Spherical sound radiation patterns of singing grass cicadas, Tympanistalna gastrica

The spatial pattern of sound radiation of grass cicadas emitting normally patterned calling songs was measured in the acoustic far field with an array of eight microphones at a distance of 15 cm. The array could be rotated to cover the sphere around the cicada. The sound was analysed in one-third-octave bands with centre frequencies from 3.15 kHz to 16 kHz, the frequency range of the calling song. The seven cicadas studied had very similar spatial radiation patterns, but somewhat different emitted sound powers (range 190-440 nW, mean 280 nW, at 22 degrees C). At low frequencies, the pattern of sound radiation was close to spherical. At higher frequencies, systematic deviations from a spherical pattern were evident. The deviations were of the order of magnitude expected for monopolar sound sources located on sound-shielding bodies. We conclude that, although the singing cicada produces a quite complex acoustic near field, it behaves as a monopole in the far field. These findings are compared with data from a singing grasshopper of similar size, which in the far field behaves as a multipole.