DG-111O-3-119
Design Guide: Band Training Facilities
March 1983
Practical Approaches for Accoustic Construction
A. Absorptive Finishes
The air distribution systems should be so designed that
at no point in the building, in any duct, does the airflow
Absorptive finishes reduce reverbation as well as loudness.
exceed 1500-2000 feet per minute.
Most are porous (fibrous or cellular), allowing sound to
Airflow velocities in the terminal necks of ducts where they
-
enter the material, where its energy is converted into heat.
connect to diffusers or grilles should not exceed 370 fpm
Fiberglass board is an excellent example. To be effective,
for NC-25, 450 fpm for NC-30, 550 fpm for NC-35, and 675
such materials must not be too thin--at least 1" --or they
fpm for NC-40.
must be backed by an airspace of at least several inches.
Some absorbers are not porous, but they are thin and free
C. Diffuser Noise. Noise at the diffusers and grilles--
to vibrate in response to the sound. For example, thin
typically a mid-to high-frequency hiss--also is caused by
wood paneling (or even furred gypsum board) vibrates and
turbulence as air is forced through restricted openings.
thus, by resonance, absorbs sound. However, resonant
It is exceedingly velocity-dependent: a doubling of airflow
absorbers are much less efficient and their absorptivity is
through a given device will increase the noise level by
limited to the low frequencies.
15 to 20 decibels. The only acoustically safe approach is
to use oversized diffusers and grilles with large free area,
Absorptivity is commonly given by the material's Noise
without integral volume control dampers, straighteners, or
Reduction Coefficient (NCR*). But like the STC (for
equalizing grids. Diffusers and grilles serving music rooms
isolation), it does not indicate low-frequency performance,
should not incorporate volume control dampers. If required,
which is of considerable interest in the design of band
these are best located at the branch duct takeoffs,
rooms. It can be used as a guide, but with this important
because their noise will then be attenuated by the acousti-
qualification: every room should have at least one major
cally lined ducts.
surface that not only has a high NRC (0.60 or more), but
also absorbs low-frequency sound. Most typically, this
D. Existing Systems. Mechanical systems should always
requirement is met by using a suspended (not glued-on)
be designed to meet the recommended criteria. If existing
systems are involved, their noise levels should be mea-
tivity, by resonance, of any furred wall is not enough. Low
sured and the feasibility of reducing any excess noise should
frequency absorption can also be provided by a large
be investigated.
air space behind wall mounted panels, as in the corner treat-
E. Equipment Location. All major equipment should
ment illustrated in Figure 5-20.
be located remote from the active music rooms. Fans,
pumps, compressors, etc. are best located on-grade,
where they can be more easily vibration-isolated. Mid-span
locations on long-span structures are unacceptable. All
rotating, reciprocating, and vibrating equipment must be resil-
iently supported or hung. All their connections to the build-
ing structure must be resilient; and ample static deflection
--up to several inches in the most critical cases--should
be specified.
F. Penetrations of Sound-lsolating Construction. Penetra
tions through sound-isolating walls and ceilings must be
perfectly sealed. The annular openings around ducts and
pipes should be either grouted solid or sealed with a non-
hardening sealant (see Figure 5-19).
5-4 Room Acoustics
Achieving satisfactory room acoustics for practice and
rehearsal is a complex matter. As discussed in 3-5.C,
the concerns include loudness and reverberation control,
clarity and communication among the members of an
ensemble, and avoidance of certain unwanted effects. Ade-
quacy in all these respects is difficult to define, since indi-
vidual musicians and bandleaders have their own standards
of reference, often based on past experience in rooms
that may or may not have been to their liking. The follow-
ing paragraphs describe the means of achieving results
that should satisfy most users of the facility.
5-22
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