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.
There are three main ways in which this can be done:

New techniques are being developed to use digital signal processing techniques to overcome the limitations of these simple detectors, but the methods used are too mathematical to explain in simple terms.

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 in use in current portable bat detectors.

Sub-miniature electret microphones

electret.gif (3632 bytes)These are made for hearing aids. 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

piezo.gif (5576 bytes)These are crystal microphones, designed mostly for intruder detection. 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.  There is considerable discussion about their suitability for various types of bat detector, but my experiments indicate that they are unsuited to  wide band applications. (See graph below) 

Capacitance microphones (specified for ultrasonic operation)

Purpose-made for bat detectors, usually by the manufacturer of the detector. They require high voltages (typically 400V) 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 for range finding applications and these can also be used.