ISO 226 Equal-Loudness-Level Contour Signal

Generates a psychoacoustic equal loudness contour (curve) as described in ISO 226.

This function will return a 29-point equal loudness contour for your desired phon level. The frequencies evaulated in this function only span from 20Hz - 12.5kHz, and only 29 selective
frequencies are covered. This is the limitation of the ISO standard.

In addition the valid phon range should be 0 - 90 dB SPL. Values outside this range do not have experimental values and their contours should be treated as inaccurate.(Limitation due to standard)

If more samples are required you should be able to easily interpolate these values using spline().

*For more information regarding this method refer to ISO 226.

------------------------------------------------------------
Usage: [SPL FREQ] = ISO226(PHON);

PHON is the phon value in dB SPL that you want the equal
loudness curve to represent. (1phon = 1dB @ 1kHz)
SPL is the Sound Pressure Level amplitude returned for
each of the 29 frequencies evaluated by ISO226.
FREQ is the returned vector of frequencies that ISO226
evaluates to generate the contour.
------------------------------------------------------------