Reduced Min Amplitude in a call definition results in fewer calls detected

This result may sound counterintuitive. One may expect to detect fewer calls after increasing the Min Amplitude parameter, that is, after applying more restrictive criteria. In fact, more calls are often detected after increasing Min Amplitude.

The reason is that when Min Amplitude is low, UltraVox XT captures large regions of the spectrogram that have amplitude higher than that value. But such large regions may not correspond to the other criteria in the call definition. For example, some regions may exceed Max Duration, and would not be considered as calls (figure below, top). When you increase Min Amplitude, it is more likely that (smaller) regions of the spectrogram will be found which correspond to all criteria in the call definition (bottom). This explains why when you increase Min Amplitude you begin to see more detected calls.

The figure below is an example of what could happen.

Suppose 50 is used for Min Amplitude. Amplitudes just above 50 are found in large regions of the spectrogram, for example that indicated with the dashed rectangle. However, the rectangle does not satisfy the other criteria set in the call definition. For example, the rectangle could be longer than 100 milliseconds, which is in conflict with the Max Duration setting. Therefore, the call within that rectangle is not found.

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After setting a higher value of Min Amplitude and clicking Detect calls in this recording, the software finds smaller regions of the spectrogram that are above that value. This time two regions are found that match the other settings of the call definition. Therefore, two calls are detected.

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The third, slightly longer call indicated with the dashed rectangle is not yet found because it conflicts with the Max Duration setting. It is detected after increasing Max Duration to for example 200 ms and clicking Detect calls in this recording.

See also

nThe Spectrogram

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