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RichP714

2: Absolute threshold of hearing

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The intensity range of audible sounds is enormous. Human ear drums are sensitive to variations in sound pressure, from a snapping twig to an exploding volcano.  The human ear doesn't respond linearly to sound pressure, however; for this reason, sound pressure level is measured logarithmically
 
As well as having a logarithmic response to sound pressure levels, the absolute threshold for hearing (ATH) varies with frequency.  By measuring this minimum intensity with test tones of various frequencies, a frequency dependent absolute threshold of hearing (ATH) curve may be derived. Typically, the ear shows a peak sensitivity (i.e., its lowest ATH) between 1 - 5 kHz.
 
The well known Fletcher-Munson and Robinson-Dadson curves have now been supplanted by ISO-226
 
20150607111039185.png 
 
ATH is shown as the bottom curve, the lowest of the so-called equal loudness countours (ELC's indicate the sound pressure level (dB SPL), over the range of audible frequencies, that are perceived as being of equal loudness
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As an aside; dig the 'scooped' nature of these contours, with the minimum usually around 1kHz
 
then lo
 
and behold
 
eq_smile.jpg 
 
It then becomes apparent that listeners that tend to enjoy a 'smiley face' equalization curve are really just trying (subconsciously) to compensate for the differences between the sound pressure level at which the music was recorded, and the sound pressure level at which the music is played back 
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Also because the roll off of most speakers is a "frown" at the extremes; so the roll off of your ears is magnified by speaker roll-off. I think this is why people like speakers (amps, cartridges, etc.) that have a flat response to above and below the audible range of frequencies, there is reduced roll off in the audible range of them.

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