Quantifying microphone array directivity

Abstract

The sound of fly-over aircraft is measured with the use of an acoustic camera to gain more information on aircraft noise. This camera can be optimised to gain data of a higher quality. Within the ANCE section this optimisation was required mainly to diminish the influence of the ground effect in the data and to increase the resolution of the eventual source plots. The increase in resolution was required to be able to separate engine and airframe noise also for the smaller aircraft. The research started with a quantification of the previous fly-over camera of the ANCE section. It appeared that the fringes from Lloyds’s mirror pattern were present, but different for each microphone. This was due to the edge diffraction and should be solved by placing the microphones at least 30 cm from the edge. The increase in resolution should be solved by doubling the array aperture to 3.4 m. These and many other findings were implemented in a new design. This design consists out of a de-attachable rigid structure of 4x4m placed on adjustable legs to which 64 microphones could be clipped in an Underbrink spiral array configuration. The design was built and tested for fly-over aircraft at Schiphol Airport. The measured data was analysed and it appeared that the fringes from the ground effect were gone. Furthermore, the resolution was improved and the engines and main landing could be separated for smaller aircraft as the B737 and A321.