Analysis of ground reflection of jet noise obtained with various microphone arrays over an asphalt surface

Abstract

Ground reflection effects on the propagation of jet noise over an asphalt surface are discussed for data obtained using a 13-in.-diameter nozzle mounted 1234 ft above grade. One-third-octave sound-pressure levels were measured for microphone heights from 0.02 to 1234 ft and at distances from 25 to 75 ft from the nozzle. Limited ground-effect data are also presented for a 2-in.-diameter jet. Analysis of ground reflection effects is made using the concept of a reflected ray transfer function in a model. The range of values of the single complex number used to define the reflected ray transfer function was larger than that expected if the range was determined only by the asphalt surface. This may indicate that the atmosphere is affecting the propagation of the reflected signal The model used as a basis for the Computer program minimized the ground reflection effects. Also discussed is the selective placement of the ground-effect reinforcements and cancellations in the planning of an experiment in order to minimize their effects.