Theatre Design & Technology - Fall 1979 - 31

the audience, but they must never be greater than NC-25.
experience is that noise levels in any critical theatre area
dience, stage, or control room) greater than NC-25 pose
hearing problems for the audience, actors, and technical
sonnel.

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Fig. 1: Background nOise In 1'10 heatres, and thea re NC curves,
The above NOise Cn ena Curves remain In use by many acoustical cOn!:.u/ants and mechanical englfleers, at hough they Will be replaced In the near future by a new set of curves, These curves show the maximum levels that can
be tolerated In each octave band for particular uses of the space. NC-) 5 IS
commonly accep ed as good for heatres, NC-20 less good, but satisfactory,
and NC-25 the maximum acceptable compromise, Both Russell Johnson Associates and Klepper Marshall King supplied noise control recommendations
for the Neely Theatre at Vanderbilt UniverSity, toward an intended goal of NCt 5, but cost-cut ing compromises dunng deSign development resulted in the
higher, but sll1l sa isfactory. background levels shown, In Thea re "0" imporant recommenda Ions by the acous leal consultan were not followed, and
he mechanical sys em IS much 00 nOIsy 0 be used a ull speed, while halfspeed opera Ion lea es some problems unsolved In some sea s.

use of the dB(A) rating system for theatres is fraught with peril
and usually admits too much low-frequency noise,
A background noise level below the threshold of hearing
would be optimal in any theatre. However, as with all other aspects of design, practical considerations (more directly, economics) force compromises. NC-15 is the criterion specified
by most acoustical consultants for critical projects, such as
most theatres, concert halls, and studios. The highest, least
restrictive (to the mechanical engineer) background noise design level that should be considered in any theatre is NC-25,
and this level only in a tight-budget, smaller-sized theatre. NC20 is a widely used criterion, since it falls between the nearsilence of NC-15 and the clearly audible, but low-level NC-25
boundaries. Note that some acoustical consultants believe a
criterion even more strict than NC-1 5 is necessary for certain
theatre projects. Not only must the audience area of a theatre
be quiet, but the stage areas and control booths open to the
theatre must also be quiet. Sometimes we permit levels in the
control booths and on stage to be 5 dB higher than levels for

Our
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Noise Sources and Transmission Paths
Noise from mechanical systems entering a theatre can be
classified in three ways: airborne, ductborne, and structureborne. The airborne component would include the noise
of machinery outside the theatre heard inside the theatre due
to poor sound-isolation qualities of the theatre's enclosing
walls, ceiling, floor, doors, windows, etc., plus noise generated at diffusers, grilles, and any return-air "mushrooms."
Ductborne noise is noise originating in the mechanical equipment room or the ducts themselves, transmitted to and from
sensitive spaces by ducts, and exiting at supply and return
grilles, diffusers, and mushrooms. Structureborne noise and
vibration are generated by vibrating sources, such as mechanical equipment, contacting structural elements common
with the theatre. Once in the building structure, vibrations may
travel long distances with little loss, radiating audible noise
along the way.
The possible sources affecting a theatre's heating, ventilating, and airconditioning system include fans, pumps, compressors, condensers, cooling towers, ductwork, dampers,
and terminal devices, such as diffusers, grilles, and mushrooms. All these sources except ductwork, dampers, and terminal devices should be located at some distance from the
theatre, and all but terminal devices are sources of both airborne and structureborne noise. Fans can be a source of
ductborne noise in most circumstances, and the design of
ductwork in the mechanical equipment room can allow even
noise from pumps, compressors, etc., to enter ductwork as
well. Upstream dampers, diffusers, and ductwork turns and divisions contribute ductborne noise alone.
Airborne noise is best controlled by locating equipment remotely. Also, the intervening structure should effectively isolate the theatre from this equipment. Structureborne noise is
controlled by spring-mounting the vibrating equipment to reduce the vibration transmitted to building structure, by insuring that all pipes, ductwork, and conduit connections to the vibrating equipment have vibration isolation measures installed,
and by building discontinuities, such as separation joints, into
the building structure to interrupt the transmission path. Care
in diffuser selection and ductwork design will prevent airborne
noise generated by diffusers in the sensitive spaces. Ductborne noise is controlled primarily by the installation of internal duct lining and commercial duct silencers, plus the use of
low, low air velocities. The lining and commercial duct silencers control ductborne transmission of fan and other equipment noise entering in the mechanical equipment room, while
low velocities are necessary to reduce air-turbulence noise.
Other measures include lagging (enclosing the ductwork in
sensitive spaces or in mechanical equipment rooms), and the
use of low-transmission-Ioss glass fiber ductwork to bleed off
low-frequency energy into non-sensitive spaces. Generally,
the high- and mid-frequency portion of mechanical system
noise is due to air turbulence at diffusers, in small ducts, and
at dampers; and the low-frequency noise is due to air turbulence in large ducts, fan noise, and vibrating equipment.

Noise and Vibration Control in Mechanical
Equipment Rooms
The task of mechanical room noise control is greatly simplified if the mechanical equipment room is located on- or below-grade, at some distance from the theatre interior. Nearby
mechanical rooms will always pose problems, but these problems can be somewhat reduced when the rooms contain only
equipment that serves the theatre, not the rest of the building.
The worst possible situation is to have fans and condensers

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