Understanding the "Equal Loudness Curves".
Admittedly a proper examination of the equal loudness curves is quite beyond the scope of a "tip of the day". But here are a few basic points.
We often hear "it takes 10dB to sound like "double" the loudness. This is true but it is referenced to 1kHz. This changes at the frequency extremes but stays remarkably accurate regardless of the loudness.
Contrary to some opinion a speaker should "not" be engineered to look like these graphs.
Notice our hearing is most sensitive in the 3kHz - 5khz range. Nails on the chalk board or a baby's cry anyone? It is no accident that our hearing is most sensitive in conjunction with our offspring expressing signs of discomfort of course. Call it divine intervention or evolution---either way....the baby is getting you up in the middle of the night..
Under 100hz it takes much less than 10dB to be perceived as a doubling of the loudness.(remember, going from one curve to the next on these graphs represent a perceived doubling of loudness.) For example, at 20hz and 90dB it only takes about 5dB for "double the apparent loudness"
Notice how insensitive our ears are in the deep bass. For example, 1kHz tone at 60dB and a 20hz tone at 100dB share the same loudness(Phon) curve.
"Decibel" is used to measure loudness of a sound". "Phon" is used to describe someone's *perception* of that loudness.
Misunderstanding the research behind the equal loudness curves is primarily responsible for the audio fallacy of "humans cannot hear under 20hz".
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