Specifications

Appendix A
Shure automatic mixers have emerged as the product
of choice for city councils, school boards, and legislative
bodies. The following are helpful hints on designing and
implementing a successful legislative system using a Shure
automatic mixer.
Mayor or Chairman Position
• Privacy switches are normally required in legislative chambers. Always use
the automatic mixer’s MUTE logic terminals to provide this feature. With the
Shure AMS automatic mixer, the Off-Attenuation control sets the “depth”
of the muting provided when the MUTE terminal is used.
• A visual indicator, like a light emitting diode (LED), showing if a council
member’s microphone is muted or active is commonly installed where the
member can easily see it. This indicator is controlled by the automatic
mixer’s GATE OUT terminal.
• An all-council mute switch gives the chairman control over every
microphone in the system. The automatic mixer’s MUTE terminals are
employed for this feature.
City Clerk
• The clerk’s position should include a switch to mute or activate the
microphone on the public lectern. This switch is normally single pole/
double throw (SPDT) and is wired to alternately ground the automatic
mixer’s MUTE terminal or OVERRIDE terminal. This configuration gives
complete manual control of the public microphone to the City Clerk. This
switch could also be mounted at the mayor’s position.
Council Member’s Position
• Standard features are a privacy switch using the MUTE terminal and an
illuminated visual indication if the microphone is muted or active.
• A gooseneck microphone or stand mounted microphone is recommended.
As the microphone is positioned above the table surface, noise from paper
shuffling or table tapping is less likely to activate the microphone or be
heard through the sound system. A low profile surface mount microphone
will always pick up a great deal of unwanted paper noise because the
microphone is on the table surface.
• A custom-made rigid tube is an attractive way of mounting a microphone.
This eliminates the possibility of a council member repositioning the
microphone incorrectly.
• If a low profile surface mount microphone must be employed, make certain
that the rear of the microphone (where the cable exits) is not facing any
surface that may reflect sound, like a nameplate or a privacy panel.
The reflective surface may interfere with the automatic activation of the
microphone or may degrade the acoustic performance of the microphone.
[A low profile surface mount microphone may be placed on top of a privacy
panel.] If it is absolutely necessary to position a low profile microphone
close to a reflective vertical surface, a sound absorbent panel (1” or 2”
thick fiberglass covered with open weave fabric) should be installed so that
the surface is made as absorptive as possible. This can often improve the
inconsistent activation that will may occur if the reflective surface is not
treated. Remember, most low profile surface mount microphones are
designed to work properly only when the rear of the microphone is facing
an open space.
P.A. Feed
• The outputs of the automatic mixer can feed a number of separate devices.
The main output generally feeds an equalizer and power amplifier to
provide the sound reinforcement needs of the room. If there is an Auxiliary
output, it generally feeds a dedicated tape machine which records the
council proceedings.
• The automatic mixer’s logic terminals are often used to control loudspeaker
muting relays for distributed loudspeaker systems. Commercially available
duckers and TTL logic-to-relay converters also are useful for loudspeaker
muting.
• In very large systems, a “mix-minus” matrix design may be appropriate.
In this type of system, a group of microphone signals are fed to all
loudspeakers except the loudspeakers closest to the microphones’ location.
This helps improve control over acoustic feedback.
Recording Applications
• A multi-track tape machine is often used to record the proceedings. One
track is often dedicated to document the entire meeting for the public record.
The remaining tracks are used to individually record the chairman, the
council members, and members of the public whom address the council.
• If it is necessary to record more than one council member microphone
on a single track, the Direct Outputs from the automatic mixer can be
paralleled to feed that track. Normally, up to twelve Direct Outputs can be
connected together.
• A “default”microphone is often employed to maintain a suitable amount of
room ambience in the recording. This microphone is activated only if all
other microphones are inactive.
Broadcast/Press Feeds
• A distribution amplifier is a useful device to provide six separate feeds from
the automatic mixer. Each output of a typical distribution amplifier is
switchable for mic or line level providing broadcasters the feed level of their
choice. As a distribution amplifier's outputs are isolated from each other, a
shorted cable plugged into one output will not affect the other feeds.
• As in Recording Applications, a default microphone should be used to
maintain room ambience to the broadcast feeds.
37
MEETING FACILITIES
Audio Systems Guide for
Appendix A:
Design hints for city councils,
school boards, and legislative chambers
Reference Information
Indiana Public Schools Council