Do Cicadas Radiate Sound through their Ear-Drums?

Sound output was investigated in males of two cicada species, Cyclochila australasiae Donovan and Macrotristria angularis Ståhl. These are large insects, about 4.5 cm in length, with a typical arrangement of sound-producing organs.

Songs produced by both species consist of continuous trains of sound pulses, with a fundamental frequency close to 4 kHz. Higher harmonics fall below the 4 kHz peak by 20–30 dB. These songs are the loudest yet recorded among insects: HOdBSPL at 20cm for the protest songs of both species, and values as high as 115 dB for the vigorous calling songs of C. australasiae (mean 113 dB).

The male tympanum (ear-drum) is between 3.3 (M. angularis) and 5.5 (C. australasiae) times greater in area than that of the female, which does not sing. The tympana and folded membranes, as well as the sound-generating tymbals, vibrate vigorously during singing; other parts of the insect do not vibrate.

Sound output is greatest at the gap between the tympana and their protective coverings, the opercula. High values are also found close to the tymbals but not over the rest of the body. When the gap between tympana and opercula is held closed, rather than open, sound output falls by 11 dB. In the field, calling males adopt a characteristic posture, which keeps this opercular gap wide open.

Ablating the tympana makes no difference to the sound output. But ablating the posterior half of the abdominal air sac produces a mean fall of 8.6 dB, together with a great broadening of the song's frequency content.

The above results support the conclusion that the majority of sound is radiated through the tympanal opening in typical cicadas, with the tympana being driven passively by the resonant vibrations of air in the air sac. This system can be modelled as a Helmholtz resonator, with the tympanal opening representing the neck of the resonator.