2017.06.13 WS Agenda C - Laserfiche WebLink

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Figure 4 Downwind sound propagation and vegetation effects Wind shadow effect in the upw ind direction While the acoustic model used in this report considers wind speed as a factor in atmospheric stability corrections to predicted average range (which includes upwind, downwind, and crosswind directions), it does not consider the additional effect of the vector addition of wind effects and temperature lapse effects di rectly in the upwind direction. Wind velocity adds or subtracts from sound velocity depending on whether the sound is moving upwind or downwind. In addition, wind velocity typically increases with increasing height, thus further augmenting the refraction of sound away from the ground. The acoustic shadow will form in the upwind direction even closer to the sound source than under calm conditions, with the shadow’s proximity to the source increasing with the speed of the wind; on the other hand, a downwind position will decrease or—given strong enough winds— totally eliminate the acoustic shadow. As a rule of thumb, summer daytime wind shadow effect is roughly proportional to the square of the wind speed up to about 15 mph. Thus, in the upwind direction, 15 mph wind speed will result in a further siren range reduction of approximately 30%; 5 mph will result in a siren range reduction of approximately 8%, 2 mph will result in a range reduction of approximately 2%. For normal wind speeds, downwind effects are typically small and are usually not considered in sound prediction models. The upwind sound propagation plus temperature lapse shadow formation is shown in the following figure. Figure 5 Upwind Sound Propagation plus Temperature Lapse Shadow Formation An average wind speed of 7 mph to 14 mph for the summer months around Ramsey County would shorten siren range directly into the wind by up to 20%; which is a maximum reduction of approximately 5,000 feet to the predicted range to 60 dB under