by Gerard Fonte
The holy grail of high fidelity sound reproduction is to recreate the listening
experience exactly. The listener shouldn’t be able to tell the reproduction
from the original. Yet even with expensive electronics and nearly distortion-less components there is never a difficulty with knowing what is a recording
and what is live. Why? Why can’t sound reproduction sound real?
It turns out that one major
component of sound is not
reproduced with any fidelity at all.
This component is phase and it is
something that is generally ignored
and often improperly understood. In
order to fathom phase, we will have to
examine how the human hearing
system operates. This is the first of two
parts. The second part will provide
the designs for speaker systems and
techniques for recording.
Sound as a Science
Unfortunately, when it comes to
sound reproduction there are many
strongly held opinions. To that end, it is
probably worthwhile to indicate that
my position is that of scientist/engineer.
I firmly believe in observation and
measurement. I have difficulty accepting data as valid when blind or
double-blind tests fail to support it.
However, it is not unreasonable to me
to consider that the ear can be sensitive
to things that are not or cannot be easily measured. Phase is just such a thing.
There are three components to
any sound: frequency, amplitude, and
phase. These three factors completely
define any and all sounds. Amplitude
and frequency are easily measured.
But phase is rarely considered in any
sound reproduction system. There are
frequency response specifications for
receivers, amplifiers, speakers, and
every stereo component. Amplitude
specifications are also presented on
the data sheet. This is seen in output
power, power-handling, gain, and input
values. But, where is phase detailed?
At most, there is a note mentioning the proper phasing of speakers.
However, this is just to make sure that
the speaker cones move in the same
direction when the same signal is
applied. It really has very little to
do with the concept of phase as a
defining characteristic of sound.
There are actually two different
aspects of phase that are important.
One in monaural phase (one ear) and
the other is binaural phase (two ears).
(References to the “ears” really means
the whole hearing process.) Monaural
phase sounds are typically delayed
sounds that are applied to one or both
ears simultaneously (more or less). A
short echo (or “reverb”) is a monaural
phase example. You can hear an echo
with either one or two ears without a
problem. In this case, there are two
identical sounds that are combined but
offset in time. This is the type of phase
we will be concerned about the most.
Binaural phase refers to the time
delay due to the difference in the path
length from one ear to the other. A
sound from the left strikes the left ear
earlier than the right ear. This type of
phase is usually associated with sound
localization. There is only one sound
but the ears hear it at slightly different
times. Obviously, both ears must participate in binaural phase perception.
Many people think that sound
perception is a very straightforward
procedure. (Note: “perception” is a
personal mental act.) They view the
ears as microphones and the brain as
a simple receiver. Nothing could be
further from the truth. Hearing is an
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