ASA 125th Meeting Ottawa 1993 May

1eID1. Noise outdoors.

Tony F. W. Embleton

80 Sheardown Dr., Box 786, Nobleton, ON L0G 1N0, Canada

Concerns about noise in the community date back to the dawn of recorded history, have continued through Roman times, the Middle Ages, and are still with us. Many of the principal wave-propagation mechanisms involved in the real atmosphere outdoors were understood at least qualitatively by the late 1800's. Today, a good understanding of all the sound propagation phenomena is of considerable economic and social importance in connection with environmental impact studies. Reality is far more interesting than simple spherical spreading in the space above a hard, flat ground. Some grounds are hard like concrete or soft as snow. Corresponding reflection coefficients are usually less than unity and vary with angle. Grounds may not be flat, leading to shadow zones or alternatively multiple reflected paths. Wind convects sound waves, and gradients of wind or temperature refract waves either upward (temperature lapse) or downward (inversion). The atmosphere is rarely still and turbulence causes fluctuations in the acoustical effects. All these features are frequency dependent, and mutually interact. Measured sound levels outdoors owe as much to near-surface weather and to ground shape as to the obvious acoustical factors to source and receiver heights and their separation. Discussion will emphasize field measurements and simple physical interpretations.