Experimental investigation of reflection of airborne noise at duct terminations

ASHRAE Transactions, July, 2008 by Alexander P. Michaud, Kenneth A. Cunefare

INTRODUCTION

When an acoustic wave propagating in a duct reaches a termination, some fraction of the energy in the wave transmits into the space beyond the termination while the remainder is reflected back up the duct. This transmission and reflection of waves is due to the impedance change presented by the termination. The relative fraction of the energy that is reflected back up the duct is referred to as the end reflection loss (ERL) and is quantified with a decibel (dB) measure. The ASHRAE Handbook contains two tables of ERL values, reproduced here as Tables 1 and 2. The two tables correspond to two different termination conditions--in a free space (Table 1) or in a rigid baffle (Table 2)--as depicted in Figure 1.

[FIGURE 1 OMITTED]

Table 1. ASHRAE ERL Values for Duct Terminated in Free Space

                         End Reflection Loss, dB

Duct Diameter, in.  Octave Band Center Frequency, Hz

                    63  125  250  500  1000  2000

         6          20  14    9    5    2      1
         8          18  12    7    3    1      0
        10          16  11    6    2    1      0
        12          14   9    5    2    1      0
        16          12   7    3    1    0      0
        20          10   6    2    1    0      0
        24           9   5    2    1    0      0
        28           8   4    1    0    0      0
        32           7   3    1    0    0      0
        36           6   3    1    0    0      0
        48           5   2    1    0    0      0
        72           3   1    0    0    0      0

Table 2. ASHRAE ERL Values for Rigid Duct Terminated Flush with Rigid
Baffle

                       End Reflection Loss, dB

Duct Diameter, in.  Octave Band Center Frequency, Hz

                       63  125  250  500  1000

         6             18   13   8    4     1
         8             16   11   6    2     1
        10             14    9   5    2     1
        12             13    8   4    1     0
        16             10    6   2    1     0
        20              9    5   2    1     0
        24              8    4   1    0     0
        28              7    3   1    0     0
        32              6    2   1    0     0
        36              5    2   1    0     0
        48              4    1   0    0     0
        72              2    1   0    0     0

The method used to determine the ERL values listed in the ASHRAE Handbook is not clearly defined or substantiated. The Handbook cites a 1981 Perdue University report, "Investigation of End Reflection Coefficient Accuracy Problems with AMCA Standard 300-67" (Sandbakken et al. 1981) and AMCA Standard 300-05, Reverberant Room Method for Sound Testing of Fans, (AMCA 2005) as sources for the listed ERL values. The Perdue report did not measure reflection coefficient values high enough to produce accurate low-frequency ERL results (< 63 Hz), but the results agreed well with analytic predictions above 125 Hz. The ASHRAE ERL values are for full-octave bands extending up from 63 Hz, and it is desirable to extend them to lower frequencies. Further, the Handbook does not define the acoustic free-space condition though it asserts, "Diffusers that terminate in a suspended lay-in acoustic ceiling can be treated as terminating in free space." This configuration is analogues to many drop ceiling applications, yet there is a large degree of configuration variability in practice that is not addressed by the configurations implied for use of the ASHRAE ERL tables. The ASHRAE ERL table values also only take into account hard termination baffles, even though many real-world applications use soft termination baffles (e.g., ceiling tiles). As a result, the possible influence of baffle hardness is not accounted for within the ASHRAE values. Another limitation of the ASHRAE ERL tables is the lack of information pertaining to different terminations (e.g., grilles and diffusers) and the impact of the presence of flex duct upstream of a termination.

In light of these considerations, ASHRAE Research Project RP-1314, Reflection of Airborne Noise at Duct Terminations, addressed a number of issues pertaining to the validity of the ASHRAE ERL values, ERL determination in general, and the impact of various duct termination conditions on ERL. Specifically, the project addressed:

1. Analytic ERL for rectangular vs. circular ducts

2. Bandwidth and spectrum shape influence on ERL

3. ASHRAE table values vs. analytic ERL

4. Flush-terminated experimental ERL vs. tabulated and analytic ERL

5. Validity of free-space duct termination condition (height above baffle)

6. Impact of high aspect ratio ducts on ERL

7. Impact of termination baffle hardness on ERL

8. Impact of flex duct on ERL

9. Impact of diffusers on ERL

10. Impact of return grilles on ERL

Implicit in each of the above was the motivation to assess their impact on, or relevance to, ERL and the scarcity of prior research that would speak to these issues. In addition to exploration of the above issues, another goal of the project was to acquire ERL values in third-octave bands down to 20 Hz, which had not been done previously in any experimental exploration of ERL of which the authors are aware. The project explored ERL analytically and experimentally, with the experimental aspect covering seven duct sizes from 6 x 6 in. (15.24 x 15.24 cm) to 18 x 54 in. (45.72 x 137.16 cm).