period four
Equipment Sound Rating
notes
Free-Field Method
Figure 61
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.
