Etymotic ER•4® microPro® Earphones - Manual (Page 9)

8
ER-4PT TO ER-4S CONVERTER C ABLE
Convert your ER•4PT earphones to ER•4S earphones with this inline
adapter cable. ER•4PT earphones have approximately 10 dB greater
output at high frequencies and 12 dB greater output at low frequencies
than ER•4S. ER•4S earphones were designed to compensate for the high
frequency emphasis in all CD recordings, in order to provide accurate sound
reproduction. ER•4S earphones can be used with a headphone amplifier
when using low-power portable devices.
TIP: Use the adapter cable with airplane sound systems when
the audio is too loud.