Handbook for Sound Engineers

Preamplifiers and Mixers 755

employ very subjective descriptions that have to be inter-
preted into quantitative engineering terms that can then
be applied, measured, and included in system designs.
Teleconference participants expect good speech intelligi-
bility, easy identification of the talker, relatively high
SNR, and other qualities. They also expect the overall
aural experience to be better than a conversation
conducted via telephone handsets.
An audio conferencing installation for voice and
program has four primary facets:

1. Conference room and building acoustics.


2. Interface with telephone/transmission system.


3. Possible secondary use as a sound reinforcement
   system.


4. Proper equipment selection and setup.

21.3.6 Room and Building Acoustics

21.3.6.1 Conference Room Noise

The first consideration for a teleconference installation
is noise in the room. Obvious noise sources, like heating

and air conditioning systems, should be evaluated and

specified for acceptable levels. External noise must also

be considered:

• Conversations in hallways or adjacent offices.


• Business machines in adjacent spaces.


• Elevators on opposite sides of the wall.


• Water flow in building services.


• Vibration of air conditioners on the roof.***


• Loading docks

There will also be unwanted noise generated in the

conferencing room itself:

• Fans in projectors and computers.


• Hum from light fixtures.


• Paper shuffling.


• Moving chairs.


• Coughing.


• Side conversations.

All of these undesired sound sources are much more

obvious, annoying, and detrimental to intelligibility at

the remote site of the teleconference than they are in the

local site where they originate. Also, as the number of

participants increases, the geographic area covered by

the participants expands and unamplified speech

becomes harder to hear due to greater distances between

the talkers and the listeners. Consequently, for comfort-

able talking and listening, the ambient noise level in a

room must be lower for larger groups.

Table 21-1 provides recommended noise level limits

for conference rooms. These are levels at the conference

Figure 21-25. Block diagram of the E-1 automatic mixing controller. Courtesy Dan Dugan Sound Design.

Auto mix

depth

Level servo

Automixgain display gain display

On

Auto

Off

Speech

Music

System

Automix

threshold

Audio

input

Typical of 8 channels

Common

Music

system

threshold

input Musicsystem

threshold

input

Override

preset

mute

Logic and

control buses

Level servo

speed

Limiter

threshold

headroom NOM gain limit threshold

NOM master

NOMgain limit gain display

Gain sense

Level servolevel set VCA

Level servo

& limiter

Audio

VCA

Audio

output

CV in

Auto mix

Fader

Auto mix

CV output

Weighting

Figure 21-26. Vortex EF2280 digital multichannel acoustic
echo and noise canceller with a built-in automatic micro-
phone/matrix mixer. Courtesy Polycom, Inc.