Given an FFT chain, this produces the spectral crest measure, which is an indicator of the "peakiness" of the spectral energy distribution. For example, white noise should produce a flat (non-peaky) spectrum, and therefore a low value for the spectral crest.
Optionally, freqlo and freqhi indicate the lower and upper frequency limits of the band to look at; by default, the whole FFT range (excluding DC and nyquist) is analysed.
In pseudo-equation form, the measure is calculated as follows:
Crest = S.maxItem / S.mean
where "S" is a list of the squared magnitudes in the spectral band. Note that this limits the value to being greater than or equal to 1. (Some research uses a logarithmic scale - you can apply the logarithm yourself if required.)