ER-4S earphones were designed to compensate for the high frequency emphasis in all CD recordings, in order to provide accurate sound reproduction. Performing musicians often use them as in-ear monitors because the response of the ER-4S matches the response of most typical monitor loudspeakers. The high accuracy and exceptional sound isolation of the ER-4S have made these earphones popular with musicians, recording engineers and audiophiles. ER-4S earphones can be used with a headphone amplifier when using low power portable devices.