In-ear monitors work by replacing monitor speakers with in-ear headphones that are shaped to seal off outside noise while delivering specific sounds to the performer, much like their inexpensive earbud cousins found with current MP3 players. These can be monitored by the sound engineer to enhance a particular portion of a band for monitoring purposes, or a full-mix feedback for overall sound such as a DJ would use. This reduces the cumbersome system of feedback from wedge-shaped speakers and eliminates the problems caused by feedback and tethering to the sound system via wires.
In-ear monitors can be hard-wired or use RF wireless in order to receive their data. DJ's will most commonly use the wired versions, as they are less expensive and a DJ is frequently stationary, while a live band will normally use the wireless models for freedom of movement. They also come in either standard or shaped variants. Standard monitors are traditionally constructed of a sound-dampening foam, while the shaped monitors are created much like a medical hearing aid and shaped to the individual ear canal.
The largest benefits of in-ear monitors are the freedom of movement and sound isolation. A wireless in-ear monitor system allows a performance to be expanded and wide open as musicians can move freely about the stage. The sound isolation can help individual musicians focus on their own parts within the music without having to drown out areas of the stage via traditional monitors. In-ear monitors also provide an excellent way to reduce the effects of feedback that come from speakers firing directly at the performer.
A full, wireless in-ear monitoring system is expensive. From the mixing console to the receiver packs, there are more parts to the system and each can carry a high cost as compared to traditional monitoring. It also needs to be kept in mind that like with all wireless systems, interference can create issues if there are other overlapping wireless sources in use. Dead zones can be created onstage since most wireless in-ear systems are omnidirectional, and as such are not focused in such a way as to guarantee complete coverage. Finally, the habit of "turning it up" is hard to break, and when the system is in-ear, hearing damage can be a result of consistent volume abuse.