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Figure 10.10 Tuning of the singing burrow of Scapteriscus acletus. a: Intensity of sound relative to normal singing volume. b: Frequency distribution of sound during construction of the burrow. [After Bennet-Clark (1970)]

ergy from muscle that is not available to power the radiation of sound at the resonant frequency, which she can hear.

As the burrow is excavated, the energy emitted in the song undergoes two progressive changes. First, the sound energy levels in all harmonics rises, because of the increasingly horn-like, and therefore amplifying, properties of the singing burrow. Second, energy is shifted away from the second and third harmonics and toward the resonant frequency. We have already encountered this phenomenon in our discussion of horns driving the vibration of reeds. The relative damping of the harmonics and enhancement of the carrier frequency in the mole cricket harp arise in the same way, and the sound becomes progressively more powerful and increasingly pure.

The excavation of the burrow has been described as a "tuning" process, which clearly is the case. Tuning a guitar string, for example, involves an alternating sequence of tightening and loosening of the string. At each step, the sound emanating from the string is compared to a standard—say, the sound from a pitch pipe—until the correspondence between the two is satisfactory. You will recognize this process as a negative feedback loop. The cricket modifies its burrow, emits a chirp (which it can hear), makes a further modification, and then repeats the process (Fig. 10.11). The operation of the feedback loop is evident

Figure 10.11 Simple model for the construction of the tuned singing burrow of the mole cricket. The cricket monitors burrow performance by how it perceives a test chirp it emits. If the perceived test chirp does not meet an ideal criterion, a round of burrow modification is initiated, which alters the energetic interaction between the sound emitter and the burrow. Note that the feedback loops extend outside the organism.

Figure 10.11 Simple model for the construction of the tuned singing burrow of the mole cricket. The cricket monitors burrow performance by how it perceives a test chirp it emits. If the perceived test chirp does not meet an ideal criterion, a round of burrow modification is initiated, which alters the energetic interaction between the sound emitter and the burrow. Note that the feedback loops extend outside the organism.

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