FIG. 4.

Panels A and B show recordings taken by pinging the array with frequency sweeps from an emitter. A shows that for a long (shallow) sweep there is more overlap, between the incident sound and the returning echo, and the beats are more prominent. B shows that for short sweeps there is less overlap. The top panel in each is the spectrogram of the bandpassed signal received at one of the array microphones, the middle panel shows the time waveform of that signal, while the bottom panel shows the envelope extracted by the array hardware. The interaction between the incident sound and an overlapping echo shows up as a beat. In both A and B, the emitter was placed in the plane of the array so as to maximize the echo returning to the microphone from the array backend. Due to limitations of the signal generator used to produce the emmited sounds, each frequency sweep has a brief glitch as it resets to the start frequency and this is visible as a vertical streak in the spectrogram. This does not change any results. C shows the envelope signal taken from an array circuit during a trial with a flying bat. In general the bat sounds recorded at the array do not show apparent effects of overlapping echoes. A detailed explanation is given in the text.