Echolocation and bat detectors

We’ve all heard the phrase ‘as blind as a bat’ but bats can actually see very well, probably better than we do at dusk! However even their eyesight needs some light so how do they manoeuvre so well in the dark? Bats have solved this problem by using a highly sophisticated system called echolocation. They emit short, loud, high frequency pulses of sound from their mouths or noses and use information contained within the echo to map a soundscape.

Bats use a range of frequencies in their calls, which tend to sweep from high to low or vary around a frequency, to help them distinguish objects from prey. The frequencies used, and the type of sweep or characteristics of the call can help us to distinguish the species of the bat when we use a bat detector.

Bat detectors

There are three systems of bat detectors that convert the ultrasonic calls of bats into the audio range of humans. These are heterodyne, frequency division and time expansion. A forth system, full spectrum, does not convert bat calls into the audio range but uses a high sampling rate to create high quality recordings.

Heterodyne bat detectors allow you to listen to one frequency at a time, whereas broadband detectors (frequency division, time expansion and full spectrum bat detectors) monitor all frequencies at the same time. This makes broadband detectors useful for sound analysis.

Heterodyne

These are affordable bat detectors perfect for beginners who want to be able to hear bats and have a go at identifying them in the field from the sound and frequency of their calls.

You use this detector by tuning the frequency, shown on the dial or a digital screen. This allows you to “listen” to different portions of the bat call and will introduce you to a cacophony of ‘smacks’, ‘warbles’, ‘chips’ and ‘chops’. With practice, you will be able to distinguish the calls of a number of bat species or families.

Listen out for the ‘feeding buzz’ which sounds like someone blowing a raspberry!

Frequency division

These are more expensive then heterodyne detectors but tend to be the cheapest broadband detectors. Most models come with a heterodyne channel which allow you to listen to bats in the field. The way these detectors work means bat calls can be recorded constantly with no gaps. Frequency division detectors preserve enough information about the bat call to be used in basic sound analysis, however some call features are lost including harmonics and quiet calls are sometimes not picked up.

Time expansion

These bat detectors are in the higher price range. Most models tend to also include heterodyne and frequency division systems. The method they use to record bat calls involves slowing the call down which gives an accurate reproduction of the bat call. This makes these detectors great for sound analysis as all the characteristics of the original bat call are kept. Unlike frequency division, there are gaps when recording so some bat calls can be missed.

Buying a bat detector

The actual sounds that you get on a bat detector depends considerably on the actual detector you are using, and similar models from the same manufacturer can often sound slightly different. If you are considering buying a bat detector it may be wise to attend a few bat walk events with more experienced people and see what they are using and try them out.

The above descriptions are basic summaries and you can find out more about the different types of bat detectors and where to buy them on The Bat Conservation website.

What do bats “sound” like?

The sounds produced by bat detectors depends on the main characteristic of the call being used by the bat. A short burst of constant frequency sounds like “smack”, a longer burst of constant frequency like a “warble”, a steep frequency sweep like a sharp “click” or a “tick” and a shallow sweep like a “tock”.

Pipistrelle call sonogram

Pipistrelles are the first bats you are likely to come across, and these are usually listened for with the bat detector set to 50kHz. Fortunately for bat workers, the three species of pipistrelle have different “best listening” frequencies. Common pipistrelle at 45kHz, soprano pipistrelle at 55kHz and Nathusius’ pipistrelle at 35kHz. Pipistrelles usually sound like irregular “smacks” that tend to vary in pitch and are at a medium repetition rate.

pipistrelle call sonagram

Note the short terminal “tail” on the call. This is what causes the call to sound like a “smack” and the frequency of which varies between the three pipistrelle species.

These are pipistrelle calls as heard on a heterodyne bat detector.

Common pipistrelle –

Soprano pipistrelle – 

Noctule call sonogram

The noctule is entirely different, and is usually best heard with the detector set to 20kHz. The sounds from the bat detector usually alternate between “smacks” and “tocks” at a fairly slow repetition rate which together sounds like a fairly irregular “chip-chop”.

noctule call sonagram

Note the alternating types of call, which gives rise to the characteristic “chip-chop” sound from a heterodyne bat detector.

These  noctule calls as heard on a heterodyne bat detector set to 19kHz.

 

Daubenton’s bat call sonogram

The Myotis bats like the Daubenton’s, all sound rather similar, generally coming out as a regular series of  dry “clicks” when listened to with the bat detector set to 35 to 50 kHz.

The Daubenton’s, whiskered and Brandt’s bats have medium repetition rates but the Natterer’s bat tends to be faster, quieter and more irregular.

The long-eared bats have a similar sound to the Myotis bats, but at a faster repetition rate, and are so quiet that they are generally nearly impossible to hear.

Daubenton's call sonagram

Note the short steep sweeps characteristic of the Myotis family of bats.

These are Daubenton’s calls as heard on a heterodyne bat detector set to 45kHz.

References:

Russ, J. (2012) British Bat Calls: A Guide to Species Identification. Pelagic Publishing, UK