Detection distances vary with frequency and loudness (amplitude) of the bat calls, atmospheric attenuation, the directional characteristics and sensitivity of the bat detector. It is also affected by the amount of structural clutter (obstacles such as vegetation) which can block the path of the signal.

The frequency and amplitude of the bat call has a major influence on how far away the call can be detected. This makes some species easier to detect from afar than others. Quiet (low amplitude) bat calls are more difficult to detect than loud (high amplitude) calls. Call amplitude can vary within an individual (as many bats will reduce the amplitude of their calls as they approach prey or clutter) and also vary between species. Species which always produce low amplitude calls (“whispering bats”) will be more difficult to detect from a distance than other species. In addition, bat calls of higher frequencies cannot be detected from as far as those of lower frequencies. Higher frequencies are often absorbed by the atmosphere (attenuate) faster than lower frequencies.

How quickly sounds attenuate in the atmosphere depend upon weather conditions such as temperature, humidity and air pressure. This relationship is complex, but in general, cool dry conditions will allow the detection of bat calls over greater distances.

The sensitivity of the detector also has a major influence on detection distance. While there is always some variability in sensitivity among units, the biggest influence is the setting of the sensitivity control.

Given all the above, it is obvious that detection distances will vary enormously. Many bats are easily detected over 30m under typical conditions, while some species which call at low frequencies may be detectable from as far as 100m. However, some species will be hard to detect from even 1m away. This is why only relative rather than direct statistical comparisons of bat activity are made between species.

Further information on detection distances and the calculation of detection fields and distances can be obtained from Chris Corben’s AnaVolumes software and the associated manual (http://users.lmi.net/corben/Beta/).

The division ratio is set at 8. The division ratio affects the amount of data which is saved in Zero Crossing recordings, the lower the division ratio, the greater the number of data points in the resulting sonogram. The division ratio for the FD audio output is 16.

Sample rate refers to the number of ‘samples per second’ taken during recording, measured in kHz (kilohertz). The sample rate of the Anabat Scout is 320ksps.

There are two main recording formats for bat files, zero crossing analysis (.zc, .zca, & .xx#) and full spectrum (.wav). Full spectrum records the full spectral information within a sound file, just like a music file. Whereas zero crossing analysis renders the spectral information down into a series of time vs. frequency dots.

The advantages of full spectrum include the ability to see intensity, harmonics, multiple bats calling at the same time, and faint bat pulses during high ambient noise. The disadvantages of full spectrum are that they are much larger files (typically 6 times the size of a zero crossing file), they require more processing power to record, and are slower to render on a computer for post-recording analysis. The advantages of zero crossing analysis is the small file size, so memory space is not an issue, and many published guides to bat calls are based on zero crossing analysis. The disadvantages of zero crossing are that spectral information is lost (which may be helpful to species identification), and that in cases of high frequency ambient noise (eg. insects), bat calls may not be recorded fully.

It is important to note that post-recording full spectrum files can be converted to zero crossing, but zero crossing files cannot be converted to full spectrum. Deciding which recording format is best will depend on the aims of your survey, the recording environment, and your budget.

All Titley Scientific detectors now save metadata in the GUANO (Grand Unified Acoustic Notation Ontology) format. This format is now the standard across bat acoustic fields. GUANO is also supported by the Anabat Insight software package.

Microphones exhibit a ‘frequency response’, which basically means they have differing sensitivities at different frequencies. A microphone may be very sensitive at 40kHz, then lower sensitivity at 60kHz, then be very sensitive again around 80kHz. Microphone sensitivity doesn’t just flat-line at a specific frequency, but has a tendency to ‘roll-off’ around a certain frequency. We refer to this as the ‘maximum frequency’, however the microphone can still detect higher frequency sounds (they just need to be louder or closer to the microphone).

To improve the GPS lock, move to an area with a clearer view of the sky for better satellite reception. You can still record without a GPS lock, but the timestamp and location data may not be accurate. Initially, your GPS data may be less accurate, but this will improve within a few minutes as more satellites are locked.

The bat counter creates a .csv spreadsheet from presses of the In and Out buttons on the detector. Each In or Out record is time and date stamped, and it records a total tally of ins and outs and the GPS location at each button press. This feature is designed to help you with roost emergence counts.