The quietness of an expensive car is one of its most immediately noticeable qualities. It is achieved through a stack of separate measures, each attacking a different route by which noise enters.
Noise arrives by three distinct paths
Airborne noise travels through the air and passes through gaps, glass and panels. Wind noise and the sound of other traffic arrive this way.
Structure-borne noise travels through solid components, entering through suspension mounts, subframes and the drivetrain, and is radiated into the cabin by panels that vibrate.
Tyre noise is a combination of both, generated at the contact patch and reaching the occupants through the air and through the suspension simultaneously.
Mass and sealing handle the airborne share
Sound transmission through a panel falls as its mass per unit area rises, which is why heavy damping sheets are applied to floors, bulkheads and wheel arches.
Laminated glass places a plastic interlayer between two sheets, and that layer absorbs vibration rather than transmitting it, which is why side windows are laminated on quiet cars.
Sealing matters disproportionately, because a small gap allows a large amount of high-frequency noise through. Multiple door seals in series address this directly.
Isolation handles the structure-borne share
Suspension components are mounted through rubber bushes or hydraulic mounts that allow the necessary loads to pass while absorbing vibration at troublesome frequencies.
Subframes are often isolated from the body entirely, adding a second stage of damping between the road and the passenger compartment.
Each isolation stage trades directly against handling precision, since a mount soft enough to absorb vibration also allows the component to move under cornering loads.
Active cancellation covers the low frequencies
Passive measures work poorly at low frequency, because blocking a long wavelength requires far more mass than blocking a short one.
Microphones in the cabin detect the residual low-frequency noise and speakers reproduce an inverted waveform, cancelling much of it at the occupants' ears.
The technique works well for steady tones such as engine order noise and road drone, and much less well for irregular sounds like an impact over a pothole.
Why silence has a diminishing return
Each reduction in noise reveals the next source, so a car that has eliminated engine noise now has audible tyre roar, and a car that has quietened tyres now has audible wind.
The measures also add weight, which harms consumption and dynamics, and the final decibels cost far more in mass and money than the first substantial reduction did.
Electric powertrains changed the priority ordering rather than solving the problem, because removing engine noise simply promoted road and wind noise to the top of the list.