A stage monitor is a speaker which is used to direct sound at the stage so that people on stage can hear what they are doing. While it might be odd to think that performers on a stage can't hear, in fact, without a stage monitor, performers hear reverberations which reflect from the walls of the venue, and these can distort the sound, tempo, and pitch, consequently throwing the performers off. Stage monitors allow performers to hold their focus so that they can keep the performance consistent.
Venues of a variety of shapes and sizes are designed to accommodate stage monitors. The traditional stage monitor, also called a fold back, is a wedge-shaped speaker placed at the front of the stage and faced towards the back. The speaker has a low profile so that the view is not obscured. It is also possible to wear headphones to hear, an option which is becoming more appealing for some artists because it also provides ear protection which can reduce the risks of experiencing hearing damage.
For large bands and big venues, stage monitors are placed on an entirely separate mixing board, and may be run by a different sound engineer. The sound engineer wants the sound loud enough to be clearly heard, without damaging the hearing of the performers. The stage monitor system is tested during setup to confirm that it is working properly and to make any needed level adjustments.
Without monitors, members of a band might start to go out of tempo or off pitch. The reverberations from the venue can be confusing, and may disorient performers and cause them to lose track. The stage monitor can also provide instant feedback about tempo and sound quality, allowing the performers to adjust their performance throughout the show to keep the sound crisp, clear, and intelligible. Since some audiences are highly demanding in live performance, performers need to be able to hear what the audience is hearing so that they can understand and react to audience responses.
The term “stage monitor” is also sometimes used in theater, television, and other types of performance to refer to a microphone on stage which delivers an audio feed to backstage. There may also be a camera which provides a video feed, depending on the situation. The stage monitor is used by performers to keep track of the performance so that they can be ready in time for their cues, and by personnel such as stage crew who need to be able to respond to events on stage.