surfaces is most effective. In speech
studios, some absorbent effective at the higher audio frequencies
should be applied at head height on the walls. In fact, material applied
to the lower portions of high walls can be as much as twice as effective
as the same material placed elsewhere. Untreated surfaces should
never face each other.
Winston Churchill once remarked that as long as he had to wear
spectacles he intended to get maximum cosmetic benefit from them.
So it is with placement of acoustical materials. After the demands of
acoustical function have been met, every effort should be made to
arrange the resulting patterns, textures, and protuberances into esthet-
ically pleasing arrangements, but do not reverse priorities!

Reverberation Time
of Helmholtz Resonators

Some concern has been expressed about the possibility of acoustically
resonant devices, such as Helmholtz absorbers, “ringing” with a
“reverberation time” of their very own adding coloration to the voice