Closed-back (or sealed) styles have the back of the earcups closed. They usually block some of the ambient noise, but have a smaller soundstage, giving the wearer a perception that the sound is coming from within their head. Closed-back headphones tend to be able to produce stronger low frequencies than open-back headphones.

Though one of the most common items in the audio chain, earphones or headphones may not always work properly. Common problems are distorted audio, low level with the volume at maximum, or high level with the volume at minimum. This bulletin will explain earphone/headphone specifications, which ones are important, and how to choose earphones for an audio system.

Let's differentiate between headphones and earphones. A headphone consists of one or two transducers (usually miniature speakers) that are attached to a metal or plastic headband. This headband provides proper placement of the transducers over the ear; the headband rests on top or behind the head. There are two common types: open-back and closed-back. The open-back type has acoustic ports on the outside that increase bass response, but has the downside of allowing background noise to bleed through into the ear. The closed type completely covers the ear, thus reducing background noise. A secondary advantage is that sound originating from the headphones will not radiate outward. Closed-back are preferred for applications such as studio recording where noise must be minimized.

Earphones are also transducers but with no headband. They can be placed on the pinna (earflap) or inside the ear canal. The former, known as "earbuds", are usually the open-air type, since small transducers can not generate low frequencies without acoustic ports. The latter, known as "in ear" earphones, use a foam or other soft material to hold the earphone in the ear canal, thus isolating it from ambient noise. Forming an acoustic seal makes the ear canal part of the earphone "system" and modifies its frequency response. The advantages are that less volume is required, and audio quality is typically better.

Most headphone and earphone specifications show actual impedance and sensitivity. Some others go a step further and show maximum/clipping level and frequency response. These specifications define how each unit will work with an audio system. On the audio system side, pertinent specifications are output impedance, power output, and/or clipping level.