Handbook of air conditioning and refrigeration / Shan K

19.14 CHAPTER NINETEEN
facing. A sound plenum effectively attenuates low-frequency noise and is often located between the
collective return grille and the return air intake of a packaged unit. A low velocity is important in a
sound-attenuating plenum because of the high local loss coefficient of square elbows.
Silencers used in HVAC&R systems can again be classified according to the means of sound
attenuation into following types:
1. Dissipative silencers. These silencers often use face-covered or encapsulated acoustic material,
such as fiberglass, mineral wool, and acrylic polymers, to attenuate noise over a broad range of
frequencies. The facing material can be made of
● Galvanized or aluminum sheet with a perforated area not exceeding 22 percent of the face
area
● Acrylic polymers or polymer sheets
2. Packless silencers. There is no fibrous fill. Noise is attenuated by means of acoustically resistive
perforations in the splitters. They are often made of sintered aluminum or acrylic plastics.
3. Reflection-dissipative silencers. These silencers use the combined effect of sound reflection and
dissipation in airflow passages of successive square elbows.
4. Active silencers. These silencers produce low-frequency inverse sound waves to cancel the unwanted
noise.
Characteristics of Silencers
The acoustic and aerodynamic characteristics of a silencer are mainly indicated by four parameters.
FIGURE 19.4 Difference in insertion loss of a rectangular silencer between
forward and reverse flow (at 2000 fpm or 10 m/s face velocity).
(Source: Handbook of HVAC Design, 1990. Reprinted by permission.)