Implements the (optimized) formula:
Higher frequencies are normally perceived as louder, which AmpComp compensates.
Input frequency value. For freq == root, the output is 1.0.
Root freq relative to which the curve is calculated (usually lowest freq).
Exponent: how steep the curve decreases for increasing freq.
Note that for frequencies very much smaller than root the amplitudes can become very high. In this case limit the freq with
freq.max(minval), or use AmpCompA.