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Welcome to Sound On Sound's indispensible, regularly updated, explanations of technical terms from the fields of Recording, Audio Production, Music Technology, MIDI, Music Software, Audio Plug-ins, Mac and PC Computing, Live Sound, Acoustics, Electronics and more...

If we do not explain a particular term below, please email and we will add it to our next update.

Last updated: 30/03/21

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A digital tape machine using a rotating head system and a tape cassette.

Rack Mount

A standard equipment sizing format allowing products to be mounted between vertical rails in standardised equipment bays.


An abbreviation for Random Access Memory. This is a type of memory used by computers for the temporary storage of programs and data, and all data is lost when the power is turned off. For that reason, work needs to be saved to disk if it is not to be lost.


An audio process that can be carried out as the signal is being recorded or played back. The opposite is off-line, where the signal is processed in non-real time.

Red Book CD

A term used to imply a standard audio CD. The name comes from the fact that the original specifications documents for the audio CD created by Sony and Philips had a red cover! Recordable CD-Rs are described as 'orange book' for similar reasons.


The way in which sound waves bounce off surfaces.


The time taken for a signal level or processor gain to return to normal. Often used to describe the rate at which a synthesized sound reduces in level after a key has been released. Also used to describe the time taken for a compressor top restore unity gain after a signal has fallen below the threshold. Also known as ‘recovery time .‘


Opposition to the flow of electrical current. Measured in Ohms.


The characteristic of a filter that allows it to selectively pass a narrow range of frequencies. See Q.


Short for Reverberation. The dense collection of echoes which bounce off acoustically reflective surfaces in response to direct sound arriving from a signal source. Reverberation can also be created artificially using various analogue or, more commonly, digital techniques. Reverberation occurs a short while after the source signal because of the finite time taken for the sound to reach a reflective surface and return - the overall delay being representative of the size of the acoustic environment. The reverberation signal can be broadly defined as having two main components, a group of distinct ‘early reflections’ followed by a noise-like tail of dense reflections.

Reverberation Time

The time taken for sound waves reflecting within a space to lose energy and become inaudible. A standard measurement is ‘RT60’ which is the time taken for the sound reflections to decay by 60dB.


An abbreviation for Radio Frequency.

RF Capacitor Microphone

An alternative form of capacitor microphone which uses the capacitive capsule as the tuning element of a radio-frequency oscillator. Sound waves arriving at the capsule change its capacitance, which varies the frequency of the RF oscillator to produce an FM signal. This is immediately demodulated by the microphone's internal circuitry to produce the audio output. The advantage of this approach is that the capsule works in a very low-impedance environment (as opposed to the very high-impedance environment of a traditional DC-biased and Electret capacitor mics), making it immune to the effects of humidity which can cause unwanted noise in conventional capacitor mics. This technology was invented by Sennheiser and is used in their MKH range of microphones.

RF Interference

Unwanted interference into an audio system from external RF signals.

Ribbon Microphone

A dynamic microphone where the sound capturing element is a thin metal ribbon diaphragm suspended within a magnetic field. When sound causes the ribbon to vibrate, a small electrical current is generated within the ribbon.


A set of requests/demands that an artist or band (or their management) ask of the hosting venue as criteria for performing. A Technical Rider would typically specify the size and layout of staging, required equipment for lighting (truss weight limits, power requirements, numer of follow-spots, lighting plots or designs, etc) and sound (input channel counts, PA power, number of monitors, effects, DI boxes, backline amps and instruments, quality of equipment etc) and possibly also the operating and rigging staff provided. A Hospitality Rider is a list of requests to ensure the comfort of the artist(s), such as the number of dressing rooms, private bathroom/shower, food and beverage requirements, a number of complimentary (comp) tickets for guests, security arrangements, and so on. There may also be additional riders covering other aspects such as a Merchandise Rider detailing the space and stands provided for selling merchandise, the rates, exclusivity rights, and so forth.

Ring Modulator

A device that accepts and processes two input signals in a particular way. The output signal does not contain any element of the original input signals but instead comprises new frequencies based on the sum and difference of the input signals' frequency components. The best known application of Ring Modulation is the creation of Dr Who’s Dalek voices, but it may also be used to create dramatic instrumental textures. Depending on the relationships between the input signals, the results may either be musical or extremely dissonant - for example, ring modulation can be used to create bell-like tones. (The term 'Ring' is used because the original circuit which produced the effect used a ring of diodes.)


Root Mean Square. A statistical measure of the magnitude of a varying quantity. Its name comes from its definition as the square root of the mean of the squares of the values of the signal.


The rate at which a filter or equaliser attenuates a signal once it has passed the turnover frequency.


Read Only Memory — A permanent or non-volatile type of memory chip containing data that can't be changed once programmed. Operating systems are often stored on ROM as the memory remains intact when the power is removed.

Room Modes

Acoustic resonances within an enclosed space or room. These occur at specific frequencies where the source sound is reflected from the room's boundaries to reinforce and/or cancel with itself to create standing waves. This results in some areas in the room with very boomy or exaggerated pitches, and others where the pitch may be almost completely absent. The resonant frequencies involved relate directly to the sound wavelength and room dimensions, and is particularly prevalent at low frequencies.

Rotary Encoder

A hardware controller comprising a knob or dial which can be rotated in either direction without end-stops. A digital encoder of some kind attached to the shaft translates the movement into a digital code that can indicate both direction and speed of rotation to the controlling software of a device.