Handbook for Sound Engineers

756 Chapter 21

table with the room in normal unoccupied operation and
at least 2 feet from any surface. Methods for achieving
low interfering noise levels are discussed in Chapter 6,
Small Room Acoustics.
More accurate assessment will result if noise criteria
(NC) are used because of the strong influence of
frequency spectrum on speech interference and listener
annoyance (see Chapter 5, Acoustical Treatments for
Indoor Areas).
Fig. 21-28 shows the maximum microphone/talker
distance for a marginally acceptable SNR of 20 dB in
transmitted speech. The graph applies to omnidirec-
tional microphones. The distance may be increased by
50% for directional microphones. If more than one
microphone is active in the system, the number of open

microphones must be taken into account by reducing the

predicted SNR by 3 dB for each time the number of

open microphones doubles. An automatic microphone

mixer will alleviate this concern.

If an acoustical survey indicates the presence of

interfering noise sources, construction techniques must

be implemented to provide adequate sound transmission

losses, or another room should be considered.

21.3.6.2 Conference Room Reverberation

Reverberation is often identified by conference partici-

pants as the “speaking into a barrel” effect. The sources

of reverberation are variable. For example, there is the

reverberation from hard surfaces in the room where

Figure 21-27. Block diagram of the Vortex EF2280 digital multichannel acoustic echo and noise canceller with a built-in

automatic microphone/matrix mixer. Courtesy Polycom, Inc.