Lightning and Thunder

We see the flash of distant lightning and hear its thunder roll simultaneously in the movie. But we all know that the lightning flash arrives before the thunder in the real world, there being about five seconds of sound delay for each mile of distance to the lightning. Suppose you were in charge of a battle scene in a war movie. When editing the scenes of the explosions on the battlefield, how would you ensure the correct experience for the theater patron?

Answer

Let’s consider identical explosions on the battlefield at two different distances from the observer (i.e., the camera) but seen simultaneously. The sound intensity emanating from the farther explosion should be less loud compared to the closer one, and the sound of the farther explosion should be delayed more with respect to its light flash than for the closer explosion. The extra distance affects both the sound intensity and its time delay. Depending on the distance and the temperature gradient in the air, there could be additional effects, such as different frequencies having slightly different speeds and/or paths en route.

For example, if the closer explosion is one-fifth of a mile away and the farther one is two-fifths of a mile away, the differences in arrival times should be clearly heard. The sound from the closer one begins to be heard one whole second after the light flash and the weaker sound from the farther explosion should be heard one second later, two seconds after the apparently simultaneous light flashes. Count these time delays aloud and you will immediately realize that most battlefield scenes do a false portrayal of the audio timing. But who really cares? The dramatic license enhances our viewing experience of the fantasy world of the cinema! But experienced military people know the difference.

Many more audio violations can be recalled from the movies. We simultaneously hear and see airplane crashes in the distance, cars falling and crashing into a chasm far below, jet airplanes passing overhead with no sound delay, etc., all for the benefit of a movie going public who should know the difference.

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