A C O U S T I C B L E N D E R S Y S T E M
21
A P P E N D I X I I : P H A S E
WHAT IS PHASE?
Phase is the relationship between two signals or sound waves, originating from the same
instrument.
For our purposes, Phase relationships are expressed as being either
“in phase” or “out of
phase”. In phase tends to enhance, while out of phase tends to suppress the natural character-
istics of an instrument. A simple way to determine the quality of Phase (in or out) of two
sounds is to compare phase switch settings at low volumes.
IN PHASE
In phase is when the waveforms
of two signals or sounds origi-
nating from the same instrument
are similarly aligned in time.
Similar phase is like looking at
yourself in a mirror; your reflec-
tion directly follows your move-
ment.
OUT OF PHASE
Out of phase is when the wave-
forms of two signals or sounds
originating from the same instru-
ment are aligned such that the
upper peak of one wave occurs
at the same moment in time as
the lower peak of the other.
Out
of phase is like looking at your-
self in a live video monitor; the
image you see is similar, but the
perspective is shifted. When
you move to the right, the image
appears to move to your left.
WHAT ARE THE BLENDER'S PHASE SWITCHES FOR?
The phase switches are useful for two reasons:
A. Due to the interactive and changing nature of phase, acoustic amplification depends on
maintaining optimum phase relationships between amplified instruments, sound systems and
venues.
B. Since an industry standard for polarity has never been established for all sound equip-
ment, the phase switches compensate for any unintentional phase differences that might
occur between components.
APPLICATIONS
When amplifying acoustic instruments with a microphone/pickup combination, there are two
crucial phase relationships:
ACOUSTIC PHASE - The relationship between the sound waves of a mic'ed acoustic instru-
ment and the speaker system.
ELECTRONIC PHASE - The relationship between pickup and microphone signals.
1. ACOUSTIC PHASE
In any situation where the mic'ed instrument faces a loudspeaker, there will be an interactive
phase relationship between the two. This usually occurs with stage amps, side fill and floor
monitors at close distances.
LOW VOLUME AMPLIFICATION
At low volumes, when a mic'ed instrument and speaker are at similar levels and are
in phase,
the sound is full and solid, with the lower frequencies emphasized.
When a mic'ed instrument and speaker are
out of phase at low levels, the bass frequencies
cancel out to some extent. The resulting sound is somewhat unnatural and unbalanced com-
pared to
in phase.
HIGH VOLUME LEVELS
At high volume levels, when a mic'ed instrument and speaker are
in phase, the sound pressure
from the speaker will excite the instrument's sound chamber, creating a feedback loop at the
instrument's lowest octave. This "cavity resonance" feedback can be dealt with by putting the
mic'ed instrument and speaker
out of phase or by adding outboard equalization.
A. PUTTING THE CAVITY RESONANCE OUT OF PHASE TO REDUCE FEEDBACK.
Inverting the MICROPHONE channel PHASE switch will put the mic'ed instrument and speaker
out of phase with each other, cancelling the low frequency feedback (not recommended for
bass instruments).
If you move from your position on stage more than a few feet, you may have to invert the
phase switch again to maintain an out of phase relationship between the mic'ed instrument
and speaker. Here's why:
A typical guitar has a cavity resonance of about 100 Hz. This is the frequency that generally
feeds back when a mic'ed guitar and speaker are in phase. 100 Hz has a wavelength of about
11 feet. phase inverts 180° for every 1/2 a frequency's wavelength. In this case, 1/2 the wave-
length is about 5 1/2 feet.
If you set your MICROPHONE channel PHASE switch to eliminate cavity resonance
(out of
phase) and then move 5 1/2 feet towards or away from the speaker, you will effectively put the
mic'ed guitar/speaker relationship at 100 Hz back
in phase; in the line of fire for low frequency
feedback.
B. USING AN OUTBOARD EQUALIZER TO REDUCE FEEDBACK
Notching out instrument cavity resonance with an outboard equalizer patched through the
Microphone channel effects loop
(See Pages 16 & 23)
will eliminate the low frequency feedback
problem completely. The advantages to using notching equalization are:
• The physical distance from the speaker will no longer be a factor for potential
low frequency feedback.
• The mic'ed instrument/speaker can remain
in phase, maintaining a more natural
and balanced response.
This is the preferred method for bass, since the mic'ed instrument and speaker can remain
in
phase, preserving low frequency content.
(See Page 20)
2. ELECTRONIC PHASE
Once the mic'ed instrument is phase aligned with the sound system, the pickup signal can be added.
When a microphone and pickup are at similar levels and are
in phase, the sound is full and
solid, with the lower frequencies emphasized.
When a microphone and pickup are
out of phase the bass frequencies cancel out to some
extent. The resulting sound is somewhat unnatural and unbalanced compared to
in phase.
It is especially important for the microphone and pickup to be
in phase when recording to attain
the fullest and most natural sound.
The PHASE switch on the BLENDER's MICROPHONE Channel in effect electronically "moves" your
instrument's position relative to the speaker by inverting the phase 180° every time you flip it.
