SONIC REALISM
with Hi-Fi? Quite simply, virtually all
the speaker systems on the market
today destroy the proper phase
relationships of the sounds that they
reproduce. The fundamental reason
for this is because of the practical
limitations of the speakers (drivers)
themselves. A single driver cannot
reproduce the full range of frequencies that the ear can hear. In order to
do so, multiple speakers are used. This
is obvious. Tweeters are hopeless at
providing booming base and woofers
fade away at two or three thousand
Hertz. The requirement of multiple
speakers is the underlying problem.
Figure 1 illustrates a typical speaker
setup. It consists of a tweeter and a
woofer. For convenience, the speaker
enclosure is placed on a support at ear
level. In this way, the direct line distance
from the tweeter and the woofer to the
ear are the same. This means that the
sounds from both speakers reach the
ear at the same time. However, a typical
ceiling is an excellent reflector of sound.
There is a second acoustic path from
the speaker to the ear that results from
a reflection from the ceiling. So a
reflected signal from the woofer, which
is lower to the floor, will travel farther
than a reflected signal from the tweeter,
which is closer to the ceiling.
This changes the phase relationships and the wave-shape (see Photos
1 and 2). The more drivers that there
are in the speaker system, the greater
this phase distortion will be. There is
also the reflection from
the floor, as well as
from other large objects
in the room. These
additional reflections
will not be considered in order to
keep things as simple as possible.
It is important to remember that
ordinary sounds are not sine waves
but complicated combinations of sine
waves that span many octaves.
Speech is a good example. In order
for speech to be intelligible, the
frequencies of about 300 Hz to 3,000
Hz must be reproduced. However,
few people think that a telephone
produces high fidelity reproduction
of someone talking. Natural speech
contains sound components from
about 100 Hz to over 10,000 Hz.
Let’s examine what happens to
speech when it is reproduced by the
speaker system in Figure 1. The speaker’s cross-over directs the high frequency
components to the tweeter and the low
frequency components to the woofer (if
no cross-over is used, the natural
response curves of the drivers perform
the same function but less effectively).
For the direct line path, the ear hears all
of the components at the same time and
their phase relationships are maintained.
However, with a typical speaker system
shown in Figure 1, the high frequency
sounds reflected off the ceiling are heard
first with the lower sounds bringing up
the rear. The phase relationships of the
reflected sounds are destroyed. In effect,
the ear hears a different signal coming
from the direction of the ceiling. Again
refer to Photos 1 and 2.
The two photos show what
happens when a signal consisting of
1,000 Hz and 3,000 Hz components
is reproduced. The 1,000 Hz signal is
routed to the woofer and the 3,000
Hz signal goes to the tweeter. The
original signal is shown in Photo 1 and
this is what is heard if both speakers
are equidistant from the ear.
If there is a path length difference
between the woofer and tweeter of
only two inches (or multiple of two
inches), the ear receives significantly
distorted signal that is shown in Photo
2. Obviously, with signals consisting of
many frequency components and
many signal paths, the reflected wave
shapes can be vastly different from the
original. These distortions are the direct
result of the loss of phase information.
Acoustic Peculiarities
One of the tell-tale indicators that
this different signal is important is seen
in the common problem of stereo
setup. If I stand next to one speaker I
will often have difficulty in determining
if the other speaker is operating. But,
this shouldn’t happen. If I’m talking to
one person next to me and someone
else 10 feet away starts talking, I certainly know that. I may not be able to
understand that person because of the
nearby person’s voice, but I surely hear
that the far person is speaking. The
same should be true of the far stereo
speaker. The sounds coming out of that
speaker are different from the close
one (otherwise it wouldn’t be stereo).
Ceiling
Longer 1 kHz path
Shorter 3 kHz path
3000 Hz
FIGURE 1. The fundamental
problem with most
speaker systems is
that multiple drivers
are required. This
separates the high and Ear
low frequencies to
different physical
locations. The result is
that reflected sounds
have different acoustic
path lengths for
different frequencies.
This is phase distortion.
Tweeter
Equal path lengths
1000 Hz
Woofer
Floor
May 2007 75