Handbook for Sound Engineers

Preamplifiers and Mixers 757

speech is originating at the moment. Requirements for
comfortable listening in the room dictate reverberation
times that are not too short, as a bit of acoustic liveli-
ness in a meeting room is desirable. If an automatic
microphone mixer is not employed to reduce the rever-
beration picked up by unused microphones, the rever-
beration heard at the remote site of the teleconference
can be excessive and intolerable. In this situation, a very
low (and uncomfortable) reverberation time at the local
site is required. The potential for reduced intelligibility
at the remote site is increased because the remote partic-
ipants do not have the advantage of separating the
speech signal from the reverberation via binaural hear-
ing, plus not having the talker in the same room also
tends to dull one’s attention.

There is also the reverberation added at the remote
site. The incoming signal is reproduced by loud-
speakers, the sound propagates around the room, and

even more reverberation is added to the talker’s signal.

So, unless there is only a telephone handset at the

remote site, both sites need to have proper acoustical

characteristics. This is often not the case as in many

conferencing rooms the visual comfort of the room

takes precedence over the aural comfort. Just ask the

interior designer!

Room dimensions should be chosen to minimize

standing waves and flutter echoes. If the room already

exists, judicious use of acoustically absorbent material

is advisable for control of the room’s acoustics.

Critical distance (Dc) is often used to predict appro-

priate talker to microphone distances. Dc is where the

direct signal of the talker is equal to reflected signal of

the talker, i.e., 50% direct signal and 50% reverberant

signal. The Dc in conference rooms is typically in the

range of 1–4 feet. Dc may be estimated from reverbera-

tion time measurements:

(21-5)

where,

Dc is the critical distance in feet,

V is the room volume in ft³,

T is the reverberation time in seconds for 60 dB decay.

For good intelligibility, an omnidirectional micro-

phone should be placed at ½ of Dc or less from the

talker. When a directional microphone is used, the

distance between talker and microphone may be

increased up to 75% of the critical distance.

Because the sound decay in the first 60–100 ms

usually is the most damaging to teleconference conver-

sations, the usual reverberation time measurement,

RT 60 , may not be the most appropriate. One manufac-

turer of conference equipment insists that the room

produce a decay of greater than 16 dB in the first 60 ms.

The one admonition to anyone faced with the design

of a teleconferencing system is do not ignore the acous-

tical characteristics of the room. Insist upon a room that

has the right acoustical environment or commit the

resources to make it right before proceeding with the

rest of the project. Acoustical deficiencies can rarely be

corrected by electronic means. If it is new construction,

work closely with the architect before the room design

is complete.

21.3.6.3 Telephone/Transmission System Interface

Fig. 21-27 shows two teleconference rooms connected

by a single two-wire telephone line. Each room has a

Table 21-1. Ambient Noise Level Limits for
Conference Rooms

Conference

Size

Maximum

Sound

Level in

dBA

Preferred

NC

Acoustic

Environment

50 people 35 20–30 Very quiet, suitable for large

conferences at 20–30 ft table.

20 people 40 25–35 Quiet, satisfactory for con-

ferences at a 15 ft table.

10 people 45 30–40 Satisfactory for conferences

at 6–8 ft table.

6 people 50 35–45 Satisfactory for conferences

at 4–5 ft table.

Figure 21-28. Acceptable ambient noise levels.

Absolute limit of

acceptability

(10 dB SNR)

Marginal limit of

acceptability

(20 dB SNR)

Acceptable

region

60

50

40

30

1 1.5 2 3 5 7 10 15 20

Maximum distance from talker to microphone—feet

Ambient noise level—dB SPL(A)

Dc 0.03V

T

= ---