End reflection loss.
The decision by AHRI to make this change was based largely on results of ASHRAE Research Project "RP-1314, Reflection of Airborne Noise at Duct Terminations." The project confirmed end reflection loss (ERL) for hard lush terminations (the type used to conduct sound testing in semireverberant test chambers) and agreed with the corresponding analytic predictions in Chapter 8 of ASHRAE Handbook--Fundamentals, Sound and Vibration.
End Reflection Loss
ERL is a calculation added to the sound data obtained in a test chamber. ERL is a function of the size of the termination compared with the acoustical wavelength and the specific location of the duct termination within the room.
ERL is a calculated value based on the discharge dimension of the air terminal unit. Table D13 from AHRI Standard 885-2008, Procedure for Estimating Occupied Space Sound Levels in the Application of Air Terminals and Air Outlets, (http://tinyurl.com/AHRI885), lists the addition to be added back to obtain the discharge power level for a given unit. It shows additions for round discharge duct diameters. The conversion from rectangular to equivalent round can be found in Section 3.1 of ASHRAE Standard 130, Methods of Testing Air Terminal Units as well as in RP-1314.
Effect of End Reflection Loss
Table 1 shows the effect end reflection loss has on the NC values of discharge sound as calculated per Table E1 in AHRI Standard 885-2008.
Actual room noise levels measured in the field will not be affected by this change. The terminal is not actually louder than before. If a specific size terminal at specified operating conditions produced an acceptable room noise level with the current sound calculation procedure (no ERL), the same perceived sound quality will be met under the new procedure (with ERL).
Designers who specify a maximum discharge sound room NC level for their projects may find that lower airflows or larger terminals are necessary to meet their specifications with the new calculation method. Another option is to increase the specified room NC level for discharge sound, recognizing that actual sound quality in the field will not be affected.
AHRI's technical committee on sound determined that sound power discharge for air terminal units should include corrections for end reflection loss was based on the results of RP-1314. All manufacturers that participate in the AHRI Certification program for variable air volume terminals now are required to add the end reflection to the discharge data submitted.
When selecting units or comparing data among competitors, check the notes that accompany the performance data to ensure the data includes end reflection. The sound power levels and resulting NC calculated per Table E1 in AHRI Standard 885-2008 may be different if the data contains end reflection.
This article is the result of AHRI's Air Control and Distribution Devices committee (ACDD) determining the need for a document detailing the change to include ERL. Mike Woodford, AHRI Staff Liaison to the ACDD committee, contributed to this article.
By David A. John, P.E., Member ASHRAE; and Luis Villegas, Associate Member ASHRAE
David A. John, P.E., is chief engineer; Luis Villegas is development and testing engineer for Metal Industries, Clearwater, Fla.
What is End Reflection?
When a duct system opens abruptly into a large room, some low-frequency acoustic energy at the exit of the duct is reflected back into the duct (end reflection).The result is the amount of acoustic energy discharged into the room is reduced. This decrease in discharged energy is larger at lower frequencies.
Therefore, when conducting sound tests to obtain discharge sound power levels for air terminal units, the sound power measured in the test chamber is less than the actual sound power in the duct. Requiring rated discharge sound data with correction for end reflection allows better prediction of the sound generated by an air terminal unit.
The 2011 ASHRAE Handbook--HVAC Applications, Chapter 48, Noise and Vibration Control, discusses end reflection. Additional information can be found in ANSI/AMCA Standard 300-08, Reverberant Room Method for Sound Testing of Fans.
Table 1: Summary of examples showing effect of ERL on discharge sound pressure levels, dB. Example 1 Small Box (8 in. by 8 in. discharge, <300 cfm) Sound Pressure Sound Pressure Level Without Level With End End Reflection Reflection Hz dB dB 125 35 43 250 30 33 500 17 18 1000 0 0 2000 -12 -12 4000 2 2 NC 16 23 Example 2 Medium Box (12 in. by 12 in. discharge, 300- 700 cfm) Sound Pressure Sound Pressure Level Without Level With End End Reflection Reflection Hz dB dB 125 29 35 250 24 27 500 14 15 1000 4 4 2000 0 0 4000 3 3 NC 10 14 Example 3 Large Box (15 in. by 15 in. discharge, >700 cfm) Sound Pressure Sound Pressure Level Without Level With End End Reflection Reflection Hz dB dB 125 33 37 250 38 39 500 24 24 1000 13 13 2000 12 12 4000 24 24 NC 27 27
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|Title Annotation:||How Does It Affect Design Engineers?|
|Author:||John, David A.; Villegas, Luis|
|Date:||Jun 1, 2012|
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