Bat Echolocation & The Use of Bat Detectors

Bats can see very well, probably better than we do at dusk, but even their eyesight needs some light and they would be unable to find their insect prey in the dark. Bats have solved this problem and can find their way about at night and locate their food by using a sophisticated high frequency echolocation system. Our hearing ranges from approximately 20Hz (cycles per second) to 15,000 to 20,000Hz (15-20Khz) depending on our age, but bat calls are generally well above this. By emitting a series of often quite loud ultrasounds that generally sweep from a high to low frequency or vary around a frequency, bats can distinguish objects and their prey and therefore avoid the object or catch the insect. 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 that turns the ultrasound into sound we can hear.

Animated illustration of echolocation

The most widely available bat detectors work on a principle known as heterodyning. The bat calls are picked up by an ultrasonic microphone and mixed with the output of a high frequency oscillator in such a way that the result is the sum and difference of the two frequencies. Thus if the bat call is at 49kHz and the bat detector is set to 50kHz then the difference is 1kHz which we can hear. The sum signal is ignored for obvious reasons! Clearly if the bat call is at 50kHz then we hear nothing, so the output of the bat detector cannot be an accurate reproduction of the original bat call.

By adjusting the tuning frequency of the bat detector we can "listen" to different portions of the bat call and with practice can distinguish the calls of a number of bat species or families.

Other rather more sophisticated bat detectors work by recording the bat sounds and replaying them at a slower rate. This can then be heard by us with the advantage that the entire call can be heard in its original form, but these "time expansion" detectors are expensive.

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 the purchased of a bat detector it may be wise to attend a few bat watching events with more experienced people and see what they are using and try them out. There are several good heterodyne bat detectors available from around £40. The Bat Conservation Trust web site has further information on bat detectors.

bats in moonlight

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

The pipistrelle is the first bat you are likely to come across, and these are usually listened for with the bat detector set to 50kHz. Fortunately for bat workers, the two newly separated species of pipistrelle have different "best listening" frequencies, one at 45kHz and the other at 55kHz. Pipistrelles usually sound like irregular "smacks" that tend to vary in the 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 two newly separated species.

These are pipistrelle calls as heard on typical heterodyne and time expansion bat detectors.

Heterodyne detector set to 55kHz  

Time expansion detector set to 10x expansion

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 are usually alternate "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 are noctule calls as heard on typical heterodyne and time expansion bat detectors.

Heterodyne detector set to 25kHz  

Time expansion detector set to 10x expansion  

Daubenton's call sonogram

The Myotis bats like the Daubenton's, all sound rather similar, generally coming out as a regular series of "clicks" when listened to with the bat detector set to 45 to 50 kHz. The Daubenton's, whiskered and Brandt's bats have medium repetition rates but the Natterer's 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 typical heterodyne and time expansion bat detectors.

Heterodyne detector set to 45kHz  

Time expansion detector set to 10x expansion