Microphone Technique Basics; How To; Reduce Feedback And Reverberation - Crown SASS Application Manual

INTRODUCTION
In this guide you'll find suggestions on using
Crown microphones effectively. The CM, GLM, and
LM microphone lines are covered in this booklet. For
application notes on the PZM®, PCC® and SASS®,
please see the Crown Boundary Mic Application Guide.
You will find that Crown microphones can solve many
of your audio problems.
MICROPHONE
TECHNIQUE BASICS
How to reduce feedback
Feedback is a squealing sound from sound-reinforce-
ment speakers that occurs when volume is too high.
To reduce feedback:
• Turn down the volume on the offending
microphone until feedback stops.
• Use as few microphones as possible. Gain-before-
feedback decreases 3 dB each time the number of
open mics doubles.
• Place the mic close to the sound source. The closer
the mic, the higher the gain-before-feedback. If close
miking causes an unnatural tone quality, try using
EQ to compensate.
• Equalize the sound system with a 1/3 octave graphic
equalizer. Notch out frequencies that feedback.
• Place speakers as far from the mic as possible.
• Place the mics behind or to the outside of the house
P.A. speakers. The house speakers should not aim at
the microphones.
• Use directional mics. Hypercardioid and supercar-
dioid patterns reject feedback better than cardioids,
and cardioids reject feedback better than omnidirec-
tional patterns.
• Use differential (noise-cancelling) mics, such as the
Crown CM-310A or CM-311A. They have the
highest gain before feedback of any mic you can buy.
The following table tells how many dB of feedback
rejection you can expect from various polar patterns,
in a reverberant sound field, compared to an omnidi-
rectional pattern at the same distance:
Omnidirectional
Cardioid
Bidirectional
Supercardioid
Hypercardioid
0.0 dB
-4.8 dB
-4.8 dB
-5.7 dB
-6.0 dB
For example, a cardioid mic provides 4.8 dB more
gain-before-feedback than an omni mic at the same
distance from the sound source.
You can place a directional mic farther from its source
than an omnidirectional mic in a reverberant sound
field and have the same gain-before-feedback. The
table below shows the distance multiplier for each
pattern:
Omnidirectional
Cardioid
Bidirectional
Supercardioid
Hypercardioid
For example, if an omni mic is one foot from a sound
source, a supercardioid mic can be placed 1.9 feet and
have the same gain-before-feedback as the omni.
The figures above apply only when the mics are in a
reverberant sound field - say, when the P.A. speakers
are distant from the mics and the sound system is set
up indoors.
How to reduce reverberation
Reverberation is sometimes loosely called "room
acoustics" or "ambience." It is a pattern of sound
reflection off the walls, ceiling, and floor. For example,
reverberation is the sound you hear just after you
shout in an empty gymnasium. Too much
reverberation in a recording can make the recorded
instrument sound distant or muddy. To reduce
reverberation:
• Place the mic closer to the sound source.
• Pick up electric instruments with a direct box or
cable.
• Use a room or studio with dead acoustics. The walls,
ceiling, and floor should be covered with a sound-
absorbing material.
• Use directional microphones. Hypercardioid and
supercardioid patterns reject reverb more than
cardioid. Cardioid and bidirectional patterns
reject reverb equally well. Cardioid rejects reverb
more than an omnidirectional pattern at the
same distance:
Omnidirectional
Cardioid
Bidirectional
Supercardioid
Hypercardioid
3
1.0
1.7
1.7
1.9
2.0
0.0 dB
-4.8 dB
-4.8 dB
-5.7 dB
-6.0 dB