Theatre Design & Technology - Fall 1979 - 30

This paper was reviewed by the Audio and Acoustics Subcommission of USITT. Comments were received from Charles Bonner,
Wally Barrow (for R. Lawrence Kirkegaard), John Bracewell, Christopher Jaffe, Dennis Paoletti, and Robert Wolff (for Russell Johnson), all
members of the Subcommission. Comments were also received from
Norman Paulhus, who is not a member of the Subcommission. Not all
comments were incorporated into the text and the following is a summary of the disagreements that remain: One consultant believes that
small theatres must be just as quiet as large theatres and that the NC15 curve should rarely, if ever, be exceeded, regardless of budget.
Another believes that it is the acoustical consultant's responsibility to
work out matters for the mechanical engineer, so that the .·confrontation" described in the article never takes place. Another prefers
to see material included on newer variable-volume energy-efficient
systems, a topic not discussed by the author because the vast majority of projects he encounters are still constant-rate-of-air-change systems. Finally, one consultant believes that the role of the acoustical
consultant is not stressed enough. Despite these reservations, I believe, along with the majority of the respondents, that the article has
considerable use for theatre people in general.
--David L. Klepper

This article discusses the unpleasant subject of unwanted
sound. The reader may be surprised to learn that the most
commonly-encountered acoustic defect in theatres is excessive noise from mechanical systems. Since background noise
is everpresent to some degree, when is it considered excessive? Obviously, it is excessive if members of the audience
and performers are distracted and annoyed by either noise or
vibration. At another level, less obvious, noise is excessive if it
significantly interferes with intelligibility. Most of us are so accustomed to mechanical equipment noise that we may not
perceive moderate levels of such noise as a distraction. In
fact, we may be unaware of a noise problem until it is pointed
out; or, if it is pointed out, we might believe that it is not severe
enough to be harmful. Nevertheless, aware of the noise or not,
the presence of continuous background noise results in degraded speech intelligibility, because intelligibility is almost directly related to the signal-to-noise ratio. Thus, the higher the
signal level, the better the intelligibility, the higher the background level, the poorer the intelligibility.
A common misconception is that the audience itself produces noise and, therefore, a moderate level of background
noise from the mechanical equipment will not be heard in an
occupied theatre. On the contrary, an audience can be, and
usually is, deathly silent during critical moments of a good
performance. When intelligibility is impaired, though, the additional concentration required is fatiguing to the listener, and
the result can be a restive audience.
Even though intelligibility is often increased by electronic
amplification, in many types of productions sound system performance can also be negatively affected by background
noise. This may require a system to be operated at close-tofeedback (if not into feedback or distortion!), resulting in very
harsh, unpleasant, and even less-intelligible sound; or require
it to be operated above a natural level, destroying realism, and
making everyone in the audience much too aware that amplification is employed. Of course, the goal for any theatre should
be high intelligibility without electronic amplification whenever
possible, and this goal demands good control of noise.
Among the reasons for the prevalence of mechanical system noise in theatres is the inadequate understanding of the
importance of quiet systems. This lack of understanding applies not only to the client and architect, but often to the mechanical engineer as well. Further, the most-frequent practice
for distributing air uniformly employs diffusers which spread

the airstream into a pattern, giving uniform coverage in a
space. For a diffuser to function as its designers intended, the
air must have sufficient exit velocity to travel some distance
beyond the diffuser; and high diffuser neck air velocities always mean air··turbulence and resultant noise.
The hiring of an acoustical consultant does not necessarily
solve these problems. Indeed, a conflict sometimes occurs
between the acoustical consultant and the mechanical engineer, because the acoustical consultant's recommendations
can effect increases to the cost of the mechanical system.
Early in the design process, the mechanical engineer may
have estimated a total price for the system, without adding to
the estimate the cost of larger ducts and diffusers, thorough
vibration isolation, silencers, and internal duct lining that a
quiet system usually demands. He may then feel compelled to
adhere to his estimate; and the acoustical consultant is then in
the position of criticizing the mechanical engineer's design
and insisting on the increase in budget necessary for a quiet
system. This conflict is not always won by the acoustical consultant, for the owner's sympathies may tend toward economy. After all, the mechanical system cannot be all that important, can it-at least as long as it has the capacity to maintain
a comfortable temperature. I hope that by now the fallacy of
this statement is recognized.
There are good mechanical engineers with experience in
theatre spaces who know the importance of quiet. The acoustical consultant can be very helpful at the start of the theatre
design process by insisting that the architect leave enough
space for the large ducts that permit low velocities and quiet
systems, and provide good locations for the mechanical
equipment rooms.

Background Noise Ratings and Criteria
In general, background noise from mechanical systems is
present for the full range of audible frequencies, and its magnitude will be different at different frequencies. Therefore, a
complete description of a particular background noise environment of a single location would indicate its level at each
frequency. For simplicity, however, background noise characteristics are most frequently described by one of two singlenumber descriptors---€ither an NC (Noise Criteria) number or
an A-weighted sound pressure level.
An NC rating is derived by comparing octave-band sound
pressure levels against a set of NC curves. The lowest-numbered curve not exceeded by the actual noise octave-band
characteristic in any octave band is the NC rating of the particular background noise. NC curves are described in steps of
five (NC-15, NC-20, NC-25, etc.), but interpolation between
these curves is certainly possible, and one occasionally hears
of such values as "NC-22" in the acoustic literature. There
has been criticism of the NC curves, especially since a sound
that exactly matches an NC curve is usually judged both rumbly and hissy. New criteria curves have been proposed and
are under consideration by the American Society of Heating,
Refrigerating, and Air-Conditioning Engineers.
A-weighted sound levels in decibels (dB) represent integrations of sound levels over the audible frequency range,
but weighted according to a curve that de-emphasizes the
low-frequency signals (as well as slightly de-emphasizing the
extreme high-frequency range). Both A-weighting and NC
curves are derived from the actual hearing response of the human ear-that is, greater sensitivity at middle and high frequencies than at low. Therefore, we should not be surprised
that the two ratings are closely related and, in fact, the difference is usually between five and ten dB, with the dB(A) rating
the larger of the two numbers. Experience has indicated that

L. Gerald Marshall holds degrees in both Music Education and Architectural Engineering. His background covers a broad range of consulting
projects and research studies in room acoustics design, sound isolating construction, and noise vibration control. Since the formation of Klepper,
Marshall, King Associates his projects have included the Rollins College Music Building, the Music and Art Building at Colorado College, the
Parks Concerts enclosure for the New York Philharmonic and Metropolitan Opera, and the Theatre Arts Building at SUNY, Purchase.

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