Tone

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Figure 10.1 A cricket song song consists of repetitive chirps. A chirp, in turn, is composed of numerous tones, which consist of groups of sound pulses.

duced by setting the harp membrane into vibration, just as the skin of a kettle drum vibrates when it is struck by a mallet. The energy to power the harp's vibration is fed into the membrane via a complicated mechanism involving two specialized structures on the cricket's wings. On one wing is the file, which is decorated with a series of ridges, called the teeth. On the other wing is a stiff plectrum. As the wings are moved relative to one another, the plectrum slides across the file, alternately catching on and releasing from the file's teeth, in a kind of catch-and-pawl mechanism. While the plectrum is held at a tooth, the harp membrane is stretched slightly by the steady pressure of the wing muscles. When the plectrum and tooth disengage, the strain on the membrane is released abruptly, and the liberated energy sets the harp membrane into a damped vibration, which dies out after 20-30 cycles or so. The sound emitted by this vibration constitutes the tone. The chirp is produced by the plectrum being dragged across the many teeth on the file, with the number of tones in a chirp corresponding to the number of teeth.

pends upon the species, ranges from roughly ten to at most a few dozen. Each chirp usually lasts about a half-second or less. Each tone in the chirp is a so-called pure tone.1 The frequency of the tone, or the carrier frequency, also depends upon the species, ranging from 1,000 to 6,000 cycles per second, or 1-6 kilohertz (kHz). For those with a musical ear, these frequencies correspond to tones from sixth C (two octaves above middle C) to eighth C (four octaves above middle C).

Cricket song is produced by stridulation (derived from the Latin stridulus, "to creak"), a process in which the cricket's wings are rubbed against one another. There is more to it than that, however: the cricket wing is essentially a means of transmission, coupling the "engine" of sound production, the cricket's wing muscles, to the sound generator, a patch of flexible membrane on the wing called the harp. Sound is pro

1. For a primer on terminology of acoustics, see Box 10A.

Energetics of Sound Production

Despite the presumed benefits crickets might derive from producing loud sounds, they face serious physical obstacles to doing so. The most serious is how efficiently work done by the wing muscles is translated into energy in sound. Acoustical physics tells us that cricket harps should do this very inefficiently.

Crickets produce sound by translating energy from one form (mechanical work by muscle) to another (resonant vibration of the harp). The harp's vibration then must set air into the oscillatory motion that radiates away as sound. Each of these transformations of energy is governed by the constraints of the First and Second Laws: each must conserve energy, and each will proceed at a characteristic inefficiency. Sound communication cannot be fully understood without understanding these constraints.

We begin by considering a simple sound emitter, a

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