How bat detectors work

Why do we need detectors to hear bats?

Most bats supplement their eyesight by echolocation, a system a bit like radar or the sonar used by submarines. Although the sounds made by bats for echolocation are very loud you can't normally hear them, because the frequency (pitch) is too high. By reducing the pitch to match our hearing range we can listen to the bats. This helps in observing them, and identifying their species.

This graph shows the range of frequencies used by humans (blue) and by bats (purple).You can see that the two frequency ranges only overlap by a tiny bit.
In fact, some lucky individuals (usually young people) can hear quite high frequencies, and are able to pick up a very small part of the bats range without equipment.

Here the original bat sounds have been mixed with a signal at 20kHz. The result is that part of the bats sound range has been "translated" down to our human hearing range. Using a heterodyne detector tuned to 20kHz enables us to hear the sounds produced by Noctule bats.

Whatever frequency you have tuned to there is always a large part of the bats spectrum that you will be unable to hear.

The sound file you are hearing is a recording of a Noctule bat through a heterodyne detector. The 'chip-chop' sound is characteristic of noctules.

How do detectors work?

All detectors pick up the sound with a sensor called a microphone or transducer (see below). The sound is converted to a changing voltage and the signal is processed electronically to change it to a sound that we can hear.
This can be done in different ways:

Heterodyning.	This involves mixing the signal with another signal and amplifying the difference signal.
Frequency division.	Digital circuits divide the frequency of the signal from the bat, usually by 16 or 32, so that it lies in the range humans can hear.
Time expansion.	The signal is literally stretched out in time, so that the frequency is reduced, and a 10ms call lasts 100ms or even 1 second.

DSP	Digital Signal Processing: once the signal has been sampled there are many possibilities in processing it to produce audible signals relating to the bat call.

What else can they detect?

Anything that makes an ultrasonic noise can be picked up with a bat detector. Keys jangling, nylon jackets rustling or gravel scrunching will all be picked up – so if you are looking for bats check your clothing is suitable. Other sources of ultrasound include dog training whistles, or gas or steam escaping. Many small animals make high pitched sounds – for example crickets, beetles and small rodents.

Sensors for bat detectors

Basically there are three main sensors that are used in current portable bat detectors - electret, piezo and capacitor microphones. However a new type of microphone looks to have very exciting possibilities for the bat sound spectrum.

MEMS	(micro-electrical-mechanical system)
	The microphone consists of a thin silicon plate which is etched to form a membrane. A special chip inside the package provides drive current for the microphone and amplifies the signal. They are even more robust than Electret microphones and produced with similar sensitivity (-42dB) but lower noise figure. These microphones can also give a digital signal output directly. Smaller than Electret types, down to 1mm square. MEMS Ultrasonic Acoustic Sensor (SPM0204UD5) has a mesh cover and a frequency response +/- 10dB between 10kHz & 200kHz so well suited to bat call detection
Electret microphones
	These are very widely used in budget audio recording equipment. Although the manufacturers usually don’t specify how they respond above audio frequencies (i.e. above 20kHz), they can be effective up to 100kHz and beyond, depending on the type used. They are very small, robust and cheap. The sensor shown here is a miniature electret microphone insert, with a diameter of about 5mm. Hearing aid microphones are even smaller.
PiezoElectric sensors
	These are crystal microphones, used mostly for intruder detection and range finding systems such as parking sensors. They have a very narrow bandwidth, peaking at 38kHz, 40kHz, 45kHz or 50kHz. This means that although they will pick up sounds from some bats such as pipistrelles, they will not pick up other bats whose calls are outside their range of frequency. They are extremely cheap, quite robust and produce a large signal output but are unsuited to wide band applications.
Capacitance microphones	(specified for ultrasonic operation)
	Purpose-made for bat detectors, usually by the manufacturer of the detector. They require high voltages (typically 400V) provided via an inverter, which tends to limit battery life. They are not very sensitive, so high gain, low noise & expensive amplifiers need to be used. They are not very robust, so need care in handling. This can be a problem when its pitch black and you are walking through a wood! Polaroid make ultrasonic capacitance sensors (shown here) for range finding applications and these can also be used.