Acoustical Noise Control 93
(4-6)
where,
a is the absorption coefficient of the lining,
Se is the plenum exit area in square feet,
Sw is the plenum wall area in square feet,
d is the distance between the entrance and exit in feet,
T is the angle of incidence at the exit (i.e., the angle that
the direction d makes with the axis of exit) in degrees.
For those high frequencies where the wavelength is
smaller than plenum dimensions, accuracy is within
3 dB. At lower frequencies Eq. 4-5 is conservative, and
the actual attenuation can be 5 dB to 10 dB higher than
the value it gives.
4.4.7 Proprietary Silencers
When space is at a premium and short runs of duct are
necessary, proprietary sound-absorbing units can be
installed in the ducts at critical points. There are a
number of configurations available, and many attenua-
tion characteristics can be expected. The extra cost of
such units may be offset by economies their use would
bring in other ways. The user should also be aware that
silencers produce a small amount of self-noise and care
must be taken to allow the air to return to a laminar flow
downstream of the silencer.
The general rule is that the air will require a length
equal to 10 times the diameter of the duct to regain a
laminar flow.
4.4.8 HVAC Systems Conclusion
The intent of this HVAC section is to emphasize the
importance of adequate attention to the design and
installation of the heating, ventilating, and air-condi-
tioning system in the construction of studios, control
rooms, and listening rooms. HVAC noises commonly
dominate in such sound rooms and are often the focus of
great disappointment as a beautiful new room is placed
into service. The problem is often associated with the
lack of appreciation by the architect and the HVAC
contractor of the special demands of sound rooms. It is
imperative that an NC clause be written into every
mechanical (HVAC) contract for sound-sensitive
rooms.
Residential HVAC systems commonly employ small
ducts and high velocity air delivery systems. Air turbu-
lence noise increases as the sixth power of the velocity;
hence, high velocity HVAC systems can easily be the
source of excessive turbulence noise at grilles and
diffusers. Keeping air velocity below 400 ft/min for
studios and other professional sound rooms is a basic
first requirement. Air flow noise is generated at tees,
elbows, and dampers; and it takes from 5 to 10 duct
diameters for such turbulence to be smoothed out. This
suggests that duct fittings should be spaced adequately.
Air flow noise inside a duct causes duct walls to vibrate,
tending to radiate into the space outside. Thermal duct
wrapping (lagging) helps to dampen such vibrations,
but even covered, such ducts should not be exposed in
sound-sensitive rooms. This oversimplified treatment of
HVAC design is meant to underscore the importance of
employing expert design and installation talent, not to
create instant experts. The overall HVAC project,
however, needs the involvement of the audio engineer at
each step.17,18
References
- L. L. Beranek, Revised Criteria for Noise in Buildings, Noise Control, vol. 3, no. 1, p. 19, January 1957.
- L. L. Beranek, W. E. Blazier, and J. J. Figwer, Preferred Noise Criteria (PNC) Curves and Their Application to
Rooms. Presented at the 81st Meeting of the ASA, Washington DC April 1971.
- L. L. Beranek, 1989. Balanced Noise Criterion (NCB) curves. Journal of the Acoustical Society of America 86:
650 – 664.
Figure 4-49. A properly designed lined plenum is a very
effective attenuator of HVAC noise and is usually located
near the equipment. Unused rooms or attic spaces may
sometimes be converted to noise-attenuating plenums.
(After ASHRAE, Reference 19.)
attenuation 10 1
Se cosT
2 Sd
------------^1 – a
Swa
©¹§·------------+
= log -----------------------------------------
Q d
