The Directivity of Sound Source Influence on Measurement of Reflection Coefficients at Oblique Incidence Using NAH

The influence of sound source directivity was analyzed when measuring reflection coefficients at arbitrary angle of incidence by NAH (near-field acoustic holography) method. Making use of piston sourcepsilas different directivities in different frequencies, the reflection coefficients of an infinite aluminum board were inversed and compared with its theoretical values. The numerical results showed that the angles of effective incidence could be inversed conditioned by the directivity of the sound source. On condition of bad directivity in low frequencies, there will be measuring errors because space wave number transform truncated sound field information and come into being wave number spectrum leakage. In higher frequencies, if the sound source directivity is better and has no side-lobes, the inversed results would be better too, and if the directivity has side-lobes, there would be enormous errors at the angles between the lobes.