One of the unexpected positives of lockdown here in London has been the peace and quiet. No one seems sure whether the birds were always there, drowned out by the sounds of the city, or whether they’re performing with special enthusiasm now that they’ve got the stage all to themselves. But the absence of other sounds is an opportunity to remember that the world around us is playing a continual symphony.
In most places, one of the most important sections in that orchestra is car tires. In a car going faster than about 25 mph, the sound of the tires is louder than the sound of the engine. If you get up to 75 mph, the tires can easily register at 80 decibels—about the same volume as a freight train heard from 50 feet away, loud enough to cause hearing damage if you’re exposed to it for a whole day.
The sound of a rolling tire mostly comes from the patch where the rubber touches the road. But the curved shape of the space between the tire and the road acts like the horn of a trumpet, amplifying the original sound by about 10-20 decibels in both directions, in front of the car and behind it.
Then we get to the more interesting question of what exactly is being amplified. Sound is a disturbance in the air which carries the signature of the object that created it. In music, the sound quality and the note you hear depend on the physical structure of the instrument. A trumpet has a tube with air oscillating inside it; a violin has a string that vibrates differently depending on its length. What does a car tire have?
The answer turns out to be a lot of different instruments, all playing together and not necessarily in tune. As the front of the tire slams into the road, air is rapidly forced out of the gaps in the tread, each acting like a miniature trumpet tube. As the back of the tire leaves the road, air is sucked back into the gaps and the slightly sticky rubber pops off the tarmac, forming a percussion section. Turn the wheel and the rubber grips and then slips and then grips again, forcing it to vibrate just like a violin string. The side walls of the tire vibrate like the body of a double bass.
The exact sound a tire makes depends on the size, shape and orientation of its various tubes, grooves and rubber blocks. And the faster you drive, the more forcefully all of these things happen, raising the volume by as much as 25 decibels. That’s why sound is the most important consideration for tire manufacturers, after road safety: Some tread patterns balance the components better, making them less unpleasant to listen to.
The tone is also affected by the road itself, which like the walls of a concert hall help to dictate sound quality. A very porous road surface reduces the violence with which air is pumped around the tire grooves, reducing the generation of sound and helping to absorb any sound that does escape.
The tire is such a complicated musical instrument that scientists are still working on fully understanding all its nuances. As electric vehicles with quiet engines become more common, perhaps we’ll pay more attention to tire sounds in the future. But for now, my minor annoyance when a car disturbs the peace and quiet is slightly softened by my appreciation for the song the tires are singing.
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June 25, 2020 at 11:05PM
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The Complex Music of Car Tires - The Wall Street Journal
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