Chapter hvac engineering Fundamentals: Part 1 Introduction

20.4.1
Transmission through ducts
The sound transmitted through ducts comes from at least three
sources: fan room equipment, air noise generated in the ducts, and
sound generated outside the ducts and transferred through the duct
walls.
Some fan room equipment noise is in the third category. Noise from
the fan is transmitted directly through the ducts and is dominated by
a frequency determined by the fan speed and number of fan blades.
Air noise is generated as part of the energy loss due to friction; more
noise is generated at points of turbulence such as elbows, transitions
Engineering Fundamentals: Part 5
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