Q&A: What is End Reflection and How Will Discharge Sound Ratings for Terminal Units Change in 2012

There is an acoustical phenomenon known as end reflection that is regularly encountered in HVAC systems. It occurs whenever air flowing in a duct reaches an outlet and suddenly expands to fill a room. Although it might not be obvious to the casual observer, sound doesn’t necessarily travel in the same direction as airflow. The greater the degree of expansion, the more sound is reflected away from the room.

An acoustician might say, “End reflection is the acoustic energy in an acoustic test duct that is prevented from entering the test space by the impedance mismatch created by the termination of the acoustic test duct”. In layman’s terms, whenever a rapid air expansion occurs, some portion of the sound energy generated by the supply device (i.e. terminal unit, fan system, air handler, room fan coil, etc) travels upstream back towards the source. This is end reflection.

All terminal unit manufacturers test their products in accordance with ASHRAE Standard 130 ‘Methods of Testing Air Terminal Units’. This standard provides testing procedures for both radiated and discharge sound. End reflection has been known to affect discharge sound readings for many years, so the standard was amended in 1994 to specify that discharge ducts in discharge sound tests must terminate flush to the inside wall of the test chamber. This was necessary because the further a discharge duct projects into the test chamber, the more end reflection occurs, effectively lowering the sound levels measured within the test chamber.

Test data measured in accordance with ASHRAE Standard 130 is used to produce catalog data in accordance with AHRI Standard 880 ‘Standard for Performance Rating of Air Terminals’. The latest version of this standard (880-2011) went into effect on January 1, 2012. It requires that manufacturers calculate the end reflection loss (ERL) and add it back to the rated discharge sound power levels of terminal unit products. A formula based on ASHRAE Research Project RP-1314 is used to calculate the ERL for the dimensions of the discharge duct used during the sound test. Although the calculated ERL is most accurately applied to 1/3 octave sound data, manufacturers may apply it to existing full octave sound data through 2014.

Here’s how the ERL is calculated:

First determine D_e, the equivalent duct diameter (ft). If the discharge duct is round, simply use the duct diameter. In the more likely situation that the discharge duct is rectangular, the equivalent duct diameter must be calculated as:

D_e = SQRT [(4 x A) / (144 x π)]

where:

A = cross sectional area of duct (in²)

So for a terminal unit with a 15 in by 12 in discharge duct:

D_e = SQRT [(4 x 180) / (144 x π)] = 1.26 ft²

Now

ERL = 10 log [1 + (0.7 x  C_o/π x f x D_e)²]

where:

C_o = Speed of sound in air (use 1128 fps)

f = Octave band center frequency (Hz)

D_e = Equivalent diameter of the duct (ft)

 

So the end reflection loss of a 15 in x 12 in discharge duct is:

2^nd Octave band (125 Hz) = 5 dB

3^rd Octave band (250 Hz) = 2 dB

4^th Octave band (500 Hz) = 1 dB

5^th Octave band (1000 Hz) = 0 dB

6^th Octave band (2000 Hz) = 0 dB

7^th Octave band (4000 Hz) = 0 dB

 

Adding this to the existing discharge sound levels of a fan-powered product would likely raise the NC level by 6 points. The smaller the discharge duct is, the greater the correction will be. Since research has shown that low frequency corrections tend to become overstated as duct sizes get very small, the maximum correction is limited to 14 dB.

 

So what does all this mean?

It means that every manufacturer will need to update all published terminal unit discharge sound performance data and selection software to meet the latest standards. Discharge sound levels will increase for all terminal units and smaller units will see the largest increases. The effect on large units could be negligible.

 

Will the actual discharge be higher?

No. The product will perform exactly as it did before, but now all of the sound energy will be properly accounted for. It would be fair to say that under the previous standard, discharge sound was in many cases being understated.

In order to change the certified performance listings posted on the AHRI website, all participating manufactures were required to resubmit all of their products to the program. It will probably be months before all of the changes are complete and posted on their new website. Although AHRI has agreed to publish a full page announcement in trade magazines to explain why these changes are necessary, it has not yet been sent out for membership approval.

Randy Zimmerman - Chief Engineer