Handbook for Sound Engineers

Sound System Design 1329

The problem with boundry mics is the users. If the
mics are loose, the users will push them out of the way
or turn them around. If they are fixed in position, users
will lay paperwork or books on top of them, spill water
or coffee on them, and otherwise abuse the micro-
phones. Boundry mics also do a very good job of
picking up the noise of paper shuffling on the table.
Thus, another school of thought suggests installing
gooseneck mics at each talker’s position. The goose-
neck mic puts the mic element up near the talker’s
mouth so it does a good job of picking up up the direct
sound. Unfortunately, the first reflection from the table
also enters the gooseneck mic causing undesirable comb
filtering. Also, it’s easy for the user to push the goose-
neck mic out of the way.
Fortunately, gooseneck mics seem to attract users
who often pull them close and talk directly into the
microphone. When this happens, the direct sound is
much louder than the first reflection from the table so
the reflection is of little consequence. Thus, the goose-
neck mic may work better than the boundry mic even
though it is technically inferior (because of the reflec-
tion from the table).
In some conference rooms, the designer may need to
try both microphone types to see how the users react. It
may also be a good idea to give each user a lighted mute
button for private conversations with nearby conferees.
Conference room loudspeaker systems are normally
laid out in a distributed fashion. To help avoid feedback
from multiple open microphones and to help avoid
pickup of paper shuffling and other ambient noises, an
automatic mixer is commonly used (see Section 34.4.3).
Often, the chair position is given an automatic priority
switch so that when the person at the chair position
speaks, all other microphones are turned off (this may
also be accomplished by a manual priority switch).
Another way to help avoid feedback is to use the
logic outputs on the back of many automatic mixers to
switch off the loudspeaker directly above the head of
the person talking. This approach, however, can hinder
the kind of back-and-forth conversation where more
than one person talks at the same time and there are
frequent interruptions. It should be noted that, in a
conference room, this kind of conversation is difficult
with any system.
A better way to reduce feedback is to use a
mix-minus system as discussed in Section 34.4.3.3. To
make this work for a large conference table, design the
system so that each loudspeaker receives its own
customized mix including all microphones except the
one nearest the loudspeaker. Taper the mix for micro-
phones located farther away from the local microphone

as shown in Fig. 34-41. Loudspeakers for inactive

participants can be on all the time unless a roving

microphone is being passed around. In this case, the

level to the local loudspeakers may need to be reduced

somewhat, and, of course, the feed from the roving

microphone to the conference table should be fed to all

loudspeakers at equal levels.

34.6.5.2 Recording a Conference

The multiple-microphone setup of a conference can be a

problem for recording. All microphones can simply be

mixed and fed to a single channel of an audio recorder,

but the multiple microphones may add unwanted ambi-

ent noise. Using the output of the automatic mixer helps

avoid this problem. Use one of the matrix outputs when

designing a mix-minus system. A courtroom-style, mul-

tichannel, logging recorder may also be used to record a

number of individual microphones on separate chan-

nels. This is especially useful when a transcript of the

conference must be made later. In some systems, the

chair is given a switch to pause the recording for

off-the-record conversations.

34.6.5.3 Audio and Video Teleconferencing

Although speaker-phone conferences over normal tele-

phone lines are still commonplace, most video/audio tele-

conferencing now takes place over the Internet or

dedicated wide area network (WAN). Whatever method

is used, the characteristics of the transmission path must

be considered. In particular, it is possible for feedback

and echoes to occur over a complex path including the

transmission line, the local microphone(s) and loudspeak-

ers and the remote microphone(s) and loudspeakers.

A digital echo canceller is built in to every modern

video/audio teleconferencing system. These devices

help solve the problems of echoes and feedback in a

teleconference. To optimize the echo canceller,

however, the local audio systems in both rooms must be

properly designed.

In most cases, this design is very similar to the

design of a conference room designed for local sound

reinforcement. Choose microphones carefully as

described in Section 34.6.5.1. Lay out a distributed

loudspeaker system. Use an automatic mixer with

matrix output to create a mix-minus system. Do this in

both conference rooms.

In operation, the echo canceller takes some time to

adjust itself to the system. After it optimizes its opera-

tion, the system should be relatively free of echoes or

feedback. The echo canceller must reoptimize itself