Determination of Aircraft Engine Speed Based on Acoustic Measurements

In order to develop more accurate aircraft noise calculation tools, more detailed source models describing the sound emission and directivity are needed. Therefore, the correlation between the actual flight configuration and the measured sound pressure level must be known. A matter of particular interest is the individual engine power setting during take-offs which is usually unknown respectively not published by the airlines. Thus, a method had to be developed that provides the capability of its determination by external measurements.A measure of the power setting is the engine speed respectively the rotational speed of the low pressure shaft on which the fan is mounted. Since its rotation causes a measurable pure tone, its fundamental frequency can be detected and tracked within a sound file recorded by a measurement station on the ground. Additionally the Doppler effect and the sound propagation time must be compensated by using an accurate position and time synchronization.This method has been evaluated by the Swiss research institute Empa for starting and landing aircrafts in the far and close range of an airport. The results contributed to the development of a new aircraft noise calculation model as part of the research project sonAIR.