A headphone is an electromagnetic transducer designed to be worn on the human head for the purpose of audio listening/monitoring, and as distinct from an earphone, or system worn in the ear. Headphones (plural) would be a pair – one for each ear. They are usually based on the principle of electromagnetic induction used to convert the electrical energy output of a headphone amplifier into acoustic energy, or sound. There are two main distinctions regarding headphone design: open-back and closed-back, generally referred to as open and closed headphones. With closed headphones, the ear is completely sealed off from outside noise (pressure chamber principle). Typical features of closed headphones are the acoustically sealed housing and the ring-shaped (circumaural) pads that completely surround the ear. The sealing around the ear has a decisive influence on the sound reproduction of closed headphones. If it is insufficient, the quality of the bass sounds will deteriorate. For this reason, the contact pressure of closed headphones is higher than that of open headphones. Sound engineers often use closed headphones, which allows them to concentrate on the music without disturbance from outside noise. The problem of sealing does not exist with open headphones. In this design, the space behind and in front of the diaphragm lets sound through. Therefore, open headphones allow music to pass straight through the diaphragm without being “muffled”, thus resulting in a more transparent and natural sound image. The distinguishing features of open headphones are their small size and low weight. These in turn make them extremely comfortable to wear, and no discomfort is felt even after prolonged periods of listening.