Fan noise data is usually plotted as Sound Power level against the octave frequency bands. These bands are usually designated by their center frequency. For specification and rating purposes, it is generally acceptable to divide the audible frequency spectrum into eight octave bands, each with an upper limit twice that of the lower limit. Since fan noise is predominantly wide band in nature with some pure tones, it is convenient to divide the audible frequency range into bands and to plot the average Sound Power Level in each band. For fans, two types of noise related to frequency are important: wide band noise, in which acoustic energy is continuously distributed over a frequency spectrum and pure tones, in which the acoustic energy is concentrated over narrow bands in the frequency spectrum. In practice, another property of noise, its frequency, is also considered. Sound Power Level, since it is a measurement of noise unaffected by such factors as the fan's distance from the hearer, is used as the basic measurement for comparing noise levels of fans, as well as noise levels at different operating points of the same fan. Sound Power Level cannot be measured directly and must be calculated from sound pressure measurements. Sound Power Level is defined similarly to sound pressure on a logarithmic scale as: This is called the Sound Power Level and is independent of the environment. In defining the noise generated by a fan, it is best to define the noise emanating from the source. Sound Pressure Level (SPL) which is environmentally dependent, is defined as: Because of this large range of hearing capability, it is convenient to express these values in decibels. The pressure variations of interest for their psycho acoustical effect vary as much as 13 orders of magnitude. Sound is perceived and measured as minute pressure fluctuations above and below the ambient pressure.
![room cooling fan decibel ratings chart room cooling fan decibel ratings chart](http://cdn.gregsmithequipment.com/images/air_compressors/images/noise_level/relateddecibels1.jpg)
The factors of greatest importance to the system designer are the psychological influences on the person rather than the physical influences of sound on the human ear. In industries that use small cooling fans, fan noise simply interferes with the ability of the people working nearby to concentrate on their work. The second is the psycho acoustical characteristic which attempts to judge the effect of noise on human beings. The first is the physical phenomenon which can be measured and thus used in technical specification. Noise does, however, have two basic characteristics. Perhaps the best definition of it is as an unwanted sound. Noise is, to a great extent, a purely subjective personal phenomena.