1 The speed of sound
In movies, sound effects are always synchronized with images. Quite normal, you may think?
This is to forget that the speed of light is a million times greater than the speed of sound. We hear sounds with delay when we are far enough from their source. This allows to count several seconds before hearing the sound of a lightning bolt.
When you attend fireworks, even at the front row, you always notice that the sound comes after the explosion.
In The Fellowship of the Ring, as in most films, sound propagates at the speed of light.
2 The sound in space
It is a well known fact: in space, as there is no air, sound does not propagate. However, most movies pretend it does.
Only a few recent productions like Gravity adopt a more realistic vision of sound in space, by making the subjective sound of the characters heard inside their spacesuit.
In Armaggedon, no realism at all. Asteroids whistle and rip the air … in space.
3 Hearing inaudible sounds
Have you ever chatted with someone in a nightclub? Generally, you are forced to shout in the ear of your interlocutor who barely understands one word out of two.
In this scene of Matrix, no problem. Despite the loud music, the characters talk from a distance of one meter, without straining their voices. Neo even hears the delicate steps of Trinity approaching.
4 Non-existing sounds
To give a danger feeling to a laser beam, nothing beats a good shrill sound, like in Goldfinger .
Obviously, a laser produces no sound. Except maybe if it has a vibrating transformer or a fan to cool it.
UI sounds (electronic sounds emitted by machines) are a classic of science fiction. Although they exist in reality, they are overused in cinema to reinforce the futuristic or technological feeling of a device.
In this Alien scene, the keyboard and screen display generate a lot of beeps. In reality, who would bear to work with such a noisy computer?
This sequence from Troy is full of swords that scrape and resonate… sometimes even without being touched.
To make a sound more efficient, we can also simplify it. For example, keeping only parts of a sound effect can give it contrast.
In this scene from Kill Bill , the weapons that spins continuously should logically make a continuous sound …
7 The imposture
It is perhaps the most widely used trick. We often replace sounds with more detailed or more spectacular effects. But sometimes we associate an image with a totally different sound.
Example: in reality, the silencer of a firearm only slightly reduces the detonation. To differentiate this sound, sound designers have invented a tubular and resonant sound effect, which recalls the cylindrical shape of the silencer. This sound, which has become a cliché, is far from reality. But as we do not hear it every day, it has become the only reference for many people.
The french movie Les tontons flingueurs delights us with very creative silencer sounds.