Abstract
In the case where the source of sound is a turbo-jet engine fan the solution of Fresnel–Kirchhoff’s diffraction integral contains an additional term. That term may be neglected when the source of sound is a single harmonic point source. Formulas, including this term, show that behind a barrier the fan noise is amplified instead of being reduced. The amplification effect occurs when the rim of the barrier is located in areas of high or rather maximum sound levels. It was found out that double-wall barriers prevent sound amplification. This additional abatement of sound in the shadow zone of the screen is partially caused by absorption of diffracted waves within the open double-wall ’’cavity.’’ A semi-empirical formula was developed to estimate this absorption. As basic configuration for experimental studies a DC-10 half model with a CF6-turbo fan engine was selected to be tested in model scale of 1 :10. All test runs were conducted in an anechoic chamber of the DFVLR, Department of Technical Acoustics, Germany. The agreement between theory and experiment is satisfactory.
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