Now imagine you’re watching the same movie and you come to the same pause in the dialogue and the soundtrack goes completely silent. Total silence is jarring and unnatural, and your first instinct would be to think there was something wrong with the audio equipment you were listening to. Your involvement with the scene would be totally lost.
There is a huge difference between ‘ambience’ and ‘silence.’ Ambience is also often referred to as ‘presence’ and for all intents and purposes the terms are interchangeable. Ambience is so important to the flow of a movie that before a scene is shot, a monoaural recording of the ambience of a room – the room’s tone – is made. During mixing, this room tone track is added to the dialogue track so that when there is a pause in dialogue or sound effect, the dead, clinical silence that would occur without a room tone track doesn’t suddenly pull the listener away from the scene.
It’s the room tone that adds to the tension of a scene. Imagine how unreal your favorite creepy horror movie or crime show would seem as the main character crept down a flight of stairs in the dark with just the small beam from a flashlight searching as something evil lurks below in the dark – in total silence. In this case ‘total silence’ refers to the complete and total absence of sound, which would occur without that pre-recorded room tone track.
Sound directors walk a fine-line between just enough ambient sound to make a scene realistic, and too much background noise that detracts from the scene.
In your home theater, there is also a room tone. The swishing of the air around you, plus the HVAC unit down the hall, maybe a distant car horn and the general disturbance of air that constantly surrounds us all factor into the tone of your listening room. When recording a scene, or a song, or a podcast, room tone is crucial to the feeling of the recording. On the flip side, when we listen back, too much room tone spoils the experience. Obviously, it’s not possible – or desirable – to rid of room of all sound. You wouldn’t last long living in an anechoic chamber, but you also don’t want the ambient noise in your theater to detract from what you are listening to.
We want to mitigate as much background noise as possible so the tone of the room in the scene on the screen shines through. Try to isolate your room from that HVAC unit (usually the biggest offender) and try to make the walls as impervious to outside sound as possible. These examples are tricky because you need active ventilation in the room (hot air out, cool air in), and sound mitigation can be crazy expensive. Try some weather stripping around doors and windows, padded floor treatments (especially in the room directly above your theater room) and other simple common sense measures all of which can make that ambient silence on the screen jump out at you and enhance your movie experience.