Bat detectors

For our bat walks we make use of our 23 bat detectors. These make the bats 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.

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 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 neighbours.

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.

What echolocation calls to listen out for at Pittville Park?

If you have a heterodyne bat detector why not wander around Pittville Park, or other parks in Cheltenham, looking out for their flight patterns, and listening out for their calls.

Common pipistrelles

These produce a series of loud, fast clicks that are often referred to as sounding like “wet slaps”. Their call will be the loudest around 45 kHz. We often begin our bat walks with our detectors set at 45 kHz as we listen out for our most common bat species.

Soprano pipistrelles

These sound very similar to the common pipistrelle but their calls are loudest at 55 kHz. You might also hear their call at 45 kHz but when you retune the detector to 55 kHz it will sound louder.

Noctule

These bats produce the loudest calls of all the British bat species. While echolocating they produce two alternating call types at different frequencies (about 19 kHz and 24 kHz) this produces a “chip-chop-chip-chop” sound. Their calls are slower than the pipistrelles.

Daubenton’s

They produce a series of very rapid clicks. The repetition rate is very fast and regular when compared to pipistrelles. The call will sound the loudest around 45 kHz but can be distinguished from the pipistrelles by tuning down below the frequencies of the pipistrelles to 35 kHz where dry “clicking” sounds can be heard. Caution is needed to identify these bats based on their calls as several related bats also have similar calls.