Fan Noise & Isolation
Fan noise is a function of the fan design, volume flow rate Q,
total pressure pt, and efficiency ht . After a decision has been
regarding the proper type of fan for a given application (keeping
mind the system effects), the best size selection of that fan
based on efficiency, because the most efficient operating range
a specific line of fans is normally the quietest. Low outlet velocity
does not necessarily ensure quiet operation, so selections made
this basis alone are not appropriate. Also, noise comparisons
of different types of fans, or fans offered by different manufacturers,
made on the basis of rotational or tip speed are not valid. The
valid basis for comparison are the actual sound power levels generated
by the different types of fans when they are all producing the
required volume flow rate and total pressure.
The data are reported by fan manufacturers as sound power levels
in eight octave bands. These levels are determined by using a
reverberant room for the test facility and comparing the noise
by the fan to the noise generated by a noise source of known sound
power. The measuring technique is described in AMCA Standard
300, Reverberant Room Method for Sound Testing of Fans. ASHRAE
Standard 68/AMCA Standard 330, Laboratory Method of
Testing In-Duct Sound Power Measurement Procedure for Fans,
describes an alternate test method to determine the sound power
duct fan radiates into a supply and/or return duct terminated
anechoic chamber. These standards do not fully evaluate the pure
tones generated by some fans; these tones can be quite objectionable
when they are radiated into occupied spaces. On critical installations,
special allowance should be made by providing extra sound
attenuation in the octave band containing the tone.
ARRANGEMENT AND INSTALLATION
Direction of rotation is determined from the drive side of the
On single-inlet centrifugal fans, the drive side is usually considered
the side opposite the fan inlet. AMCA has published standard
nomenclature to define positions.
In air-conditioning systems, ducts should be connected to fan
outlets and inlets with unpainted canvas or other flexible material.
Access should be provided in the connections for periodic removal
of any accumulations tending to unbalance the rotor. When operating
against high resistance or when noise level requirements are low,
it is preferable to locate the fan in a room removed from occupied
areas or in a room acoustically treated to prevent sound transmission.
The lighter building construction that is common today makes it
desirable to mount fans and driving motors on resilient bases
designed to prevent transmission of vibration through floors to
the building structure. Conduits, pipes, and other rigid members
should not be attached to fans. Noise that results from obstructions,
abrupt turns, grilles, and other items not connected with the
fan may be present.