Headphones (or head-phones in the early days of telephony and radio) are a pair of small listening devices that are designed to be worn on or around the head over a user's ears. They are electroacoustic transducers, which convert an electrical signal to a corresponding sound in the user's ear. Headphones are designed to allow a single user to listen to an audio source privately, in contrast to a loudspeaker, which emits sound into the open air, for anyone nearby to hear. Headphones are also known as earspeakers, earphones or, colloquially, cans. Circumaural and supra-aural headphones use a band over the top of the head to hold the speakers in place. The other type, known asearbuds or earphones consist of individual units that plug into the user's ear canal. In the context of telecommunication, a headset is a combination of headphone andmicrophone. Headphones connect to a signal source such as an audio amplifier, radio, CD player, portable media player, mobile phone, video game console, or electronic musical instrument, either directly using a cord, or using wireless technology such as bluetooth or FM radio. The first headphones were developed in the 1800s from telephone receivers for use by telephone operators, to keep their hands free. Initially the audio quality was mediocre and a step forward was the invention of high fidelity headphones.