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--- doppler_effect [2026/02/09 10:00] – ultracomfy
+++ doppler_effect [2026/02/09 10:51] (current) – ultracomfy
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 ====== Sound Physics ======
-What our ears pick up are vibrations of <del>ear</del> air molecules.
+What our ears pick up are vibrations of <del>ear</del> air molecules. Vibration of air molecules means that air molecules reverberate, physically, between two or more locations. The way our ears "hear" is by measuring how often that happens, ie. "reverberations per unit of time". 7kHz for example is 7000 reverberations in a second. Like waves on water, air reverberations expand through the air - that's what the speed of sound is: Mach 1 - 1234.8km/h.
+Now, the cool thing is that if a sound emitter, ie. a thing that makes air molecules vibrate, "chases it's own vibrations", it too has a doppler effect. Even if your car generates 7000 reverberations per second, if your car drives towards someone then the total amount of reverberations arriving at the listener per second will be higher than 7000. Just like the water in the pool, the vibrations will begin to bunch up behind each other and then arrive all at once at the listener.
+Now if you've ever played with a frequency tone generator, you know that higher tones correspond to higher frequency. Since your ear only measures the total number of vibrations, it does not matter if what you're hearing is 5000 stationary reverberations or 3500 reverberations that move towards you - it will sound the same to your ear - depending on the speed at which the moving sound emitter is moving of course! To make 3500 reverberations sound like 5000 you would have to figure out some fancy maths to determine an exact speed.\\
+Either way, if a sound source is moving towards you, what happens in your brain is that your ear picks up a lot of reverberations coming it at once, and therefore you hear a higher pitch of the sound, in accordance with the frequency at which the reverberations arrive at you.
+====== Relative Movement ======
+The key term here is "frequency at which the reverberations arrive at you". This is important because this isn't really about the speed of the sound emitter - it's only about how many revererations arrive at your ear. Say you are the car and drive towards and past a construction. The construction isn't moving, but if you are racing into the reverberations emitted by the construction then you will pick up much more than you would if you stood still. Then, when you go past it and drive away, you are basically racing away from the reverberations and will pick up less each second. Lower frequency means lower pitch.
+The last example here is //two// moving things. Two cars next to each other will hear each other normally as they aren't moving towards or away from each other - they have no relative movement. If both the water puncher and the watcher moved in the same direction, the waves would arrive at the watcher at their original frequency.
+Most things in our world have movement relative to us, even if only on a molecular level, so the doppler effect is around us all the time. It's just that in most cases the relative movement is so negigible that we don't consciously pick it up (or our ears aren't precise enough). But, doppler is good and useful! It is useful for spacefaring radio signals from spacecraft don't just transmit their radio signal but can also be analysed to give us useful information about their movement relative from us. NASA uses the doppler effect to cross-reference the speed data of their spacecraft.
+And there is so much more! Have you heard of relativistic doppler? Grand worlds will be opened up to you with the amazing data gathering of other planets and stars you can do by applying physical principles that go beyond the elementary conceptual understanding I can provide here. Hell, I didn't even give you the math, that's how little I actually know. Time will tell if I am ever gonna learn more about this.