If you sit in a car and it's driving $300{m \over s}$ and you clap your hands for a very very short sound, will it's wave be $600{m \over s}$ fast, because it adds the speed of the car? Or will it be $300{m \over s}$ because it is just the point of origin that matters and from that point it travels away?

If the answer of the title question would be yes, that would also mean that light can also travel faster than speed of light, right? (I assume sound and lightwaves can be compared as both contains electeomagnetic waves)

  • $\begingroup$ No, the velocities are not additive. See sonic boom $\endgroup$
    – lemon
    Jan 25, 2018 at 20:04
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    $\begingroup$ Is your question about sound or about light? Sound waves are fundamentally different from EM waves/light. The similar mathematics doesn't mean they are physically similar. And pure sound is not electromagnetic. $\endgroup$
    – Bill N
    Jan 25, 2018 at 20:04
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    $\begingroup$ Sound waves are like what most physicists/philosophers thought light was, before Einstein & Co. Sound must move in a medium, similar to the "aether" that was once hypothesized for light. But light is different, and does not require an "aether" to propagate. $\endgroup$
    – Hot Licks
    Jan 25, 2018 at 20:33
  • $\begingroup$ This example explains why physicists considered the aether to be a stationary medium through which light propagated. It turned out this mechanical model didn't wor and the aether concept was replaced by the field. $\endgroup$ Jan 25, 2018 at 21:15
  • $\begingroup$ @BillN, they are more similar in behaviour than you might think. For example, sound waves exhibit time dilation when the air moves around stationary objects. $\endgroup$
    – Steve
    Jan 25, 2018 at 22:11

2 Answers 2


The speed of sound of 340 m/s is measured with respect to air. So here we can mention two cases:

  1. If you are inside a car that is moving at 340 m/s in the highway and the windows are closed (so that the air inside the car is also moving at that speed), the sound of your clap will move at 680 mm/s with respect to the ground. This is because it moves at 340 m/s with respect to the air inside the car.

  2. Imagine the same situation but with a car that has no roof nor windows. Then you, inside the car, will find that the air moves at 340 m/s backwards. If you clap in this situation the sound waves will move at 340 m/s in the air, but because this air moves at 340 m/s backwards you will see that the sound waves do not move at all.

The second case is exactly what happens in this picture: enter image description here When the supersonic aircraft is moving exactly at (or very near) the speed of sound (both with respect to the air), the waves travel at the same speed and they accumulate to form that peculiar cone.

In the case of light the case is different because light speed is 300000 km/s with respect to space itself. And you cannot move with respect to space, you move in space with respect to other things. Motion is relative. So the speed of light is the same for everyone independent of the speed of the observer (or the source).

  • $\begingroup$ Worth noting that the air will only move 680 m/s if you ignore relativistic effects. Realistically, relative to the ground frame it will be moving a little slower than 680 m/s if you account for relativity. This might help OP wrap his head around why this fails when you try to apply it to light speed. $\endgroup$
    – JMac
    Jan 25, 2018 at 20:32
  • $\begingroup$ @JMac Thanks for the note, apart from your interesting fact I realized (with the help of the answer below, above and your comment) that sound and light is very different. $\endgroup$ Jan 25, 2018 at 20:36
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    $\begingroup$ The speed of light is not the same for everyone independent of their speed - it is the same locally for everyone (relative to their local clock). That's a subtle difference. $\endgroup$
    – Steve
    Jan 25, 2018 at 22:22
  • $\begingroup$ Wrong, on many counts. The sound waves in the car do move. postulating a car with no doors or windows is equivalent to just moving a sound generator (like a speaker) through the air at the same speed. Sounds waves generated will move backwards through the air mass at exactly 340 m/s, although since the speaker is moving forwards at 340m/s, the speed of these backwards sound waves will be 680 m/s relative to the speaker. Sound waves moving forwards will also move through the air mass at 340m/s, but as the speaker is also moving forward at the same speed the relative speed will be zero. $\endgroup$ Jan 25, 2018 at 23:20
  • $\begingroup$ The speed of Light is different. It will be measured at exactly the same speed (186,000 miles/sec) no matter how fast or slow the emitter, or the observer is traveling, with respect to each other or to any frame of reference. $\endgroup$ Jan 25, 2018 at 23:20

To answer the second question: sound waves are not electromagnetic waves, they're pressure waves.


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