Abstract
Strong vertical gradients of temperature in the atmosphere in the first few centimeters above a hot ground surface, such as a roadway in the sun, are known to significantly refract light, producing optical mirages. Since the velocity of sound is 1700 times more sensitive to temperature, proportionally, than the velocity of light, powerful acoustic mirages and their associated shadow zones should appear somewhere, due to this thin layer of hot air, although they have not been reported explicitly. A measurement program using an acoustic point source was undertaken to find whether such a mirage could influence standard compliance testing of noise sources, particularly vehicular, at distances of 7.5 and 15 m. For both distances shadow zones were found which produced excess attenuations of l0 to 20 dB for frequencies of 2–10 kHz when the source and microphone were sufficiently close to the ground. These measurements throw doubt on existing measurements of tire noise from vehicles.
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