Page 145 - The Master Handbook Of Acoustics

120 CHAPTER SIX

impossible. This chapter is a very brief overview of both analog and
digital sound processing principles and practice, especially the prin-
ciples.

Resonance

The man on the stage is doing the old trick of breaking the wine glass
with sound. Instead of doing it with the diva’s voice, he holds the gob-
let in front of the loudspeaker, which shatters it as a high-intensity
tone is emitted. The secret lies in the brief preparation made before the
audience was assembled. At that time he placed a small coin in the
goblet and held it in front of the loudspeaker as the frequency of the
sine generator was varied at a low level. He carefully adjusted the gen-
erator until the frequency was found at which the coin danced wildly
in the glass. During the demonstration no tuning was necessary, a blast
of sound at this predetermined frequency easily shattered the glass.
The wild dancing of the coin in the glass in the preliminary adjust-
ment indicated that the excitation frequency from the loudspeaker was
adjusted to the natural frequency or resonance of the goblet. At that
frequency of resonance a modest excitation resulted in very great
vibration of the glass, exceeding its break-
ing point. As shown in Fig. 6-1, the ampli-
f o tude of the vibration of the glass changes
as the frequency of excitation is varied,
going through a peak response at the fre-
quency of resonance, f o.
Such resonance effects appear in a
Amplitude wide variety of systems: the interaction of

mass and stiffness of a mechanical system,
such as a tuning fork, or the acoustical res-
onance of the air in a bottle, as the mass of
the air in the neck of the bottle reacts with
the springiness of the air entrapped in the
Frequency
body of the bottle. See Helmholtz res-
FIGURE 6-1
onators, Chap. 9.
The amplitude of vibration of any resonant system is Resonance effects are also dominant in
maximum at the natural frequency or resonant fre-
quency (f) and is less at frequencies below and above electronic circuits as the inertia effect of
that frequency. an inductance reacts with the storage