[71 dB]

It doesn’t slowly drop, it stays constant (assuming a constant speed) and then drops quite quickly as it passes and then stays constant again. The pitch change actually occurs over a period typically of 0.5 - 1 second, although obviously depends on the speed of the vehicle.

We are splitting hairs here. Slowly dropping means incredibly slow (hardly noticeable by the observer), but it is a mathematical fact. When the car is x meters from the observer approaching, the frequencies emitted by the car get multiplied due to Doppler effect by

c/(c-v*cos(𝜶)),

where 𝜶 is the angle from the car to the observer compared to the street the car is driving. This angle changes while the car drives by the observer causing the multiplier effect dynamically change. The angle 𝜶 is calculated as

tan(𝜶) = y/x ⇒ 𝜶 = arctan(y/x).

Or

cos(𝜶) = x/(√(x²+y²)) ⇒ 𝜶 = arccos (x/(√(x²+y²))).

Let's assume a car is driving by with a minimum distance of y=5 m at a speed of v=72 km/h (20 m/s) in conditions were the speed of sound was c=345 m/s and these was no wind. When the car is x=50 meters away approaching, the Doppler multiplier is

(345/(345-20*cos(arctan(5/50)))) = 1.061214363...

When the car is only x=40 meters away, the Doppler multiplier is

(345/(345-20*cos(arctan(5/40)))) = 1.06103425...

When the car is x=30 meters away, the Doppler multiplier is

(345/(345-20*cos(arctan(5/30)))) = 1.060650382...

When the car is x=20 meters away, the Doppler multiplier is

(345/(345-20*cos(arctan(5/20)))) = 1.059591584...

When the car is x=10 meters away, the Doppler multiplier is

(345/(345-20*cos(arctan(5/10)))) = 1.054686387...

Not a dramatic change for sure, but not "constant" either.

It is sometimes but it’s quite rare in music production. It’s very common in film sound though, obviously there are often vehicle passbys, including aircraft. If fact there are entire sound libraries of passby but sometimes we need to create our own for various things, from artillery and other weapons or projectiles to flying insects. There are a number of plugins to help with the process and all sound designers/mixers have at least one in their collection.

G

Yeah, good point! This is more relevant in movie sound for obvious reasons...

 

[XTF1]

Another classic Doppler-based device: speed radar/guns. Next time I receive a ticket , I’m gonna try that illusion argument and see if the judge buys it !!!

One point about cars & airplanes approaching and passing by: the sound they emit is not a pure, single, frequency: could the “illusion” come from perceiving the dominant fundamental first, then more (lower SPL) harmonics as it approaches, then the opposite as it moves away? And all of this affected by the brutal change in frequency when it passes by.

 

[gregorio]

One point about cars & airplanes approaching and passing by: the sound they emit is not a pure, single, frequency: could the “illusion” come from perceiving the dominant fundamental first, then more (lower SPL) harmonics as it approaches, then the opposite as it moves away?

There isn’t one dominant fundamental, the engines have more than one and even at a distance there will be a complex waveform with at least one several harmonics, otherwise it wouldn’t be recognisable as a vehicle. However, you’re right that as the vehicle approaches we would hear a greater number of higher order harmonics, although I don’t think that would cause the illusion of an increasing pitch, as more higher order harmonics creates the illusion of higher volume rather than higher pitch.

G

 

[71 dB]

Today while walking out I payed attention to how I hear Doppler effect. For the most part I hear it the way it happens without much rise in the frequency. However, this is not simple. Does it matter if turn my head with the bypassing car? I didn't try this. My head stay still and the cars drove past me from behind me. The noise from the cars is very broadband and not very tonal because of the noise from the tires. At the point of bypass the spectrum of the noise sounds most bright. This can be because the cars are on one side of my head at that moment (HRTF-related?). It can be because there are least high frequency attenuation. It can be because of how the environment reflects the sound from the car. Doppler effect may be quite simple mathematically, but how we experience it seems to be very complex issue depending on many many variables...

 

[gregorio]

At the point of bypass the spectrum of the noise sounds most bright. This can be because the cars are on one side of my head at that moment (HRTF-related?). It can be because there are least high frequency attenuation. It can be because of how the environment reflects the sound from the car.

At the point of passby the spectrum sounds more dense/bright because that’s the point of highest SPL and lowest loss of high/mid freqs, due to both air absorption and a higher proportion of direct sound to reflected sound (which will have more high/mid freq loss due to absorption coefficients).

G

 

[Ryokan]

 

[sander99]

I remember really noticing the doppler effect most in the case of vehicles with siren passing by (police car, ambulance, fire truck). And I mean noticing spontaniously, without thinking about it in advance and at the same time very consciously, impossible to ignore.

 

[bigshot]

It’s clear in car racing too.

 

[gregorio]

I remember really noticing the doppler effect most in the case of vehicles with siren passing by (police car, ambulance, fire truck).

Yes, as well as car/train horns and for the same reason. Sirens and vehicle horns/whistles are obviously designed to be in an optimal hearing range, with very well defined pitches (a very clear fundamental and set of harmonics) and extremely predictable, so it is particularly obvious when that sound pattern changes. It is less obvious with most vehicle sound because the sound has a number of simultaneous non-harmonically related pitches and a more noise like spectrum.

An exception can be race cars, for two reasons: Firstly, they tend to be going a lot faster and therefore the Doppler shift is greater and secondly, they have a much higher rev range and the exhaust systems of race cars are tuned for performance rather than to muffle the engine sound. So the engine sound of race cars tends to be both a lot louder and have a more well defined pitch compared to standard road vehicles.

G