Calculates formant bandwidth as a function of formant frequencies using a
modified version of TMF-63 formula. Namely, above 500 Hz it follows the
original formula from Tappert, Martony, and Fant (TMF)-1963, and below 500 Hz
it applies a correction to allow for energy losses at low frequencies. See
Khodai-Joopari & Clermont (2002), "Comparison of formulae for estimating
formant bandwidths". Below 250 Hz the bandwidth is forces to drop again to
avoid very large values near zero (just guesswork!)