The subject of tyre noise is often raised in your words Doctor, and in Diesel Car road tests and long-term test car reports. Can you offer any general information regarding the situation, such as to why some tyres are actually significantly noisier than others, how to interpret the EC tyre noise ratings, and why the legislation to reduce tyre noise by imposing limits does not seem to be all that effective ñ or at least to my ears, anyway. Or are road surfaces just getting noisier?
Ah! A tough subject to address. Let’s start with the EC tyre noise testing and limits. Noise is a form of pollution and, particularly in some urban areas, it can be quite annoying, even distressing, when perceived tyre noise is excessive. In this case, however, we are talking of external tyre noise, as measured in standard tests using a noise meter to quantify the emitted noise level at a specific distance from a vehicle travelling past at a specified speed. These tests involve the use of a standard road surface that is constructed using specified road-making materials, as one might expect, and these tests establish the noise ratings for named tyres of a specific tread pattern carcase, construction, size, speed rating, and load rating. You cannot, by inference, presume that these ratings, using noise symbols or decibel ratings, are transferable from one size of tyre brand and name to another of the same tyre designation, but of a different size or profile altogether, for example.
You also need to be aware that the tyres tested are brand new tyres that are put through their paces after a specified amount of conditioning to remove surface lubricants, and that the generated tyre noise level changes with wear. Generally, it appears that tyres often get noisier with wear, most probably as the noise-insulation properties reduce with loss of tread depth, although quite possibly illegal tyres with minimum depth or worn out slicks might in some cases, be quieter than new tyres!
In addition to all this data relating to the tyre itself, in the real world there are huge differences between road surfaces, and the amount of noise that they generate in conjunction with the tyres. Concrete surfaced roads are frequently noisy, not so much due to the nature of the concrete itself, but from the rippled surface finish that is usually applied to the top surface to improve grip and drainage. But this all relates to external noise, as perceived by a pedestrian, or maybe another vehicle driver.
What usually causes most concern to drivers, rather than pedestrians, is the cabin noise that is transmitted into their own car, from the same basic noise source of tyre and road surface, but somewhat insulated by the suspension and bodywork of the car. I recently discovered that the construction of wheel arches and their liners can be, and is sometimes, modified to create a directional property in the road/tyre generated noise, which can thus be designed to reduce the noise measured in the standard external noise test procedure. In doing this, it therefore may in process direct the generated noise elsewhere, thereby possibly increasing the cabin noise, or perhaps cunningly retaining and absorbing the noise within the wheel arch! As a result of this, there may be little correlation between the regulatory measured external noise figures and ratings and the noise level perceived by the in-car driver and any passengers, although not necessarily.
I feel that cabin tyre noise is very much related to the noise insulation of the car itself, in terms of the amount and quality of soundproofing materials applied to areas such as the door panels, the roof, the floor pan, and the boot area. I have had some success in applying additional purpose-made sound insulating material in such areas, and there’s little doubt in my mind that some model ranges employ more and better insulation material in their top-level model variants. Possibly the same applies to group variants using similar or identical floorpan or modular construction units, which might account for why an Audi could be quieter than a Skoda or SEAT with the same power unit under the bonnet. I think that suspension bushing, and the use of noise and vibration insulating bushes in subframes may have similar beneficial effects, and that cars using air suspension (mostly prestigious top drawer models) may have some advantages in terms of reduced noise transmission.
At the end of the day though, the best solution is reduction of noise at source, and higher profile tyres with deeper sidewalls probably have an advantage over wide low-profile tyres, although I am less certain whether low rolling resistance tyres designed for improved fuel economy enjoy any benefits, or alternatively suffer negative effects, in terms of noise generation. Certainly some of such tyres have some of the lowest regulatory noise level ratings.
I think I must leave it at that, but my own experiences suggest that you certainly become more aware of tyre noise as the tyres wear. Regarding what you see as little improvement in road noise, it gives me some pleasure to quote a statement from a government document issued by DEFRA ìNoise Action Plan: Roadsî which has a plan to tackle the problems. Helpfully it says ìNoise is a natural consequence of a mature and vibrant society. Noise, however, can have major implications for quality of life (well being), human health, economic prosperity and the natural environmentî. Too right! Perhaps our younger readers are part of the ìvibrant societyî and the older ones part of the ìmatureî society that they see fit to bulk together. But I can assure you that there is, or was, a government plan to reduce road noise, although whether there is money now left in the kitty to do anything about it is anybody’s guess! Thanks for your letter. Regards,