Voice communications in the cockpit noise environment : the role of active noise reduction
This thesis addresses the topic of voice communications in the cockpit noise environment of modern `fast-jet' aircraft and helicopters, and in particular, describes research undertaken in support of the development of a system for reducing the noise level at the operators' ear by acoustic cancellation within the ear defender, known as Active Noise Reduction or ANR. The internal noise spectra of today's high performance `fast-jet' aircraft and military helicopters is described, and the complex interaction of acoustic noise transmission, speech, and microphone noise pick-up, which produces the total acoustic environment at the aircrews' ears, is discussed. Means of mathematically modelling the audio channel, quantifying the components identified above, and identifying areas of shortfall in performance are derived, leading to a procedure for the development of attenuation requirements, described as the communications audit. A model of the electroacoustic characteristics of the ANR ear defender assembly is presented and the sound field distribution within the ear defender/ear cavity, and its effect upon cancellation performance, is discussed. The extensive laboratory and flight testing of the ANR system that has been undertaken is reviewed, paying particular attention to the measurement and analysis techniques employed in such testing. Finally, the performance characteristics of ANR are discussed and compared with the requirements previously established. Design limitations placed upon the system by the constraints of its area of application are described, and the scope for future improvements is considered.