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End reflection loss.

To better predict the sound generated by air terminal units, the AHRI variable air volume terminals certification program now requires air terminal unit manufacturers to include end reflection in their cataloged discharge sound power levels. The published discharge sound power levels at the low end of the sound spectrum will increase, typically from 1 db to 8 db, which means the published NC levels also will increase by several points, especially in the medium and small boxes (1 to 4 point increases in the medium and up to 7 points in the small boxes are expected).

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.

Conclusion

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.

Acknowledgments

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
COPYRIGHT 2012 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2012 Gale, Cengage Learning. All rights reserved.

 
Article Details
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Title Annotation:How Does It Affect Design Engineers?
Author:John, David A.; Villegas, Luis
Publication:ASHRAE Journal
Geographic Code:1USA
Date:Jun 1, 2012
Words:924
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