- Generated by the interaction of the tyre with the road. The noise reaches the driver's ear via two fundamentally different routes:
- Structureborne via the suspension components to the vehicle cabin and then airborne to the observer - dominant mechanism below 500Hz
- Airborne from the tyre/road interface through the panels of the body or through holes in the body structure to the observer - dominant mechanism above 500Hz
- The classical tyre to road interface noise generation mechanisms are:
- At the leading edge the tread blocks strike the road surface and subsequently vibrate. The frequency of these impacts is the pattern repeat frequency. The block impacts lead to the production of a sound wave, the form of which has been shown empirically to be of sine form with some third harmonic influence.
- On a smooth road the pattern repeat frequency of the tread blocks would be quite dominant although modern tread pattern design reduces this effect. The block impact effect on a rough road is similar, however there also exists some further influence from the road surface which breaks up the repeat effect.
- In the contact patch, area of the tyre in contact with the road surface at any given time, there is some movement of the tread blocks relative to the road which generates a slip noise. This mechanism will also be affected by road surface roughness as a more coarse surface tends to reduce the effect of slip.
- The tread blocks that are in the contact patch are caused to vibrate by their impacts with the asperities in the road surface. This mechanism is greater on coarse road surfaces and this is partly the reason for the reduction of block impact and slip effects on coarse roads.
- The groove organ pipe noise is the resonance of the air within the tread grooves in the contact patch. Grooves running circumferentially around the tyre are particularly significant because their length is such that the first mode of vibration of the air column is low enough to be coincident with other vibration mechanisms. This mechanism tends to be of low importance on coarse roads.
- As the grooves progress through the contact patch they close up which results in the air which is ordinarily trapped within the tread grooves being compressed. This squeezing and then releasing of the air acts as a pump which generates noise, known as air pumping. The pressure increase in lateral grooves will create a noise source to the side of the contact patch. When traveling on a porous road surface the air, rather than being compressed within the tread grooves, is squeezed into the void spaces of the road. This is a major reason for the reduction in tyre - road interface noise when traveling on porous roads.
- At the trailing edge, the blocks vibrate after they have been released from the ground contact, this phenomenon is known as "Block snap out". The tangential forces which exist in the contact area apply significant deflections to tread blocks and it is as these forces are removed, and the block is released, that it has a tendency to vibrate.
- Further mechanisms of noise generation exist and include:
- Noise generated by the sidewalls of the tyre. The deflection of the sidewall is greatest in the region of the contact patch.
- The air cavity resonance is the resonance of the air within the tyre. The first circumferential mode of this air space is known to have a frequency of 200-300Hz, the actual frequency being controlled by tyre size and wheel dimensions. At higher frequencies, typically around 1100 - 1200 Hz, there is a lateral mode of the air cavity. Numerous researchers have investigated the cavity resonance.
- At the front and rear of the contact patch a 'horn' is formed by the tyre and road surfaces.
- There are a number of factors that influence the interior road noise:
- Body transmissibility
- Suspension type and geometry
- Suspension bushing tuning
- Shock absorber tuning
- Tyre Size and construction
- Weight distribution
M. Heckl, "Tyre noise generation", Wear, Volume 113, Issue 1, 1 December 1986, Pages 157-170
D.J. O'Boy, A.P. Dowling, "Tyre/road interaction noise--A 3D viscoelastic multilayer model of a tyre belt", Journal of Sound and Vibration, Volume 322, Issues 4-5, 22 May 2009, Pages 829-850