Why do movies think the speed of sound is as fast as the speed of light?

Question

In action movies where things blow up or heroes are shot by snipers from a long distance, or where fireworks are shown, the sound and the flash of the thing making the sound usually occur simultaneously.

But sound only travels at a little over ~300m/s (this is about a million times slower than light). So explosions happening a safe 1,000m away will be heard about 3 seconds after they are seen; the flash from a distant gun will be seen well before the bullet hits or the discharge is heard. This is discussed, for example, in this YouTube video.

We don't insist that movies have to obey the rules of physics (we wouldn't have many superhero or SciFi movies if we did). But surely there is some room for realism for the simple stuff?

Why don't movies have a realistic speed of sound?

Answer 1

Because they fail test screenings. The general movie audience does not respond well to realistic depictions of explosions or cannons at a distance.

From the other SE, a partial excerpt of a great answer (that you should upvote as well):

Most film is art, not life/reality. Sound designers have to match the visual art on screen with the sonic art of their mix, and that usually includes a certain amount of "realer than real", suspending some of what we "know" about physics in order to tell the story in a way that translates to this 2-dimensional, 2-sense media.

Case in point, when we see some big impact on screen, such as a lightning bolt, we as audience members expect to hear it, much as we expect to hear the starship Enterprise whooshing by at warp speed, even though there's no air (and thus no air movement to create the "whoosh") in space. We also expect to see it match up with the visual effect on the screen; this both increases the overall impact of whatever just happened, and keeps us suspending disbelief that we're not watching a movie, we're looking through a window into another universe. Hearing a delayed sound effect might jerk us out of the director's world and wonder what's going on with the media system, and once we've left, it's hard to get us back.

The speed of sound delay would also, even in the ideal, make us feel further from the action. You see a lightning bolt hit a distant mountainside. 3 seconds later, you hear an ear-splitting thunderclap. While that might be totally accurate to the scope of the shot, in the ultimate ideal of the audience totally suspending reality and believing they're physically in this world you've created, they'll be thinking "damn, I'm glad I'm not over there", when in cinema the location of the camera can jump from place to place as needed, and the very next jump cut could put the audience right in the thick of the aftermath of that thunderbolt.

You'll also see sound designers architecting a "double explosion". They'll engineer a big, powerful explosion sound right at the moment you see it, then the visual effects guys will put in a massive shock wave, and as that passes by the camera the sound designers will add an extra "whoosh". Or, with a slight delay (and a really fast shockwave), the designers will engineer a small crescendo to the big boom, so you know it's coming. These little audio tricks aren't what you hear in comparable "real life" events, but then again, neither is the sound of a movie punch or a movie explosion compared to the real thing.

Slightly related, the Mythbusters episode 165 "Blow your own Sail" had a clip titled "Sounds Bogus", where they compared actual explosions to movie ones:

In a recent episode of "MythBusters" on Discovery Channel, show hosts Adam Savage and Jamie Hyneman worked with Meyer Sound to investigate whether the "big bangs"—specifically gunshots and explosions—heard in movie soundtracks are sonically faithful to the real events. The episode, entitled "Blow Your Own Sail," was aired on May 11, 2011.

For the recent episode, [the hosts] reunited with "Honorary MythBuster" and Meyer Sound Staff Scientist Roger Schwenke, with the goal to compare highly accurate, on-the-scene audio recordings to their Hollywood counterparts.

"Real gunshots and explosions are too loud to reproduce exactly in a theater and would damage people's hearing," says Schwenke. "So the question is: are movie sound effects simply the real sound played back at a safe level, or have they been changed in ways other than just level."

The results showed that the sounds are faked in movies. So aside from making the sounds seem "real", there's a safety concern too.

Answer 2

The position of the microphone is often independent from the position of the camera, both physically (the location of the apparatus used for shooting) and subjectively (the perceived location of sound). It's common, for example, for the audience to be able to hear characters speaking even when the camera is so far away that there's no way their voices would be audible, and for the sound of the characters to remain relatively consistent while the camera moves closer. Typically, the audience should hear sound as though from a microphone planted near the intended focus of the audience's attention.

If a group of characters were speaking near the camera and a flash of lightning were visible in the distance, the audience should hear the thunderclap when and how the characters would hear it. If the characters are in an underground bunker and see the lightning flash on a closed-circuit monitor, for example, the sound should not only be delayed, but muffled. If, however, the lightning strike is not observed by a character but only by the audience, the subjective microphone should be very close even if getting a good bunch of storm clouds in frame would require the camera to be far enough away that it would induce a noticeable lag.

Answer 3

I would compare it to books, where you have two kinds of narrators: A character-based narrator, who is bound to the physical limitations of the character. And an omniscient narrator, who is free to even read thoughts and be at different places at the same time or make connections across centuries.

So in movies you can have two kinds of camera. A character based camera, which sees things from the perspective of a character. Far away sounds are muffled, he can only guess who is hiding in the dark and far away explosions are usually depicted in the realistic fashion of hearing the blast wave much later.

But in many cases the movie will switch to the omniscient/omnipresent camera perspective. So you can watch a huge battle from high above, or see an explosion from a perspective which no character could have, like watching a space battle from the outside. In this case we are not a physical observer with a predetermined location and physical limitations. The camera represents an omniscient observer, which can hear characters speak in their space ships in real time, and can hear an explosion in exactly the same moment it happens. Like we can hear the battle-cries of soldiers on the ground in a huge army as they open their mouths.

Because our focus of attention / our virtual presence is right there at the center of the action. Our virtual persona is standing right beside the bomb and can hear the explosion instantly, while also being able to observe the whole scene around like a godly being would, not bound by having an eye and an ear at the same place.

So in many modern movies it goes like this:

1. Characters sitting together in shelter (character camera)
2. The bomb ticks it final tick (omniscient camera right at the timer)
3. The bomb explodes cineastically - instant, clear explosion sound (omniscient)
4. The characters hear the muffled blast wave with a delay (back to character camera)

So we even hear the explosion twice, first from the omniscient instant perspective and than a second time with the limited sensors available to the characters from the character camera perspective.