IEM stands for in-ear monitor, which is a device used by musicians, audio engineers, and audiophiles to listen to music or to hear a personal mix of vocals and stage instrumentation for live performance or recording studio mixing. IEMs are similar to earbuds in that they are small, portable audio devices inserted directly into the ear, but the biggest difference between the two is that IEMs are inserted into the ear canal and create a seal, while earbuds simply sit in the outer part of the ear. IEMs have some specific features and design choices to be aware of, such as blocking outside sound well, using passive acoustic isolation, and containing tiny drivers that convert electrical signals to sound. IEMs come in two different designs: universals and custom in-ear monitors (CIEMs) . Universals are a fixed shape and usually come with silicon or foam ear tips, while CIEMs are custom-fitted to the contours of the users ears to provide the best level of comfort. IEMs are used by audio engineers, musicians, and audiophiles because sound is transmitted directly into the ear, resulting in a tighter, less sibilant bass response, in addition to less disruption from unwanted noise.