True high-fidelity requires reproduced sound to be as close as possible to the sound of a live performance. Etymotic's earphones have the highest response accuracy of all in-ear earphones. ER4S earphones are designed to compensate for the high frequency emphasis in recordings. Performing musicians often use them because the response matches that of most monitor loudspeakers. Popular with musicians, recording engineers and audiophiles, ER4S can be used with a headphone amplifier if desired. Included with every pair of ER4S in ear monitors is an industry-exclusive channel matching compliance graph signed by the Etymotic engineer who precision matched and custom tuned the balanced-armature drivers. Every pair of ER4S earphones is assembled and tuned in the United States.