Acoustic Foam – A specific type of open-celled expanded polyurethane foam that allows sound waves to enter and flow through the foam, absorbing their energy and preventing them being reflected. The density and depth of the foam affects the frequency range over which it is effective as an absorber.
Acoustic Treatment – A generic 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 an acceptable reverberation time and overall sound character.
Active Loudspeaker or Monitor – 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.
Amp/Amplifier – An electrical device that increases the voltage or power of an electrical signal. The amount of amplification can be specified as a multiplication factor (e.g. x10) or in decibels (e.g. 20dB).
Analogue (cf. Digital) – The origin of the term is that the electrical audio signal inside a piece of equipment can be thought of as being ‘analogous’ to the original acoustic signal. Analogue circuitry uses a continually changing voltage or current to represent the audio signal.
Arming (e.g. for recording) – Arming a track or channel on a recording device places it in a condition where it is ready to record audio when the system is placed in record mode. Unarmed tracks won’t record audio even if the system is in record mode. When a track is armed the system monitoring usually auditions the input signal throughout the recording, whereas unarmed tracks usually replay any previously recorded audio.
Audio Interface – A device which acts as the physical bridge between the computer’s workstation software and the recording environment. An audio interface usually connects to the computer via FireWire or USB to pass audio and MIDI data to and from the computer. Audio Interfaces are available with a wide variety of different facilities including microphone preamps, DI inputs, analogue line inputs, ADAT or S/PDIF digital inputs, analogue line and digital outputs, headphone outputs, and so on. The smallest audio interfaces provide just two channels in and out, while the largest may offer 30 or more.
Automation (e.g. of faders) – Automation refers to the ability of a system to store and reproduce a set of control parameters in real time. Fader automation is a system involving moving faders (virtual or physical) in which adjustments made by the user are recorded and can be reproduced in exactly the same way at a later time, or modified if necessary. Most fader, mute, routing and plug-in parameters can be automated in most DAW software.
Auxiliary Sends – A separate output signal derived from an input channel, usually with the option to select a pre or post fader source and to adjust the level. Corresponding auxiliary sends from all channels are bussed together before being made available to feed an internal signal processor or external physical output.
Balanced/Unbalanced Cables – Most audio equipment operates internally with unbalanced signals. These are transferred between devices using a single-core screened cable. The signal voltage is passed on the inner core while a zero volt (ground) reference voltage is conveyed by the outer screen (an all-encompassing metal or conductive plastic braid). The screen also serves to ‘catch’ any radio frequency interference (RFI) and prevent it from influencing the audio signal.
Where greater protection from electromagnetic interference and freedom from earth references are required, a balanced interface is used. The term ‘balanced’ refers to identical impedances to ground from each of two signal carrying conductors which are enclosed, again, in an all-embracing overall screen. The screen is grounded, as with the unbalanced interface, but this connection plays no part in passing the audio signal or providing a voltage reference. Instead, the two signal wires provide the reference voltage for each other – the signal is conveyed differentially and the receiver detects the voltage difference between the two wires. Interference instils the same voltage on each wire (common mode) because the impedance to ground is identical for each, and thus any interference is ignored completely.
Signals conveyed over the balanced interface may appear as equal half-level voltages with opposite polarities on each signal wire – the most commonly described technique. However, modern systems are increasingly using a single-sided approach where one wire carries the entire signal voltage and the other a ground reference for it. Some advantages of this technique include less complicated balanced driver stages, and connection to an unbalanced destination still provides the correct signal level, yet the interference rejection properties are unaffected. For interface balancing to provide effective interference rejection, both the sending and receiving devices must have balanced output and input stages respectively.
Back Electret – A form of electrostatic or capacitor microphone. Instead of creating an electrostatic charge within the capacitor capsule with an external DC voltage, an electret microphone employs a special dielectric material which permanently stores a static electric charge. A PTFE film is normally used, and where this is attached to the back plate of the capsule the device is called a ‘back electret’. Some very early electret microphones used the dielectric film as the diaphragm but these sounded very poor, which is why later and better designs which used the back electret configuration were specifically denoted as such. Some recent designs attach the PTFE film to the diaphragm and these are known as Front Electrets. Modern electret capsules compare directly in quality with traditional DC-biased capacitor capsules and are available in the same range of configurations – large, medium and small diaphragm sizes, single and dual membrane, fixed or multi-pattern, and so on.
Bass Response – The frequency response of a loudspeaker system at the lower end of the spectrum. The physical size and design of a loudspeaker cabinet and the bass driver (woofer) determine the low frequency extension (the lowest frequency the speaker can reproduce at normal level) and the how quickly the signal level falls below that frequency.
Bass Tip-up – see Proximity Effect
Bass trap – A special type of acoustic absorber which is optimised to absorb low frequency sound waves.
Bit Rate (see also Sample Rate) – The number of data bits replayed or transferred in a given period of time (normally one second). Normally expressed in terms of kb/s (kilo bits per second) or Mb/s (mega bits per second). For example, the bit rate of a standard CD is (2 channels x 16 bits per sample x 44.1 thousand samples per second) = 1411.2 kilobits/second. Popular MP3 file format bits rates range from 128kb/s to 320kb/s, while the Dolby Digital 5.1 surround soundtrack on a DVD-Video typically range between 384 and 448kb/s.
Blumlein Array – A stereo coincident microphone technique devices by Alan Blumlein in the early 1930s, employing a pair of microphones with figure-eight polar patterns, mounted at 90 degrees to each other with the two diaphragms vertically aligned.
Boom – A mechanical means of supporting a microphone above a sound source. Many microphone stands are supplied with a ‘boom arm’ affixed to the top of the stand’s main vertical mast. The term may also be applied to larger, remotely controlled microphone supports used in film and TV studios, or even to the handheld ‘fishpoles’ used by film and TV sound recordists.
Boundary – A physical obstruction to sound waves, such as a wall, or a large solid object. When sound waves reach a boundary they create a high pressure area at the surface.
Buffer (in reference to computer memory and processing) – A buffer is essentially a short-term data storage facility used to accommodate variable data read or write periods, temporarily storing data in sequence until it can be processed or transferred by or to some other part of the system.
Cabinet – 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’ or ‘vented’ in various ways, the precise design influencing the bass and time-domain characteristics.
Cabinet Resonance – 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. Cabinets are usually braced and damped internally to minimise resonances.
Capacitor – 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. If a voltage is applied across the terminals of a capacitor a static electric field energy 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 resistance that reduces with increasing signal frequency.
Capsule – An alternative term for a transducer which converts acoustic sound waves into an electrical signal.
Channel – A portion of an audio system dedicated to accommodating a single audio signal. Normally used in the context of an audio mixer, where each channel provides a range of facilities to process a single audio signal (gain, EQ, aux sends, fader etc). A mixer might incorporate 6, 12, 32 or more channels.
Click Track – A rhythmic audio signal, normally comprising clicks or plops, intended as an audible cue to assist musicians in keeping accurate time during a performance. It would not normally be heard by the audience.
Clipping – When an audio signal is allowed to overload the system conveying it, clipping is said to have occurred and severe distortion results. The ‘clipping point’ is reached when the audio system can no longer accommodate the signal amplitude – either because an analogue signal voltage nears or exceeds the circuitry’s power supply voltage, or because a digital sample amplitude exceeds the quantiser’s number range. In both cases, the result is that the signal peaks are ‘clipped’ because the system can’t support the peak excursions – a sinewave source signal becomes more like a squarewave. In an analogue system clipping produces strong harmonic distortion artefacts at frequencies above the fundamental. In a digital system those high frequency harmonics cause aliasing which results in anharmonic distortion where the distortion artefacts reproduce at frequencies below the source fundamental. This is why digital clipping sounds so unlike analogue clipping, and is far more unpleasant and less musical.
Clocking – The process of controlling the sample rate of one digital device with an external clock signal derived from another device. In a conventional digital system there must be only one master clock device, with everything else ‘clocked’ or ‘slaved’ from that master.
Coincident – A means of arranging two or more directional microphone capsules such that they receive sound waves from all directions at exactly the same time. The varying sensitivity to sound arriving from different directions due to the directional polar patterns means that information about the directions of sound sources is captured in the form of level differences between the capsule outputs. Specific forms of coincident microphones include ‘XY’ and ‘MS’ configurations, as well as B-format and Ambisonic arrays. Coincident arrays are entirely mono-compatible because there are no timing differences between channels.
Colouration – A distortion of the natural timbre or frequency response of sound, usually but not always unwanted.
Common Mode Signal – A signal that appears equal in amplitude and polarity on both wires of a balanced interface, and consequently is rejected by the differential receiver.
Comping – Short for ‘compilation.’ The process of recording the same performance (e.g. a lead vocal) several times on multiple tracks to allow the subsequent selection of the best sections and assembling them to create a ‘compilation’ performance which would be constructed on a final track.
Compressor – A device (analogue or digital) which is designed to reduce the overall dynamic range of a complex varying audio signals by detecting when that signal exceeds a defined threshold level, and then reducing the amplitude of that portion of signal according to a defined ratio. The speed of response and recovery can usually also be controlled.
Cone – A specific shape of drive unit diaphragm intended to push and pull the air to create acoustic sound waves. Most bass drivers use cone-shaped diaphragms, where the electromagnetic motor of the drive unit is connected to the point of the cone, and its outer diameter is supported by some form of flexible membrane.
Converter – A device which transcodes audio signals between the analogue and digital domains. An analogue-to-digital (A–D) converter accepts an analogue signal and converts it to a digital format, while a digital-to-analogue (D–A) converter does the reverse. The sample rate and wordlength of the digital format is often adjustable, as is the relative amplitude of analogue signal for a given digital level.
CPU – Central Processing Unit – the number-crunching heart of a computer or other data processor.
Crossover – A set of audio filters designed to restrict and control the range of input signal frequencies which are passed to each loudspeaker drive unit. A typical two-way speaker will employ three filters: a high-pass filter allowing only the higher frequencies to feed the tweeter, a low-pass filter that allows only the lower frequencies to feed the woofer, and a second high-pass filter that prevents subsonic signals from damaging the woofer.
Crossover Frequency – The frequency at which one driver ceases to produce most of the sound and a second driver takes over. In the case of a two-way speaker the crossover frequency is usually between 1 and 3kHz.
Daisy Chain – An arrangement of sharing a common data signal between multiple devices. A ‘daisy chain’ is created by connecting the appropriate output (or through) port of one device to the input of the next. This configuration is often used for connecting multiple MIDI instruments together: the MIDI Out of the master device is connected to the MIDI In of the first slave, then the MIDI Thru of the first slave is connected to the MIDI In of the second slave, and so on... A similar arrangement is often used to share a master word clock sample synchronising signal between digital devices.
DAW – Digital Audio Workstation. Elaborate software running on a bespoke or generic computer platform which is designed to replicate the processes involved in recording, replaying, mixing and processing real or virtual audio signals. Many modern DAWs incorporate MIDI sequencing facilities as well as audio manipulation, a range of effects and sound generation.
dB (deciBel) – The decibel is a method of expressing the ratio between two quantities in a logarithmic fashion. Used when describing audio signal amplitudes because the logarithmic nature matches the logarithmic character of the human sense of hearing. The dB is used when comparing one signal level against another (such as the input and output levels of an amplifier or filter). When the two signal amplitudes are the same, the decibel value is 0dB. If one signal has twice the amplitude of the other the decibel value is +6dB, and if half the size it is –6dB.
When one signal is being compared to a standard reference level the term is supplemented with a suffix letter representing the specific reference. For example, 0dBu means the signal is the same as the standard line-level reference of 0.775mV rms.
The two most common standard level references are +4dBu (1.223V rms) and –10dBV (0.316V rms). The actual level difference between them is close to 12dB.
Decca Tree – A form of ‘spaced microphone’ arrangement in which three microphone capsules (usually, but not always, with omnidirectional polar patterns) are placed in a large triangular array roughly two metres wide, with the central microphone one metre further forward. Sounds approaching from different directions arrive at each capsule at different times and with slightly different levels, and these timing and level differences are used to convey the directional information in the recording. The timing differences between channels can result in unwanted colouration if they are combined to produce a mono mix.
Decoupler (also isolator) – A device intended to prevent the transmission of physical vibration over a specific frequency range, such as a rubber or foam block.
Delay – The time between a sound or control signal being generated and it auditioned or taking effect, measured in seconds. Often referred to as latency in the context of computer audio interfaces.
DI Box – Direct Injection Box. A device which accepts the signal input from a guitar, bass, or keyboard and conditions it to conform to the requirements of a microphone signal at the output. The output is balanced and with a low source impedance, capable of driving long mic cables. There is usually a facility to break the ground continuity between mic cable and source to avoid unwanted ground-loop noises. Both active and passive versions are available, the former requiring power from internal batteries or phantom power via the mic cable. Active DI boxes generally have higher input impedances than passive types and are generally considered to sound better.
Diaphragm – the movable membrane in a microphone capsule which responds mechanically to variations in the pressure or pressure gradient of sound waves. The mechanical diaphragm vibrations are converted into an electrical signal usually through electromagnetic or electrostatic techniques such as ribbon, moving coil, capacitor or electret devices.
Differential Receiver – A differential receiver is used to convert a balanced input signal to an unbalanced on at the output. It has two inputs which are summed together in opposite polarities so that common mode signals cancel each other out and are ignored, while differential signals are passed intact. A transformer can be used as a passive differential receiver, but most systems use active electronics.
Differential Signal – A signal which is applied between the two wires of a balanced interface, and which consequently is passed by the differential receiver. A differential signal can be presented as two signals of identical amplitude (half the total amplitude each) but opposite polarities – as would appear at the output of a dynamic microphone or a balancing transformer. However, it can also be presented as a full level signal on one line and no signal at all on the other, which is how 'impedance balanced' outputs appear.
Digital (cf. Analogue) – Digital audio circuitry uses discrete voltages or currents to represent the audio signal at specific moments in time (samples). A properly engineered digital system has infinite resolution, the same as an analogue system, but the audio bandwidth is restricted by the sample rate and the signal-noise ratio (or dynamic range) is restricted by the wordlength.
Double-lapped Screen – (Also known as a Reussen screen) The signal-carrying wires in a microphone cable are protected from external electrostatic and RF interference by a ‘screen’ which is a surrounding conductor connected to earth or ground. The Reussen screen is a specific form of cable screen, comprising two overlapping and counter-wound layers which are unlikely to ‘open up’ if the cable is bent, yet remain highly flexible
Dynamic – An alternative name for microphones that employ a electromagnetic system for converting diaphragm movement to an electrical system, i.e. moving coil and ribbon microphones.
Dome – A specific shape of drive unit diaphragm intended to push and pull the air to create acoustic sound waves. Most tweeters use dome-shaped diaphragms which are driven around the circumference by the drive unit’s motor system. ‘Soft-domes’ are made of a fabric – often silk – while metal domes are constructed from a light metal like aluminium, or some form of metal alloy.
Drive Unit/Driver – A physical device designed to generate an acoustic sound wave in response to an electrical input signal. Drive units can be designed to reproduce almost the full audio spectrum, but most are optimised to reproduce a restricted portion, such as a bass unit (woofer) or high-frequency unit (tweeter). A range of technologies are employed, with most being moving-coil units, but ribbon and electrostatic drive units also exist, each with a different balance of advantages and disadvantages.
Equaliser (cf. Filter) – A device which allows the user to equalise, balance or adjust the tonality of a sound source. Equalisers are available in the form of filters, shelf equalisers, parametric equalisers and graphic equalisers – or as a combination of these basic forms.
Equivalent Input Noise – A means of describing the intrinsic electronic noise at the output of an amplifier in terms of an equivalent input noise, taking into account the amplifier’s gain.
FET – Field Effect Transistor. A solid-state semiconductor device in which the current flowing between source and drain terminals is controlled by the voltage on the gate terminal. The FET is a very high impedance device, which makes it highly suited for use in impedance converter stages in capacitor and electret microphones.
Fidelity – The accuracy or precision of a reproduced acoustic sound wave when compared to the electrical input signal.
Filter (cf. Equaliser) – Filters remove unwanted parts of the spectrum above or below a turnover frequency, and with the rate of attenuation versus frequency is called the slope. A high-pass (or low-cut) filter removes frequencies below the turnover frequency and usually has a slope of 6, 12 or 18dB/octave.
Filter Frequency – The ‘turnover’ or ‘corner’ frequency of a high- or low-pass filter. Technically, the frequency at which the signal amplitude has been attenuated by 3dB.
FireWire – A computer interface format based upon the IEEE 1394 standard and named FireWire by Apple computers (Sony’s i.Link format is also the same interface). FireWire is a serial interface used for high speed isochronous data transfer, including audio and video. FireWire 400 (IEEE 1394–1995 and IEEE 1394a–2000) or S400 interface transfers data at up to 400Mb/s and can operate over cables up to 4.5metres in length. The standard ‘alpha’ connector is available in four and sixconnector versions, the latter able to provide power (up to 25V and 8 watts). The FireWire 800 format (IEEE 1394b–2002) or S800 interface uses a 9-wire ‘beta’ connector and can convey data at up to 800Mb/s.
Flash Drive (cf. Solid-state Drive) – A large capacity solid-state memory configured to work like a conventional hard drive. 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’. Some computers are now available with solid state flash drives instead of normal internal hard drives.
Flutter Echoes – Short time-span sound echoes which can be created when sound waves bounce between opposite parallel walls in a small or moderately sized room. A shorter version of the ‘slapback’ echo which can be experienced in a larger hall when sound from a stage is reflected strongly from the rear wall.
Frequency Response – The variation in amplitude relative to the signal frequency. A measurement of the frequency range that can be handled by a specific piece of electrical equipment or loudspeaker, often expressed with a variation limit, such as −3dB.
General MIDI/GM – A universally agreed subset of the MIDI standard, created to enable manufacturers to build synthesizers, synth modules and plug-in instruments that exhibit an agreed minimum degree of compatibility.
Ground Loop/Ground-Loop Hum – A condition created when two or more devices are interconnected in such a way that a loop is created in the ground circuit. This can result in audible hums or buzzes in analogue equipment, or unreliable or glitch audio in digital equipment. Typically, a ground loop is created when two devices are connected together using one or more screened audio cables, and both units are also plugged into the mains supply using safety ground connections via the plug’s earth pin. The loop is from one mains plug, to the first device, through the audio cable screen to the second device, back to the mains supply via the second mains plug, and round to the first device via the building’s power wiring. If the two mains socket grounds happen to be at slightly different voltages (which is not unusual), a small current will flow around the ground loop. Although not dangerous, this can result in audible hums or buzzes in poorly designed equipment.
Ground loops can often be prevented by ensuring that the connected audio equipment is plugged into the same socket or mains distribution board, thus minimising the loop. In extreme cases it may be necessary to disconnect the screen connection at one end of the audio cables or use audio isolating transformers in the signal paths. The mains plug earth connection must NEVER be disconnected to try to resolve a ground loop problem as this will render the equipment potentially LETHAL.
GUI – Graphical User Interface (pronounced ‘Gooey’) – a software program designer’s way of creating an intuitive visual operating environment controlled by a mouse-driven pointer or similar.
Hard Disk Drive (cf. Solid-state Drive) – 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 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.
Headroom – The available ‘safety margin’ in audio equipment required to accommodate unexpected loud audio transient signals. It is defined as the region between the nominal operating level (0VU) and the clipping point. Typically, a high quality analogue audio mixer or processor will have a nominal operating level of +4dBu and a clipping point of +24dBu – providing 20dB of headroom. Analogue meters, by convention, don’t show the headroom margin at all; but in contrast, digital systems normally do – hence the need to try to restrict signal levels to average around −20dBFS when tracking and mixing with digital systems to maintain a sensible headroom margin. Fully postproduced signals no longer require headroom as the peak signal level is known and controlled. For this reason it has become normal to create CDs with zero headroom.
High-range (high, highs) – The upper portion of the audible frequency spectrum, in a technical sense typically denoting frequencies above about 1kHz, but musically associated with frequencies over 3kHz.
Hub – Normally used in the context of the USB computer data interface. A hub is a device used to expand a single USB port into several, enabling the connection of multiple devices. Particularly useful where multiple software program authorisation dongles must be connected to the computer.
Hz/kHz – The standard abbreviation for Hertz (kilohertz) – a unit of measurement for frequency. 10Hz means ten complete cycles of a repeating waveform per second.
Impedance – The ‘resistance’ or opposition of circuit to the flow of current, when encountered in the context of electrical connections or the resistance that a medium presents to air flow, in the context of acoustics. Although measured in ohms, the impedance of a ‘reactive’ device such as a loudspeaker drive unit will usually vary with signal frequency and will be higher than the resistance when measured with a static DC voltage. Signal sources have an output impedance and destinations have an input impedance. In analogue audio systems the usually arrangement is to source from a very low impedance and feed a destination of a much higher (typically 10 times) impedance. This is called a ‘voltage matching’ interface. In digital and video systems it is more normal to find ‘matched impedance’ interfacing where the source, destination and cable all have the same impedance (e.g. 75 ohms in the case of S/PDIF).
Microphones have a very low impedance (150 ohms or so) while microphone preamps provide an input impedance of 1,500 ohms or more. Line inputs typically have an impedance of 10,000 ohms and DI boxes may provide an input impedance of as much as 1,000,000 ohms to suit the relatively high output impedance of typical guitar pickups.
Insert Points – The provision on a mixing console or ‘channel strip’ processor of a facility to break into the signal path through the unit to insert an external processor. Budget devices generally use a single connection (usually a TRS socket) with unbalanced send and return signals on separate contacts, requiring a splitter or Y-cable to provide separate send (input to the external device) and return (output from external device) connections. High-end units tend to provide separate balanced send and return connections.
Input Impedance – The input impedance of an electrical network is the ‘load’ into which a power source delivers energy. In modern audio systems the input impedance is normally about ten times higher than the expected source impedance – so a typical microphone preamp has an input impedance of between 1,500 and 2,500 ohms.
Isolator (also decoupler) – A device intended to prevent the transmission of physical vibration over a specific frequency range, such as a rubber or foam block.
Latency (cf. Delay) – The time delay experienced between a sound or control signal being generated and it being auditioned or taking effect, measured in seconds.
Lay Length – The distance along the length of a cable over which the twisted core wires complete one complete turn. Shorter lay lengths provide better rejection of electromagnetic interference, but make the cable less flexible and more expensive.
Limiter – An automatic gain-control device used to restrict the dynamic range of an audio signal. A Limiter is a form of compressor optimised to control brief, high level transients.
Loop – The process of defining a portion of audio within a DAW, and configuring the system to replay that portion repeatedly.
Loudspeaker (also Monitor and Speaker) – A device used to convert an electrical audio signal into an acoustic sound wave. An accurate loudspeaker intended for critical sound auditioning purposes.
Loudness – The perceived volume of an audio signal.
Low-range (low, lows) – The lower portion of the audible frequency spectrum, in a technical sense typically denoting frequencies below about 1kHz, but musically associated with frequencies below 300Hz.
Magnetic Shielding – Also called magnetic compensation (which is usually a more accurate description). A means of restricting the radiation range of the stray magnetic field from a drive unit’s permanent magnet which might otherwise interfere with the correct operation of moving-coil meters or CRT television monitors. While it is possible to enclose a magnet in a soft-metal case to prevent a stray magnetic field this becomes very expensive for large magnets, and so a more common approach is to affix additional small external magnets with opposite polarities to cancel out the unwanted stray field.
Maximum SPL – The loudest sound pressure level that a device can generate or tolerate.
Metering – A display intended to indicate the level of a sound signal. It could indicate peak levels (e.g. PPMs or digital sample meters), average levels (VU or RMS meters), or perceived loudness (LUFS meters).
Mid-range (mid, mids) – The middle portion of the audible frequency spectrum, typically denoting frequencies between about 300Hz and 3kHz.
Microphone – A device used to convert an acoustic sound wave into an electrical signal.
MIDI – Musical Instrument Digital Interface. A defined interface format that enables electronic musical instruments and computers to communicate instructional data and synchronise timing. MIDI sends musical information between compatible devices, including the pitch, volume and duration of individual notes, along with many other aspects of the instruments that lend themselves to electronic control. MIDI can also carry timing information in the form of MIDI Clock or MIDI Time Code for system synchronisation purposes.
Mineral Wool – Made from natural or synthetic minerals in the form of threads or fibres tangled together to form a moderately dense ‘blanket’ which permits but impedes air flow and is useful in the creation of sound absorbers, often employed as a cheaper and more efficient alternative to polyurethane form.
Mirror Points – The positions on the walls or ceiling where, if the surface was covered with an optical mirror, one or both loudspeakers could be seen in the reflection. The mirror point is essentially any position on a boundary where sound waves from a sound source – usually a monitor loudspeaker – will be reflected directly to the listening position. This is therefore the ideal location to place an acoustic absorber to prevent audible reflections.
Mixer – A device used to combine multiple audio signals together, usually under the control of an operator using faders to balance levels. Most mixers also incorporate facilities for equalisation, signal routing to multiple outputs, and monitoring facilities.
Modal Distribution – The characteristic distribution of low frequency resonances within a confined space such as a room.
Modes (room) – Specific patterns of low frequency sound reflection between surfaces in a room, resulting in resonant peaks.
Modelling – A process of analysing a system and using a different technology to replicate its critical, desired characteristics. For example, a popular but rare vintage signal processor such as an equaliser can be analysed and its properties modelled by digital algorithms to allow its emulation within the digital domain.
Monitor (also speaker and loudspeaker) – A device which provides information to an operator. Used equally commonly in the context of both a computer VDU (visual display unit) – such as an LCD screen – and a high quality, accurate loudspeaker intended for critical sound auditioning purposes.
Monitor Controller – A line-level audio signal control device used to select and condition input signals for auditioning on one or more sets of monitor loudspeakers. Some monitor controllers also incorporate facilities for studio talkback and artist cue mixes.
Mono – A single channel of audio.
Moving Coil – a technology used to convert energy between the mechanical and electrical domains. A coil of wire is allowed to move within a magnetic field. If the coil is caused to move it will generate an electric current proportional to the rate of movement (as in a microphone). If a varying electric current is passed through the coil it will move in proportion to the amount and direction of the current (as in a loudspeaker).
M-S (Mid-side) – A specialist form of coincident microphone array which, when decoded to left-right stereo, creates an equivalent XY configuration. In the MS array one microphone is pointed directly forward (Mid) while the second is arranged at 90 degrees to point sideways (Side). The Mid microphone can employ any desired polar pattern, the choice strongly influencing the decoded stereo acceptance angle. The Side microphone must have a figure-eight response and be aligned such that the lobe with the same polarity as the Mid microphone faces towards the left of the sound stage. Adjusting the relative sensitivity of the Mid and Side microphones affects the decoded stereo acceptance angle and the polar patterns of the equivalent XY microphones.
MTC – MIDI Time code – a format used for transmitting synchronisation instructions between electronic devices within the MIDI protocol.
Multi-timbrality – The ability of an electronic musical instrument to generate two or more sounds simultaneously.
Mutual Angle – the physical angle between two microphones, used to specify various microphone array configurations (e.g. 90 degrees for a Blumlein pair, or 110 degrees for an ORTF array).
Near-coincident – A means of arranging two or more directional microphone capsules such that they receive sound waves from the directions or interest at slightly different times due to their physical spacing. Information about the directions of sound sources is captured in the form of both level differences between the capsule outputs, generated by aiming directional polar patterns in different directions, and the timing differences caused by their physical spacing. Specific forms of near-coincident microphones include the ORTF and NOS arrangements.
Near Field – Describes a loudspeaker system designed to be used close to the listener. Some people prefer the term ‘close field’. The advantage is that the listener hears more of the direct sound from the speakers and less of the reflected sound from the room.
NOS – A specific form of near-coincident microphone array devised by the Nederlandse Omroep Stichting (NOS), the Dutch national broadcaster. The technique employs a pair of small-diaphragm cardioid microphones mounted with a mutual angle of 90 degrees and spaced apart by 30cm. The theoretical stereo recording angle is 81°.
Ohm – Unit of electrical resistance.
Off-/On-axis – Directional microphones are inherently more sensitive to sound from one direction, and the direction of greatest sensitivity is referred to as the principle axis. Sound sources placed on this axis are said to be ‘on-axis’, while sound sources elsewhere are said to be ‘off-axis’
Optimisation (of computer) – The concept of configuring a computer in such as way as to maximise its performance for certain tasks. In the context of a machine being used as a DAW, optimisation might involve disabling sub-programs that access the internet regularly or intermittently, such as email hosts, automatic program update checkers and so on. It might also include the structure of the hard drive, or the separation of program data to a system drive and audio data to a separate drive to minimise access times and maximise data throughputs.
ORTF – A specific form of near-coincident microphone array devised by the Office de Radiodiffusion Télévision Française (ORTF) at Radio France, the French national broadcaster. The technique employs a pair of small-diaphragm cardioid microphones mounted with a mutual angle of 110 degrees and spaced apart by 17cm. The theoretical stereo recording angle is 96 degrees.
Output Impedance – The effective internal impedance (resistance which many change with signal frequency) of an electronic device. In modern audio equipment the output impedance is normally very low. Microphones are normally specified with an output impedance of 150 or 200 ohms, although some vintage designs might be as low as 30 ohms.
Output Sensitivity – The nominal output voltage generated by a microphone for a known reference acoustic sound pressure level. Output sensitivity is normally specified for a sound pressure level of one Pascal (94dB SPL), and may range from about 0.5mV/Pa for a ribbon microphone, to 1.5mV/Pa for a moving coil, and up to 20 or 30mV/Pa for a capacitor microphone.
Overdubbing – Recording new material to separate tracks while auditioning and playing in synchronism with previously recorded material.
Passive Loudspeaker or Monitor – A loudspeaker which requires an external power amplifier, the signal from which is passed to a passive cross-over filter. This splits and filters the signal to feed the two (or more) drive units.
Patch (cf. Bank) – A specific configuration of sounds or other parameters stored in memory and accessed manually or via MIDI commands.
PCI Card – Peripheral Component Interconnect: an internal computer bus format used to integrating hardware devices such as sound cards. The PCI Local Bus has superseded earlier internal bus systems such as ISA and VESA, and although still very common on contemporary motherboards has, itself, now been superseded by faster interfaces such as PCI-X and PCI Express.
Phantom Power – A means of powering capacitor and electret microphones, as well as some dynamic microphones with built-in active impedance converters. Phantom power (P48) provides 48V (DC) to the microphone as a common-mode signal (both signal wires carry 48V while the cable screen carries the return current). The audio signal from the microphone is carried as a differential signal and the mic preamp ignores common-mode signals so doesn’t see the common-mode power supply (hence the ghostly name, phantom). This system only works with a balanced three-pin mic cables. Two alternative phantom power specifications also exist, with P12 (12V) and P24 (24V) options, although they are relatively rare.
Phono plug (RCA-phono) – An audio connector developed by RCA and used extensively on hi-fi and semi-pro, unbalanced audio equipment. Also used for the electrical form of S/PDIF digital signals, and occasionally for video signals.
Pitch Bend – A means of temporarily changing the tuning of an audio signal generator, such as a synthesizer oscillator, either manually via a control wheel or under MIDI control.
Plug-in – A self-contained software signal processor, such as an Equaliser or Compressor, which can be ‘inserted’ into the notional signal path of a DAW. Plug-ins are available in a myriad of different forms and functions, and produced by the DAW manufacturers or third-party developers. Most plug-ins run natively on the computer’s processor, but some require bespoke DSP hardware. The VST format is the most common cross-platform plug-in format, although there are several others.
Plug-in Power – Consumer recorders, such as MP3 recorders, are often equipped with a microphone powering system called ‘Plug-In Power’. This operates with a much lower voltage (typically 1.5V) and is not compatible with phantom powered mics at all.
Polar Pattern – The directional characteristic of a microphone (omni, cardioid, figure-eight, etc.).
Polyphony – The ability of an instrument to play two or more notes of different pitches at the same time.
Pop Shield – A device placed between a sound source and a microphone to trap wind blasts – such as those created by a vocalist’s plosives (Bs, Ps and so on) – which would otherwise cause loud popping noises as the microphone diaphragm is over-driven. Most are constructed from multiple layers of a fine wire or nylon mesh, although more modern designs tend to use open-cell foam.
Power Amplifier – A device which accepts a standard line-level input signal and amplifies it to a condition in which it can drive a loudspeaker drive unit. The strength of amplification is denoted in terms of Watts of power.
Powered Loudspeaker or Monitor – A powered speaker is a conventional passive loudspeaker but with a single power amplifier built in or integrated with the cabinet in some way. The amplifier drives a passive crossover, the outputs of which connect to the appropriate drive units.
Pre-amp – Short for pre-amplification: an active gain stage used to raise the signal level of a source to a nominal line level. For example, a microphone pre-amp.
Project Studio – A relatively small recording studio facility, often with a combined recording space and control room.
Proximity Effect – Also known as ‘Bass tip-up’. The proximity effect dramatically increases a microphone’s sensitivity to low frequencies when placed very close to a sound source. It only affects directional microphones – omnidirectional microphones are immune.
Quantisation – Part of the process of digitising an analogue signal. Quantisation is the process of describing or measuring the amplitude of the analogue signal captured in each sample, and is defined by the wordlength used to describe the audio signal – e.g. 16 bits.
Rack Mount – A standard equipment sizing format allowing products to be mounted between vertical rails in standardised equipment bays.
RAM – Random Access Memory: the default data storage area in a computer, normally measured in Gigabytes (GB).
Reflection – The way in which sound waves bounce off surfaces.
Reverb – Short for reverberation. The dense collection of echoes which bounce off of 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.
Reverb Decay – 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.
Ribbon – A type of electromagnetic microphone in which the diaphragm is also an electrical conductor which is placed within a strong magnetic field. As the diaphragm moves it generates a small proportional current.
Satellite (speaker) – Normally used in the context of a loudspeaker system employing a subwoofer to reproduce the lowest frequencies, with smaller ‘satellite’ loudspeakers to reproduce the higher frequencies.
Self Noise – A term used to describe the electronic noise contribution of the active impedance converter in an electrostatic microphone, and specified in terms of the equivalent acoustic sound pressure level required to create the same signal voltage as the amplifier’s noise floor. The self-noise figure dictates the lower limit of the microphone’s total dynamic range.
Sensitivity – The efficiency of a loudspeaker in converting an electrical input signal to an acoustic output signal, or of a microphone converting sound to an electrical signal.
Sample – Either a defined short piece of audio which can be replayed under MIDI control; or a single discrete time element forming party of a digital audio signal.
Sample Rate (cf. Bit Rate) – The rate at which a digital audio signal is intended to operate, normally denoted either in terms of kilo-samples per second (kS/s) or kilo-Hertz (kHz). The audio bandwidth must be less than half the sample rate, which is high quality audio systems operate at 44.1 or 48kS/s to provide an audio bandwidth of at least 20kHz.
Sampler – A hardware device or software program which replays (and possibly captures) short audio excerpts under MIDI control.
Sequencer – A device which records and replays MIDI instructions. Original sequencers were hardware devices but most are now software and are integrated into most DAWs.
Shockmount – a mechanical isolator intended to prevent the transfer of vibrations which may be transmitted through a microphone stand from reaching a microphone where they would otherwise produce unwanted low frequency sound.
SMPTE TimeCode – A means of affording recordings with reliable positional information coded to resemble clock time, originally used to identify individual picture frames in video and film systems.
Solid-state Drive (cf. Hard Disk Drive) – A large capacity solid-state memory configured to work like a conventional hard disk drive. 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’.
Sound Card – A dedicated interface to transfer audio signals in and out of a computer. A Sound Card can be installed internally, or connected externally via USB2 , USB3, Firewire or Thunderbolt. They are available in a wide range of formats, accommodating multiple analogue or digital audio signals (or both) in and out, as well as MIDI data in and out.
Soundproofing – The use of materials and construction techniques with the aim of preventing unwanted sound from entering or leaving a room.
Spaced Array – A means of arranging two or more microphone capsules such that they receive sound waves from different directions at different times – these timing differences being used to convey information about the relative directions of those sound sources. The technique is usually used with omnidirectional microphones, although directional mics can also be employed. The best known form of spaced array is the Decca Tree. Mono-compatibility is often reduced because the timing differences between channels often results in comb-filtering colouration when the channels are summed to mono.
S/PDIF – Sony/Philips Digital Interface. Pronounced either ‘S-peedif’ or ‘Spudif’. A stereo or dual-channel self-clocking digital interfacing standard employed by Sony and Philips in consumer digital hi-fi products. The S/PDIF signal is essentially identical in data format to the professional AES3 interface, and is available as either an unbalanced electrical interface (using phono connectors and 75ohm coaxial cable), or as an optical interface called TOSlink.
Speaker (also Loudspeaker and Monitor) – An accurate loudspeaker intended for critical sound auditioning purposes.
SPL – Sound Pressure Level. A measure of the intensity of an acoustic sound wave. Normally specified in terms of Pascals for an absolute value, or relative to the typical sensitivity of human hearing. One Pascal is 94dB SPL, or to relate it to atmospheric pressures, 0.00001 Bar or 0.000145psi!
SRA – See Stereo Recording Angle
Standing Waves – Resonant low frequency sound waves bouncing between opposite surfaces such that each reflected wave aligns perfectly with previous waves to create static areas of maximum and minimum sound pressure within the room. (See also Modes and Modal Frequencies)
Stereo – By convention, two channels of related audio which can create the impression of separate sound source positions when auditioned on a pair of loudspeakers or headphones.
Stereo Recording Angle – The angle over which sound sources can be captured by a microphone array. For a stereo array with a stereo recording angle of 90 degrees, sound sources can be placed ±45 degree relative to the array’s centre front axis, with a source at the extreme angle appearing fully left or right in the stereo image.
Subwoofer – A specific type of efficient loudspeaker system intended to reproduce only the lowest frequencies (typically below 120Hz).
Surround – The use of multiple loudspeakers placed around the listening position with the aim of reproducing a sense of envelopment within a soundstage. Numerous surround formats exist, but the most common currently is the 5.1 configuration in which three loudspeakers are placed in front of the listener (at ±30degrees and straight ahead), with two behind (at ±120 degrees or thereabouts), supplemented with a separate subwoofer.
Synthesis – The creation of artificial sound.
Synthesizer – A device used to create sounds electronically. The original synthesizers were hardware devices and used analogue signal generation and processing techniques, but digital techniques took over and most synthesizers are now software tools.
Talkback – A system designed to enable voice communication between rooms, such between an engineer and producer in a control room and performers in an adjacent recording room.
TRS – A type of quarter-inch jack plug with three contacts (Tip, Ring and Sleeve), used either for stereo unbalanced connections (such as on headphones) or mono balanced connections (such as for line-level signals). Physically compatible in size with the TS quarter-inch jack plug used for electric guitars and other instruments.
Tube – See Valve
Tweeter – The colloquial term to describe a loudspeaker drive unit optimised for the reproduction of high frequencies. (See Woofer).
USB – Universal Serial Bus. A computer interface standard introduced in 1996 to replace the previous standard serial and parallel ports more commonly used. The original USB1.0 interface operated at up to 12Mb/s, but this was superseded in 2000 by USB2.0 which operates at up to 480Mb/s. Most USB interfaces can also provide a 5V power supply to connected devices. USB3.0 was launched in 2008 and is claimed to operate at rates up to 5Gb/s, but it is only now (2011) starting to appear on hardware.
Valve – Also known as a ‘tube’ in America. A thermionic device in which the current flowing between its anode and cathode terminals is controlled by the voltage applied to one or more control grid(s). Valves can be used as the active elements in amplifiers, and because the input impedance to the grid is extremely high they are ideal for use as an impedance converter in capacitor microphones. The modern solid-state equivalent is the Field Effect Transistor or FET.
Vibrato – A cyclical variation in pitch often employed in musical performance.
W (Watt) – Unit of electrical power.
-way (as in, 2-way, 3-way) – A colloquial way of denoting how many separate frequency bands are reproduced by a loudspeaker. Most are two-way systems with a woofer and tweeter, but some are three way with a woofer, midrange and tweeter.
XLR – A very robust and latching connector commonly used to carry balanced audio signals such as the outputs from microphones or line level devices. An XLR is a type of connector developed by US manufacturer, Cannon, and used widely in professional audio systems. The company’s original X-series connector was improved with the addition of a latch (Cannon XL) and a more flexible rubber compound surrounding the contacts to improve reliability (Cannon XLR). The connector format is now is available in numerous configurations, from many different manufacturers, and with several different pin configurations. Standard balanced audio interfaces – analogue and digital – use three-pin XLRs with the screen on pin 1, the ‘hot’ signal on pin 2 and the ‘cold’ signal on pin 3.
XY – A specific way of mounting two directional microphone capsules such that they both receive sound waves from any direction at exactly the same time. Information about the direction of a sound source is captured in the form of level differences between the two capsule outputs. Commonly, the two microphones in an XY array are mounted with a mutual angle of 90 degrees, although other angles are sometimes used. The two capsules will have the same polar pattern, the choice of which determines the stereo recording angle (SRA). The XY configuration is entirely mono-compatible because there are no timing differences between the two channels.