Noctules are large, fast and high-flying bats that typically use a two-tone echolocation system. The lower notes being qCF calls from 17-23kHz and the upper notes FM/qCF calls starting between 24 and 50kHz and coming down to 21-30kHz. Durations can range from 9-30ms and intervals from 120-800ms. On a bat detector they are heard as a distinctive chip-chop at a much slower pace than a Pipistrelle. Leisler's are quite similar, although alternation is not as reliable and have slightly higher frequencies. Noctules typically have low notes below 20kHz although they can go higher and there is some overlap with Leisler's.
In practice you may find that the alternation is irregular or absent (or sometimes just inaudible in which case the pulse spacing may be extended the odd note simply being too faint to see. Alternatively the bat may just be using a single note, especially in the build up towards an interception buzz. Occasionaly the high notes can develop into a sequence of short pure fm calls.
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