Handbook of air conditioning and refrigeration / Shan K

19.6 CHAPTER NINETEEN
Branch ducts should have air velocities about 80 percent of the listed value. Airflow velocity of the
final runout to outlets should be 50 percent of the listed value. The presence of the elbow, diffuser,
and other fittings can increase airflow noise substantially, and therefore the duct velocity should
reduce accordingly.
Diffusers and Grilles. The manufacturer’s sound rating of a diffuser is obtained with a uniform
velocity distribution throughout the neck. If a volume damper is installed immediately before the
diffuser, the turbulent airflow at the neck will be significantly increased compared to the manufacturer’s
laboratory-tested data. This turbulence can be considerably reduced by adding an equalizing
grid in the neck of the diffuser. Volume dampers should not be located closer than 5 ft (1.5 m) from
a supply outlet.
The sound power level added to the diffuser sound rating is proportional to the damper pressure
ratio (DPR). The DPR is equal to the throttled pressure loss across the damper, divided by the minimum
pressure loss across the damper when its blades are fully opened. According to the data
provided by ASHRAE Handbook 1999, HVAC Applications, if a volume damper is installed in the
neck of a linear diffuser, when the DPR varies from 1.5 to 6, a corresponding 5 to 24 dB should be
added to the diffuser sound rating. If the volume damper is installed in the inlet of the plenum of a
linear diffuser, when DPR varies from 1.5 to 6, a corresponding of 2 to 9 dB should be added. If the
volume damper is installed at least 5 ft (1.5 m) from the inlet plenum of a linear diffuser, only a corresponding
of 0 to 5 dB should be added.
Airflow noise generated at the diffusers or grilles at the end of duct-borne paths is difficult to
attenuate except by reducing their neck or face velocities. Imbalance of volume flow between various
branch runouts may create a greater branch duct velocity and grille face velocity in one of the
branch runouts which has the lowest flow resistance.
Poor Fan Entry and Discharge Conditions. Noise is generated because of the abrupt air passage
constrictions and sudden changes in airflow direction. Both cause flow turbulence and flow separation
as well as energy losses. Such low-frequency noise results in a duct rumble and is very difficult
to attenuate. The designer should carefully design the fan intake and discharge connections. Doing
so is the best way to control this kind of airflow noise as well as to provide an energy-efficient air
system.
19.3 SOUND ATTENUATION ALONG DUCT-BORNE PATH
Sound Attenuation in Ducts
Sound attenuation is the reduction in the intensity of sound, expressed in watts per unit area, as it
travels along a sound transmission path from a source to a receiver. Sound attenuation is achieved
by (1) the absorption of sound energy by the absorptive material, (2) spherical spreading and
scattering, and (3) reflection of sound waves incident upon a surface.
Sound attenuation in duct sections, duct fittings, silencers, and other sound-reducing elements
can be indicated by insertion loss. Insertion loss (IL) at a specific frequency is the reduction in
sound power level, in dB re 10 �12 W, measured at the receiver when a sound attenuation element is
inserted in the transmission path between the sound source and the receiver. Noise reduction is the
difference in sound pressure levels between any two points along the sound transmission path.
Sound absorptivity is the ability of a material to absorb sound energy. When a sound wave
impinges on the surface of a porous sound-absorbing material, air vibrates within the small pores.
The flow resistance of air and its vibration converts a portion of the absorbed sound energy to heat.
The fraction of the incident sound power that is absorbed is called the sound absorption coefficient
�. Most sound-absorbing materials have a low � at low frequencies and a higher � at high frequencies.
For a typical sound absorption material, � might be equal to 0.15 in the octave band whose
center frequency is 63 Hz, and � � 0.9 at 1000 Hz.