Bat detector

There are other types of detectors which record bat calls so that they can be analysed afterward, but these are more commonly referred to by their particular function.

Some bat calls have components around 20 kHz or even lower and sometimes these can be detected at 2 or 3 times the usual range.

A heterodyne bat detector simply shifts all the ultrasound frequencies downward by a fixed amount so we can hear them.

In this case the bat signal is mixed with a high frequency oscillator, typically around 450–600 kHz.

It is also possible to use a 'comb spectrum' generator as the local oscillator so that the detector is effectively tuned to many frequencies, 10 kHz apart, simultaneously.

Some early bat detectors used ex-Navy low frequency radio sets, simply replacing the aerial with a microphone and pre-amplifier.

It is also possible to modify a portable Long Wave radio to be a bat detector by adjusting the tuning frequencies and replacing the ferrite rod aerial with a microphone and pre-amplifier.

Species like Pipistrelles which end their call with a "hockey stick" CF component can be recognised according to the lowest frequency which gives the clearest "plop" sound.

It is easy to recognise a doppler shift in CF calls of flying bats due to their speed of flight.

The disadvantages of a heterodyne bat detector are that it can only convert a narrow band of frequencies, typically 5 kHz, and has to be continually retuned, and can easily miss species out of its current tuned range.

Square waves sound harsh and contain harmonics which can cause problems in analysis so these are filtered out where possible.

Alternatively, listening to the FD output gives an audible rendering of the bat call at 1/10 frequency.

Dual FD/heterodyne detectors are useful for cross country transects especially when there is a function provided for recording voice notes such as times, locations and recognised bat calls.

Also with some species such as the Lesser Horseshoe bat with a call around 110 kHz, the resulting frequency is still quite high although it can be recorded.

Surprisingly, this is not a great disadvantage when analysing a recording later Time expansion (TE) detectors work by digitising the bat calls at a high sampling rate using an analog-to-digital converter and storing the digitised signal in an on-board memory.

Since both frequency and amplitude information are preserved in the recorded call, more data is available for species analysis.

Early units were equipped with small memories which limited the length of time that could be digitised.

Some units also include a pre-buffer feature to capture events that happened shortly before the 'record' button was pressed which can be useful for manual surveys.

TE detectors are typically used for professional and research work, as they allow a complete analysis of the bats' calls at a later time.

[3] The original bat calls are digitised and the zero crossing points used to produce a data stream which is recorded on a memory card.

There are sophisticated timing and trigger controls and the device can be set to respond to bat calls, so that many hours of recording are available in unmanned situations.

[citation needed] Research is in progress for analysing many types of ultrasound calls and sounds besides those of bats.

Visual observation is the obvious means of detecting bats, but of course this can only be done in daylight or crepuscular conditions (i.e., dusk and dawn).

In lower light conditions a night vision device can be used but the more affordable generation 1 type has a lag time which fails to provide a suitable image of a flying bat.

The size of the roost entrance determines the number of beams necessary and thus the power required and potential for off-mains use.

Single beam DIY systems are available for bat boxes but these do not log the direction of transit.

The Countryside Council for Wales (CCW) uses two similar systems with beams spaced close enough together that every bat transiting the entrance is logged along with the temperature.

It is important that data is analysed using a methodology which takes light sampling behaviour into account.

Passive infrared sensors are slow with a response speed of the order of a tenth of a second and will normally not detect a small fast mammal like a bat.

Radar has been used to detect bats beyond the acoustic limit, but is very costly in equipment and man hours.