Why do we need detectors to hear bats?
Most bats supplement their eyesight by echolocation, a system a bit like radar or the sonar used by submarines. Although the sounds made by bats for echolocation are very loud, you can't normally hear them, because the frequency (pitch) is too high. Bat detectors pick up the bat calls and convert them to a pitch within our hearing range, so that we can listen to the bats. This helps in observing them, and identifying their species.
This graph shows the range of frequencies used by humans (green) and by bats (blue). You can see that the two frequency ranges only overlap by a tiny bit. In fact, some lucky individuals (usually young people) can hear quite high frequencies, and are able to pick up a very small part of the bats range without equipment.
How do detectors work?
Detectors pick up the sound with a special microphone (see below). The sound is converted to a changing voltage and the signal is processed electronically to change it to a sound that we can hear. This can be done in different ways:
Real time recording: the bat call is recorded ("sampled") at very high speed, without any modification. Once the signal has been sampled there are many possibilities in processing it to produce displays and audible signals relating to the bat call.
Heterodyning: This involves mixing the signal with another signal and amplifying the difference signal.
Frequency division: Digital circuits divide the frequency of the signal from the bat, often by 16 or 32, so that it lies in the range humans can hear.
Time expansion: The signal is literally stretched out in time, so that the frequency is reduced, and a 10ms call lasts 100ms or even longer.
Microphones for bat detectors
The performance of your bat detector depends on the type and quality of microphone fitted. Early inexpensive bat detectors used a piezo type microphone, which gave good sensitivity at the expense of very limited frequency response (less than 35 - 50 Khz). Good frequency response was provided by capacitance microphones - but these were very expensive and fragile. Since the 1970's electret types have been used, which are robust and relatively cheap, but mainly intended for the audio (hearing aid) market, so characterised only up to 15kHz or so. This century has seen the evolution of MEMS (micro-engineered mechanical system) sensors providing a new type of microphone with good response and sensitivity - and in the last few years types particularly suitable for ultrasonic applications have become available.
What else can they detect?
Anything that makes an ultrasonic noise can be picked up with a bat detector. Keys jangling, nylon jackets rustling or gravel scrunching will all be picked up – so if you are looking for bats check your clothing is suitable. Other sources of ultrasound include dog training whistles, or gas or steam escaping. Many small animals make high pitched sounds – for example crickets, beetles and small rodents.