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Hello All,

I have a perhaps strange question. First let me introduce it with an observation i made today during field recording. I just got back to my studio and I am listening to the recordings and I am quite happy with the overall sound, but i am bothered by something.
Today I was recording bypassing cars on a relatively busy street with cars driving at 50 km/h speed. I was monitoring through my headphones and using an m/s setup (mkh30/20). I noticed that all the cars approaching had a distinct shorter arrival time in comparison to their passing time. Or better said: the envelope was about 1:2 being 5 seconds fade in and 10 seconds fade out. (I hope anyone understands this :)
My microphones where pointing at the road in a 90 degree (horizontal) angle. So the sounds theoretically should be approaching 1:1. In the studio I am still hearing this effect. Oh and yes I am using an MS matrix to fold to stereo :)

OK so now finally my questions:

  1. Am I going crazy or is this just Doppler playing with my senses?
  2. Is this why SFX recordists' normally point there mic's at the vehicle?

I will upload a short snippet via Soundcloud asap. Just need the find an isolated by-pass.

Thanks in advance!

Greetings,

Arnoud Traa

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Welcome, by the way. Really good first question too. –  g.a.harry Jul 27 '11 at 22:16
    
Thanks! I wanted to make a good impression :) –  Arnoud Traa Jul 28 '11 at 8:28
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5 Answers

up vote 4 down vote accepted

The answer of g.a.harry could explain a large part of what you're hearing, but it can't be just the doppler effect:

alt text

The doppler formula describes just frequency (f) dependent on speed (v), not sound intensity or energy.

There is at least another aspect that plays a role: from the front, most cars are much quieter than from the back. The exhaust radiates most of its sound away from the car in a relatively narrow beam whereas the engine radiates in all directions. So the energy from the exhaust is concentrated in a smaller area which means you will hear it longer when you are in that area.

2: Recordists point their mics at the vehicle when they want to capture the sound on-axis all the time, rather than an extreme spatial effect.

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(hits his forehead) ofcourse, the most logical reasons of all, the exhaust! boy i feel like an idiot ;) thanks for clarifying! oh and the pointing of mics, i always thought they did that to get a center image of the vehicle and reflections in the sides.the on-axis reason is much more likely. i will experiment with it and see who much difference there is between on and off axis. i've got 2 dpa 4060's they might do the job (although being near perfect omni's).. will report back. thanks! –  Arnoud Traa Jul 30 '11 at 12:28
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Damn. @Joel beat me to the Doppler explanation.

Yeah, I guess the 1:2 makes sense given that the sound on approach ends up all smooshed, and therefore higher pitched, and the ones on departure are elongated, thus lower pitched.

The other thought is that it could be psycho-acoustic. If you're listening for the beginning of a fade in from 0dB when your noise floor is 50-60dB, unless it was really distinct, the sound would have to get up to say 40dB before it would be able to distinguish itself in any way. I'm pulling numbers out of nowhere here, but it seems to make sense.

Take a look at the waveform again. Your ambient sound will be a fairly thick bar of pink-ish noise. Where does each passby begin to be visible out of the noise? Where would it become visible on a less busy street, or even on a really quiet one?

Essentially, your brain is unable to distinguish one sound from another until something specific gets loud enough for your brain to catch on. Even if you know exactly what you're listening for, if it's similar enough to everything else that's going on, it'll still take a while.

The really psycho-acoustic bit comes in when you are listening to the car depart. Because your brain has latched onto that particular sound, it can remain focused on it much further down into the ambient noise. In fact, I'd wager that it's likely the case that your brain will continue to think it's hearing that unique car long after the sound is actually indistinguishable.

In turn, because your brain is so focused on the sound that just passed by, it doesn't notice the next one sneaking up behind. So it's not until the next is almost here that your brain looks up from the pudding to see the clown with the great big enginey knife standing over it.

I may be completely wrong on this one, but it feels pretty accurate. I've done a lot of that kind of stuff while mixing. Focusing on one sound to the exclusion of everything else and forgetting that the rest of the mix is even there.

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No no, you win. ;) –  MtL Jul 28 '11 at 3:01
    
Oh wow... this makes total sense to me! It's exactly the answer i was waiting for. I had the feeling i was leaving something out of the equation. Thanks! And upvote! –  Arnoud Traa Jul 28 '11 at 8:30
1  
If you reverse the recording do the same time periods exist or reverse? That would be a test of the psycho acoustics. –  ofa Aug 1 '11 at 11:23
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That makes sense to me. The sound approaches you, as the vehicle approaches you. (Call that a double approach.)

Then, the source passes you, and the sound just reaches back to you, at a slightly impaired speed, due to the velocity of the source AWAY from you. Hence doppler pitch shifting. Pitch (or frequency) is measured in speed, cycles per second. And if the vehicle is moving towards you, (and you are stationary) the frequency is faster (higher). As the source moves away, it will be slower (lower) as it reaches you.

I am CERTAIN, someone else can explain this better. With equations and stuff.

Can't answer your second question. Sorry.

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Wow SSD is already great resource! Joel, thanks for your answer! I must admit 'g.a. harry' got the upvote, but your answer sure helped get my head around it! Arnoud –  Arnoud Traa Jul 28 '11 at 8:43
    
by extension, the same car has a bluish tint in colour as it approaches you, and then a reddish one as it turns away. and by that same extension, it's dark at night, as the universe expands :) –  georgi Jul 29 '11 at 22:04
    
"it's dark at night, as the universe expands :)" which in turn makes the asphalt wobble nicely! –  Arnoud Traa Jul 30 '11 at 12:29
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SFX recordists point the mic at the vehicle so that they get an even frequency response. Off-axis response, even on omni-directional mics, can be uneven.

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there is a sound monster that eats sound and spits it out when something is going past another thing

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