Page 241 - The Master Handbook Of Acoustics

216 CHAPTER NINE

coefficients, the idea was conceived to measure the absorption of Coca
Cola bottles. A tight array of 1,152 empty 10-oz bottles was arranged in
a standard 8 9 ft space on the concrete floor of the reverberation
chamber. It was determined that a single, well-isolated bottle has an
absorption of 5.9 sabins at its resonance frequency of 185 Hz, but with
a bandwidth (between 3dB points) of only 0.67 Hz! Absorption of 5.9
sabins is an astounding amount of absorption for a Coke bottle! This is
about what a person, normally clothed, would absorb at 1,000 Hz, or
what 5.9 sq ft of glass fiber (2 in thick, 3 lb/cu ft density) would absorb
at midband. The sharpness of this absorption characteristic is even
more amazing. This would correspond to a Q of 185/0.67 276! As
interesting as these data are, they tell us that leaving an empty Coke
bottle in a studio will not devastate the acoustics of the room, but it
might have a tiny effect at 185 Hz.
In Helmholtz resonators, we have acoustical artifacts that far ante-
date Helmholtz himself. Resonators in the form of large pots were
used in ancient times by the Greeks and Romans in their open-air
theaters. Apparently they were used to provide some reverberation
in this nonreverberant outdoor setting. Some of the larger pots that
have survived to modern times have reverberation times of from 0.5
to 2 seconds. These would also absorb sound at the lower frequen-
cies. Groupings of smaller pots supplied sound absorption at the
higher frequencies.
More recently (that is, in medieval times) such resonators were
used in a number of churches in Sweden and Denmark. 11 Pots like
those of Fig. 9-30 were embedded in the walls, presumably to reduce
low-frequency reverberation that is often a problem in churches.
Ashes have been found in some of the pots, undoubtedly introduced to
“kill the Q” of the ceramic pot and to broaden the frequency of its
effectiveness.
If bottles and ceramic pots are not suitable forms of Helmholtz
resonators for a studio, what is? Figure 9-31 shows a conveniently
idealized square bottle with a tubular neck. This bottle alone would
produce its characteristic tone if one were to blow across the open-
ing. Stacking these bottles does not detract from the resonator
action, but rather enhances it. It is a small step to a box of length L,
width W, and depth H that has a lid of thickness equal to the length
of necks of the bottles. In this lid are drilled holes having the same