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Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs (2016)

Chapter: Appendix B - Sound Intensity: Vector Output Compared to Screen Output

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Suggested Citation:"Appendix B - Sound Intensity: Vector Output Compared to Screen Output." National Academies of Sciences, Engineering, and Medicine. 2016. Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs. Washington, DC: The National Academies Press. doi: 10.17226/23473.
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Page 121
Page 122
Suggested Citation:"Appendix B - Sound Intensity: Vector Output Compared to Screen Output." National Academies of Sciences, Engineering, and Medicine. 2016. Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs. Washington, DC: The National Academies Press. doi: 10.17226/23473.
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Page 122

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B-1 A P P E N D I X B The sound intensity instrument with visualization used in this study has two forms of output, the sound power portrayed on the screen and the vector output in a spreadsheet table. The output to the screen, as stated in Chapter 4, is the sum of the powers of all the vectors that go through the area selected by the user for the screen display, each vector being a separate estimate of the power flow. So, the average power flow is the sum divided by the number of vectors. The effect of this operation can be different than one might expect. For example, if one scans a high intensity area that may be half the total area and then displays the data on the screen using that same area as the area for scanning, one would get a sound power. If one had the screen display for the entire area, double the scanning area, the screen would display a sound power that is 3 dB higher. That is, the machine is taking the power in the defined area that the measurement was made in and treating that as an estimate of the intensity of any bigger area the user selects. Likewise, if one scans a low intensity half and then has a screen display for the whole area the output would be much lower than the true power. So the power displayed on the screen really comes from estimates of the intensity times the area. The only way the research team found to regularize this was to take the screen display based on the entire area of the wall. Then all the measurements are normalized to the same area and all the measurement episodes are each an estimate of the power flow. Inexplicably, the same vectors take on a new dimension when output in the spreadsheet table. The vectors in the spreadsheet table have units of energy, joules. Each vector is a separate esti- mate of the energy during a measurement episode. The vectors are normalized such that the area under consideration is the entire party wall defined by the user, not any subset. To get the power flowing through the wall one must divide by the number of vectors, since each is a separate esti- mate, and also divide by the time duration of the measurement episode to convert from joules to watts. Table 4-19 in Chapter 4 shows these power calculations for the five out to in and the first of the three in to out episodes at the Champaign, IL, residence. Table B-1, in its last column, for reference, contains the power estimates from the display given in Table 4-19. When one adds the five out to in power measurements from the screen display, and adds the same five for the vector calculation, the average is the same. The small scatter from one case to the other is believed to be the difference in the total area between what one can control on the screen and what one can select using a mouse. In summary, the screen display is calculating power from intensity times the area, and the vector output is calculating power from joules divided by time. Sound Intensity: Vector Output Compared to Screen Output

B-2 Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs (min:s) (s) 1 4:24 1,242 2:54 174 93.5 30.9 22.4 40.2 41.2 -1.0   2 4:58 1,896 3:37 217 92.6 32.8 23.4 36.4 38.9 -2.5 Ins ide 3 5:08 1,966 3:51 231 89.6 32.9 23.6 33.0 31.0 2.0   4 5:22 1,437 2:28 148 89.5 31.6 21.7 36.2 35.6 0.6 5 5:52 1,540 2:33 153 95.8 31.9 21.8 42.1 41.1 1.0   Average difference (Wa s) 0.0   1 7:11 1,687 2:47 167 106.4 32.3 22.2 51.9 50.6 1.3 Outs ide 2 7:20 1,034 1:38 98 not avai lable 30.1 19.9 not avai lable 55.7 not available   3 7:27 1,642 2:51 171 not avai lable 32.2 22.3 not avai lable 51.5 not available Power from Table 4.19 Wa s (dB) Difference Wa s (dB) Posion measured Episode Time data collected (hr:min; PM) Number of vectors, N Measurement episode duraon Sum of vector energy esmates Joules (dB) 10*log(N ) (dB) 10*log(s) (dB) Average power Wa s (dB) Table B-1. Comparison of vector outputs.

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TRB's Airport Cooperative Research Program (ACRP) Report 152: Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs provides guidance for selecting and implementing methods for measuring noise level reduction in dwellings associated with airport noise insulation programs. The report complements the results of ACRP Report 89: Guidelines for Airport Sound Insulation Programs.

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