Interior vehicle noise is an important factor in the perceived product quality where tyre noise is a dominant source. The perceived tyre noise that the driver can experience is not only a product of the level but also by the character of the sound and the subjective expectations of the sound. In November 2012 the EU introduced a requirement for the labelling of tyre quality. The tyre label takes into account three important factors: fuel consumption, wet grip and external rolling noise. This has hastened the need to reduce tyre noise. In this project the sound quality parameters of tyres are studied as well as noise generation mechanisms and noise transfer paths.
Testing is important in vehicle and tyre development. In the CASTT project (Test Services for Winter Tyres) work automotive test entrepreneurs and LTU together to increase the knowledge in test methodology and measurement techniques. The aim of this project is to develop new services for the test entrepreneurs to meet the demands of the car and tyre manufacturers.
One objective of this project is to develop a model capable of predicting the tyre noise in the cabin of the car. Tyre noise is generated from the tyre/road interaction and transferred into the car compartment through structure borne and airborne paths.
Simulations or operational data of tyre/road interaction is filtered through experimentally measured transfer functions from the hub of the car into the car compartment to create auralizations (analogous to visualization for the ears). These tyre noise auralizations can then be used for sound quality evaluation. By varying parameters such as compounds and components in simulations, tyre noise can be auralized in different cars in an early design phase. For operational data, the model can be implemented according to the following procedure.
The method makes it faster and easier to evaluate the interior tyre noise for a large number of tyres and cars. Tyre noise predictions and auralizations are relevant both in tyre and car development. This method will be used in the development of quieter tyres.
The vibrations generated by the tyre/road interaction are transferred through the hubs of the car and into the cabin as structure borne sound. The hub acts as the coupling element and describes the boundary condition for the rim. In order to couple simulations (or operational data) with the experimentally measured transfer functions, the full structural behaviour is measured. The mechanical mobility of the hub is measure in 6-dof simultaneously in the following measurement set up.
The frequency response functions are measured from the hub to an artificial head positioned at the driver location in the car. To validate the model, a study was made to compare auralization of structure borne tyre noise based on operational data against artificial head recordings made under the same conditions. Results from a listening test showed that audible differences existed in the auralizations compared to the artificial head recordings. These audible differences were caused by low coherence in the measured binaural transfer functions below 150 Hz and in the frequency range 1000-1600 Hz. This issue will be dealt with in future studies.
Studies has also been conducted to see how the resonance frequencies of the tyre vary with parameters such as tyre pressure and the loading of the tyre. Vibrations of the tyres sidewall were measured using a Laser Doppler Vibrometer (LDV) while the tyre was excited with a shaker from below.
This project and will lead to international publications and a PhD thesis in the end of 2013.
The project is a collaboration between Nokian Tyres, Technical Acoustics LTU, Arctic Falls and Icemakers AB.
PhD candidate: Magnus Löfdahl.
Examination of the variability between artificial head recordings made in different cars of the same brand and model. INTER-NOISE 2009.
Assessment of changes in automotive sounds caused by displacements of source and listening positions. Noise control engineering journal
Model development for auralization of interior tyre noise. Licentiate thesis
An auralization model for structure borne tyre noise based on operational data. ISMA 2012
Mobility measurement in 6-DOF applied to the hub of a car (Applied Acoustics paper, submitted 2013)
Link to some of the publications: