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56 TRG-TRC007-EN
notes
period four
Equipment Sound Rating
The free-field method is commonly used for HVAC equipment that is too
large to be tested in a reverberant room. This includes water chillers, cooling
towers, and the outdoor sound from packaged rooftop air conditioners and
air-cooled condensers.
This type of equipment is placed on a hard surface in an anechoic (or sound-
absorbing) room or on a large parking lot outdoors. This approximates the
characteristics of sound in a free field above a reflecting plane. That is, the
sound-pressure waves travel evenly in a hemispherical pattern, away from the
equipment. Sound-power levels are determined by measuring sound-pressure
levels on an imaginary hemispherical surface surrounding the equipment.
Free-Field Method
Figure 61