To consider the quality of sound, we must consider the frequency that sound pressure variations occur within. Frequency is the number of pressure variations per second and is measured on hertz. Fan sound is broadband, meaning it contains a large range of frequencies. It can also contain higher levels at specific frequencies called tones.

Fan sound power levels are tested and oftern published in eight separate octave bands. Octave bands are group of frequncies where the center frequency of each octave is twice the frequency of the previous octave, similar to musical octaves. The first octave band represents low frequency-a rumbly sound that you can actually feel if the amplitude is high enough. The eighth octave represents high frequency, or hissing sound.

Although reporting fan sound levels in eight octave bands gives the best description of sound quality, single number ratings are often used to simplify analysis. The A weighted network approximates the response of the human ear to sound. Since we do not hear low frequency sound in the first octave bands very well, we reduce the sound power value by 25 decibels. In the fifth through eighth octave bands that we hear the best, we make very little correction to the sound power.

Acceptable sound levels can be assured in installations by selecting equipment with high efficiencies and low sound power levels, keeping sufficient distance between the equipment and occupied spaces and installing sound attenuation between the equipment and occupied spaces.

In summary sound is a sensation perceived by the ear, resulting from rapid fluctuations in air pressure; sound pressure is the acoustic pressure at a point in space where a listener’s ear is situated and measured in decibels; sound power is the acoustic power radiating from a sound source and is measured in decibels or acoustic watts and is independent of the environment and frequency is the number of pressure variations per second and is measured in hertz.