Directivity as a discriminant in monitoring of air-acoustic environmental noise

Abstract

Directivity is useful in the automatic monitoring of air-acoustic environmental noise because discrimination is possible not only between ground and airborne sources, but between different ground sources as wall. The experimental program compared omnidirectionals, gradients, cardioids, donuts, cardonuts, and second-order donuts as vertical discriminators between ground and airborne sources. DIFAR, DARAS, and CODAR arrays were the means examined for resolving ground sources. The data were gathered in flat, sandy, scrub-pine woods (New Jersey) and under a 50-ft jungle canopy (Panama). Nineteen different microphones were mounted at zero, 10- and 40-ft heights. Each height contained an omnidirectional, downward-looking cardioid and pair of horizontally crossed gradients. The 10-ft level included separate DIFAR, 2-ft baseline DARAS, and 75-ft CEDAR arrays. Vertical directivity discriminated between ground and airborne sources provided the microphone pattern nulled both upward and downward to attenuate the multipath. Cardonuts and second-order donuts were most effective, flyover noise being reduced 25 dB even in the presence of severe ground reflectivity. DIFAR and DARAS gave bearing angles to vehicles consistently within a few degrees in spite of the multipath. CODAR was not useful. Adaptive DIFAR enhanced capability by improving or rejecting bad bearings.