Broadly speaking, an earpiece refers to anything that functions by either covering the ear, or hanging near the ear. In this broad category are the parts of an eyeglass frame that extend over the ear, and any material specifically designed to fit into the ear and block off the entry of sound or water. The most common meaning of an "earpiece" however is an electronic device that converts electronic signals into sound waves, and directs them into the ear. In this context, the earpiece can also be called an electronic-acoustic transducer.
Originally, the earpiece was the only way of listening to electronically-generated audio signals. Thereafter, amplifiers were developed, and the role of earpieces became more a matter of personal preference than necessity. Nowadays, earpieces are used for convenience — where privacy is desired, or where mobility is important. Depending on its design, an earpiece can also be variously known as headphones, ear buds, stereo phones or headsets.
Earpieces can be organized broadly into two categories — wired earpieces and cordless or wireless earpieces. The wireless types use either radio waves or infrared signals to connect to the signal sources. Earpieces for CD players, computers or mobile phones are some examples. A signal transmission link is used, like Bluetooth® or Wi-Fi.
The wired earpiece has to be physically attached to a signal source. To facilitate this, jacks at the end of the cords are inserted into the signal source. Jack sizes have been standardized, and the most common size is the 3.5 millimeter (about 0.2 inches) that was brought into the market by the Sony Company in 1979. The much older, 1/4 inch (about 6.4 mm) jack is now used only in professional equipment. When wiring up the stereo jacks, the tip is always the left channel, the portion in between is the right channel, and the outer sleeve is the ground.
The use of ear buds has a profound impact on how one perceives sound. This is because an earpiece feeds the sound directly into the eardrums, without the normal acoustic contouring by the listener’s head and outer ear that normal loudspeakers give scope for. Lacking these spatial references, the brain then imagines the sound to be in a straight line within the head and between the ears. This strips the sound of its 3 dimensional effect, otherwise produced by the normal loudspeakers.
From their shapes and sizes, earpieces can be placed into four categories. The circum-aural types have large pads that surround the outer ear. They are most commonly used in recording studios. The supra-aural headphones fit just over the ear, without fully surrounding it. Ear buds are designed to fit inside outer ear’s canal. However, they don’t fully cover the canal, and exterior sounds can still be heard around them. In contrast, canal phones, the fourth category, fit snugly into the ear’s canal, and effectively block off the external sounds.