Noise is undesirable or unwanted sound. With better understanding of the effects of environment on the inmates of the dwellings and factory workers, noise has become an important subject in the design, installation, and operation of the fans.

In a well-balanced properly installed fan, the mechanical noise originating from bearing and vibration of various parts is not as prominent as the aerodynamically generated noise. The latter is due to the various flow phenomena occurring within the fan.

The main causes of aerodynamically generated noise are:

1. The flow at entry and exit of the fan. i.e., suction and exhaust noise,
2. Rotation of blade through air or gas,
3. Turbulence of air,
4. Shedding and vortices, from blades,
5. Separation, stalling and surging.

Some methods of reducing fan noise are:

1. Operation of fans at their maximum efficiencies,
2. Use of low speed and low pressure fans,
3. Employment of uniform flow in ducts,
4. Use flexible fan mounting,
5. Use of sound absorbing walls; duct should be lined by sound absorbing material,
6. Use of silencers at the suction and exhaust,
7. Reinforcing fan casings.

Fan Laws for Sound

Dimension analysis indicate that the sound power ratio SWR = SW/W of geometrically similar series of fans should be dependent only on Mach number, Ma = V/Va, (ratio of a typical fan velocity to the velocity of sound) and Reynolds number, Re = DVp m which is the ratio of the dynamic to viscous forces involved, SWR = constant x (Ma)xx (Re)y

Taking the rotational speed N, diameter D, and output power W of the impeller as the typical quantities:

W µ N³D⁵   Maµ ND   Re µ ND² 

Therefore SW = constant x N³ + x + y D⁵ + x +2y

Acoustical theory, considering only the Mach number effect, yields the following relations for the source types mentioned

Monopole Source x = 1 so that SW varies as V⁴

Dipole Source x = 3 so that SW varies as V⁶

Quadrupole Source x = 3 so that SW varies as V⁸

Fan noise is principally dipole in origin and, for the simplest cases of boundary layer separation, there is reason to believe that the Reynolds number exponents y should be about - 0.4, leading to:

SW = constant x N^5.6 D^7.2 

Axial fans seems to follow this relationship quite closely (e.g. N5.5 D7.5), but other type may well introduce more complex generation mechanism. For example, centrifugal fans are usually considered to follow the law:

SW = constant x N⁵ D⁷

Geometrical similarity as its affects Reynolds number is not well maintained in practice from size to size (thickness, clearances, even number of blades). It is wise to keep the index of D2 greater than the experimental index of N and to treat small changes with size as a scale effect on the constant. The influence of atmospheric conditions is small -generally within 1 dB.