Specifications

EFFECTS OF ACOUSTIC ENVIRONMENT ON GATING
Room Noise:
The louder the room noise, the greater the
talker's sound pressure level must be at the microphone for
it to gate on reliably. Generally this is not a problem because
people tend to talk louder in noisy environments. In addition,
the sensing circuitry has been equalized to reduce sensitivity
to room noise. If room noise is causing erratic gating, in-
struct the talker to stand or sit closer to the microphone.
The quieter the room, the farther the talker can be from the
microphone before gating becomes erratic.
Gating action is degraded most by continuous noise [such
as from air-moving equipment]. Noises of a transient
nature, including outside-acceptance-angle talking, have little
interfering action.
Reverberation:
If the talker is far enough from the
microphone so that the sound field at the microphone is dif-
fuse, the microphone will not stay gated on. For example, in
highly reverberant environments the microphone may gate
on initially for a distant sound source, then gate off when the
reverberant sound field builds up.
The less reverberant the room, the farther the talker can
be from the microphone before gating becomes erratic.
Typically, a source will gate the microphone on reliably up to
2 to 6 meters away [6 to 20 ft], depending on the
reverberation time and noise level of the room.
Talker Distance and Angle:
As the talker moves away
from the microphone and his speech becomes weaker in
comparison to room noise and reverberation, the accep-
tance angle for reliable gating narrows.
If the talker speaks just outside the acceptance angle, the
microphone will gate on occasionally. It will not gate on at all if
the talker is well outside the acceptance angle.
Reflective Surfaces: Sound reflections from a hard sur-
face behind the microphone can hamper proper gating. The
microphone should be at least 1 meter
[3 ft] from a wall
behind it, and at least 0.3 meters
[I ft] from objects behind
it such as large ashtrays or briefcases. For this reason, do
not place the AMS mixer on the table near the rear of the
microphones.
Early reflections from nearby walls may trigger occasional
microphone gating for sound sources outside the accep-
tance angle. This is more likely to occur in smaller rooms ex-
cited by narrow frequency range loudspeakers. This effect is
minor and should not cause any operational difficulty.
Operation on the verge of feedback [ringing], with sustain-
ed feedback or with test tones, can be expected to cause
some gating due to standing wave patterns in the room.
APPENDIX II
THEORY OF THE SHURE AUTOMATIC MICROPHONE
SYSTEM
Two problems associated with conventional multi-
microphone installations result from unwanted sound pickup
from temporarily unused microphones. These microphones
contribute excess room noise and reverberation, reducing
clarity and intelligibility. They also increase the total gain of a
sound reinforcement system, pushing the system close to
feedback [howling] and reducing the gain-before-feedback
available to individual microphones.
An idealized multi-microphone installation would have the
microphones spaced in a uniform, diffuse sound field, and
operated at identical effective gains. Under these condi-
tions, the increase in system gain compared to a single
microphone is given by:
G[dB] = 10 log1 n
Where
G is the system gain increase in dB over a single
microphone and n is the number of "on"
microphones.
In a sound reinforcement application, total system gain
must be adjusted below the feedback threshold with all
microphones operating. In a conventional mixing system,
each microphone would have G decibels less gain-before-.
feedback available than it would operating alone. G also
represents the increased pickup of room noise and
reverberation compared to a single microphone.
A solution to these problems is to gate on only the
microphone or microphones immediately in use. The Shure
AMS does this effectively and reliably by gating an individual
microphone on only in response to a sound source within its
acceptance angle. A microphone will not gate on for diffuse
room noise and reverberation, or for sound sources outside
its acceptance angle. While gated on, the AMS microphone
has a standard cardioid [unidirectional] pickup pattern
[half-
or hemi-cardioid in the surface-mount AMS221.
With multiple talkers, or a talker within the acceptance
angle of more than one microphone, a number of
microphones may be gated on simultaneously. In a
sound-
reinforcement application, if the system gain is set below the
feedback threshold with a single microphone gated on, the
increase in system gain with additional "on" microphones
could cause feedback. The AMS prevents
this by
automatically reducing all "on" microphone gains by at least
G decibels as additional microphones are gated on. This
maintains constant total system gain, avoiding feedback and
permitting the maximum microphone gains at all times.
This constancy of system gain means that pickup of room
noise and reverberation also remains constant at the level of
a single microphone. Except for transitions between zero
and one "on" microphone, gating action does not result in
audible modulation ["pumping" or "breathing"] of the room
noise. In applications where room-noise modulation caused
by the gating action of the first microphone is objectionable
[e.g., critical recording or broadcasting], the logic terminals
can be used to ensure that at least one microphone is always
gated on. An Override In terminal can be used to keep a
commonly used microphone gated on. Alternatively, the con-
nection shown in Figure 16 can be used to force one of the
microphones on only when all the other microphones are
gated off.
As mentioned in the description of the Off-Attenuation
controls, partially rather than fully attenuating the "off"
microphone makes the gating action significantly less
noticeable. However, if insufficient attenuation is used, the
increased system gain from the unused but not fully off
microphones will result in room noise and feedback problems
approaching those experienced with all microphones fully on.