Page 240 - The Master Handbook Of Acoustics
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ABSORPTION OF SOUND
expanded metal, metal lath, hardware cloth, or even perforated vinyl
wall covering can be used as a cover. Do not be surprised by absorption
coefficients greater than 1.0. This manufacturer elected to publish the
coefficients actually obtained from laboratory measurements rather
than arbitrarily reducing those over 1.0 to 0.99 or 1.0. The greater
absorption of the standard 8 9 ft sample results from edge diffraction
and other effects that make the sample appear larger acoustically than
it really is.
Helmholtz Resonators
The Helmholtz type of resonator is widely used to achieve adequate
absorption at lower audio frequencies. There is nothing particularly
mysterious about such resonators; in fact they pop up in various forms
in everyday life. Blowing across the mouth of any bottle or jug pro-
duces a tone at its natural frequency of resonance. The air in the cav-
ity is springy, and the mass of the air in the neck of the jug reacts with
this springiness to form a resonating system, much as a weight on a
spring vibrating at its natural period. Change the volume of the air cav-
ity, or the length or diameter of the neck, and you change the fre-
quency of resonance. Such a Helmholtz resonator has some very
interesting characteristics. For instance, sound is absorbed at the fre-
quency of resonance and at nearby frequencies. The width of this
absorption band depends on the friction of the system. A glass jug
offers little friction to the vibrating air and would have a very narrow
absorption band. Adding a bit of gauze across the mouth of the jug or
stuffing a wisp of cotton into the neck, the amplitude of vibration is
reduced and the width of the absorption band is increased.
The sound impinging on a Helmholtz resonator that is not absorbed
is reradiated. As the sound is reradiated from the resonator opening, it
tends to be radiated in a hemisphere. This means that unabsorbed
energy is diffused, and diffusion of sound is a very desirable thing in a
studio or listening room.
Bottles and jugs are not appropriate forms of a Helmholtz resonator
with which to apply the resonance principle in studios. An interesting
experiment conducted many years ago at Riverbank Acoustical Labo-
10
ratories bears this out. To demonstrate the effectiveness of a continu-
ously swept narrow-band technique of measuring sound absorption