We were doing some physics problems with sound and the Doppler effect today. One problem had two scenarios where one was source moving away and the other was the listener moving away. The math gave two answers that were close but not the same. I don’t understand how they can be different when speed is relative.
In: Physics
Sound is a pressure wave in air, and that means the air is a necessarily privileged reference frame. While you might be able to consider the moving object as being stationary that would imply a bulk movement of the air in relation to it which would impact the calculation.
Sound moves at… well, the speed of sound in that substance. If the substrate of the sound, typically air, is moving then the speed of sound would seem to change depending on the direction.
It’s been a long time since I took physics; could the discrepancy be from the relative speed difference between receiving the sound waves while standing still (343m/s) and while moving away (say at 3m/s making a difference of 340m/s instead)?
The source of sound traveling away won’t change the speed in which sound arrives at the stationary target, but if the target is moving would that create your relative difference?
Because when something moves, it warps the sound wave around itself, whereas when you move, you just perceive the sound wage differently.
There is a great episode of Malcolm in the Middle where Dewy explains this to one of the kids in his special needs class, when he realizes he’s just as much a genius as Malcolm but decides not to tell anyone.
The basic notion of a doppler effect is that when a source and observer move towards one another, each successive wave crest/trough is generated closer so the frequency rises. The same principle applies for moving further away. In this case, it doesn’t matter whether the source or observer is moving. Indeed, they are fundamentally the same thing because all you have is a relative velocity between the two.
This gets muddied when you add a medium because you have two relative velocities instead of one. You have the relative velocity of the observer to the medium and the relative velocity of the source to the medium. So you have a doppler effect from source to medium, multiplied by a doppler effect from medium to observer.
Latest Answers