explainlikeimfive
ELI5: Why do "musical roads" not have Doppler shift?
[removed]
Discussion
The video you linked does sound like it has a Doppler shift though.
This post is like a trap for AI users. The user posts a video that obviously shows Doppler shift and all the commenters just answer the question as if the shift isn't there, and all give completely wrong answers. AI will explain whatever you want to hear, it will give good arguments for wrong answers.
Each bump produces a pressure wave, but the source of each pressure wave is in a different place so the main melody will be Doppler shifted.
That was unintentional on my part but now you mention it...
The music is coming from the road, which is stationary. The music is moving, but the sound isn't. So you don't get a Doppler shift. Imagine a trumpet player in a car travelling along the road vs 100 trumpet players lined up on the road and playing a note one after each other.
It doesn't really matter if the sound is coming from a stationary thing, just that the source of the sound is moving. Imagine each groove in turn sending out a pulse of pressure that moves slightly faster than each groove making its pulse. The pulses will be bunched up going one direction, and spread out going the other direction, which is the Doppler effect.
That's backwards. The car vibrates, not the road. It's all about reference frames. If the source of the sound is moving toward or away from you, you get a doppler shift. If you are moving towards or away from a stationary sound source, you will also get a doppler shift.
If you are moving at the same speed relative to the source of the sound, no doppler shift.
If you're in the car, you are moving the same speed as the source of the sound, so no shift. Someone on the side of the road would hear the shift.
Except the OP is literally describing a bystander on the side of the road experiencing no Doppler shift. You're wrong, the car is not vibrating on a musical road. The sound is created at the interface between the wheel and the road, as the wheel hits each carved ridge in the road that it is creating a pulse, which when combined with many other pulses, creates an audible frequency. There is no single origin of the sound moving towards or away from an observer so there is no way for the sound waves to be compressed or stretched. There are many individual origins that are created wherever the rubber meets the road. The commenter above you'd analogy of many trumpeters arranged along the road seems quite apt actually.
watch the video with sound on there is clearly doppler shift.. lol
I hear a distinct Doppler shift, especially as the second car passes (the only one where the clip captures a car both approaching and retreating).
The wave.crests are created by the bumps on the road, and are tuned to sound right inside the car. When the car is approaching the stationary listener, the wave crests are bunched together because each bump is closer than the last, so the note is higher than what those inside the car hear.. When the car is retreating, the wave crests are spread out, so the note is lower.
The sound is created at the interface between the wheel and the road, as the wheel hits each carved ridge in the road that it is creating a pulse, which when combined with many other pulses, creates an audible frequency.
This is like saying that the sound on a guitar is produced at the interface of the pick and the string. That's what initiates the vibrations that produce the sound, that's not what's producing the sound. The string still produces the sound.
The car is what is vibrating and causing the air to move, thereby making sound.
There is a doppler shift for the bystander. Even if what you were saying were true (it's not) and the road were making the noise, that source of noise would still be moving toward or away from the bystander, thus you would still get the doppler shift.
I was gonna say... Listening to the video - there's a pretty clear doppler shift based on when the car passes the camera...
So, I do think you hear it?
You dont hear it inside the car because the car is triggering the source of the audio so its moving with you, but on the side of the road that changes.
I don't know what you mean. In the video you linked there is still a very clear drop in pitch when the vehicles go from approaching the camera to moving away from the camera. Maybe you are today years old when you found out you're tone deaf (medically).
Each indent in the road produces one sound "shockwave". When approaching the camera, these shockwaves are closer together because the car travels in the same direction. When past the camera, the car is moving in the opposite direction of the shockwaves, so they are spaced further apart.
Am I the only one who can hear a strong doppler shift in the video?
Yeah, I agree with you. I can hear a bit of it on the approach, and those last notes from the standing perspective are super long and drawn out.
I hear a doppler shift in the video... BAH BAH BAH becomes BUH BUH BUH as the car passes the camera
Because the road is playing the sounds not the car
Car = moving
road = stationary
The speaker (source of the noise) is not in the car. So no doppler.
Actually the source of the noise is coming from the car. The car vibrates, not the road. If you are in the car from your frame of reference, you aren't moving relative to the source of the noise.
Someone standing on the side of the road would hear a doppler shift.
It's why pilots that are supersonic can still hear stuff around them, or why being in a police car with the sirens on you can't hear the doppler shift. But if a police car drives by you, you do hear the shift.
The part of the tire hitting the bumps on the road is stationary too. (otherwise it would be slipping)
OP: take a good listen to the final notes from the standing perspective, and tell me that's not Doppler shift.
Super interesting and unintuitive question btw. I learned something new today.
If you watched the video they linked, the only thing to be learned is that OP doesn't understand what a Doppler shift is. You can clearly hear it in the video.
A doppler shift happens because the sound is produced by a moving object. In this case, the sound is produced by carved shapes in the road surface, which aren't moving.
Compare it to running a mallet down a xylophone: the notes sound the same no matter if the hammer is moving left or right.
the way that grooves in the road and a xylophone make noise is completely different. The xylophone produces a whole note from itself, the grooves only make part of the waveform each, you need many to make a note.
Doppler shift happens when the source of the sound is moving away or toward you. You hear a doppler shift from the engine because it is moving, but the music effect comes from the road, which is not moving relative to a bystander
I don't think it comes from the road, isn't it coming from vibrations in the tire and car suspension?
Vibrations between the tyre and the road, both of which are stationary
Only a very small portion of the tire is stationary relative to the ground. The whole tire will vibrate.
Your submission has been removed for the following reason(s):
Loaded questions, and/or ones based on a false premise, are not allowed on ELI5. ELI5 is focused on objective concepts, and loaded questions and/or ones based on false premises require users to correct the poster before they can begin to explain the concept involved, if one exists.
If you would like this removal reviewed, please read the detailed rules first. If you believe this submission was removed erroneously, please use this form and we will review your submission.