Sound Synthesis of Fan Noise and Modeling of its Perception in Car Passenger Compartment

E2 - Sound Quality

In the context of either hybrid or electric cars, motorization sound has become an issue of less importance as compared to thermal motor sound. However, other sound sources that used to be masked by the motor sound may now be audible and cause annoyance to users. The CEVAS project in which the present study takes part addresses the problem of modelling the sound produced by either air conditioning (HVAC) or electric battery cooling (BTM) systems from the definition of the sound sources to their perception by users. The main aim of the project is to build a reliable and efficient tool for designing new systems according to high quality standards in terms of acoustic comfort. In the scope of the CEVAS project the present work addresses the question of how the HVAC and BTM sounds are perceived. This includes the synthesis of an audio sound corresponding to acoustic characteristics defined at the dashboard (for example the air vent outlets), the modelling of sound propagation inside the passenger compartment and the sound perception by users.
This paper focuses on the two first points. The sounds at the dashboard are obtained through an additive synthesis algorithm that can generate both broadband noise and tonal components. Several resolutions of spectrum description were tested. A perceptual similarity test was conducted with both recorded and synthesized sounds. It revealed that 1/3-octave-band resolution was sufficient for a reliable reproduction of the recorded broadband noise, while separate modelling of tonal components was also necessary to take into account any possible tonal emergence.
For modelling the sound propagation the passenger compartment transfer functions between specific points on the dashboard and the driver's ears were measured with a specifically designed electroacoustic source and a binaural head recording system. These transfer functions were then convolved with the anechoic synthesized sounds in order to recreate the sound spatial aspect as it is perceived in a real environment. In the end the tools presented here enable the reproduction of sound sources defined at the dashboard only by spectral characteristics. HVAC and BTM sounds can thus be synthesized as they would be perceived at the listener's ears in a real inner car compartment.