Theatre Design & Technology - Fall 1979 - 33

cases, even this measure will not be adequate for removing
noise picked-up in the equipment room through the thin sheet
metal of the duct, and ductwork in the equipment room should
then be encased in drywall or plaster (plus glass fiber separation) to control this noise pick-up.
Since there is considerable difference in cross-sectional
area between lined and unlined ducts, the construction documents must make clear the interior free-area requirements,
with penetration and other details planned for the full size of
each duct.

Some Practical Considerations
Short of radical surgery, requiring a complete theatre rebuilding, what might be done to drop the noise level of noisy
heating, ventilating, and air-conditioning systems? Let's return to the basics for a moment. The three types of system
noise are airborne, structureborne, and ductborne. If the
problem is clearly airborne, check for obvious sound-transmission leaks in both the theatre and mechanical equipment
room construction. Possibly packing and calking small areas
or airtight baffling with drywall for larger areas can be helpful.
If obvious leaks are not apparent, the problem might not be
easily solved, because heavy additional construction may be
required-an additional free-standing wall, or a resilientlysuspended airtight drywall ceiling, for example.
If the problem appears to be structureborne (if you actually
feel building vibration in any part of the theatre), check for vibration isolation of the equipment. If entirely missing, or supplied only as a gesture (typically, ribbed-neoprene and/or
cork pads where springs should have been provided), a partial
fix may be supplied by a local representative of vibration-isolating equipment for a relatively modest expenditure. If spring
isolation is present, check the integrity of the existing isolators. You should be able to move the unit. Common faults are
springs bottomed-out by excessive weight or shorted-out by
excessive latera/deflection. Again, a local representative of a
manufacturer of vibration isolation devices can be very helpful. Sometimes all that is necessary is to pull loose chunks of
plaster, wood, or concrete from underneath the mounting
frame to remove the "short circuit" from the vibration-isolation system.
If noise is coming out the duct terminals, either supply or
return, check whether it is rushing air noise or low-frequency
fan hum. If rushing air noise is present, consider reducing air
velocities, either by two-speed motors for fans, allowing the
room to be fully conditioned before a performance and then
supplied with some air at reduced velocity and turbulence during the performance, or by permanently reducing fan speeds
by changes in pulleys.
Removing diffusers is another check to make. If this results
in a noticeable reduction in noise, then a quieter diffuser can
be selected and installed. In certain theatres diffusers are out
of sight in an upper volume and are likely candidates for removal without replacement.
If volume dampers are near or part of diffusers, see if their
full-open position reduces noise. If so, a complete system
rebalancing, in which noise is considered as important as air
delivery, will usually be helpful.
If low-frequency noise is the problem, examine the largesize ductwork, especially near the fan, for turbulence-producing geometry. Can the ductwork be reoriented to remove
sharp turns, especially near fan inlets and outlets? If the noise
source is the fan, not duct turbulence, consideration should
be given to the insertion of manufactured silencers in the appropriate ducts.
At the risk of sounding patronizing, I will conclude by pointing out the wisdom of obtaining the assistance of an acoustical consultant when trouble-shooting and when evaluating
possible remedial measures.O
USITI /Fall, 1979

Sound Control
Techniques for the
Legitimate Theatre
and Opera*
by Harold Burris-Meyer
and Vincent Mallory

Theatre Design & Technology

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