2017.06.13 WS Agenda C - Laserfiche WebLink

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Assumptions Used The acoustic model uses a ground classification of 0 for soft ground, 0.5 for medium ground and 1 for hard ground. Various ground factors were used in the prediction model to provide an accurate estimate of the Ramsey county ground conditions. Temperatures and wind speeds vary considerably over the diverse terrain. For modeling purposes, the average summer daytime temperature, wind speed, pressure and humidity were used. The weather conditions used for the siren system sound coverage prediction were determined using the average meteorological data measured by the NOAA NWS for daytime conditions in Ramsey County MN The atmospheric conditions chosen to model are the following (Refer to Appendix B):  Temperature at 66.22°F  Relative Humidity at 67.03%  Barometric pressure at 1017 mbar Siren Types, Output Verification, and Siren Site Coordinates Ramsey County MN is the home of 19 independent cities which have selec ted various sirens to meet the alert and notification needs of the communities they serve. Each siren type and the maximum sound power output for each siren location is listed in Table 1. Each siren site was surveyed and each siren type and output power rating were noted in the survey report provided by the survey team. The information provided by the survey report wa s used to verify siren output power, siren type, siren height, and siren location used in the acoustic model. The data is listed in Appendix C. Atmospheric Conditions Effect on Siren Range In a calm daytime atmosphere, temperature decreases with increasing height above the ground and is known as a temperature lapse. The speed of sound through a medium is proportional to its density. The speed of sound through the air has been found to be proportional to the square root of air temperature; therefore , sound velocity decreases with height above the ground, and, as a result, sound waves have a tendency to bend upward under calm conditions with the sun high in the sky and warming the ground, which is typical for summer day time conditions. A further result is that an acoustic shadow is formed at the ground level, an area where the sound reaching a listener is greatly reduced because the sound wave is being bent upwards and away from the listener. Under extreme conditions, such acoustic shadows can form very close to the source in the upwind direction. Acoustic shadow zone formation due to temperature lapse is shown in the following figure.