In several cricket species (Gryllus campestris L., Gryllus bimaculatus de Geer, and Acheta domesticus L.) the sound-radiating parts of the tegmen have been determined for the calling song. The oscillatory properties of these structures have been analysed.
Extirpation of one harp-area (Fig. 2) lowered the sound pressure level of the first maximum (4–5 kHz) in the sound frequency spectrogram by 18 dB on the average (Fig. 4). This drop was partially reversed by installation of plastic substitutes (Fig. 5).
The radiated sound can be changed by excising teeth on the pars stridens. By this means the damping characteristics of the tegminal resonator were calculated. The logarithmic decrements were between 0.086 and 0.262, depending on the animal.
The sound level distribution on the surface of the tegmen of stridulating males was measured for the 5 kHz-component of the calling song. The fact that a distinct maximum of sound pressure was located on the harp (Fig. 7) demonstrates the sound-radiating function of this structure.
The sound distribution around stridulating animals was measured in the equatorial and median planes (Fig. 8). The sound-radiation is directed. Microphone probe measurements in the 12 mm long cavity under the elevated tegmina showed a caudad directed sound-level-gradient of 9.3 dB on the average.
The cavity under the tegmina has no resonator properties.
The resonance frequency of the harp was calculated to be 5.6 kHz. This value corresponds closely to the measured resonance frequency of the harp, which varies with different specimens of Gryllus campestris from 4.2–5.4 kHz.
Motion pictures of the wing covered with cork powder and excited with sound showed in a direct way the resonator properties of the harp and the mirror (Fig. 9).
The resonance curve of the harp was determined (Fig. 10), and it was shown that the harp is a uniform and linear resonance system (Figs. 11 and 12). The harp frame has no resonator function (Fig. 13).
The amplitude of the harp displacement in response to a sound pulse is an exponential function for the building-up-transient as well as for the dying-out-transient (Figs. 14 and 15).—From the resonance curve and the dying-out-transient the expected phaseshift between the driving force and the resonator displacement can be calculated. Calculated and experimentally determined values for the phase-shift are in good agreement (Fig. 17).
Characteristics of the harp resonator (Q-factor, resonance frequency, response-threshold) were determined. The resonators of the right and left tegmen are differently tuned.
There is a close correlation between the resonance frequency of the harp and the location of the first maximum in the sound frequency spectrogram of the calling song (Table 3).
In comparison with the pressure level of the radiated sound, the response threshold of the harp is rather low. For this reason a stridulating cricket can throw the harp of another animal into resonance (Fig. 18). This has been shown experimentally over distances up to 150 cm.
The harp was artificially damped with a mass and the displacement of the resonance frequency was determined (Fig. 19). From this displacement the mass of the harp was calculated.
In comparison with the harp the mirror is more strongly damped and it is tuned to a higher frequency of 7.2 kHz (Fig. 20 and Table 2).
The tegminal resonators (harp and mirror) of Gryllus bimaculatus and Acheta domesticus (Fig. 21 and Table 2) are comparable to those of Gryllus campestris which have been studied in detail.