Bat sonar recording
The availability of low power, fast analog-to-digital converters and cheap digital storage devices has opened up new possibilities for recording and analysing bat sonar. It is now possible to record high-quality full-spectrum ultrasound signals without interruption for 12 hours or more with the latest small very high capacity (64 GB) data storage cards. However, it is not so easy to decide how best to process, analyse and present this enormous amount of data.
Sonar from bats covers an enormous frequency range, from about 9 kHz (eg Euderma maculatum) to well over 200 kHz (eg Kerivoula hardwickii). Many common bats such as Pipistrellus pipistrellus produce significant energy in their fundamental frequencies up to 120 kHz, and in their harmonics to over 140 kHz. If the user is interested only in locating and identifying bats, simple heterodyne and/or frequency-division detectors may be sufficient, but in order to carry out more detailed or scientific analysis it is necessary to capture as much as possible of the full frequency spectrum.
Since the 1970's, electret microphones have been available which are adequate for basic studies of many European and north American bats (typically up to about 120 kHz), and more recently MEMS (MicroElectroMechanical Systems) microphones have appeared which are responsive up to about 170 kHz. However, the highest performance is produced by polarised capacitance microphones, and the recently developed NEUmic from Ultra Sound Advice represents a further advance. Recordings made of Kerivoula hardwickii in Brunei using polarised capacitor microphones reveal significant acoustic energy beyond 210 kHz, with greater sensitivity and detail than a MEMS microphone. (Top is NEUmic, bottom is MEMS mic).