The Handbook of Acoustic Bat Detection by Runkel Volker; Gerding Guido; Marckmann Ulrich

Author:Runkel, Volker; Gerding, Guido; Marckmann, Ulrich
Language: eng
Format: epub
Publisher: Pelagic Publishing

9.4 Triggering systems

The significance of call recognition or triggering of the recording for the assessment of the recording range of a detector has already been mentioned. As well as the actual sensitivity of the microphone and amplification, the production of recordings also depends on the recognition of a positive signal. The exact workings of a trigger function are not known in detail for any of the detectors, even though it may be one of the possible unique selling points. The reliability and objectivity of a trigger mechanism is important for the evaluation of a device. If a bat call triggers a recording once, it must do so for repetitions of the same call. This means that the trigger algorithm must set off a recording even if, for example, there is a change in background noises, provided the latter do not superimpose on the call itself.

Devices that have an adaptive recording threshold, instead of a fixed one, must be treated with caution. A fixed recording threshold means that, irrespective of the signal input level, this threshold has to be exceeded in order to trigger the device. It is generally a threshold which is relative to the maximum recording level. Both the Avisoft USG and the batcorder have this type of fixed threshold. Ideally the system is calibrated for sensitivity, as it is only in this way that a fixed threshold can be implemented meaningfully. According to its specifications, the Pettersson D500 also has a fixed threshold. This can only be set at three levels, however, and no precise details are provided on the actual trigger threshold levels. Moreover, the device is not calibrated. The Batlogger has single triggering modes (Period, SD-Trigger), which presumably have a fixed threshold, but this is not clear from the settings or documentation. Neither the microphone nor the device is calibrated. According to the manufacturer, both triggers are less sensitive to species with low-frequency calls. That means that, under certain circumstances, the system will not be triggered. There is therefore a possibility that species from the genera Nyctalus, Eptesicus, Vespertilio, Tadarida and Plecotus will be missed.

The problem with a variable or adaptive recording threshold is that the triggering of the recording is unreliable or not always reproducible. The detector changes the recording threshold according to the level of ambient or background noise. A sound will trigger the device once it exceeds the set value. If, for example, bush-crickets are active during the summer, or the location is ‘loud’ because of other noise sources, any bat calls will have to be correspondingly louder (Figure 9.5). Adaptive thresholds are used in the SMxBat devices from Wildlife Acoustics as well as in the Batlogger models (crest, crest-advanced triggers).

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