Quietness is one of the clearest markers separating a luxury car from an ordinary one, and it is also one of the most expensive things to achieve. It is not a single property but three distinct noise sources that have to be attacked in different ways.
Three sources with three cures
Noise inside a cabin arrives as wind noise from air moving over the body, road noise transmitted through the tyres and suspension, and mechanical noise from the powertrain.
Each travels by a different route. Wind noise comes through seals and glass, road noise comes up through the structure, and powertrain noise arrives through both mountings and air.
Treating one does nothing for the others, which is why a car can be smooth mechanically and still be tiring on a coarse surface.
Why wind noise starts with shape
Air separating from the body creates turbulence, and turbulence near the cabin becomes audible sound. Mirrors, A-pillars and the wiper area are the usual offenders because they sit close to the occupants' ears.
Shaping those areas so airflow stays attached is more effective than any amount of insulation added afterwards, because the noise is never generated in the first place.
Sealing does the rest. Double door seals, thicker or laminated glass and careful attention to every aperture in the body all reduce what gets through.
Why road noise is the hardest to remove
Road noise enters as vibration through the tyre, into the suspension, and then into the body structure, which radiates it into the cabin as sound.
The tyre is the largest single variable, and a quiet compound and tread pattern usually trade away some grip or wear life in exchange.
Isolating the suspension from the body with rubber bushes reduces transmission, but softer bushes allow more movement and blunt the steering response the same car is expected to have.
What insulation actually costs
Mass is the traditional answer. Heavy damping sheets on floor and bulkhead panels stop them acting as loudspeakers, and thick absorptive material soaks up airborne sound.
All of it adds weight, which affects efficiency, braking and handling, so the amount used is a deliberate decision rather than a maximum.
Modern practice targets material precisely where measurement shows it works, instead of covering large areas uniformly.
Why electric cars complicated the problem
Removing the engine removed the noise that previously masked everything else. Road and wind noise that were always present simply became audible.
Electric drivetrains also produce high-frequency whine from gears and inverters, which the ear detects readily even at low volume.
The result is that quietness has become harder rather than easier at the top of the market, and active systems that generate cancelling sound through the audio speakers are now doing work that mass alone used to do.