: speakers in unison, sounds appear to come from between
If the sound level in both the right and left speakers are equal, the
sound will seem to come from a point in space half-way between the speakers.
Figure 2: If a sound is louder in one speaker than the other
the sound will be heard closer to the louder speaker
If one speaker is louder than the other, the sound will seem to come
from a point in space closer to the louder speaker.
When a stereo signal is recorded, microphones are placed so that the
sounds in each speaker is at the correct level to make sounds seem to come
from the directions they originated form. Because of this all of the instruments
of a symphony orchestra will seem to come from their proper places between
the speakers; violins, trumpets and timpani toward the left; violas, woodwinds
and horns toward the center; cellos, basses and trombones toward the right.
Stereo sound, as described above, works only if the speakers are in
phase, meaning that the speakers are wired to work in unison. To be in
phase, the speaker cones must move together; they must move toward the
listener at the same time and must move away from the listener at the same
If you accidentally wire one (but only one) speaker backwards, the
speakers will work out-of-phase; one speaker cone will be moving toward
the listener while the other speaker cone is moving away from the listener
and vice versa. This causes a total collapse of the stereo effect. All
sounds will seem to come from everywhere between the speakers at once or
may seem to come from both speakers at once. To be sure your speakers are
in phase; tune to an AM radio station (or use another non-stereo signal);
adjust the volume to be equal in both speakers. The sounds should seem
to be coming from a single point in space between the speakers and nowhere
else. If this is not the case, the speakers are out-of-phase. To fix the
phasing, reverse the wires on one (and only one) speaker.
Figure 3: speakers out of phase, listeners can't tell where
sound comes from
Using Phasing to Add a Third Channel to a Two Channel System
Encoding surround sound is very simple. The third channel is simply
mixed into the left channel as if it were any other microphone channel.
However, before being mixed with the right channel, the phase of the third
channel is reversed. This has the same effect as if the wires on the right
speaker were reversed, but only for sounds encoded for the third channel.
Therefore, sounds intended for the left and right speakers will sound normal.
Sounds encoded for the third channel will be hard to locate and may even
be lost in the shuffle unless deliberately listened for.
Decoding the Surround Channel
In order for a speaker to work there has to be an electric current
flowing through it. As the current through the speaker changes, the speaker
cone moves back and forth. The more current flowing through the speaker,
the farther the cone moves. The current actually reverses direction so
that the speaker cone is alternately pushed out and pulled in, creating
[missing diagram of typical speaker]
To decode surround sound is also very simple. All you have to do is
add a third speaker. The secret is how the third speaker is hooked up to
the system. Normally a speaker is hooked up to the red and black (or +
and -) connectors on the stereo amplifier. The third speaker has both wires
hooked to the red (or +) connectors on the amplifier. This is the simplest
way to create a surround sound system.
Figure 5: Surround sound with sounds for front channels
Notice in Figure 5 that current flows through the front speakers, in
one side and out the other. Current doesn't flow in the third speaker because
current is trying to flow in both sides at once.
Figure 6: Surround with sounds for the rear channel
With sounds encoded for the third channel, the current flows out of
phase for the front speakers. Notice in Figure 6 that when the current
flows out of phase in the front speakers, it is presented to the rear speaker
in such a way that it is able to flow through the speaker. Therefore, sound