TECHNICAL FEATURE Sound Pressure Level: The ear's response to sound waves in air and are variations in pressure above and below atmospheric pressure. NC: A single number for rating the sound of a space. The measured octave bands for a space are compared with the NC curves. These curves were developed with lines that represent constant perceived loudness by the human ear. The NC rating is the value of the highest NC curve touched by measured sound in Octave Bands 1 through 8. Noise: Unwanted sound. The actual sound that an air terminal generates is the sound power level and is measured for radiated and discharge sound per the method of test as outlined in ASHRAE Standard 130-2008, Methods of Testing Air Terminal Units. NC is actually a calculation to predict the sound heard by occupants in a space and is based on the response of the human ear. The method of predicting NC from sound power levels is outlined in the AHRI Standard 885, Procedures for Estimating Occupied Space Sound Levels in the Application of Air Terminals and Air Outlets. AHRI Standard 885 is available at no cost at www.ahrinet.org. As of this date, the most current release of this standard is Standard 885-2008 with Addendum 1. To establish fair competition among air terminal unit manufacturers, AHRI formed the Air Control and Distribution Devices (ACDD) section, which governs the air terminal unit certification program for AHRI. The standard used by the terminal unit manufacturers to obtain their air terminal unit certified data is ANSI/AHRI Standard 880-2011, Performance Rating of Air Terminals. Because NC is a calculated value based on attenuation assumptions, the air terminal unit manufacturers agreed on the attenuation factors to use when publishing their air terminal unit catalog data. The first agreedupon factors were given in the ARI Standard 885-90, Appendix E. Appendix E was modified again in 1998 with the release of Standard ARI-885-98. Once again, manufacturers were required to publish cataloged NC values www.info.hotims.com/49808-31 28 ASHRAE JOURNAL ashrae.org SEPTEM BER 2014