Evaluation of a Correction Factor for Flyover-Noise Ground Plane Microphones

This paper explores ground plane microphones used to acquire flyover-noise measurements. A test method is presented to evaluate a potential free-field correction factor to translate a ground plane microphone measurement to a free-field like measurement for flyover-noise applications. Various ground plane geometries, microphone installation methods, and ground impedance parameters are evaluated for their component level influence on the system response. Field-test like installation effects are simulated using an acoustically treated test platform in a large anechoic chamber. This work supports on-going efforts within the Committee on Aviation Environmental Protection (CAEP) and the International Coordinating Council of Aerospace Industries Associations (ICCAIA) to define an improved method for this type of measurement for use in noise certification testing of civil airplanes. The work leverages similar studies and serves the effort to develop a correction factor that is representative of the test environment. Laboratory test results are presented for various ground plane microphone configurations. Individual attributes in the response of a ground plane are compared to the corresponding free-field correction factor for contribution analysis. The results of the study demonstrate the importance of treating a free-field correction factor as a system-level response for a specific microphone ground plane installation.