Study of the feasibility of estimating combustion noise radiation in reverberant environments

Frank R Eberhardt, Purdue University

Abstract

The current procedure for measuring combustion noise transfer functions requires performing measurements in semi-anechoic environments where the pressure measured at the four exterior microphone locations is comprised only of the sound that is radiated directly from the engine and a reflection from the floor. To make this procedure more versatile, the goal of this research was to extend the measurement procedure to reverberant environments, such as hard-walled engine test cells, in which there is no acoustic treatment utilized to mitigate the reflections due to hard surfaces. Reflections created by this type of environment cause the transfer functions to become more variable across the frequency range and the impulse responses to last longer than those in a semi-anechoic environment. By employing time-domain windowing, these reflections can be removed from the estimated impulse responses resulting in smoother transfer functions that emulate those measured in a semi-anechoic environment. Another challenge, present in both hard-walled and semi-anechoic test cells, is the correlation of engine cylinder pressures due to the nature of multi-cylinder engines. Measuring a transfer function from a single cylinder pressure transducer to an exterior microphone location while in the presence of the other operating cylinders causes the path impulse response to have time-delayed and attenuated artifacts from other cylinders in addition to the desired path impulse response. This problem can also be overcome by use of the time-domain windowing technique in conjunction with operating the engine at continually varying load and speed conditions (sweeps instead of steady state runs). The engine variation helps to de-correlate the engine cylinders and improve the estimation of the in-cylinder pressure to microphone path transfer functions. These transfer functions are used to predict pressure levels measurements for specific engines; thus, the results of this research will help improve simulations and assessments of combustion noise and enable NVH engineers to address combustion noise issues early in the development cycle.

Degree

M.S.M.E.

Advisors

Davies, Purdue University.

Subject Area

Mechanical engineering

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