excerpted... > (The loudness level in phons is a subjective sensation---
this is the level we perceive the sound to be at) From about 500hz to
roughly 1,500 hz the line is flat on the 10b scale.
> Does the above not indicate that human hearing of sound begins around 500
hz?

No, it does not indicate that human hearing begins around 500 hz.

The threshold of human (and any other) hearing is a combination of the
frequency (measured in Hertz - also called cycles per second) and
amplitude.  Frequency is the pitch of the sound.  Think of the keys on a
piano.  The amplitude is the intensity or energy of the sound wave
(measured in decibels).  Think of hitting the same key harder or softer.

Your perception of loudness is measured in phons.  The perception of
loudness is non-linear.  The human ear is optimized to hear best in the
middle frequencies.  As you get further to the ends of the frequency
spectrum, the sound has to have more energy for you to hear it (or to
perceive it to be the same loudness).  So, a tone at 5,000 Hz will require
more energy (more decibels) to sound the same loudness as a tone at 500 Hz.

The frequency limits of human hearing are the highest and lowest
frequencies that can be heard regardless of how loud the sound is.  This
varies by person, by age and by a number of other factors.  I remember
learning that the average frequency range for human hearing is
approximately 50 to 50,000 Hz.  A 10 Hz sound (elephants communicate in
this range) can not be heard by human ears even if you're standing right
next to it.  A 100,000 Hz sound (dog whistle?) can't be heard by a human
ear (most people anyway) even though an oscilliscope can show that it is
putting out as many decibels as a police whistle.  Okay, that's not exactly
true.  The 50-50,000 range was defined at a constant but low energy level.
Put enough energy into a very low sound and you might hear something even
though it's "outside" the frequency range.

What the original quote says is that within a certain range, you can
approximate hearing as linear.  If you make a 10 decibel tone, it will
sound approximately the same loudness whether it is at 500 Hz or 1,500 Hz.
A graph would make this much easier to understand.  Here's a couple of good
links.
http://www.rwc.uc.edu/koehler/biophys/9d.html
http://www.engr.uky.edu/~donohue/audio/fsear.html

Sounds in the 250-300 Hz range are well within the range of human hearing.
If I remember right, that's about middle C to about D sharp.



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