Car noises

Tux

Tux

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Ok, maybe the thread topic is wrong

I've always wondered, why does a car that's approaching you sound different than one going away from you? A different pitch if that's the correct term?
Now I know there's probly a fancy and complicated as heck scientific term for it, but could somebody explain it to me in idiotese?
 
Gru said:
Ok, maybe the thread topic is wrong

I've always wondered, why does a car that's approaching you sound different than one going away from you? A different pitch if that's the correct term?
Now I know there's probly a fancy and complicated as heck scientific term for it, but could somebody explain it to me in idiotese?
Click to expand...

I think its called Doppler Shift. Got to do with the compression of the sound waves as the car nears and the "de-compression" as it goes past.

If I remember correctly, scientists also use it to determine if a galaxy is moving away from earth or towards earth by looking at the "Red Shift".
 
Bernie said:
I think its called Doppler Shift. Got to do with the compression of the sound waves as the car nears and the "de-compression" as it goes past.

If I remember correctly, scientists also use it to determine if a galaxy is moving away from earth or towards earth by looking at the "Red Shift".
Click to expand...

Correct and correct, see here http://en.wikipedia.org/wiki/Doppler_effect
 
Thanks. More than anything the eyewatering graphics on that wiki page explains it, sorta. Will have to do an in depth study of it and get back to you guys on it if it's still not clear to me :)
 
Yip the Doppler Effect. It's the increase in pitch when a source approaches us (or we approach a source) and a decrease in pitch when the source recedes from us (or we recede from the source). It's basically a change in frequency of two objects relative to one another.

Successive waves emitted by a source moving towards an observer are closer together than normal because the source is advancing (hence it's at a higher frequency since the separation of the waves is the wavelength of the sound). Likewise when the object recedes, it emits waves that are further apart from each other (lower frequency).

dablakmark8 said:
is it not the wind resistance sound from the aerodynamics of the cars front and back.
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Nope it's got to do with compression and decompression of the wavelength of the sound emitted from the one object relative to another i.e. the car/bike/plane relative to a stationary person etc.
 
Bernie said:
I think its called Doppler Shift. Got to do with the compression of the sound waves as the car nears and the "de-compression" as it goes past.

If I remember correctly, scientists also use it to determine if a galaxy is moving away from earth or towards earth by looking at the "Red Shift".
Click to expand...
Technically there's only redshift if the object is moving away from us, otherwise the light would be blue (violet?) shifted.
 
Another thing ... :D (there's always another question with me)

take that same car and keep an eye on it's wheels. Soon as it starts decelerating. At a certain point it appears as if the wheels start spinning backwards. What causes this optical illusion? Is it something to do with the errmm "framerate" your brain process it at or what?
 
Gru said:
Another thing ... :D (there's always another question with me)

take that same car and keep an eye on it's wheels. Soon as it starts decelerating. At a certain point it appears as if the wheels start spinning backwards. What causes this optical illusion? Is it something to do with the errmm "framerate" your brain process it at or what?
Click to expand...
Exactly. At some point the wheels move at a speed where they move more than halfway between the two spokes between the time slices that your brain processes the image. Your brain assumes that the wheel spokes moved the lesser of the two distances and voila, you think the wheel is spinning the wrong way around when you look at it.
 
Not quite related to the Doppler effect, but can you imagine how complex bats radar systems must be to successfully avoid flying into each other, if a group of them is in an enclosed area? Each bat using a different pitch and frequency, and is able to distinguish the echoes of its calls from the calls of the other bats, using them to calculate the speed and direction of travel of different obstacles...
 
doppler effect, plus the fact that the exhaust faces backwards so the sound waves exit to the rear! (doh!)

Xarog said:
Exactly. At some point the wheels move at a speed where they move more than halfway between the two spokes between the time slices that your brain processes the image. Your brain assumes that the wheel spokes moved the lesser of the two distances and voila, you think the wheel is spinning the wrong way around when you look at it.
Click to expand...

only on tv...only on tv...our brains/eyes dont work in "frames"
 
I'd love to see you source that claim.
 
Xarog said:
I'd love to see you source that claim.
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http://en.wikipedia.org/wiki/Temporal_aliasing (yes yes, wiki isnt the be all and end all of life, but in this case the article is accurate)

anyway, if you see it in real-life, it is usually because of the lighting around you..incandescents and fluorescent lights can cause this "flicker"
http://www.scif.com/safety/losscontrol/Article.asp?ArticleID=320
point #2 in the link.


older turntables had a calibration printed on the side of them, so you could adjust the speed accurately, using this exact same principle. they either had a little light built into it, or you had to use them in some other artificially lit environment. they wouldnt work in sunlight as the effect wasnt noticeable.
http://en.wikipedia.org/wiki/Phonograph#Belt_drive_system

http://www.earthsky.org/faq/50023/stroboscopic-effect


maybe your brain works in frames..but you should blame your parents for that.

of course, you can test his out.
Tomorrow, when it is nice and sunny, go stand somewhere, like, on the N1 or the N3 and see if you can see the wheels spinning backwards...if at first you dont see it happen, go stand closer to the cars, if that still doesnt work, go lie down in the middle of the road and try again. repeat this a few times to get an accurate result:D .
 
None of those links deal with how the brain actually works, only how the 'reverse motion' illusion is aggravated by the use of strobe lights or fluorescent lights.

So I'm still waiting for you to source your claims. Frankly I cannot see how your brain is capable of making basic speed judgements unless it calculates what it sees on a frame by frame basis.
 
Xarog said:
None of those links deal with how the brain actually works, only how the 'reverse motion' illusion is aggravated by the use of strobe lights or fluorescent lights.

So I'm still waiting for you to source your claims. Frankly I cannot see how your brain is capable of making basic speed judgements unless it calculates what it sees on a frame by frame basis.
Click to expand...


it isnt aggravated by using lights that strobe, it only exists under those circumstances (or any "frame" based medium like film/tv). when you say aggravated you make it sound like it happens all the time, but is just easier to see under specific lighting. that isnt true.

tell you what, why dont you go out of your way to prove me wrong, and then i'll go down on you? k.
 
Renamed20070410 said:
it isnt aggravated by using lights that strobe, it only exists under those circumstances (or any "frame" based medium like film/tv). when you say aggravated you make it sound like it happens all the time, but is just easier to see under specific lighting. that isnt true.

tell you what, why dont you go out of your way to prove me wrong, and then i'll go down on you? k.
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Common sense proves me right. The brain can only calculate so much at any given time, and it can only calculate a thing at a given speed. Either you have to accept that the brain can calculate things at an infinite speed (which means that you'd be able to think *anything* through instantly without a moment's hesitation), or the brain has to follow a logical process which occurs at a set rate (i.e. each step takes a certain amount of time to complete before the next step can be addressed). Since we value people for their quick thinking, it stands to reason that we do not in fact think infinitely fast; thus there will come a point where things will move too quickly for the brain to follow them.

Strobe lights aggrivate the problem because they make it harder for the brain to follow the progression, however, if something moves fast enough this interruption will occur naturally.

But you can save the 'going down' on me, I'm pretty sure you're not my type.
 
your common sense would require adjusting then.

seeing as you havent proven me wrong, besides giving me your 2c worth, let me take this opportunity to prove me right.

your eye, sir, isnt a a digital signal processor. it doesnt have a synchronised clock rate. the cones and rods receive and process the light and pass it on to the brain, sure. but they dont all do it at the same time like a digital camera. as such, and due to the unsynchronised messaging going on, things that happen too fast are still captured, but only by some parts of the eye (while the rest are capturing processing and sending on the next batch of photons), and commonly, those of us in the real world refer to what we see as a "blur"

do you see fast things as a blur? if so, I am right.

or, look at it this way, if your eye could (which it cant) work on a frames basis (or even your brain) then there should be some magic setting on your monitor, somewhere above 100hz that you can select, where the screen would flicker like mad, and have a slowly progressing horizontal line, same as when you try to film a pc monitor at anything other than 50hz or 60hz (depending on yoru mains frequency)

but there isnt such a magic setting. oh no sir, when things happen too fast, our eye still captures it and sends it on and what we see is a blurry image. not a flickering image.

there is one situation where you may see a stroboscopic effect in natural daylight, and that is if the car is on the other side of a picket fence..the fence posts would create the "frames" required to split the image up into parts and make it look like the wheels are spinning backwards.

i'm gonna quote wiki again (coz i am in the mood)
"A critical part of understanding these visual perception phenomena is that the eye is not a camera: there is no "frame rate" or "scan rate" in the eye: instead, the eye/brain system has a combination of motion detectors, detail detectors and pattern detectors, the outputs of all of which are combined to create the visual experience."


if we have 5 million rods and cones in our eyes (thumbsuck figure) and they are all sending info at different times, then it stands to reason we do not have a frame rate. your argument may be that the brain can only process info at a certain speed, and this does stand to reason, but as we process parts of our vision as a first-in first-out system due to the way the cones and rods send the info, we end up with blurry motion. so your wheels, on the highway, will be blurry, and not spinning backwards.
 
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Renamed20070410 said:
your common sense would require adjusting then.
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Nope.

seeing as you havent proven me wrong, besides giving me your 2c worth, let me take this opportunity to prove me right.
Click to expand...
I proved myself right, and I'm still waiting for you to source your claim about the way the brain works.

your eye, sir, isnt a a digital signal processor. it doesnt have a synchronised clock rate. the cones and rods receive and process the light and pass it on to the brain, sure. but they dont all do it at the same time like a digital camera. as such, and due to the unsynchronised messaging going on, things that happen too fast are still captured, but only by some parts of the eye (while the rest are capturing processing and sending on the next batch of photons), and commonly, those of us in the real world refer to what we see as a "blur"
Click to expand...
Blur is caused by the light moving over the various parts of the retina faster than they can react to it; i.e. using a very rough numerical explanation : if the retina reacts to light chemically for no longer than 5 nanoseconds, and a particular beam of light hits 5 different spots for 1 nanosecond each in a 5 nanosecond burst, then you will see 5 different spots of light even though there was only one beam of light. Blur is caused by light moving across your retina faster than the retina can complete its chemical excitation.

Whether or not the eye processes an entire image at a time or only parts of an image at a time is entirely irrelevant to the existance of blur (a simple experiment to prove this : look at the sun and then look away from the sun, and then close your eyes - you will see a long blur even though there was no quick movement, simply because your retina was excited to such an extreme rate that they don't 'shut off' immediately).

do you see fast things as a blur? if so, I am right.
Click to expand...
Nope.

or, look at it this way, if your eye could (which it cant) work on a frames basis (or even your brain) then there should be some magic setting on your monitor, somewhere above 100hz that you can select, where the screen would flicker like mad, and have a slowly progressing horizontal line, same as when you try to film a pc monitor at anything other than 50hz or 60hz (depending on yoru mains frequency)
Click to expand...
Partially correct. Correct enough that I should recant my comment about the reverse motion effect occuring naturally. But this still passes no comment on the way the brain works.

but there isnt such a magic setting. oh no sir, when things happen too fast, our eye still captures it and sends it on and what we see is a blurry image. not a flickering image.
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True, but not for the reasons you claim.

You originally said :

only on tv...only on tv...our brains/eyes dont work in "frames"
Click to expand...
So why are you now totally ignoring how the brain works?

What I said :
Exactly. At some point the wheels move at a speed where they move more than halfway between the two spokes between the time slices that your brain processes the image.
Click to expand...
Is correct and in fact backed up by ALL the links you posted, even if you are technically correct about the eye's analogue nature, because the statement itself made absolutely no indication whether the timeslices were natural, or whether they were artificially induced.

So I'm still waiting for you source the claim that the brain doesn't work in 'frames' or 'time slices', which is what I was originally asking you to prove.
 
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ah, i see...now the phrase used is time slices.....interesting.

that sun bit you mentioned is called persistence of vision. go look it up.
i'm sure you are just being a bit pedantic here for the sake of it.
 
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