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Your vehicle’s tires are rolling kick drums. Keith Moon played two; your car plays four. Expansion joints, potholes, and heaves pound the rubber, which compresses the air inside and transmits hollow thuds to the cabin.
As cars get quieter, engineers are working to stifle the drumbeat of tire impacts. Their current favored method is polyurethane insulation glued around the tire’s inner liner. These acoustically insulated tires have stealthily spread through the market to the point that they are now offered by all the major tire manufacturers and come installed from the factory on several high-end luxury vehicles, including those hallmarks of tranquility, the Mercedes-Maybach S600 and the Tesla Model S.
Tire manufacturers see big potential for the technology as electrification silences powertrains, making tires one of the most obvious sources of sound intrusion. The main function of inner tire insulation is to improve the tire sound quality, specifically by absorbing impact noise. In actuality, it reduces overall cabin volumes only slightly. It has the greatest effect on frequencies near 200 hertz (Hz), a sound that is low in pitch, relative to what the human ear can register, but is at the high end of the frequencies a tire generates. Think of the deep ping of a bouncing basketball. “You hear it if the tire goes over an expansion joint,” says Scott Pajtas, technical sales director of Michelin. “It can also be a constant tone if there’s a road-surface irregularity or a uniformity irregularity in the tire.”
We put the technology to the test with two sets of Continental ContiSportContact 5 tires delivered from Tire Rack. The tires were identical save for the ContiSilent label etched into the sidewall and the polyurethane insulation glued to the insides of one set. We mounted them on a Tesla Model S, both to minimize the influence of powertrain noise and because the insulated version is a factory fitment on the Tesla.
For our first test, we averaged the cabin sound level over 15-second samples, driving on both a freshly paved, perfectly smooth asphalt road and an older, coarser concrete road with expansion joints, taking data at both 45 and 70 mph. In all conditions, the insulated tires had a negligible effect on the overall loudness.
Subtle differences did appear when we recorded laps on the ride road of Fiat Chrysler’s Chelsea, Michigan, proving grounds. As the chart above shows, the insulated tires performed better through the first half of the 4.3-mile loop. Over various imperfect surfaces—including expansion joints, rumble strips, and sine-wave ripples in the pavement that repeat at increasingly greater frequency—there’s a small but noticeable difference in overall loudness. However, we saw a more pronounced difference when looking specifically at the one-third-octave band centered on 200 Hz, which covers frequencies from 178 to 224 Hz. Even if the overall loudness hasn’t dropped much, it indicates that the tone of the sound has changed.
That advantage narrows in the second half of the ride road, which includes more features you would expect to encounter in the real world. Several stretches are replicas of actual Michigan roads with lumpy, cracked, and pocked asphalt. Other sections include concrete slabs with slight vertical offsets—a condition that seems tailor-made for these tires—and yet there’s only a marginal difference between the two sets.
If the effect appears small by our sound-meter measurements, it seemed even smaller when measured with our eardrums. We struggled to discern any significant improvement, although it probably didn’t help that our back-to-back drives were separated by a half-hour tire swap. If you really strain to hear it—squint hard, cock your head, and clench your teeth—you just might notice the slight change in the tone of tire impacts. But if you can’t tolerate the uneven drumming of tires, your best option is still drowning it out with The Who.
Chasing the same 200-Hz tire-impact noise as tiremakers, Honda engineers developed injection-molded plastic chambers that wrap around the barrel of the Acura RLX’s wheels. They are tuned so that sound waves reflecting in the tire cavity cancel out the undesired frequencies. Mounting the noise-canceling device to the wheel, rather than the tire, presents a particular challenge. The same centrifugal force that presses acoustic insulation against the tire liner pulls the plastic chambers away from the wheel. At 186 mph, the forces can be as great as 1500 g’s. Honda’s solution involves minimizing the weight and maximizing the rigidity of the resonators, which are then press fit under metal ridges cast into the wheel.