Bats navigate in the dark by using “ultrasonic echolocation”. This means they make a series of sounds at frequencies which are (normally) beyond the range of human hearing (the “ultrasonic” part), and they listen to the echoes which bounce back off objects around them (which is the “echolocation” part).

 

Bat echolocation calls are in the range of about 20kHz to 120 kHz. For humans to be able to hear them they have to be converted to our hearing range which is about 20Hz to 12kHz (adult range). This is the function of a “bat detector”.

 

There are 3 main types of bat detector commonly available. All use an ultrasonic microphone to “hear” the call, some sort of processing to convert it, and a speaker to “play” the converted sound so we can hear it. Almost all have at least one headphone / tape / etc socket.

 

Heterodyne.

This mixes the bat call with a known frequency, which effectively subtracts one from the other and results in a lower frequency that we can hear. These detectors are the simplest to design and build and therefore the cheapest to buy, and they are also the simplest to operate. Some have a narrower range than others, such that if one is tuned to 120kHz it may not detect a bat using 20kHz. Others (conveniently, the cheaper ones) will detect any bats whatever frequency is set, allowing the user to “tune in” to the bats in any given situation. The “shape” of the sounds of the converted calls are not affected by the conversion, so it is possible to hear the differences between different species, and different types of call, however the frequency after conversion is not “accurate” so computer analysis, while possible, is not particularly useful.

 

Frequency Division.

This cuts the call into tiny slices and only plays a few of them (typically every 8th, 10th, or 16th depending on the setting). This means that all frequencies can be heard at once so there is not the danger of missing a bat as with some heterodyne detectors, and a recording will be “accurate” so computer analysis can be used. However, the “shape” of the sound is lost so it is not possible to reliably distinguish by ear between similar species, different types of call, etc. As most of the call has been lost in the sampling, computer analysis can be somewhat limited.

 

Time Expansion.

This records the call for a short time, then plays it back at a slower rate. The frequency is “accurate” so computer analysis can be used, the call “shape” is slightly distorted but in a consistent way so different species and calls can be distinguished, however there is a delay between recording and playback so the sound is not “real time”. Operation also depends on a push-button or similar, so you have to know there is a bat about, and when it is calling. These are the most complex to design and build, and thus the most expensive to buy, and they can also be difficult to operate reliably. The entire call is recorded, in it’s original form, so computer analysis results can be as detailed as is required.

 

 

 

Some models have the ability to use 2 or even all 3 of the conversion methods, some can record calls internally for later playback and analysis on a computer, some have multiple headphone / tape / etc sockets, built in lights to make them easier to use in the dark, and several other useful facilities for specialist jobs.

 

Prices (in 2008) are about :-

Heterodyne  -  £40 to £200

Frequency Division  -  £150 to £400

Time Expansion  -  £300 to £500

Specialist models  -  £1,000 to £3,000