How Sound Is MeasuredSound is the result of small rhythmic variations in atmospheric pressure spreading outward like ripples in a pond. These variations travel in air at approximately 1,100 ft/sec. When they reach the human ear, rhythmic variations are interpreted as sound. Sound is measured in sound pressure and sound power. Sound pressure is a measure of the variations in atmospheric pressure; sound power is the energy emitted by a source independent of the surroundings. The difference between sound pressure and sound power can be illustrated by comparing a sound source to a light source such as a 75 W light bulb. Sound pressure changes as the surroundings change, just as a light bulb is brighter the closer one is to the source and dimmer the farther away one is. By contrast, sound power remains constant independent of the surroundings; the bulb's power remains 75 W.
Occupational Safety and Health Administration (OSHA) publishes a table of permissible noise exposures. It associates exposure time in hours to sound levels in decibels. The higher the sound level, the shorter the time a person should be exposed to that sound level.
Fan manufacturers commonly use an eight-octave band scale with center frequencies doubling from 63 to 8,000 Hz to convey sound expectancy. Energy is separated electronically into the eight-octave bands with the values shown as sound power levels (Lwi) in decibels. These values are converted to a weighted sound pressure level (dBA) at various distances to account for how the human ear responds at various frequencies. Sound power level ratings indicate the amount of sound generated by a fan when operating at various points within the normal operating range. Ratings typically are derived from tests conducted using methods described by the Air Movement and Control Association (AMCA) Standard 300 Test Code for sound ratings.
Account for SurroundingsSound pressure levels presented by fan manufacturers reflects a theoretical free-field environment. Free field allows the acoustical energy to radiate in all directions - imagine a fan floating in mid-air, if that were possible. Theoretical free-field conditions are approximated in laboratories with sound-absorbing walls, floors and ceilings. A reverberant-field environment accounts for noise reflection. Laboratories with hard walls, floors and ceilings are used to simulate reverberant-field conditions.
Installations fall somewhere between free- and reverberant-field conditions. Using fan manufacturers' published sound expectancies without accounting for variations from a free-field environment can lead to unacceptable sound levels. Installation factors that will influence sound level include distance from the sound source; distance from reflective surfaces; whether the fan inlet is open or connected to duct work; thickness of duct walls and presence of additional sound-generating sources.
Meaured vs. Perceived SoundFor critical sound level environments, acoustical consultants should be employed to determine the appropriate fan sound level requirements. Acoustical consultants will identify requirements for the measured sound, perceived sound and attenuation.
Measured sound requirements refer to the free-field level required to meet OSHA standards for a specific installation. Per-ceived sound is what the worker or neighbor actually hears. A sound source can be within the OSHA standards and still be perceived as objectionable.
Attenuation is the reduction of sound levels from the source to the listener. It is measured as the reduction of sound energy via insertion of an object between the source and listener. Often, a fan is not capable of meeting the required sound levels. In this situation, some type of sound attenuation is used to meet the required levels.
Types of AttenuationAttenuation for a fan can be accomplished by changing the fan location, adding personal devices or employing acoustic materials. Altering fan location by moving the fan farther away from the listener or changing the direction of the sound path can de-crease perceived sound. Personal de-vices are used for intermittent noise and include protective ear devices, ear-plugs and headphones. Acoustic materials and devices include vibration isolation, fan silencers, duct silencers, sound blankets, acoustical barriers and acoustical enclosures.
Sound levels and how the sound will be affected by the surroundings are important considerations when selecting any piece of process cooling equipment. The fan manufacturer, acoustical consultant and process cooling equipment manufacturer can work together to ensure the proper equipment is supplied to meet any measured or perceived sound requirements.