The following lesson was created by Vimeo for its Vimeo Video School . It’s used here with permission. Look for a new lesson each week.
A quick Google search for the term “room tone” brings up, among other things, this photo of Vimeo’s very own Casey D.:
Photo via Andrea Allen on Flickr 
You might be asking yourself the following questions: What’s Casey doing all alone with that microphone? And why does he have his game face on?
The answer, my friends, is simple: Casey is performing an often overlooked, yet crucial role of film and video making: recording the presence, or tone, of the room. Later, during the editing process, Casey will use this recording to help create the sound track of his video, intercutting it with dialogue to smooth out any rough or jarring editing points.
Why do I give a hoot?
One of the most important aspects of film making is achieving the illusion of absolute continuity. As video creators, we know that movies are made up of many takes. Sometimes what ends up as a single scene is filmed over the course of several days, yet it is imperative that each day, every condition of the set, costume, etc. is identical. That way, the different takes can eventually be edited together seamlessly. Sound is a critical factor in this equation!
What you think of as a silent room is never actually silent– there may be the subtle buzz of a fluorescent light, or the distant rumble of traffic outside. The way that these sounds bounce around in the space against objects, walls, the floor and ceiling, plays a huge part in what the room tone, or presence, sounds like. This is almost always unique, and it is this distinct “aural fingerprint” that you must be conscious of when making a video.
Recording 5-10 seconds of room tone gives you an audio texture to lay behind dialogue from several different takes, relieving the risk of ambient inconsistency. It also has the bonus of adding realism to the environment on screen!
How do I do it?
During production of a shoot, simply place a microphone in the same position and orientation as the original dialogue recording, and record 15-30 seconds without any of the actors or crew are speaking or creating any additional noise. Like dialogue, presence is usually recorded in mono.
We put together a little video to help clarify and demonstrate room tone and ambience. Without further ado, feast your eyes upon our awkwardness. And learn, my friends, learn!