Etymotic Research er-4b Guida Utente

ER-4P (Portable) earphones have enhanced bass and higher sensitivity.
ER-4Ps can be used with portable CD, MP3, DVD and other players without
requiring an additional amplifier. The ER-4P has 10 dB greater sensitivity 
at high frequencies and 13 dB more at low frequencies than the ER-4S.
The higher sensitivity and enhanced bass of the ER-4P have made it the 
most popular earphone for most uses.

ER-4S (Stereo) earphones have the most accurate frequency response for
reproducing CD and MP3 recordings. The sensitivity is comparable to home
and professional earphones. ER-4S earphones are ideal for home stereo use
and computer use. ER-4S earphones can be used with a headphone amplifier 
when using low power portable devices. ER-4S is the earphone preferred 
by musicians and audiophiles.

ER-4B (Binaural) earphones are for the binaural recording enthusiast.
The ER-4B is the reference earphone for use with material that has not been
equalized for loudspeaker playback.