Anabat - Frequently Asked Questions
There are several types of bat detector available that use different types of sound analysis to process the bat calls and make them audible and/or visible. The most commonly used techniques for processing bat calls and make them audible are Heterodyne, Frequency Division and Time Expansion. Zero-Crossings Analysis, Fourier analysis, and Instantaneous Frequency Analysis are commonly used to make bat calls visible.
The AnaBat is equipped with a broad spectrum microphone and uses the Frequency Division and Zero-Crossings Analysis (ZCA) techniques. Unlike other types of bat detector, the AnaBat does not require exceptional hearing or tunning to hear particular frequencies (and hence species). There is no lag between call detection and audio or visual display. The AnaBat also uses little power and has highly efficient data storage capabilities which allow the system to be used for long term Passive Monitoring.
Frequency Division and ZCA techniques have been criticised in the past for their relatively lower sensitivity and incomplete representation of bat calls (no display of harmonics and amplitude). In reality, all bat detectors have limitations and are a trade-off between desirable qualities, technological limitations and cost. The AnaBat was proven to be sufficiently sensitive for its purpose which includes observation, survey and identification of free-flying bats etc. In fact, ZCA is ideal for species identification because the most important differences between species lie in the way frequency varies in time. The accompanying software (AnalookW) also allows rapid and sophisticated analysis of the bat call data.
HF output on the SD2 also provides a buffered version of the original bat call (prior to Frequency Division). This allows full-spectrum signals to be passed through an external high speed A/D card and saved to a separate storage device such as a laptop.
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 AnaVolumes and the associated manual (supplied on the distribution disk).
The AnaBat detector unit itself does not 'decide' what is and isn't a bat call. All ultrasonic signals detected by the AnaBat are saved to the DATA.DAT file on the CF card. It is the CFC Read program or AnaPocket on the PDA which 'decides' what are bat calls.
An ultrasonic signal must meet certain criteria before it is recognised as a bat call. These criteria depend on the MaxTBC, Min Line Length, and Smooth parameters. These parameters can be changed in the download options window of CFC Read when downloading data, or on the Record page under Options in the AnaPocket menu. While this will affect the AnaBat sequence files produced, it will not change the raw data files CFC Read automatically downloads from the CF card.
Thus you can feed the raw data file through CFC Read again with different settings if you wish.
The dots within the call (points crossing zero in ZCA) must be smoothly connected, which means that each dot must be close to the average of the dots either side of it. The closeness is decided by the Smooth parameter. Normally, this would be set to 50. You could set it lower if you want only the best quality calls or higher if you are willing to accept worse calls. But 50 seems a good compromise for most purposes.
Min Line Length
This parameter specifies the number of dots in sequence which must be smoothly connected for the signal to be considered a bat call. The sequence of dots must have a length greater than the Min Line Length parameter. Five is usually a good choice, but if you are using a Data Division Ratio of 8 you could go a bit larger. The problem is that smaller values make you more vulnerable to noise, but larger values increase the likelihood of missing some types of short duration calls.
This is simply the maximum time in seconds between the start of one call and the start of the next call. A file will only be saved if at least two calls are detected within the time set by MaxTBC. Five seconds is usually a good choice. If only one call is detected within MaxTBC then no file will be saved. If two or more calls are detected with MaxTBC then a file will always be saved.
A. Once again, the AnaBat detector unit itself does not 'decide' what to save. All ultrasonic signals detected by the AnaBat are saved to the DATA.DAT file on the CF card. It is the CFC Read program or AnaPocket on the PDA which 'decides' when to save an AnaBat file.
Any of the following four criteria must be met if a file (containing bat calls or noise) is to be saved automatically.
- 1. If the buffer is filled, the file will be saved immediately. The buffer can hold up to 16,384 data points, which is a fair bit. Therefore, this criterion is not commonly met. However noisy files or files containing many bats could reach this limit. If files contain lots of data but are shorter than 15 seconds, it is likely that the buffer is full. Files which have met this criterion will always be large (> 16 KB).
- 2. A file will be saved if there has been NO data for MaxTBC seconds after the last signal was received. This criterion can result in files of short duration, potentially much shorter than 15 seconds, and can be changed by altering the MaxTBC setting.
- 3. A file will be saved if the time since the first recognised bat call is greater than 15 seconds. This ensures a file can never be longer than 15 seconds.
- 4. The file size itself is limited to 32K bytes and it is possible to have data which contains less than 16,384 points but requires more than 32 KB to store. In this case the file would mostly be noise. The file will be cut off and saved when it gets to 32 KB in size.