New tyres cause a humming sound in my car

Tyre noise -- also known as tyre roar or hum -- doesn't have much to do with the tyres being new; it is rather a problem with the design of the tyre itself. Your five senses are very good at tuning out any sort of constant input, and that includes the sound of your vehicle's original tyres. Your car's new tyres may very well be louder, but there's also a chance that they're simply producing a different sound frequency from the one produced by the old tyres.

Tyre construction

Tyres have three main components: the rubber compound, the tire tread and the internal reinforcing bands. A softer rubber compound grips the road better, but wears out faster. A perfectly slick racing tyre puts the most rubber on the road, but lacks channels to ferry water away from the tread. Strong reinforcing bands keep the tyre round, but they don't allow it to deform to fit the road conditions.

The nature of tyre noise

Tyre noise isn't the sound of the tyres hitting the road, per se. Sound isn't a constant stream of energy, it's the result of the rapid and constant compression and decompression of air. If one of a tyres' tread blocks hits the road at low speed, it's almost soundless because air has the chance to escape from the space between the tread blocks (the rubber between the grooves) and the road without compressing. However, at high speed, that same bit of tread compresses the air, then the air expands between the blocks of rubber on the tyre tread. When the next tread block comes along, it compresses the air again; on and on this goes, until you wind up with the constant roar of air being constantly compressed and released.

Rubber compounds

The hardness or softness of a tyre's rubber compound plays a crucial role in determining tyre noise, as a rock-hard rubber compound compresses air more than a softer one. Think of it as the difference between dropping a large dictionary flat on the floor and wrapping that dictionary in a pillow before dropping it. The pillow absorbs much of the dictionary's energy, itself compressing instead of allowing the dictionary to compress air trapped beneath.

Off-road tyres and noise

Off-road tires have large, "knobby" tread blocks instead of large, smooth blocks. These knobs increase the tyre's point-contact pressure at the ground, allowing it to dig into soft earth instead of distributing the load and skimming over the top of it. Unfortunately, when on a smooth surface, these knobs do the same thing with air: They squeeze the air against the ground, compressing it. The large, open areas between the knobs allow the air to expand more than it otherwise would. This extreme compression-decompression cycle is what makes off-road tires roar at motorway speeds.

Low-noise tread

There are two ways to reduce tread noise, either by reducing air compression or reducing expansion. Tyre manufacturers will often engineer tyres with a few solid rubs of tread going all the way around the tread. This solid band effectively bears more of the car's load than the open tread elements and -- since no air can get underneath -- helps to make it quieter. The opposite approach is to carve a number of solid grooves around the tyre that give air a place to vent instead of fully compressing. All else being equal, the former approach is better, in most cases, since it also puts more rubber on the road to enhance dry-road traction.

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About the Author

Richard Rowe has been writing professionally since 2007, specializing in automotive topics. He has worked as a tractor-trailer driver and mechanic, a rigger at a fire engine factory and as a race-car driver and builder. Rowe studied engineering, philosophy and American literature at Central Florida Community College.