I’m trying to understand the concept of relative velocity in the context of sound waves. When sound travels from a source to a receiver, I’m confused about how to calculate the wave’s velocity relative to the source (vwm). The equation Vfm = 80 m/s represents the velocity of the medium moving at a constant relative to the medium. Now, if I want to find the velocity of the medium moving from the source, why do I have to add up the velocity of the wave relative to the source and the velocity of the source relative to the medium? Also, when should I use the velocity of the medium relative to the source, and when should I use the velocity of the source relative to the medium?”
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>Now, if I want to find the velocity of the medium moving from the source, why do I have to add up the velocity of the wave relative to the source and the velocity of the source relative to the medium?
It’s the same process as if you were to hop on a train doing 50, and ride a bike on top of it doing 10. The speed relative to the ground is 60, the sum of both velocities.
Sound propagates as a wave. A sound wave propagates through the medium of air. The wave’s velocity relative to the sound source is only one part of its velocity, the other being how fast the wave itself is propagating in the medium, and that is determined by the properties of the medium itself (air pressure and temperature).
I’m not quite sure what you are trying to say here but the formulas for using the doppler effects are quite simple.
If you wish to calculate the observed frequency; f_obs = (1+v_object/v_sound)*f_0
With f_0 being the original frequency. V_object is the difference in velocity between the source and the observer, with moving closer to each other being positive, and moving away being negative. v_sound is the speed of sound, or you can even change it to speed of light if you want to see doppler effects for light.
For calculating the speed of an object based on the frequency you measure(assuming you know the original frequency);
f_obs-f_0 = (v_object/v_sound)*f_0
I think the key take away is that you use the difference in velocity, not absolutes. Motion is relative. But if you know the motion of one object, you can calculate the motion of the other if you have all this information.
I can’t tell if you understand this already, but when you’re thinking about the concept of relative velocity you’re asking about:
A sound wave is a wave of pressure moving through the air. If you were standing still, the sound wave from the source will pass by you in the air. The pitch that you hear the sound at is a result of the frequency of the wave, or technically how fast the air pressure is varying around your ears as the wave passes by. Now you can think of a sound wave behaving just like a wave in the ocean. If you’re just in the water floating, the wave passes over you quickly. If you were to swim in the direction of the wave you’ll be “in it” for a longer time. This is basically the Doppler effect – when you’re moving away from the source, as the sound wave is passing you you are moving along with it, so the pressure is changing a bit slower around your eardrums than if you were standing still and it drops the pitch.
I don’t know if you already understood that, sorry if you did, but it might help in understanding why those equations from the other great posts work.
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