Crown GLM-200 Specifications download pdf (Page 22)

1. ACOUSTIC PHASE

In any situation where the mic'ed instrument faces a loud-

speaker, 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 relation-

ship between the mic'ed instrument

and speaker. Here's why:

A typical guitar has a cavity reso-

nance of about 100 Hz. This is the fre-

quency 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

degrees for every 1/2 a frequency's

wavelength. In this case, 1/2 the

wavelength is about 5 1/2 feet.

If you set your MICROPHONE channel

PHASE switch to eliminate cavity res-

onance (

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 FEED-

BACK

Notching out instrument cavity resonance with an outboard

equalizer patched through the Microphone channel effects loop

(See Page 20) 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 instru-

ment and speaker can remain

in phase

, preserving low frequen-

cy content.

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 fre-

quencies 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.

FISHMAN POCKET BLENDER

APPENDIX II: PHASE Continued

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 degrees every time you flip it.

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Crown GLM-200 Specifications Page 22

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