An open-celled expanded polyurethane or melamine foam that allows sound waves to enter and flow through the foam to dissipate their energy and thus prevent the sound waves from being reflected. The density and depth of the foam affects the frequency range over which it is effective as an acoustic absorber.

Alternating Current (cf. DC). Audio signals are represented in the electrical domain as currents flowing alternately forward and back in the circuits as an analogue of the compression and rarefaction of acoustic air pressure due to the passing of sound waves.

see 'Close Miking'.

Also known as a 'condenser microphone'. This is a specific form of electrostatic microphone which operates on the principle of measuring the change in electrical voltage across a capacitor which contains a constant static charge. The capacitor is formed from two metal electrodes, one fixed (the back-plate) and the other a thin conductive membrane that flexes in response to sound pressure, and the static charge comes either from a DC-bias voltage or a dielectric material. (See also 'Back Electret', and 'RF Capacitor Microphone'.)

A passive, two-terminal electrical component which stores energy in the form of an electrostatic field. The terminals are attached to conductive ‘plates’ which are separated by a non-conductive dielectric. Capacitance is measured in Farads and the amount of capacitance depends upon the size of the plates and the spacing between them.  If a voltage is applied across the terminals of a capacitor a static electric field develops across the dielectric, with positive charge collecting on one plate and negative charge on the other. Where the applied voltage is an alternating signal, a capacitor can be thought of as a form of AC resistance that reduces with increasing signal frequency. The old-fashioned term is a ‘condensor’.

Any box-like construction will resonate at one or more frequencies. In the case of a loudspeaker, such resonances are likely to be undesirable as they may obscure or interfere with the wanted sound from the drive units. As resonance also involves the storing and releaseing of energy over time, cabinet resonances can result in 'time smearing', causing a sharp impulsive sound like a 'thud' to be reproduced as a prolonged 'boom'.  Cabinets are usually braced and damped internally to minimise resonances.

The physical construction which encloses and supports the loudspeaker drive units. Usually built of wood or wood-composites (although other materials are often used including metal alloys and mineral composites). Cabinets can be ‘sealed’ (often referred to, misleadingly, as an 'infinite baffle') or ‘vented’ in various ways (ported, bass-reflex, ATL, transmission line), the precise design influencing the bass and time-domain characteristics.

The amount by which a circuit amplifies a signal, normally denoted in decibels (dB).

A large capacity solid-state memory configured to work like a conventional hard disk drive, referred to as SSD. Some computers are now available with solid-state flash drives instead of normal internal hard disk drives. Also used in digital cameras and audio recorders in formats such as SD and CF2 cards, as well as in ‘pen drives’ or ‘USB memory sticks’.

The conventional means of computer data storage. One or more metal disks (hard disks) hermetically sealed in an enclosure with integral drive electronics and interfacing. The disks are coated in a magnetic material and spun at high speed (typically 7200rpm for audio applications). A series of movable arms carrying miniature magnetic heads are arranged to move closely over the surface of the discs to record (write) and replay (read) data.