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?

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    1) because movies are written by screen writers, not by scientists. 2) Because it looks and sounds "cooler", much like having noisy explosions in space. – Paul L May 23 '16 at 17:55
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    Also, 3) Because plenty of movies don't think the speed of sound is as fast as light. There are plenty of war and action movies where we see things happen before we hear them. Although I admit there are definitely some movies where there is a delay in the sound for certain explostions but not others - probably for purely cinematic reasons. – Todd Wilcox May 23 '16 at 17:56
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    So basically, it's only done when it's a plot device. I wonder how many movies that explain it, are consistent with it outside of the scene where it is explained. – cde May 23 '16 at 21:22
  • Do you have some specific movie or movies in mind? If not, this question is too broad, because some movies do this while others adhere to the accepted physics. – TylerH May 24 '16 at 19:34
  • @TylerH When this was first pointed out to me I struggled to think of any examples where it wasn't true. So I think the question is asking about a very broad, if not quite universal, movie trope. – matt_black May 24 '16 at 19:43

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.

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    I don't think the issue is one of disregarding physics, but rather one of shifting POV. The "double explosion" often occurs as a result of a shift from a third-person omniscient POV (focusing attention on the explosion, listening through an imaginary microphone there) to a character's POV (listening through an imaginary microphone nearer the character). – supercat May 23 '16 at 21:15
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    In mythbusters, the team lamented that post-production chose to align the visuals and sounds of several of their larger explosions rather than playing them out in real time (from a mile away). They commented that it made things look very different from how they actually played out. Even in a science show, this sort of adjustment of sound shows up! – Cort Ammon May 24 '16 at 1:50
  • @CortAmmon not sure if that's the same as the "Sounds Bogus" episode, if you know which episode or an interview it is from, please let me know. – cde May 24 '16 at 2:06
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    Real gunshots are also considered too loud for delicate recording equipment... Unless you like taking the risk of blowing out your mics. – Catija May 24 '16 at 4:35
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    having heard real gunshots (with hearing protection) from the minimum safe distance (behind a gun), I can totally understand why real gunshots are too loud. Also, they're boring sounding, like a loud pop rather than the somewhat elongated sound they have in the movies. – Journeyman Geek May 24 '16 at 9:33

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.

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    This entire question is complaining that the delay is rarely there... even if it's from the characters' POV... – Catija May 23 '16 at 21:16
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    OP knows why it happens like that, he's asking why isn't it fixed to match reality. You leave off the why not. – cde May 23 '16 at 21:18
  • @Catija: It's possible, and not terribly uncommon for a director to try to shift the audience's attention to different places even during a continuous camera shot, or to have a soundtrack that combines multiple virtual microphones. The thing I would consider odd would be if characters reacted to the sound (e.g. "Did you hear that!?") without a noticeable delay (as opposed to e.g. reacting to a flash and ignoring the sound). – supercat May 23 '16 at 21:23
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    @cde: Only in those cases where the audience is supposed to be hearing what a particular character is hearing is the location of the "virtual microphone" fixed. Otherwise it will generally shift with the desired audience focus. – supercat May 23 '16 at 21:25

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 to 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 and eye and an ear at the same place.

So in many modern movies it goes like this:

  1. Charaters 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.

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