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Discharge Sound Power and End Reflection

Starting on January 1, 2012, there will be a change in the discharge sound power levels of all VAV terminal units reported by participating AHRI members. The terminal units have not undergone a design or option change, but the discharge sound data has been updated to include End Reflection, which is the sound energy that is reflected back into the duct rather than entering the reverb room. The new AHRI required data will show increases in the discharge sound levels for nearly all products, especially in the second and third octave bands. But again, nothing changed from the equipment perspective; the rating process has changed. This increase DOES NOT mean that the equipment got louder. It did not. In fact, it did not change.


Physics teaches us that you can neither create nor destroy energy. Within a system the base amount of energy at the beginning plus the input must equal the base amount at the end plus the output. When the air traveling through the duct from the terminal unit into the reverberant chamber enters the chamber abruptly, there is a pressure wave set up at the end of the duct. This pressure wave reflects some sound energy back into the duct; therefore it does not enter the reverberant chamber and cannot be measured there. The reflected waves are 180 degrees out of phase from the original waves, and that allows them to cancel out some of the sound inside the duct. That canceling process converts the sound energy to heat and the heat is absorbed into the system. Once again, you cannot measure this with the sound meter in the reverberant chamber. But it can be mathematically calculated and added back to the measured sound to give you a new resultant. The amount of this sound is dependent on the duct size and the cross sectional area as well as the air density. The interesting thing about this increase is that it becomes a decrease further down the duct (like at the diffuser in the room) when the unit is installed. AHSHRAE Standard 130 tells us how to measure sound in a reverberation room. But it seems that those of us who wrote that standard overlooked the energy conservation issue. ASHRAE RP-1314, a research project to calculate end reflection, gave us hard application numbers to prove out the theory. The equation to calculate end reflection by octave band is shown below.

C0 = 1128 @ 70° F.

End reflection was not included in ASHRAE 130 or AHRI 880. Consequently the numbers in all of our catalogs are artificially low, and how much varies by discharge duct size. So the differences vary somewhat from unit to unit.


As of January 1, 2012, the data has to be corrected on the AHRI web site,, and in each manufacturer’s software for equipment performance and selection. It has to be in printed catalogs at the next printing.


The Discharge Sound Power will change in the second and third octave bands. There may be very slight changes in the fourth octave band, but this would be unusual. There will be slight or no changes to the discharge NC. The amount of change here depends on which band sets the NC. As these changes become public, your engineers and contractors will be asking why our sound levels changed. They did not. They should not expect anything different from the low sound and noise levels they have always gotten from Nailor equipment. It is the rating process and its subsequent reporting that has changed.

Source: Nailor Product Bulletin, January 1, 2012