Noise Weather

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cmet

The sound weather can help to decide what the level difference is between favorable and long term average level. It was mentioned above that the ISO 9613-2 introduces the adjustment cmet which is just this difference. Let N denote the number of hours (and n as one particular hour out of N) in the time period that is representing one sample of the so-called ‘long-term’ time period in ISO 96113-2. Let M denote the number of hours (and m as one particular hour out of M) for which it is assumed that favorable propagation conditions apply. Hence, cmet is the difference of the sound weathers for a given location and a given scenario for the two relevant time periods, Equation 2.

Normally the hours M are part of the hours N. The condition whether or not an hour n belongs also to M is not clear. However, the distribution of the hourly sound weather offers an objective criterion to specify this condition. It is possible to say that all hours belonging to M must have sound weather levels LSW greater than the mean value of the distribution or – as another example ‑ greater than the energy average of the sound weather of all N hours. These are only two reasonable criteria. The second is used in Figure 4.

It is obvious that the hours N can be also any selection out of a larger ensemble. N can only contain the hours for the assessment time day or evening or night or even only the hours of the nighttime in winter. Hence, sound weather is an appropriate tool to investigate meaning, the reliability and the uncertainty of such corrections.

Figure 4 shows two examples of a so-called ‘cmet rose’. The left cmet rose indicate the distance and angle dependent cmet if N covers all hours of a year and M covers only the hours for which the sound weather exceeds the energy averaged sound weather of N. The right cmet rose shows the results if the M hours in the left rose are restricted to the assessment time ‘night’, that is from 10 pm to 6 am of the next day. Obviously, the meteorological corrections differ depending on the time of day.

Figure 4– cmet roses
favorable condition: LSW greater than the energy average
left: for all hours of a year, right for all hours 10 pm to 6 am next day

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