MEMORANDUM

This memo addresses the questions and comments you forwarded on to HMMH from the December DECIBEL meeting held at Aurora City Hall.  I will address them below by first stating the question or comment and then following with HMMH’s response to the question or comment.

1.                  Measurement Report – Table 4, Site 1 indicates the measurement range was 61-66 DNL.  66 seems awful low, wanted verification that this was correct, and if so further explanation of how those number were produced.

Response:  61 dB to 66 dB DNL is correct for the range of aircraft DNL calculated from the measurements obtained at Site 1.  Please review Cumulative Aircraft Noise Exposure section of the measurement report (page 5) to understand how these numbers were produced.  I also call your attention to Figure 2 of that report (Page 8), which shows the total measured DNL at Site 1 ranged from about 62 dB to 68 dB DNL.

2.                  Measurement Report – Conclusions, #2; the statement that aircraft noise adds 0-4 dB to the community’s cumulative noise exposure seems low.  Would like explanation of how that is determined and supported.

Response:  HMMH measured the daily total DNL at each measurement site.  HMMH also calculated the daily aircraft DNL for each measurement site.  We then determined how much of the daily total DNL was attributed to aircraft.  For example, if the total measured DNL was 65 dB, and we calculated an aircraft DNL of 54 dB, then the aircraft noise adds 0 dB to the total noise exposure because the aircraft noise is more than 10 dB lower than the measured total.  Recall that noise is reported in a logarithmic scale.  Therefore, 54 dB plus 65 dB equals 65 dB, not 119 dB.  HMMH subtracted the aircraft noise exposure from the total measured noise exposure to obtain an estimate of the community noise exposure if the aircraft noise was not present.  We then determined how much the aircraft noise increased the community noise exposure.  At Site 6, we determined the aircraft noise contributed 0 to 4 dB as stated in the measurement report.  Although aircraft noise was present, it was not high enough nor it did not last long enough over the 24-hour period to increase the community noise exposure on the days when aircraft noise contributed 0 dB to the community noise exposure.

3.                  Noise Modeling Report – Table 4; DECIBEL wants to know if Touch and Go’s flight track utilization rates can be added to this table.

Response:  Yes.  We added touch-and-go flight track utilization rates to Table 4 in the final report.

4.                  Noise Modeling Report – Table 6; for the year 2007 DECIBEL would like to know how HMMH forecasts the number of homes that will be inside a particular DNL.  How do you forecast homes that will be built in the future?  Or do you?

Response:  For this study, we did not propose to predict future land use development as a part of this study.  Therefore, we determined the number of existing homes that would be inside the future DNL contour.  Clearly, if more homes were to be built within the noise contours, the number of impacted homes would be higher than estimated.

I trust this satisfies your needs at this time.  Please respond if you have further questions or comments, otherwise I will see you on January 29th at the DECIBEL meeting in Aurora.

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