es. Most engineering specifications, however, are single number ratings, most typically a maximum noise criteria (NC) requirement at the occupant location (not in a reverberant chamber where AHRI Standards 880 and 260 are measured). The specified NC value is an estimate of how much sound energy will enter the space after passing through several duct connections and other building elements, and represents the maximum sound level expected by occupants in that space. Because the manufacturer of an HVAC component cannot control all the other building elements that contribute to attenuation and amplification of sound, it cannot guarantee the actual noise level that will occur in the space. NC estimates include a number of calculated sound attenuation factors. AHRI 885-2008 with Addendum 1: Procedure for Estimating Occupied Space Sound Levels in the Application of Air Termi- Figure 3: Preferred background sound level. nals and Air Outlets was developed to standardize the methodology of sound calculations, and to provide some AHRI Standard 885 includes a spreadsheet for easy calculaattenuation element data that is not available elsewhere. An im- tion of estimated room sound levels from octave bands of sound portant sound element included in AHRI Standard 885 is ceiling power generated by either AHRI Standards 880 or 260. plenum/room reduction and is based on a joint ASHRAE/AHRI AHRI Standard 885 also provides a table of recommended research project (RP 755) conducted at the Canadian NRC lab. default space reductions for both radiated and discharge sound www.info.hotims.com/41645-24 22 ASHRAE Journal www.info.hotims.com/41645-13 November 2012