Sound is the sensation perceived by the ear, resulting from rapid fluctuations in air pressure. A vibrating or rotating object creates areas of high and low pressure which causes the eardrum to move back and forth. Our brains process this motion as sound.

Sound is an element of a building’s operation that affects occupant comfort. Stringent standards have been mandated to achieve better indoor air quality and occupant comfort. To meet these standards, buildings are now required to have more ventilation and filtration, causing fans to run at higher speeds, which can produce higher sound levels. Acceptable sound levels may differ in different location like offices, libraries, bathrooms, and warehouses. In addition, local noise ordinances often specify maximum sound levels at property lines.

Understanding the basics of sound is necessary to specify equipment that creates a comfortable environment for occupants and meets code requirements.

Sound pressure is defined as the acoustic pressure at a point in space where a listener’s ear or microphone is situated. It is measured in decibels which are on a logarithmic scale that starts with zero at the threshold of hearing.

Background sound pressure levels inside and outside buildings are often specified to ensure occupant comfort. In some areas like office cubicles, some background noise is actually desired to help mask private conversations.

To the human ear, three decibels is considered barely perceptible. A five decibel increase is a clearly perceptible change to most people. The difference of a ten decibel increase in sound is perceived as being twice as loud.

While sound pressure levels are normally specified for occupied spaces, sound power levels are typically specified for equipment such as fans. Sound power level is defined as the acoustic power radiating from a sound source. Fan manufacturers use a calibrated sound source with known sound power levels, and a semi-reverberant laboratory to measure sound power values. Sound power is the purest form of sound measurement. It is independent of the installed environment and distance from the sound source.

To summarize, sound power is independent of the application. This value presents the acoustic energy created by a piece of equipment and it is the responsibility of the manufacturer to test, certify, and publish these values. Specifying engineers like to use sound power so they can do an apples-to-apples comparison between different manufacturers. Sound pressure, or what your ear can hear, depends on the installation and resulting acoustic environment surrounding the fan or other sound source.