The compact Koss 168121 is an earbud-style headphone set that comes with a wind-up case for easy storage. The Koss headphones sit in-ear to deliver sound output and passively reduce external noises. The wind-up case for these in-ear headphones wraps the earbud wires in a spool inside of a case with lid that can be closed up and stored in a pocket or purse. The Koss 168121 has a 4-foot cord that end in a 3.5 mm L-shaped plug, and the unit will work with most devices that accept this style of plug. These reliable Koss headphones use a minimum frequency response of 70 Hz with a maximum frequency response of 20 kHz. The impedance for these in-ear headphones is 32 Ohms with 90 dB sensitivity. The Koss headphones provide binaural sound output.