Comair Rotron uses four rating methods for describing the noise levels in the fans it manufactures.

PSIL
The first system used is Preferred Speech Interference Level. The PSIL is determined as the arithmetic average of the sound pressure level in the three octave bans with centre frequencies of 500, 1000 and 2000 Hz. This rating is a good guide to the effect of noise on spoken communications.

dBA
A second rating system is the " A" weighted sound pressure level (dBA) often used by government agencies in determining compliance with such regulations as the the Occupational Health and Safety Act ( OH&S). The dBA rating is determined directly by a sound level metre equipped with a filtering system which de-emphasizes both the low and high frequency portions of the audible spectrum. This measurement is recorded at a distance of 3 feet from the source.

NPEL
A third rating system is the "A" weighted sound power level reference to a 1 picowatt and expressed in Bels. This is also referred to as the Noise Power Emission Level (NPEL). NPEL was adopted by the Institue of Noise Control Engineering (INCE) as the preferred unit of measure. The INCE "Recommended Practice for Measurement of Noise Emitted by Air Moving Devices (AMDs) for Computer and Business Equipment" is a guideline for the description and control of noise emitted by components.

Freely Suspended
The fourth rating system used is a method known as Freely Suspended. In this method a fan is suspended from springs in the middle of a Calibrated Reverberate Room. The fan is run at nominal voltage, free delivery, and at a distance of 1 metre. The sound prssure level (dBA) is recorded. (For comparison dBA @ 1 metre + .7778 = dBA @ 3 feet).

Cause of Fan Noise
Since noise in most measuring systems is specified in decibels (dB), it is useful to see how dB changes relate to perceived loudness: dB change Apparent Change in Loudness
3dB Just noticeable
5dB Clearly noticeable
10dB Twice (or half) as loud
Noise emanating from axial fans is a function of many variables and causes:Vortex Shedding
This is a broad band noise source generated by air separation from the blade surface and trailing edge. It can be controlled somewhat by good blade profile design, proper pitch angle and notched or serrated trailing blade edges.Turbulence
Turbulence is created in the airflow stream itself. It contributes to broad band noise. Inlet and Outlet disturbances, sharp edges and bends will cause increased turbulence and noise.Speed
The effect of speed on noise can best be seen through on of the fan laws:
dB₁ = dB₂ + 50log₁₀ (rpm₁/ rpm₂)

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