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<br /> <br /> <br />Siren Sound Output <br /> <br />The sound power of each siren is characterized by the maximum sound pressure level at a standard <br />100 ft distance from the siren on its axis (at the height of the siren). <br /> <br />Meteorological Conditions <br /> <br />In accordance with the guidance of FEMA-43, the “average daytime weather conditions” were used <br />in the sound propagation calculations. The resultant air absorption attenuations used in the model <br />were therefore based on a temperature of 66.22°F and a relative humidity of 67.03%. Refer to <br />Appendix B to review average monthly values used to calculate these numbers. <br /> <br />Ground Cover and Topography <br />Ground cover <br />Ground cover and ground surface between source and receiver is characterized as one of four <br />possible types: <br /> <br />1. Water/Pavement (very hard surface) <br /> <br />2. Urban (nominally soft – lawn and grass) <br /> <br />3. Rural (very soft uncompacted ground) <br /> <br />4. Normal Asphalt (hard surface) <br /> <br />The Urban ground profile dominated the landscape in Ramsey County MN. <br /> <br />When the ground cover in all directions around a siren is the same, and if no barrier or <br />prevailing wind effects are present, the sound level contours are circles, the radius of which <br />depends on the source type and the ground cover. This ideal condition shown in figure 1 <br />never occurs in nature. Varied ground conditions and topography create irregular sound <br />propagation contour lines. <br /> <br />Topography <br /> <br />The barrier effects of hills and mountains between the source and receiver were determined by using <br />ground contour information obtained in the form of detailed elevation maps from the Geological <br />Surve y, U.S. Department of the Interior. These maps, which were produced by the Defense Mapping <br />Agenc y Topographic Center, provide a grid of terrain elevation values at 10 meter intervals. <br /> <br />The effect of considering barrier attenuation is illustrated in the Figure 1, which compares the ideal <br />circular siren sound level contours with the actual contours that result when barrier attenuation and <br />varied ground cover are included. Figure 1 illustrates that large errors can be made by ignoring the <br />effects of topography, ground cover, and surface variances. <br /> <br /> <br /> <br /> <br /> <br />