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<br /> <br /> <br />Figure 2 Sound transmission in temperature lapse condition <br /> <br />The opposite effect occurs in a temperature inversion (that is, in those instances when the temperature <br />decreases the closer one is to the ground). Because of the ground capacity to retain heat absorbed <br />during da ylight hours, temperature inversions typically occur at night when this heat is being released, <br />and is a contributing factor as to why the same sound source usually sounds louder at night than in the <br />daytime. This effect is shown in the following figure. <br /> <br /> <br /> <br /> <br /> <br />Figure 3 Sound transmission in temperature inversion condition <br /> <br />Ground effects on sound absorption and attenuation <br /> <br />Differences in attenuation between ground with and without trees are small for siren frequencies (for <br />example, 500 Hz octave band) when the ground is modeled to match the conditions found in the <br />Ramsey county area. The area in and around Ramsey county consists of various ground contour <br />conditions that may act as barriers to sound energy. Specific effects of the barrier attenuation were <br />determined by using ground contour information that was obtained from elevation maps developed by <br />the Geological Survey, U.S. Department of the Interior. This is consistent with the recommended <br />practice of limiting any additional tree attenuation to a short distance (less than approximately 200 <br />feet) of canopy penetration since many other sound rays can “hop” over the tree belt. An illustration <br />of this phenomenon is shown in the following figure. <br /> <br /> <br /> <br /> <br />