New Psychoacoustic Criteria for Turbocharger Aero Noise 2019-01-1489

With ever more stringent regulations related to air pollution and CO2 emissions, there is a growing trend to address this through the downsizing of automotive engines. Turbochargers are used to improve engine power by using exhaust gas energy to provide boosted air pressure for better efficiency. They are a key element in powertrains of today and in the future. During real operating conditions, the rotational speed of a turbo varies greatly making this rotating machinery run in “continuous transient” phases and produces unwanted noises as a result. Recently, we identified a new source of aero noise while developing a new type of turbocharger. Our typical in-house procedure for characterizing usual aero broad band noise source was to use the 3-microphone methodology for measuring the acoustic intensity. However, this methodology is not well adapted if the noise source is tonal, not constant over time and includes high frequency content, which is the case with the new type of turbocharger design and which could create bad noise perception in case of noise transfer into the vehicle cabin. As a result and first step, a set of new psychoacoustic criteria was studied to better understand the relationship between the turbocharger design and the noise content characteristics in addition to the existing methodology. Two new metrics called « Fluctuation Standard Deviation Ratio » and « Low Frequency Peak Emergence » were developed to fulfill this need. The “Fluctuation Standard Deviation Ratio” is assessed in the time domain and can be connected to the human perception of a «steam locomotive», which for a usual compressor flow noise can lead to connotations such as “feeling of unsafety”. The low frequency peak emergence is assessed in the frequency domain and relates to a perception of an annoying whistling sound, which is easily captured by human ears when the peak frequency is below 2 kHz - 3 kHz. Both criteria will be detailed in this paper. Their relevance with human perception will also be shown using practical examples.