TECHNICAL FEATURE based on the attenuation for both radiated and discharge sound based on this appendix. The air terminal unit manufacturers revised Appendix E in 2008 with the release of AHRI Standard 885-2008 (Figure 1). Printed air terminal unit catalogs in engineers' libraries may include NC values that were calculated using attenuation factors that are from the outdated AHRI 885 Standard. The same terminal unit will have different cataloged NC rating, depending if the printed catalog uses the procedures outlined in Standards 885-90, 88598 or the current 885-2008. It is recommended that designers check the notes pertaining to the NC calculation in a printed catalog to make sure the values are based on the most current Standard 885. The purpose of Appendix E is to allow a comparison of sound levels between manufacturers at common operating points and with common attenuation. The actual sound levels on installed units, unless the conditions are exactly the same as shown in Appendix E, will vary. Designers also should make sure they are using NC data obtained by the methods required per Standard 885-2008 or the most current release. One way to avoid any confusion is to specify sound power levels. Beginning in 2012, terminal unit manufactures were required to add end reflection into the discharge sound power levels for their catalog and certified data. This does not change the attenuation values found in AHRI Standard 885 but it does increase the discharge sound power levels found in the manufacturers' catalogs. More information on end reflection can be found in the June 2012 ASHRAE Journal article, "End Reflection Loss." Specifying Flow Rates There are two types of controls for air terminal units; they are pressure dependent controls and pressure independent controls. Pressure-Dependent Control System: a control system in which the airflow through the air terminal varies with system pressure. Pressure-Independent Control Systems (Pressure-Compensated Control System): control system in www.info.hotims.com/49808-14 30 ASHRAE JOURNAL ashrae.org SEPTEM BER 2014