Noise is nuisance
By PTA News Bureau
The noise from the tyre, both inside the vehicle and radiated into the general environment, is definitely an unwanted element. In an urban environment it can be the dominant noise and the one that stops people sleeping, enjoying their gardens and even walking in the street it has been shown to seriously degrade many people’s quality of life, says Alan Bennets, Director of UK-based Bay Systems, which has come up with a unique tyre cavity resonance measurement system. “For tyre makers good tyre performance factors such as wet grip, energy efficiency and wear rate need to be maintained and improved while reducing noise. This is likely to be a legislated necessity and not an option over the next 20 years,” Bennets told Polymers & Tyre Asia in an interview.
“To reduce this noise we are confronted with the standard alternatives,” he pointed out. “One way is to reduce the source, which requires intimate knowledge and understanding of the mechanisms that generate tyre noise. A second option is to break the transmission path. This typically requires the insertion of higher mobility components in the transmission path to reduce the forcing function’s effectiveness. Another alternative is to alter the radiation efficiency – reduce the velocity at the radiating surface possibly by adding mass or a decoupling layer.”
It is accepted that the most elegant solution is the reduction of the noise at source, particularly if this can be done at zero net cost, Bennets said. “All other approaches inevitably add cost weight and usually some other trade off, e.g. less precise steering and suspension control due to softer rubber bushings.
“Tyre noise is made up of multiple interactions: Road with tyre tread, with tyre carcass, with air in tyre, with wheel rim, with suspension members, with vehicle body responses and not to forget the direct radiation from the tyre surfaces.
“All of the above has been well understood – different from being solved – by tyre engineers for at least the last 30 years if not since the birth of the pneumatic tyre and the automobile.
“What was clear to the researchers at Cooper Tire and to Bay Systems was that the reduction in tyre noise, after the most recent European Union reduction of approximately 2dBA, was not going to be possible without a much deeper understanding of the transmission of noise through the tyre.”
The air inside the tyre plays a part in almost every aspect of tyre noise except the interaction of tread and road surface, which is a function of the tread pattern design, Bennet stressed. The stiffness and hence the transmission of vibration from the road to the axle is approximately divided 60 per cent through the air and 40 per cent through the tyre walls (run-flats with stiffer walls will be different).
“If the air is major transmission path, then changes in its stiffness are likely to be reflected in the noise transmitted into the vehicle cabin. Furthermore at the tyre cavity resonances, typically at 210Hz, 420Hz, 630Hz etc. +/-20Hz, the amplitude of the standing wave stiffens the air in the tyre. So when the cavity modes are high the tyre can transmit more noise and in some cases the primary cavity mode can be clearly heard in the vehicles cabin.”
According to Bennets, a change in the cavity mode amplitude of 6dB has been observed to make a noticeable difference to the cabin noise. A change in primary cavity mode amplitude of 6dB can be easily achieved by adding absorption (acoustic foam) inside the tyre or more elegantly by subtle changes to the tyres construction. The direct measurement of the tyre cavity mode amplitudes will tell the tyre and vehicle engineer which tyre/wheel combination will deliver the lowest road and tyre cavity noise.
(Full text in PTA Aug/Sep issue)