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The ultrasonic call has been "heterodyned" – multiplied by a constant frequency to produce frequency subtraction, and thus an audible sound – by a bat detector. A key feature of the recording is the increase in the repetition rate of the call as the bat nears its target – this is called the "terminal buzz".
Laryngeally echolocating bats, in general, produce ultrasonic waves with their larynx that is specialized to produce sounds of short wavelength. The larynx is located at the cranial end of the trachea and is surrounded by cricothyroid muscles and thyroid cartilage. For reference, in humans, this is the area where the Adam's apple is located ...
To avoid this type of jamming, bats typically wait enough time for echoes to return from all possible targets before making the next sound. This can be seen clearly when a bat attacks an insect. The bat produces sounds with progressively shorter time intervals, but always allowing enough time for sounds to travel to the target and back. [6]
Microbats and a few megabats emit ultrasonic sounds to produce echoes. Sound intensity of these echos are dependent on subglottic pressure. The bats' cricothyroid muscle controls the orientation pulse frequency, which is an important function. This muscle is located inside the larynx and it is the only tensor muscle capable of aiding phonation ...
In any case, the most sensitive range of bat hearing is narrower: about 15 kHz to 90 kHz. [25] Bats navigate around objects and locate their prey using echolocation. A bat will produce a very loud, short sound and assess the echo when it bounces back. Bats hunt flying insects; these insects return a faint echo of the bat's call.
Frequency division (FD) bat detectors synthesise a sound which is a fraction of the bat call frequencies, typically 1/10. This is done by converting the call into a square wave, otherwise called a zero crossing signal. This square wave is then divided using an electronic counter by 10 to provide another square wave.
The potential for ultrasonic imaging of objects, in which a 3 GHz sound wave could produce resolution comparable to an optical image, was recognized by Sergei Sokolov in 1939. Such frequencies were not possible at the time, and what technology did exist produced relatively low-contrast images with poor sensitivity. [34]
A frequency division bat detector analyses each bat call and re-synthesises it at typically a tenth of its frequency to make them audible as explained in the bat detector article. It can produce anomalies with sounds with a random structure and can only process periodic or tonal calls with a measurable frequency, but a recording can be used to ...