High resolution bat sonar analysis

Full-spectrum analysis of bat sonar typically makes use of short-time discrete Fourier transforms (STFT, more commonly known as DFT). There are quite a few alternative analysis methods, but none are as generally robust with short time-varying signals, or as computationally efficient, as DFT. [Zero-crossing analysis can be a useful tool for identification, but all harmonic information is discarded.] The digitised ultrasound signal is analysed by DFT in short samples, and the result is a frequency vs. time spectrogram. For DFT the frequency resolution is roughly 1 part in N (the sample size). Frequency resolution can be refined by increasing the length of the sample, but the sample is then acquired, and averaged, over a longer time. Furthermore, processing time increases proportionally to Nlog2N. Faster A-to-D converters help, but at the expense of higher power consumption (important in hand-held devices) and larger data files.

Typically, bat sonar is analysed with DFT sample sizes of 256 or more, and sampling rates of up to 400 kS/s, giving spectrogram frequency resolutions of about a kHz, and a time resolution of around a millisecond. This is barely sufficient for displaying sonar from many European and US bats, but seriously inadequate for detailed study of sonar from some tropical bats operating with very high frequencies, or from common bats producing very short sonar pulses.

Standard DFT algorithms discard phase information, but this can be re-introduced to produce dramatic improvements in time and frequency resolution, so that DFT sample size can be much reduced. For instance, a 64-point DFT incorporating this “phase integrated” technique produces frequency and time resolutions of 0.05 kHz and 50 µs (with a 12-bit AtoD converter sampling at 600 kS/s). Surprisingly, this technique has been ignored by bat researchers, although it has been applied in other research fields for over 50 years.

The bat sonar gallery shows spectrograms produced by a hand-held analyser using this technique. Here the frequency resolution is limited by the 320x240 pixel screen - much more detail is revealed by expanding the frequency scale (see HD spectrograms) or transferring data to Excel (see HD Excel spectrograms).

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