The Koss CC_01 isolating in-ear monitor is the first stereophone to create a perfect fit with a unique Custom Fit dial. Rotate the crafted knurled knob and the ear cushion expands to provide a custom fit. Koss in-ear monitors are designed to produce a perfect seal. This seal allows the variable matched transducer to directly couple with the eardrum. This direct response system means greater accuracy and efficiency across the entire frequency response range. No other in-ear monitor provides the ability to dial in comfort or performance. It's an entirely new state of the art in ergonomic and acoustic design.