October 2011
In 1991, Etymotic Research launched the first commercial earphone for music playback that was designed to be inserted in the ear canal. This created a whole new product category that goes by various names: in-ear headphones, canal phones, insert phones, etc. I prefer the widely used term in-ear monitors (IEMs). Whatever you call them, these types of earphones have a few basic characteristics. Insertion into the ear canal avoids many of the variations in outer-ear (pinna) shape and headphone positioning that can affect the perceived sound of normal headphones. Depending on the design of the eartip, IEMs can offer a significant amount of isolation from ambient noise, which allows for much lower and thus safer listening levels, as well as increased perception of details. Many IEMs are also highly efficient, so they can be adequately driven directly from the output of a portable audio device. Those characteristics, combined with their very small size, make IEMs an excellent choice for listening on the go.
From their extensive work with developing tools for audiologists, the engineers and scientists at Etymotic came up with a target curve that represents the ideal frequency response for earphones. They then tweaked that curve to account for the 5dB boost in the top octave on most CDs, and an equivalent 5dB cut that they found was characteristic of most professional studio monitor speakers. The result was the ER-4 S -- their first product, launched in 1991 and still available today. The only trouble with that original design was that its low sensitivity -- 0.1V for 90dB at 1kHz -- requires dedicated headphone amplification. In 1994, Etymotic introduced the ER-4P. It had a higher sensitivity -- 102dB at 0.1V -- and could therefore be driven directly from the output of almost any portable audio device.