Many times, I had already had the impression that what I was hearing was desynced from what I saw.

Could this be caused by the speed difference between light and sound? Or is sound still fast enough that this shouldn't be possible without any tools, and what I'm observing is caused by something else?

Most commonly I'm observing this, when I'm in an area where it is ..... quiet quiet.

And seeing someone walking on solid ground at like ~300 meters away with something sandy on it gives a crunchy noise when the shoe touches the ground and crunches the sand. But when observing this, I just hear the noise when I see the foot raising.

  • $\begingroup$ weatherwizkids.com/weather-lightning.htm $\endgroup$ – CoffeeIsLife Mar 10 '16 at 9:18
  • $\begingroup$ Sound travels slowly for human standards. You easily notice the delay if you're observing something that makes a "short" sound more than 50-100 meters away, e.g. a man hitting a pole with a hammer. If the sound is more "extended", such as the sound of a footstep in sand, it is more difficult, since you don't know exactly when you'd expect the sound to be hearable. $\endgroup$ – pela Mar 10 '16 at 9:19
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    $\begingroup$ Look at the sky in a thunderstorm... $\endgroup$ – CoffeeIsLife Mar 10 '16 at 9:19
  • $\begingroup$ @user97554: not kinda usefull, since I'm interested about why it took that long to get an idea of what is sound like on an pre mideval point. Where as I know of weather lightning they simply considered it beeing 2 diferent things caused by deitys. $\endgroup$ – Zaibis Mar 10 '16 at 9:27

Light travels at 300,000 km/s while sound travels only about 330 m/s (1/3 km/s). In other words, light travels one million times faster than sound.

Say you see something at a distance of 1 km. That means it really happened 1/300,000th of a second ago. In other words, to your eye and brain, as far as you can tell you see it at the exact time it happens. However, sound from that event will take about 3 s to reach you. So the sound will be clearly delayed. \Even a difference of 0.1 s (33 m) is noticeable if you pay close attention.

You can use this to measure the distance of lightning (or other events). When you see the flash, start counting seconds. When you hear the thunder, divide the number of seconds by 3 and you will have the distance to the lightning in km.

The speed of sound is also the reason why echoes are delayed: the sound has to travel to something that reflects it, and then back to you. If the reflecting wall is 330 m away, you will hear the echo after 2 s (2 x 330 m).


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