Laserfiche WebLink
<br /> <br /> <br /> <br />Evaluation of Current S ystem <br /> <br />Acoustic Criteria of Siren System <br /> <br />To evaluate the coverage of the outdoor warning sirens in Ramsey County MN, a single consistent <br />mathematical model for sound propagation was used to predict the attenuation that occurs when sound <br />propagat es from a siren to a receiver. The model includes attenuation that occurs due to the following: <br /> <br /> Spreading of the sound over a larger area as it travels ou tward from the source. <br /> <br /> Absorption of acoustic energy by the air. <br /> <br /> Scattering of acoustic energy out of the directional beam of a rotation siren by atmospheric <br />turbulence. <br /> <br /> Absorption of acoustic energy at the ground surface as the sound wave propagates in a <br />nearly horizontal path. <br /> <br /> Reflection and diffraction of acoustic energy by barriers formed by hills between the source <br />and receiver. <br /> <br /> Refraction by the temperature gradient that exists near the surface of the ground . <br /> <br /> Refraction by the wind speed gradient that exists near the surface of the ground when a wind <br />is present. <br /> <br /> The locations of the sound level contours for each siren depend on the following: <br /> <br /> The sound power being emitted by the siren. <br /> <br /> The frequency of the signal. <br /> <br /> Local meteorological conditions, including temperature, relative humidity, and average <br />vertical gradients in sound velocity. <br /> <br /> The nature of the predominant ground cover between the source and the receiver . <br /> <br /> The ground elevation between the source and the receiver.