Monday, November 28, 2011

End Reflection

Soon, every manufacturer of VAV terminals will be changing their discharge sound data to comply with a change in the way discharge sound in the reverb room is measured. This change came about as the industry started to measure octave band sound generation for many products. VAV terminals have been certified through AHRI for sound levels (for over 20 years); several years before that, it was through the Air Diffusion Council. The new test method for non-VAV terminals is AHRI 260. Part of the procedures call for adding the calculated “End Reflection” to the measured sound levels. End Reflection is a phenomenon that occurs when sound travels through a duct and encounters a rapid change in area. Some of the low frequency sound is reflected back, effectively cancelling some of the sound waves coming down the duct.

End Reflection has been a part of AHRI 885’s sound calculations from its inception in 1989. Originally, 885 used an ISO equation, but when ASHRAE completed a thorough analysis in a research project a few years ago, 885 adopted the ASHRAE equation (which was very similar to ISO). When testing discharge sound in a reverb room, the duct connecting the unit to the room is specified to terminate flush with the inside wall of the test chamber. There is an end reflection in this installation, but the ADC and AHRI procedures for VAV boxes were developed long before the ASHRAE or ISO equations had been verified. Now, however, it’s time to play catch-up.

AHRI has decided that all member manufacturers shall now include end reflection in their discharge sound data and must update the Certified Directory and on-line catalogs by January 1, 2012. The next printing of their catalogs shall also be corrected.

The impact will be to show a reported, calculated (estimated) NC increase of as much as 10 NC. The effect is greatest for the smallest units. The units won’t be any louder, but the data will show an increase. This will, no doubt, cause much concern. However, our experience is that discharge sound is seldom a cause of complaints. Engineers will need to modify their specifications accordingly and I expect there will be a lot of discussions. It should be an interesting start to the new year.

Authored by: Dan Int-Hout, Chief Engineer Krueger