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.