Early in-ear monitor systems were quite primitive. Sometime in the mid-80s, standard earbuds were set in basic earmolds and hard-wired to amplifiers. These soon developed into complex, wireless in-ear systems with custom-molded earpieces. Usually having two or three speakers in each earpiece, these devices were able to handle mid, high, and low frequencies separately, resulting in pristine sound quality for the user.
The purpose of in-ear monitoring is to deliver a high-quality signal to a musician's ear. This signal is comprised of a musical mix of the performer's choosing; a signal that allows him to clearly hear a mix at a certain tone and volume.
The user simply plugs in a cable on the monitor output on the mixer and plugs it into the IEM transmitter. The transmitter then sends the musical signal to a receiver, usually a belt pack, into which the personal listening device is plugged.
In-ear monitoring is beneficial to the artist on stage because she can have her own personal mix at their own desired volume. It also benefits the sound engineer because loud stage monitors can bleed into adjacent microphones and affect the sound coming off of the stage. Eliminating this gives them more control over the mix.
Perhaps the most obvious benefit, though, is that monitoring can help protect the hearing of performers. The use of monitor speakers that have to compete with drums and assorted amplified instruments, can lead to ear-splitting volume levels on stage. Using IEMs allows for monitoring at much lower volume levels, which can help performers lower their risk of sustaining irreversible hearing damage.
From a performance standpoint, singers often find that they have fewer pitch problems and have to strain less to hear their own voice, which reduces vocal fatigue. They are also free to roam the stage and still hear their own custom-designed mix.
Despite the significant up-front cost, IEMs can still be a money saver for touring acts. Couple the cost of expensive wedge or floor monitor speakers (and the amplifiers needed to power them) with the added long-term costs of transportation, setup and teardown of these bulky items, and it becomes apparent that travelling with IEM systems may prove to be a sound, long-term financial investment for touring musicians.
One downside of the use of IEMs is the isolation effect that can occur with their use. The tight fit and external noise-canceling effect of the units can make it a little difficult to hold a conversation with someone whose voice isn't amplified and being fed directly to your in-ear monitor. This can also separate the performer from audience noise, which can reduce the interaction with the audience, especially between songs. This issue can be addressed by the placement of extra microphones that mix crowd noise into the monitor mix.
Although IEDs are hailed for their noise-canceling capabilities, they are still devices that can deliver high volume levels directly to the eardrum. To avoid potential hearing loss and tinnitus (ringing in the ears), special care needs to be taken with their use; ideally under the guidance of an experienced audiologist.