A device which converts an analogue audio signal into a digital representation. (Cf. D-A Converter.) (An explanatory video is available when subject link is clicked.)

A loudspeaker system in which the input signal is passed to a line-level crossover, the suitably filtered outputs of which feed two (or more) power amplifiers, each connected directly to its own drive unit. The line-level crossover and amplifiers are usually (but not always) built in to the loudspeaker cabinet.

The term typically describes an electronic circuit containing transistors, integrated circuits (ICs), tubes or other devices that require power to operate, and which are capable of amplification. Also used to describe loudspeaker monitoring systems which employ separate amplifiers to power each drive unit individually. 

A general term embracing a range of products or constructions intended to absorb, diffuse or reflect sound waves in a controlled manner, with the intention of bestowing a room with acceptable reverberation times at all frequencies, and a neutral overall sound character.

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.