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Sound effects are a great way to add "color" to your video and add necessary sounds that you were unable to get on location. They are synthetic and are created specifically to match the events on the screen. If you do include these you could either record real sounds that would fit with the action or you could use synthetic sounds that are more designed to elicit an emotional response that seem realistic. There are a number of web resources to find pre-recorded sound effects, but it is important to find out whether or not they are copyrighted. Usually copyrights are not held on sound effects because it would be difficult to prove ownership, but if they are very unique it is possible for the author of the sound to maintain licences on it.
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When trying to get all the sounds recorded or inserted later to sound as though they fit and are realistic, you need to consider sound fidelity. This is the concept that looks at how faithful the sound is to the source that creates it. For example, if you hear a hammer hitting a nail it is important to see it happening as well if you want to maintain sound fidelity. Non-fidelity sound is when you hear a certain sound and see a conflicting image, like if you heard the sound of a hammer hitting a nail and then saw a two people hitting each other. This is another way that sound is used to elicit an emotional response instead of trying to maintain realistic continuity.
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Another aspect that must be considered when using sound in your digital video project is timing. This can be hard because the time represented on the sound tracks for a video may be different than the actual events on the screen. One of the ways to avoid this is through the use of "Synchronous" and "Non-Synchronous" sound. Synchronous sound is Diegetic sound that matches the source perfectly, like when you see a car driving and you hear the acceleration and engine in perfect unison with the movement of the vehicle.
Non-Synchronous sound is Diegetic sound that does not match up perfectly, like badly dubbed foreign films. Each one of these styles is used to ensure a certain effect, which is decided by the filmmaker. The same type of thing is present with the use of "Simultaneous" and "Non-Simultaneous" sound. Simultaneous sound is sound that takes place at the exact same time as when the sound is being created, as with Synchronous sound. Non-Simultaneous sound takes place either before or after the source of the sound is making it. An example of this is if you see someone turn a key in an ignition and then several seconds later you hear the car start. "Sound Advance" is Non-Simultaneous sound that happens prior to the event, and "Sound Follow" is sound that happens afterward. There are a number of reasons a producer and editor might choose to do this, mainly based on stylistic ways of offsetting the audience or changing their expectations. One of these purposes is called a "Sound Bridge" and is mainly used to smooth over the transition between two video clips where there is an awkward delay between the visuals. This is the same principle as "Split Edits," which do this in a very minor way to smooth together visual edits and maintain visual continuity.