Why do parked cars emit doppler shifted sounds when you drive by them?

H2Bro

Hello,

This one has been bugging me for a while. When you drive a car past parked cars that are somewhat close, with the window down, there is a rhythmic whooshing sound. This sounds very much like numerous dopplershifts, but I have difficulty seeing it as a doppler shift because the parked cars are not emitting any sound.

My current guess is that the speed of the moving car causes the air between the moving and parked vehicle to be temporarily disturbed, lowering its pressure and causing the noise.

Is it a doppler shift or something else?

Simon Bridge

When you drive, your car is making a noise which is reflected off the parked cars.
For this, noise = any pressure wave ... engine noise, road contact noise, "bow" wave from pushing through the air and the turbulence around your car.

You can model the effect in a shallow tray of water.

H2Bro

That is interesting to think about. thanks for the reply

sophiecentaur

If the cars are not making any noise then what would be the difference between them and buildings? Also, the buildings are bigger and would produce much stronger echos.

Are you convinced that the effect you hear is 'doppler', predominantly, and not just a varying amount of reflection as you drive past the gaps between the cars?

Simon Bridge

Gaps are a good point ... gaps between buildings should do the same thing.

H2Bro

I'm not at all convinced what I hear is a doppler shift, if you read the first post I say it only sounds like one, in that its a quick transition from high pitch to low pitch as I drive by each car.

I'm fairly certain the sounds are synchronized with the cars as I drive past, also, if I pass only one car, it only makes one sound. So it may not be the gaps.

I haven't noticed the effect from buildings perhaps because they are considerably set back from the road, but you can check this yourself as the sound occurs from pretty much any object you pass at high speed so long as it is relatively close to your window. i.e. lamp posts, telephone poles, neither of which have considerable reflecting surface.

H2Bro

If I imagine myself stationary wrt to the objects alongside of the road, and they go whizzing past me, the reason they make a sound as they go is clear - they are moving at high velocity wrt. to the air around my ears and therefore cause pressure waves that are audible.

What gets me here is, my car is moving at velocity through air that is stationary w.r.t to the parked cars, so they are not really impacting the air to cause audible waves.

Could it be that there is a standing pressure/bow wave of air around the front of the car is being deflected by nearby objects as I go by?

sophiecentaur

I might suggest that the apparent change in pitch of the noise as you drive past gaps between cars and buildings could be because the resonance peaks are higher in level and frequency when the distance between car and object is less. This change in 'colouration' of the engine / road noise could account for what you hear.

Simon Bridge

I've heard it off bollards... it's not a bad question.
Wonder how we'd go about testing the ideas?

sophiecentaur

Record the sound as you drive past, measure the spacings involved and then fft selected passages of the recorded sound. See if the peaks in the spectrum correspond to (multiples of) the resonant frequency you would expect due to car-reflector separation.
The results will have loads of spread in them so you would need to do a lot of measurements and then look for correlation.

H2Bro

Nvm, I see Simon had said that earlier. Thanks guys!

Edit: disregard that comment, made it without refreshing the browser window.