Koss's miniature portable earphone, affectionately dubbed "The Plug," makes use of an ear-cushion material (similar to that used in sound-blocking earplugs) that slowly expands in your ear canal to provide a custom fit and maximum sonic isolation from external noises. A detachable, extrawide headband makes these Koss-termed "stereophones" easy to carry or store.
For accurate sound reproduction, these earbud-style headphones are made with oxygen-free copper voice coils and neodymium iron boron magnets. You can enjoy them with either a portable or home audio device, since both 0.125-inch and 0.25-inch adapter jacks are included.
"The Plug" weighs a mere quarter ounce and has a frequency response rated at an impressive 16 to 23,000 Hz (though its unlikely you'll hear bass output anywhere close to what you'd get from a similarly rated loudspeaker!).
Impedance is 16 ohms, with less than a 0.3 percent distortion and reasonably high sensitivity of 112 dB SPL/1 mW.
The Koss Plug revolutionized the world of in-ear headphones. A first of its kind, these are stereo, noise isolating in-ear headphones that deliver the kind of accurate acoustical reproduction and isolation typically found in high-end, closed-style high fidelity headphones. A dynamic element positioned just outside the ear connects to a tubular port structure that inserts into a soft, expandable cushion. The soft, foam cushion expands for a custom comfort fit with every listening experience. It also improves isolation, plugging out ambient sounds, and enhances deep bass and treble clarity.