HOW CAN WE HEAR STEREO SOUND
Our ears react to changes in air
pressure. Every source of noice
creates changes in air pressure
that spread away in waves (Like
waves formed by a stone that was
thrown to water).
We can hear the noise only if it
vibrates in 20 - 20,000 Hz per second.
It's important to know, that the
shape of sound wave created from
musical instruments is a very complicated
shape (only the flute has quite
simple shape that can be compared
to a sinus).
Sound waves reach our ears and there
transfer a signal that our brain can
"read". How can our brain locate
the source of the sound wave?
Brain receives TWO independent
"pictures" of sounds wave, a picture
from each ear. Brain continuity
compares the "pictures" from ears.
While doing that, brain finds all
the elements of every "picture".
(Not like our eyes, receiving full
spectrum gives us white color, hearing
system can "see" every source of
noice although we hear many sources
at the same time). If brain sees
the same sound wave in both pictures, brain
"checks" :
1. To which ear the sound
wave came first.
2. How much time it took
to wave sound to appear in the other ear.
3. Brain compares the level
of sound wave to a table based on life time
experience.
Knowing this information, brain
tells us where the noise comes from. In simple words, if a noise came first
to our left ear, the source of noise is on left, the time it took to the
noise to arrive to the second ear, tells us if the noise is from
the left or somewhere from the left/right (if it comes from the center,
noise appears to both ears at the same time), the loudness of noise gives
us an estimation on how far the noise is from us.
It's beautiful to see how accurate
and fast our hearing system is:
Noise speed is about 330 meters
per second.
Distance between ears is about
25 cm.
If a noise comes directly from
left or right, it takes less then 0.001 of a second
to reach the other ear, if a noise
comes from somewhere between the ears, it takes less time until it reaches
the second ear.
And it's amazing because our hearing
system doesn't deal with one sound at a
time - but with many different
kind of noises at the same time.
Hearing system is much more accurate
then any electronic measuring system
we know - but many of us rely on
outcome of such instruments when buying
a stereo system.
Another important feature of our
hearing system (will be explained later why)
is that it reacts to level of noise
in a logarithmic way. It means that we can
figure out that a noise is louder
then another, only when actually it's much louder.