DIRECTIONAL SOUND PROCESSING AND INTERAURAL SOUND-TRANSMISSION IN A SMALL AND A LARGE GRASSHOPPER

Physical mechanisms involved in directional hearing are investigated i n two species of short-horned grasshoppers that differ in body length by a factor of 3-4. The directional cues (the effects of the direction of sound incidence on the amplitude and phase angle of the sounds at the ears) are more pronounced in the larger animal, but the scaling is not simple. At high frequencies (10-20 kHz), the sound pressures at t he ears of the larger species (Schistocerca gregaria) differ sufficien tly to provide a useful directionality, In contrast, at low frequencie s (3-5 kHz), the ears must be acoustically coupled and work as pressur e difference receivers, At 3-5 kHz, the interaural sound transmission is approximately 0.5 (that is, when a tympanum is driven by a sound pr essure of unit amplitude at its outer surface, the tympanum of the opp osite ear receives a sound pressure with an amplitude of 0.5 through t he interaural pathway), The interaural transmission decreases with fre quency, and above 10 kHz it is only 0.1-0.2, It still has a significan t effect on the directionality, however, because the directional cues are large. In the smaller species (Chorthippus biguttulus), the intera ural sound transmission is also around 0.5 at 5 kHz, but the direction ality is poor, The reason for this is not the modest directional cues, but rather the fact that the transmitted sound is not sufficiently de layed for the ear to exploit the directional cues, Above 7 kHz, the tr ansmission increases to approximately 0.8 and the transmission delay i ncreases; this allows the ear to become more directional, despite the still modest directional cues.