Loudness weighting coefficients

Calculate loudness weighting coefficients based on ISO 226

NOTE: this function is now available from the IoSR Matlab Toolbox as iosr.auditory.loudWeight.
-------------------------
Calculate loudness weighting coefficients based on ISO 226
g = loud_weight(f)
g = loud_weight(f,phon)
G = LOUD_WEIGHT(F) returns loudness weighting
coefficients for frequencies F (Hz). The function is
based on the loudness at 65 phons defined in ISO
226:2003.

G = LOUD_WEIGHT(F,PHON) returns loudness weighting
coefficients at the loudness level PHON. PHON should be
a scalar and, according to the standard, is only valid
at all frequencies such that 20<=PHON<80 (although the
function will return SPL values outside of this range).

The input f may be an array of any size. The outputs
will be the same size as f, with coefficients calculated
for each element.

NB: since ISO 226:2003 only reports values up to 12.5
kHz, frequencies are limited to 12.5 kHz for the
purposes of calculating weighting coefficients.