Testing is conducted according to Air-Movement and Control Association (AMCA) Standard 300, “Reverberant and Room Method for Sound Testing of Fans.” AMCA, a trade organization comprising fan, damper and silencer manufacturers, specifies test methods and requirements so that all members use the same rating guidelines.
Fan manufacturers can provide laboratory test results for the total sound power levels of the fan that includes the fan inlet, discharge, motor drive train and casing radiated noise. This analysis starts with the fan sound power levels (per octave band) and ends with the sound absorption or reflection effects of the receiving room.ĭesign professionals should be aware that there are multiple noise propagation paths that should be considered: the noise that travels through both the supply and return duct system paths, and the airborne noise that radiates away “breaks out” through the walls of the duct into the neighboring spaces.įans generate the primary noise source in HVAC systems. In order to determine if a duct system requires additional attenuation, all the duct components along the critical duct path must be analyzed. Table courtesy of Kinetics Noise Control, Inc. Specific requirements and guidelines may be found in OSHA Standard 29CFR, Part 1910, Subpart G “Occupational Health and Environmental Control.” Since A-weighting accounts for the sensitivity of human hearing and it is a relatively simple metric, Occupational Safety and Health Administration (OSHA) has adopted it to set limits on noisy work environments. Example of A-weighting using rule-of thumb guidelines.
Once adjustments are made, decibel addition can be used to yield a single “A-weighted” sound pressure level (dBA). Adjustments are made in all frequencies except 1000Hz. Human beings are so sensitive to sound in the 2,000 Hz to 4,000 Hz levels that A-weighting, the most common weighting system, actually increases those levels to more closely reflect the human ear response. An A-weighting filter on the sound meter is commonly used to emphasize frequencies from 1000 to 8000 Hz where the human ear is most sensitive, while attenuating very high and very low frequencies to which the ear is less sensitive. Weighting is a useful means of adjusting a linear noise spectrum to closely reflect the human ear response. Loudness is a function of sound pressure and frequency. Equal loudness contours have been developed which show the increase and decrease in sound level energy required at various frequencies for the average human to perceive sound in a particular frequency as sounding just as loud, say, as that of a 20 dB sound pressure level at 1000 Hz. However, the frequency of a sound will have substantial bearing on how loud the receiver perceives it to be. It is a function of frequency and sound pressure level. Loudness, a subjective measure, is an attribute of auditory sensation.
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