Towards Roughness as an Additional Metric for Aircraft Noise Containing Multiple Tones

This paper extends existing research knowledge in the field of auditory roughness as it applies to the particular kinds of modulated sounds produced by contra-rotating propellors which may be fitted to new designs of advanced open rotor aircraft engines. The internationally standardised metric for aircraft noise certification (EPNL) is insensitive to the possible additional noisiness contributed by the superposition of close spaced tonal components in typical open rotor aircraft engine noise. We propose a simple analytical model which is capable of estimating auditory roughness directly from the predicted narrow-band frequency spectrum of both existing and future open rotor aircraft engines without having to extract an envelope frequency spectrum from a synthesised (or measured) signal, such as is done when applying existing procedures devised by Terhardt [1] and then Aures [2]. Listening tests showed how the auditory roughness of super-positioned multiple pure tones varied as expected in comparison to the auditory roughness of otherwise equivalent amplitude modulated tones, thereby lending some support for the analytical model.