[Petrochemicals (OP)]

When travelling at a sedate 30 mph in a car it suddenly occoured to me passing cars are incredibly noisy for some reason, yet the car I was in was quiet. All passing cars in the opposite direction made a high pitched din. Why is this ?

[Bill S]

Could it have something to do with soundproofing?   

[Bored chemist]

I think the first question to ask is  whether the effect is real or not.
Do you have a smartphone?
If so you can probably find some sort of sound level meter app.

It's entirely possible that you simply get used to the noise from your own car, and that's why the others sound so loud.

[alancalverd]

Most of the noise nowadays is from the tyre/road interface, and is radiated outwards from the vehicle. There is plenty of metal between you and your tyres but only air between you and the other vehicles. If you drive close to a wall you will hear your own tyre noise reflected, and the lack of tyre noise on fresh snow is remarkable.

[Petrochemicals (OP)]

Could it have something to do with soundproofing?   

no, it is windows down, the oncoming cars are excruciating to the ear.

I think the first question to ask is  whether the effect is real or not.
Do you have a smartphone?
If so you can probably find some sort of sound level meter app.

It's entirely possible that you simply get used to the noise from your own car, and that's why the others sound so loud.

Most of the noise nowadays is from the tyre/road interface, and is radiated outwards from the vehicle. There is plenty of metal between you and your tyres but only air between you and the other vehicles. If you drive close to a wall you will hear your own tyre noise reflected, and the lack of tyre noise on fresh snow is remarkable.

Directional sound ? I will stick my head out the window next time !

[chiralSPO]

If you drive close to a wall you will hear your own tyre noise reflected, and the lack of tyre noise on fresh snow is remarkable.

Yes. Actually, I think a significant amount of the noise "from" the other car is noise generated by one's own car which is reflected off of the other car. I remember noticing this effect as a child when driving with the windows open: I could tell when we were passing by parked cars, even with my eyes closed.

[Colin2B]

I could tell when we were passing by parked cars, even with my eyes closed.

Yes, I remember doing that, a line of parked cars can be almost musical.
You get a similar effect walking along the pavement, a brick wall, hedge, and fence all have a different reflected sound, shiplap fencing particularly distinctive.

[alancalverd]

The "twanging" sound of an overlap fence is an interesting analog of the Pound-Rebka experiment.

[evan_au]

Could it be that the cars that are most likely to overtake you with the greatest speed are those cars whose owners are very proud of their cars, and the way they perform, and the way they sound?

[vhfpmr]

If you drive close to a wall you will hear your own tyre noise reflected, and the lack of tyre noise on fresh snow is remarkable.

Yes. Actually, I think a significant amount of the noise "from" the other car is noise generated by one's own car which is reflected off of the other car. I remember noticing this effect as a child when driving with the windows open: I could tell when we were passing by parked cars, even with my eyes closed.

Snow is very anechoic, too.

[Petrochemicals (OP)]

Could it be that the cars that are most likely to overtake you with the greatest speed are those cars whose owners are very proud of their cars, and the way they perform, and the way they sound?

Nope law obiding citizens travelling in the opposite direction at a constant moderate speed.