Handbook for Sound Engineers

1330 Chapter 34

when there is any change in the system so try not to
move any microphones once a conference has started.

34.6.6 Sound Masking (Noise Masking), Speech
Privacy Systems

Sound masking is an electronic system that creates a
low-level “masking sound” to improve speech privacy
and mask irritating noises in a work space. A well-
designed sound masking system provides protection for
confidential conversations and creates a more pleasant
work environment. A sound masking system may also
be called a noise masking or white noise system.
Open-plan offices are the most common application
for sound masking. Doctors’ exam rooms, and other
facilities that fall under HIPPA privacy regulations, are
also good places for sound masking. Sound masking
may also be used in courtrooms, law offices and in
high-security government or industrial facilities.

34.6.6.1 Criteria for the Environment

A limited range of environments is suitable for
sound-masking. The environment must be relatively
quiet since the masking sound will need to be louder
than the noise to be masked. A general criterion for
ambient noise level is about NC-35 (noise criteria curve
number 35) or about 45 dB on the A scale. The environ-
ment must have a low-reverberation time and as few
hard, reflecting surfaces as possible. A well-designed
open-plan office will usually meet these criteria.

34.6.6.2 Mechanical Noise Reduction

Noise from computer printer areas, copying machines,
and so on should be reduced by mechanical and acousti-
cal means since the sound masking system cannot be
expected to overcome these relatively high-level sources.
In addition, mechanical barriers in the form of open-plan
office-type acoustical dividers are normally installed
between offices to help attenuate noises and speech from
point to point. Nonreflective ceilings (preferably dropped
acoustical tile) and carpeted floors are almost mandatory.
Without these mechanical aids, the sound masking sys-
tem cannot perform its function successfully.

34.6.6.3 Masking Sound

Once higher-level noises have been reduced and an
acceptable acoustical environment has been created,
low-level noises become more irritating, and speech pri-

vacy becomes more important. Masking sound, created

by a sound masking, speech-privacy system, can help

solve these problems, Fig. 34-88.

Masking sound systems should be located in the

vicinity of the listeners, not near offending noise

sources. Masking systems must be completely unnotice-

able to the listeners. That is, no one should know there

is a sound-masking system in operation unless someone

turns it off. The reason for this, of course, is that the

purpose of the system is to help reduce distracting

noises and to aid speech privacy. If the system itself

becomes a distraction, one of its primary purposes has

been defeated.

The masking sound itself is created by an electronic

random-noise generator similar to that used for equal-

ization, and it is fed through an equalizer to a set of

power amplifiers and loudspeakers. The loudspeakers

are normally hidden in the ceiling plenum, above the

acoustic tiles.

34.6.6.4 Criteria for the Loudspeaker System

To meet the primary goals of sound masking and speech

privacy and to remain unnoticed by listeners, the loud-

speaker system must produce random noise that does

not change in level as a listener moves from place to

place within the environment. A traditional down-

ward-facing distributed system could achieve this goal,

but only with an extremely high density of loudspeak-

ers. Thus, an upward or sideways facing system is more

common, Fig. 34-89. The upward and sideways systems

use mechanical structure in the ceiling plenum to reflect

and help randomize the noise distribution. In a typical

open-plan office with a ceiling height of 8–9 ft, the

loudspeakers would be spaced in a square or hexagonal

pattern with approximately 10–20 ft spacing between

individual loudspeakers. Care must be taken to avoid

placing a loudspeaker too near a hard reflecting object,

such as an airduct, which might cause an audible hot

spot in the room below. Various ceiling materials and

Figure 34-88. Factors affecting speech privacy. Courtesy

Atlas Sound.

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