Bat detectors

For our bat walks we make use of our bat detectors, these make the bat’s calls, which are normally at a too high a frequency for humans to hear, audible. Some of our bat detectors can also record the call allowing us to analyse them at a later date.

Different bat species have characteristic calls – the structure of the call can differ, the number of calls per second can differ and they can call at different frequencies.

We have a range of detectors that you can borrow from us.

How bat detectors work

Sound spreads through the air, or other substance, as a mechanical vibration. Technically, the frequency is the number of waveforms that are generated in one second. The sensation of a frequency is commonly referred to as the pitch of a sound.

A high pitch sound corresponds to a high-frequency sound wave and a low pitch sound corresponds to a low-frequency sound wave. When we say someone has a deep voice they are producing low-frequency sounds and when someone has a high-pitched voice they are producing higher frequencies –they are producing lots of sound waves.

Humans can normally hear in the 20 to 20 kHz (20,000Hz) range, but we will lose the ability to hear higher frequencies as we age. Any sound with a frequency below the audible range of hearing (i.e. less than 20 Hz) is known as an infrasound and any sound with a frequency above the audible range of hearing (i.e. more than 20 000 Hz) is known as an ultrasound.

Bat detectors have a microphone and a speaker. Our bat detectors also have a frequency dial that allows us to set the incoming frequency to the bat species we might be looking for; during our bat walks we will change the incoming frequency during our walks looking for the different species of bats.

Bat detectors can work in a number of different ways. Our bat detectors are known as heterodyne bat detectors.

They work by filtering the ultrasonic (and therefore inaudible) bat sound with an ultrasonic signal from the detector and producing the audible difference between the two sounds. If you set the bat detector to 50 kHz and the incoming bat call has a peak frequency of 49 kHz the detector will play the call at 1 kHz which we can hear.

Bats though usually call across a range of frequencies creating audible sounds that can be characteristic of the species. To confirm our ID we can also look at the call using computer programmes to analyse the structure of the call.

Why do bats call?

Bats detect their prey using echolocation. They shout out sounds and listen to the echoes; these give them details of the size, shape and direction of their prey. Bats also have social calls for communicating with their neighbors.

Typically echolocation calls have a well-defined structure and are generally consistent; if not exactly species-specific they can produce characteristic frequencies and generic call structures.