Page 242 - The Master Handbook Of Acoustics

217
ABSORPTION OF SOUND

FIGURE 9-30
Pots embedded in the walls of medieval churches in Sweden and Denmark served as
Helmholtz resonators, absorbing sound. Ashes, found in some of the pots, apparently
11
served as a dissipative agent. (After Brüel. )
diameter as the holes in the neck. It is just a bit harder to realize that
partitions between each segment can be removed without greatly
affecting the Helmholtz action. In this
way, a Helmholtz resonator of the perfo-
rated face type can be related to oddly
shaped bottles, giving something of a
visual picture of how perforated face res-
onators perform.
In a similar way, Fig. 9-32 illustrates
another bottle with an elongated slit
neck. These, too, can be stacked, even in H
multiple rows. It is but a short step to a
L
slot-type resonator. The separating walls
in the air cavity can also be eliminated
without destroying the resonator action. W
A word of caution is in order, however.
Subdividing the airspace can improve FIGURE 9-31
the action of perforated face or slit res- Development of perforated-face Helmholtz resonator
onators but only because this reduces from a single rectangular-battle resonator.