Interior quietness is among the most reliable markers of an expensive car, and it is achieved through several unrelated disciplines rather than by adding insulation.
Noise arrives by three different routes
Airborne noise travels through the air and enters through glass, seals and panel apertures. Wind noise and traffic noise are the obvious examples.
Structure-borne noise travels through the metal itself, entering at the suspension mounts and engine mounts and radiating from interior panels.
The third route is regenerated noise, created inside the cabin by vibrating trim and by air moving through ventilation ducting.
Sealing and glazing address the airborne path
Multiple door seals in series create separated air gaps, each of which reduces transmission, which is why premium cars have two or three seals per door.
Acoustic laminated glass places a viscoelastic layer between two glass sheets, damping the frequencies that ordinary glass transmits most readily.
Aerodynamic detailing matters as much as sealing. Mirror housings, A-pillar shapes and screen surrounds are refined to prevent the turbulence that generates noise in the first place.
Mass and damping treat the structural path
Damping material bonded to large panels converts panel vibration into small amounts of heat, which stops the panel radiating sound into the cabin.
Hydraulic engine mounts and suspension bushes are tuned to isolate specific frequency ranges, and active mounts can counteract vibration using an electromagnetic actuator.
Body stiffness underpins all of it, since a stiffer structure has fewer resonances in the frequency range the ear finds objectionable.
Weight is the usual cost. Damping sheets, additional seals and laminated glass all add mass, and a heavily treated car can carry a considerable quantity of material that exists purely to stop sound, which is one reason quiet cars tend to be heavy ones.
Tyre noise is the hardest remaining source
At steady cruising speed, tyre noise commonly dominates. It is generated at the contact patch and enters both through the air and through the suspension.
Manufacturers specify tyres with acoustic foam bonded inside the carcass, which absorbs the cavity resonance responsible for a substantial part of the sound.
Wheel arch liners, underbody panels and the shape of the suspension components all contribute, which is why fitting different tyres to a quiet car can change its character noticeably.
Active control handles what remains
Microphones in the cabin detect low frequency noise, and the audio system generates an inverted waveform that cancels part of it.
The technique works well at low frequencies, where wavelengths are long enough for the cancellation to hold across the space occupied by an occupant's head.
It becomes ineffective at higher frequencies, which is why active systems supplement physical treatment rather than replacing it, and why the passive engineering still determines the result.