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TRG-TRC007-EN

41

period three

This section introduces several terms that are fairly specific to the science of 
acoustics.

Attenuation refers to the reduction in sound level as sound travels along the 
path from a source to a receiver. It is typically used to refer to the reduction 
of sound as it travels through a duct system. Straight ducts, elbows, junctions, 
and silencers are examples of elements that attenuate sound.

Regenerated sound results from components of the duct system that create 
turbulence in the air stream. Turbulence is caused by an abrupt change in 
airflow direction or velocity with a corresponding static-pressure loss. 
Regenerated sound increases with air velocity or when the air is forced to 
make sharp turns. Elbows, junctions, diffusers, silencers, and dampers are all 
examples of elements that regenerate sound.

Notice that some elements can both attenuate and regenerate sound. For 
example, as air makes a 90-degree turn in a rectangular duct elbow, some 
of the sound is reflected back upstream, attenuating the airborne sound 
downstream of the elbow. At the same time, however, the turbulence created 
by the air turning the sharp corner causes some regenerated sound.

supply

supply

duct

duct

return

return

duct

duct

diffuser

diffuser

heat pump

heat pump

(source)

(source)

return

return

-

-

air

air

grille

grille