This is the phenomenon that occurs when a signal that is higher than half the sample rate (referred to as the Nyquist frequency) of a recording system is recorded without being completely filtered out. The result is an upside-down version of a signal that was actually higher than you would have been able to record. In most modern recording systems, signals above the Nyquist frequency are filtered out to prevent or reduce aliasing.

The graphics below show two recordings of the lower frequency fundamental and higher frequency first harmonic of a lesser horseshoe bat call. In the first recording the 108 kHz upper harmonic has been accurately captured by a sample rate of 256 kHz. In the second recording the sample rate was 128 kHz and the upper harmonic has been inverted and captured as a lower frequency alias.

In the spirit of reconciliation, Faunatech acknowledges the Traditional Custodians of Country throughout Australia and their connections to land, sea and community. We pay our respect to Elders past and present and extend that respect to all Aboriginal and Torres Strait Islander peoples today.