ACTIVE CROSSOVER: A dividing network that splits a fullrange signal into two or more frequency groups and routes them to feed the various components (e.g., woofers and tweeters) in a speaker system. Active crossovers divide a line-level output signal from a mixer or other sound source and route the resulting signals to individual amplifiers that drive different speaker components. Passive crossovers usually are built inside speaker cabinets where they divide an amplifier's output signal for routing to different speaker combinations. See Bi-amp.

AES/EBU DIGITAL STANDARD: developed jointly by the Audio Engineering Society (AES) and the European Broadcast Union (EBU). The standard describes a format for transmitting two channels of digital audio along a serial cable. See S/PDIF.

AES: the Audio Engineering Society, a professional association of sound engineers headquartered in New York City, with chapters throughout the world. Sponsors large annual conventions showcasing new audio technologies and products.

AMPLIFIER: a device that tracks the amplitude of an incoming signal and proportionally increases the voltage, current or power of the signal by adding power from another source.

AMPLITUDE: magnitude, or level, of an electrical signal (voltage) or acoustical signal (volume).

ANALOG-TO-DIGITAL CONVERTER (ADC): a circuit or hardware peripheral that converts audio signals from the analog domain into a digital form.

ANALOG: in recording, a signal that is stored as a continuously varying electrical representation (analog) of the input signal.

ATTENUATION: the process of decreasing the amplitude of a signal as it passes from one point to another. Analog attenuation circuits typically use resistors to reduce the voltage of a signal. In audio, the effect of such attenuators is usually expressed in decibels.

AUX SEND: alang for auxiliary send, a circuit pathway in a mixing console that supplies an independent mix, which can be routed to an external (auxiliary) device such as an effects processor or monitor system. Most modern consoles have several aux sends on each input so several devices can process a sound. See bus.

BAFFLE: typically, a term referring to the board on which a speakers are mounted. Due to weakening of the board caused by mounting holes cut into it for speakers, ports, vents, etc., the front baffle of a cabinet is sometimes thicker than the other sides of an enclosure.

BALANCED LINE: an audio line comprising three conductors: two carrying signal and a ground (shield) wire, in which one of the signal wires carries the sound, while the other carries an inverted copy. When the signal reaches the destination, the inverted copy is flipped and added to the original. Any noise that has been induced into the signal is also inverted. When this is combined with the "un-inverted" noise, it cancels it out. Thus, balanced lines are less susceptible to hum and can carry audio signals over longer distances. Balanced audio lines typically use 3-pin XLR or 1/4-inch tip-ring-sleeve phone connectors.

BASS-REFLEX: a popular enclosure design that uses a tuned port to extend a loudspeaker's bass response by allowing some of the air movement from a woofer cone's rear motion to combine with the bass frequencies from the cone's front movement. However, to avoid phase cancellation from occurring when these signals are combined, the port length and diameter must be carefully matched (tuned) to the speaker's free-air resonance and the enclosure volume.

BI-AMP: short for "bi-amplification." A two-way audio system in which the signal is divided by frequency into two signals that are independently amplified and fed to separate speaker system components (e.g., woofers and tweeters). In a "tri-amped" system, signals are divided by frequency into three individually amplified groups. See crossover.

BIT: Short for binary digit, this is a single piece of information (expressed as either 0 or 1) in a computer or digital system.

BRIDGED MONO: a method of combining both channels of certain stereo power amplifiers to create a doubly powerful single-channel (monaural) amp. See monaural.

BUS: an internal pathway for audio signals in a console. Frequently misspelled "buss."

CAPACITOR MICROPHONE: an alternate, less-used term for condenser microphone.

CARDIOID MICROPHONE: a microphone with a directional pickup pattern that is most sensitive to sounds coming from the front and sides while rejecting sounds coming from the rear. The pickup pattern is roughly heart-shaped when viewed from above, hence the name "cardioid."

CLIPPING: a distortion condition in which the top of a waveform is cut off ("clipped"). This is usually caused when a signal overloads a stage of the device being driven.

CLOCK: a source of timing information, especially important when two or more devices must be synchronized. See synchronization.

COMPRESSION DRIVER: a specialized mid- or high-frequency speaker comprising a small diaphragm and voice coil coupled to a large magnet structure. The unit is mounted to a horn, which acoustically matches the impedance of the driver to the impedance of the air and shapes the signal. Compression drivers tend to be expensive due to the precision tolerances required in their manufacture, but they deliver many times more sound-pressure-per-watt of input power than traditional direct-radiating cone speakers.

COMPRESSOR: a device that smoothes the level of an input signal by regulating its dynamic range. A compressor prevents the signal from rapidly exceeding or falling below a selected amplitude threshold. Beyond the threshold, the ratio of the signal's input level to output level (e.g., 2:1, 4:1,= and so on) can be user-selected. Compression is commonly used to keep mic levels within an acceptable range. Because it can slow a signal's rate of decay below the threshold, compressors are also used to add sustain to instruments such as electric guitar and bass.

CONDENSER MICROPHONE: a microphone that picks up sounds via an electrically charged, metallized diaphragm, which is separated from a conductive back plate by a thin air layer. Sound waves striking the diaphragm cause a minuscule voltage change, which is increased by a tiny amplifier circuit within the mic body. Because power is required by both the microphone capsule and the amplifier, condenser microphones must have a power source, which can be a battery inside the mic body or "phantom" power from a mixing console or external power supply.

CROSSOVER: a dividing network that splits a full-range signal into two or more frequency groups and routes them to feed the various components (e.g., woofers and tweeters) in a speaker system. Passive crossovers are usually built inside speaker cabinets where they divide an amplifier's output signal for routing to different speaker combinations. Active crossovers divide a line-level output signal from a mixer or other sound source and route the resulting signals to individual amplifiers that drive different speaker components. See bi-amp.

CUE MIX: See monitor mix.

CUT-OFF FREQUENCY: the frequency point above or below that a filter strongly attenuates a signal. (Usually, the signal's output level at the cut-off frequency is 3 dB below its input level.) In a lowpass filter, a high cut-off frequency allows most of a sound through and generally produces a bright sound, while a low cut-off frequency blocks most of the sound and produces a muted or plain sound. See highpass filter, lowpass filter.

DAW: abbreviation for digital audio workstation, a generic name for a computer-based system of recording, editing and manipulating audio in the digital domain.

dB: abbreviation for "decibel." See decibel.

dBm: a term expressing an electrical power level, referenced to 1 milliwatt (i.e., 0 dBm = 1 mW). Originally, dBm was used to express the power dissipated in telephone applications with 600-ohm impedances, but it is not necessarily referenced to a particular impedance.

dBu: a means of expressing voltage, referenced so that 0 dBu equals 0.775 volts, regardless of impedance. 1 mW of power is dissipated if 0.775 volts is applied to a 600-ohm load, so when the load impedance is 600 ohms, 0 dBu = 0 dBm.

dBV: a means of expressing voltage, referenced so that 0 dBV equals 1-volt RMS, regardless of impedance.

dBv: synonymous with dBu, but rarely used due to confusion with dBV. See dBu.

DCA: abbreviation for "digitally controlled amplifier," which is an amplifier circuit whose output gain can be varied by a digital signal.

DECIBEL: a unit of measure used to logarithmically express ratios of change in power or signal levels. Equal to one-tenth of a Bel (named for Alexander Graham Bell).

DIGITAL: computer technology in which information is described as a series of (usually binary) numbers.

DIGITAL-TO-ANALOG CONVERTER (DAC): a device that converts audio from the numeric string of a digital representation to an analog signal of constantly fluctuating voltages.


DIM SWITCH: a switch near a console's master section that reduces (attenuates) playback level by a set level (usually 15 or 20 dB), and is often used to momentarily cut the playback level during control room conversations, answering phone calls, etc.

DIP SWITCH: A bbreviation for Dual In-line Package, this is one or a series of compact switches designed for mounting directly on a circuit board. DIP switches are often found inside computers and other electronic gear as a method of setting user preferences and defaults, usually in "set-and-forget" applications.

DYNAMIC MICROPHONE: a transducer that relies on the law of induction, with an output proportional to the velocity of a moving element within a magnetic field. The most common type is the moving-coil microphone, which picks up sounds when sound waves strike a diaphragm attached to a coil of wire. When the coil moves within the magnetic structure of the microphone, it creates an output voltage. The process is exactly the reverse of the way a speaker operates. Moving-coil dynamic microphones tend to be extremely rugged, making them well suited for most sound reinforcement applications. The other common type of dynamic microphone is the ribbon mic.

DYNAMIC RANGE: a ratio (expressed in decibels) of the difference between the softest and the loudest sound that can be produced, reproduced or captured by a musical instrument or audio device.

ECHO: the effect of a sound reflecting off a distant surface and returning to the source. Typically, an echo is defined as a delay lasting more than 50 milliseconds (0.05 seconds).

ECHO SEND: a console bus that allows the engineer to route a variable part of a channel's signal to an effects device. Also sometimes called reverb sends or post-fader sends.

EQUALIZATION: a circuit that allows the frequency-selective manipulation of a signal's amplitude. The simplest equalizers are shelving types, offering the ability to cut or boost gain above or below a given frequency. Examples include the treble and bass controls found in home stereo systems or guitar amps. More complex circuits that allow tonal shaping in multiple frequency bands include graphic and parametric equalizers. Abbreviated "EQ."

FEEDBACK: a condition in which the output of a circuit recycles through its input. Acoustic feedback is a whine or howl that occurs in live audio situations when an amplified sound re-enters a sound system through the same microphone (or guitar pickup) that reproduced the original source, creating a loop.

FREQUENCY: the number of times a periodic waveform cycles, or repeats, over a period of time. See hertz.

FUNDAMENTAL FREQUENCY: the lowest root frequency component of a periodic waveform. The fundamental frequency of a sound usually is perceived as its pitch. (Sometimes, this is true even when the fundamental's amplitude is lower than that of its harmonics.)

GAIN: a ratio expressing the difference between the input and output power, level or current in a circuit.

GATE: a device that opens or closes a pathway by stopping signals that fall below a user-defined level. Audio gates often are used to salvage noisy tape tracks and silence "dirty" sound systems: The gate stays closed—blocking residual, low-level noise—until the audio signal's level exceeds a user-determined threshold. Then, the gate opens, allowing the sound to be heard. Gates can also be used to create effects such as gated reverb.

GRAPHIC EQUALIZER: a frequency-shaping device having multiple filter bands, each operating at a fixed frequency and bandwidth.

GROUND LOOP: a condition that occurs when several ground pathways exist between two devices, resulting in hum and increased noise.

HEADROOM: the margin of safety (usually expressed in decibels) between nominal operating levels and a signal-overload condition.

HERTZ: a unit of measure of the frequency of a vibrating object, such as a guitar string, speaker cone or electrical signal. Equivalent to cycles per second, it is named for Heinrich Hertz and abbreviated "Hz."

HIGHPASS FILTER: a circuit designed to attenuate, or cut, frequencies that fall below some designated point, while allowing higher frequencies to pass unaffected.

HYPERCARDIOID: a variation of the cardioid microphone pickup pattern. A hypercardioid microphone is most sensitive at the front and sides, while rejecting sounds entering 120° to the rear.

IMPEDANCE: Measured in ohms, this is a way of expressing a circuit's opposition (resistance and reactance) to a signal or current attempting to pass through. The practical difference between impedance and resistance is that impedance changes as a function of frequency.

IN-EAR MONITOR: a system allowing onstage players to listen through earphones rather than floor monitor speakers. Often abbreviated as IEM.

INFINITE BAFFLE: alternative term for a sealed speaker enclosure. Because the sound waves created by a speaker cone's rear motion cannot interact with those emanating from the front of the cone, the effect of the sealed box is akin to having a front baffle of "infinite" size, hence the phrase "infinite baffle."

I/O: abbreviation for "input/output."

JACK: a "female" connector designed to "mate" with a "male" connector or plug.

K: symbol for "kilo" in computer applications, in which a kilo represents 1,024 rather than 1,000. For example, one kilobyte (1 KB) equals 1,024 bytes. See byte.

k: scientific symbol for "kilo" (1,000). For example, a standard test tone is 1,000 Hz, which also can be stated as "1 kHz."

LIMITER: a device that severely restricts the upper dynamic range of a signal, regulating the rate of increase of an input signal's amplitude to keep it from exceeding a pre-determined threshold. Limiters are closely related to compressors but apply much higher compression ratios, usually in excess of 20:1.

LINE-LEVEL: an input or output operating level, typically -10 dBV for home and semi-pro equipment and +4 dBm for professional gear. Typical line-level audio signals include synth outputs, mixer outputs and signal processor outputs.

LOWPASS FILTER: a circuit designed to attenuate frequencies that occur above some designated point while allowing lower frequencies to pass unaffected.

MADI: acronym for "multichannel audio digital interface," a professional standard for transmitting up to 56 channels of digital audio data over a single cable.

MATRIX MIXER: a specialized submixer built into sound reinforcement consoles that allows the engineer to create multiple dedicated mixes for applications such as lobby audio, subwoofers, underbalcony fills or separate feeds to individual clusters or zones in a venue.

MEGABYTE: a quantity of computer data or memory capacity equal to 1,024 kilobytes, or 1,048,576 (1,024x1,024) bytes. Abbreviated "MB."

MIDI MACHINE CONTROL: a protocol for using MIDI commands, often from a console/controller surface, to control the transport functions (stop/play/record/locate/rewind/fast-forward) of a tape recorder/workstation.

MILLISECOND: one one-thousandth of a second. Abbreviated "ms."

MONAURAL: having one audio channel.

ms: See millisecond.

MONITOR MIX: one or more simultaneous mixes—live or in the studio—created to help performers hear while they perform. Sometimes referred to as "cue mixes" in the studio.

MUTE: a control that interrupts ("mutes") the flow of a signal. For example, during a mixing session, a console with muting would allow the engineer to silence a noisy guitar track during a quiet introduction and activate it just before the guitar part begins. Some mixers offer the ability to automate mutes.

NOISE GATE: See gate.

NYQUIST THEOREM: the basic tenet in digital audio systems, stating that the highest throughput frequency in a sampling system must be equal to or less than one-half the sampling frequency. Theoretically, the highest frequency reproducible (without generating undesirable aliasing noise) by a system operating at a sampling rate of 48 kHz is 24 kHz. In practice, with the need for brickwall anti-aliasing filters, the actual upper-frequency response of such a system is somewhat less.

OHM'S LAW: The basis for nearly all electronic and electrical theory, this law states a constant relationship between voltage, current and resistance. In a circuit, the voltage across an element is equal to the current in amperes through the element, multiplied by that element's resistance in ohms. Mathematically, this is expressed as E = IR, where E is the voltage, I is the current and R is the resistance.

OMNIDIRECTIONAL MICROPHONE: a microphone that is equally sensitive to sounds coming from all directions.

PANNING: the ability to place a monaural signal at any point in the stereo soundfield of a stereo signal using amplitude clues for localization. Short for "panorama," a pan control on a mixing board is a rotary control where the extreme counterclockwise position routes a signal to the left-channel output, the extreme clockwise position routes the signal to the right and the center position sends the signal equally to the right and left sides.

PARAMETRIC EQUALIZER: a circuit designed for frequency-selective attenuation or boosting of a signal's amplitude, with independent controls for gain, center frequency and bandwidth (including continuously adjustable Q). A quasi-parametric EQ may provide full frequency and gain adjustment, but only two or three Q settings. Sweepable EQs have an adjustable (sweepable) center frequency, but operate on a fixed bandwidth.

PASSIVE CROSSOVER: a dividing network that splits a full-range signal into two or more frequency groups and routes them to feed the various components (e.g., woofers and tweeters) in a speaker system. Passive crossovers usually are built inside speaker cabinets, where they divide an amplifier's output signal for routing to different speaker combinations. Active crossovers divide a line-level output signal from a mixer or other sound source and route the resulting signals to individual amplifiers that drive different speaker components.

PATCHBAY: a central routing center for signals, such as audio, video or MIDI data. While patchbays have traditionally used short cables (called patch cords) to route signals to various points on a panel of jacks, they have been evolving slowly into microprocessor-controlled devices that handle and store signal routings electronically.

PHANTOM POWER: a method of powering condenser microphones by sending DC current (typically 9 to 52 volts) over the same mic cable that carries the audio signal. "Phantom" is derived from the fact that there is no visible power cord and the voltage is not perceptible in the audio path.

PHASE: the relative measurement of a period of time referenced to the start point of a cycle of a periodic waveform. In one complete period, a wave's polarity fluctuates 360 degrees (180 degrees positive and 180 degrees negative). Absolute phase is a reference point in time within one cycle; e.g., halfway through one period, the waveform's phase is 180 degrees. At one-quarter of the waveform, the phase is 90 degrees. Relative phase is an instantaneous ("snapshot") measure of the difference in time between two acoustic or electronic waveforms of the same waveform and frequency. For example, if one waveform is one-quarter of the way through its cycle (90 degrees at its peak positive value) and the other is three-quarters of the way through its cycle (270 degrees at its greatest negative value), they are 180 degrees out-of -phase with respect to each other. The two signals are "in phase" if their amplitudes are identical at the same point in their cycles. Console inputs often have "phase" switches (more correctly, these should be referred to as "polarity" switches), which invert the phase of the signal 180 degrees.

PHASE CANCELLATION: an attenuation of signal components resulting from combining out-of-phase waveforms. When two waveforms are mixed, their harmonics are added. If the signals are out-of-phase with each other, the amplitudes of the harmonic components differ at various times (as determined by the phase relationship). If the added harmonics have the same polarity, then the signal is reinforced at those frequencies. If harmonics with positive values are added to harmonics with negative values, then the signal is attenuated (canceled) at those frequencies.

PHASE SHIFT: a slight time difference between two similar waveforms, which puts them out-of-phase with respect to each other.

PHONE CONNECTOR: a connector based on a plug having a shaft 1/4 inch in diameter. Commonly used as audio connectors on electric guitars, synthesizers and "semi-pro" signal processors and mixers. Originally developed by Bell Telephone; hence, the "phone" name.

PHONO CONNECTOR: Sometimes referred to as an RCA connector, these are generically known as pin-jack connectors. Commonly used on home stereo equipment, the "phono" designation for these audio connectors comes from the fact that they originally were universally used for the outputs on phonographs.

PINK NOISE: a test signal comprising noise that has been shaped to provide equal intensities of sound in each octave band. Pink noise is used for test signals because its spectral balance closely compensates for the frequency sensitivity of the human ear.

POLAR PATTERN: A circular, 2-D plot that indicates the directional response of a transducer. While polar patterns are commonly used to show microphone pickup patterns, they can also indicate the dispersion of a speaker. Interpreting polar patterns, even with something as simple as a common cardioid microphone, can be fairly complex. For one, the polar response shown on paper doesn't indicate the fact that a microphone's pickup pattern is actually a 3-D space around the mic. Second, the polar response of any microphone is frequency-dependent. For example, an ultradirectional "shotgun" microphone is extremely directional at high frequencies and much less so at low frequencies.

POST-FADER SEND: Sometimes called an "effects send," this refers to a console auxiliary send that is connected after ("post") a channel's output fader. For example, if a post-fader send is used to route part of a vocal track to a reverb, when the vocal channel's level is raised with the fader, the amount sent to the reverb also would increase. See pre-fader send.

POTENTIOMETER: an electronic component comprising two terminals connected to either end of a resistive element and a conductor that can be moved between the two ends, thus allowing the creation of a variable resistor or voltage divider. Potentiometers are most often rotary controls (such as volume controls), but also exist as linear controls, such as faders or sliders. Often called a "pot."

PRE-FADER SEND: Sometimes called a "monitor" (or "foldback") send, this refers to a console auxiliary send that is connected before ("pre") a channel's output fader. For example, if a pre-fader send is used to route part of a vocal track to a stage monitor, when the main vocal channel's level is raised, the amount sent to the monitor would be unchanged. See post-fader send.

PROXIMITY EFFECT: a boost in the low-frequency response of a directional microphone that occurs when the sound source is relatively close to the microphone. The phenomenon begins when the source is about two feet away from the mic capsule and becomes more noticeable as the subject gets closer to the mic. Used properly, a singer can use the proximity effect as a means of adding fullness to a voice; however, the effect can also emphasize nondesireable low-frequency noises such as breath sounds and popping consonants ("p" and "b" sounds).

PSYCHOACOUSTICS: the study of how sounds affect the human brain. During the years, psychoacoustic research has resulted in an improved understanding of human hearing, such as the way the brain processes sounds picked up by the left and right ears and translates subtle differences between the two into cues that indicate directionality.

Q: in filters, the ratio of a bandpass or band-reject filter's center frequency to its bandwidth. Thus, assuming a constant center frequency, Q is inversely proportional to bandwidth (i.e., higher Q values indicate a narrower bandwidth). For this reason, the term is often used to denote bandwidth. See parametric equalizer, resonance.

RACKMOUNT: equipment designed to fit industry-standard, 19-inch-wide equipment racks and cases. The height of rackmount gear is standardized in 1.75-inch "units": a 2U (2-unit) device is 3.5 inches tall, a 10U mixer is 17.5 inches tall, etc.

RCA CONNECTOR: See phono connector.

RESISTOR: an electronic component that opposes the flow of electrical current. Resistance is measured in ohms.

RESONANCE: the property possessed by a simple vibrating body of oscillating more strongly in sympathy with a regular oscillatory disturbance at the same frequency. When a vibrating object (such as a guitar body) is stimulated by a second oscillator (such as a vibrating string), its pattern of vibration may be altered. If the two vibrate at the same (or a harmonically related) frequency, they tend to phase-lock together in sympathetic vibration at this common resonant frequency. The amplitude of their vibration is thus greatly increased. Oscillations at nonharmonic frequencies have far less effect. The same principle holds with electrical signals.

REVERBERATION: the decaying residual signal that remains after a sound occurs, created by multiple reflections as the original sound wave bounces off walls, furniture and other barriers within a room or other acoustical environment. Modern reverb effects processors use digital signal processing (DSP) techniques to simulate acoustical spaces. Analog methods of simulating reverberation include spring reverbs (commonly used in guitar amps), plate reverbs (which use a mechanical transducer to vibrate a large metal plate) and reverb chambers (where signals from a speaker sounding in a room are picked up by one or more microphones).

RIBBON MICROPHONE: a type of dynamic microphone that uses a thin metal ribbon placed between the poles of a magnet. Ribbon mics are typically bidirectional, meaning they pick up sounds equally well from either side of the mic.

RMS: abbreviation for Root-Mean-Square, a formula to describe the level of a signal. RMS is derived by squaring all the instantaneous voltages along a waveform, averaging the squared values and then taking the square root of that number. When used to describe power amplifiers, RMS power (in watts) is considered a more useful measure of power output than "program" or "peak" power. (A power amp's performance depends on the nature of the input signal. "Peak" power ratings don't account for this, whereas RMS ratings, because they are derived from multiple points in a sine wave, more closely reflect the actual energy content of the input signal.) RMS also is used to measure input sensitivity (in volts) in a preamp or line amplifier.

ROLL-OFF FILTER: a circuit that attenuates a signal that is above (lowpass filter) or below (highpass filter) a specified frequency. For example, microphones frequently have a bass roll-off filter to remove wind noise and/or excessive breath pops.

ROM: acronym for read-only memory, a non-erasable chip or circuit capable of storing computer data.

RTA: An abbreviation for real-time analyzer, RTA is an acoustical measurement device that displays the frequency response of a signal played back through P.A. or studio speakers. RTAs typically divide the audio spectrum into 16 or 31 bands that represent individual frequencies that correspond with the frequency sliders used on graphic equalizers, allowing the user to compensate for room or system anomalies.

RT60: a means of expressing the measurement of the reverberation time in a room or acoustical space that indicates the elapsed time required for the reverbereration to decay by 60 decibels.

SAMPLING FREQUENCY: See sampling rate.

SAMPLING RATE: Also referred to as sampling frequency, this is the rate at which "snapshots" of an analog signal are converted into digital representations. The higher the sampling rate, the greater the accuracy of a digital reproduction. See Nyquist Theorem.

SENSITIVITY: a measure of the relative efficiency of a speaker or loudspeaker system, often expressed as the number of decibels the unit will produce fed from a 1-watt signal measured at a distance of one meter.

SIGNAL-TO-NOISE RATIO: a ratio (in decibels) that expresses the difference between the level of a signal at a reference point in a circuit and the level of electrical noise at the same point.

SINE WAVE: a continuous periodic waveform whose amplitude varies as the sine of the linear function of time. Occasionally referred to as a sinusoidal wave, it has no harmonics except the fundamental.

SOLO: a feature on a mixing console that automatically routes one or more selected channels to the recording monitors or headphones without disturbing the main audio mix. For example, if a buzzing sound is heard while recording a drum/piano/bass rhythm section in the studio, an engineer can push the solo button on several tracks to locate the offending instrument. Some studio consoles use destructive solo, where the solo’ed instruments replace the mix in the main stereo bus. On sound reinforcement mixing consoles, solo functions are normally routed to headphones (non-destructive solo), allowing the engineer to check console channels while the concert is in progress. In-place solo is a function that permits the user to hear individual channels but in the correct stereo perspective as defined by that channel's pan control.

SPACED PAIR: a stereo microphone technique where two microphones are pointed directly at the source separated by two feet or more. Depending on the mic-to-source distance, this method can provide an extremely wide (occasionally exaggerated) stereo perspective. See X-Y miking.

S/PDIF: acronym for Sony/Philips Digital Interface, a "consumer" digital interface using either fiber-optic or coaxial (RCA) connections that allow the transfer of digital audio data from one device to another.

SPL: abbreviation for "sound pressure level," a means of expressing sound levels. Frequently used as a comparative measure of speaker efficiency or maximum system output.

SUPERCARDIOID MICROPHONE: a variation of the cardioid microphone that is most sensitive at the front while rejecting sounds entering 150° to the rear.

THD: abbreviation for "total harmonic distortion," a condition where a circuit adds additional unwanted harmonics (e.g., second and third harmonics) that were not part of the original signal. THD represents the effect of all the harmonic components (hence, the "total" part of the name, as opposed to second- or fifth-harmonic distortion) and is usually expressed as a percentage of the signal.

TRANSDUCER: a device that transforms energy from one form to another. Examples of electromechanical transducers include microphones (which convert acoustic pressure into electrical voltage) and loudspeakers (which convert voltages into acoustic pressure).

TRS: abbreviation for "tip, ring, sleeve." A type of three-conductor ("stereo") connector used on some phone and TT connectors. The tip and ring carry the program signal and the sleeve is ground. TRS connectors are commonly used for stereo jacks (left, right, ground), console channel insert points (send, return, ground) and monaural balanced lines (in-phase, out-of-phase, ground). See balanced line.

TT CONNECTOR: Abbreviation for "Tiny Telephone," TT connectors use miniature phone plugs with a 0.173-inch diameter shaft. Due to their compactness and reliability, TTs are often used for professional console and outboard patchbays in studios and live sound applications, in which a single patchbay may require hundreds of patch points in a limited space. The TRS versions of TT connectors are capable of handling balanced line signals and are preferred in pro audio installations. See phone connector, patchbay, balanced line.

TWEETER: a loudspeaker designed to produce high frequencies.

VCA: abbreviation for voltage-controlled amplifier, an amplifier circuit whose output gain can be varied by an external voltage.

VOLT: a unit of measure of electromotive force (resulting from a difference in electrical potential) equal to the force required to produce a current of one ampere through an element having a resistance of one ohm.


WATT: unit of measure of electrical power dissipation, formally defined as one joule (a unit of energy) per second, which is equal to the power absorbed by one ohm of resistance when one ampere of current is in the circuit. Electrical power, measured in watts, can be derived in three ways: the voltage squared divided by the resistance (V2/R), the current squared times the resistance (I2R) and the product of the voltage and the current (VI). Named for James Watt, inventor of the steam engine and the speed governor.

WAVEFORM: a 2-D graph of one period of a signal, showing changes in pressure (amplitude) as a function of time.

WAVELENGTH: the distance from the beginning to the end of one cycle (or from equivalent points in two consecutive cycles) of a waveform. A wavelength is equal to the speed of sound times the frequency of the waveform.

WHITE NOISE: a test signal comprising random noise, providing constant energy at all frequencies, similar to the sound heard when an FM radio is set between stations.

WOOFER: a speaker designed to reproduce low frequencies.

XLR: Developed by ITT/Cannon, XLRs are rugged, locking, multi-pin connectors frequently used in professional audio equipment. While 3-pin XLRs are most commonly seen on microphones and console inputs, other configurations also exist, such as 4-pin XLRs (a standard for stage intercom systems) and 5-pin XLRs (often used on stereo microphones).

X-Y MIKING: a stereo microphone technique where two directional mics (typically cardioid) cross at an angle from 90° to 130°, with their capsules placed closely together. In most cases, X-Y miking provides good stereo separation with a well-balanced image. See spaced pair.

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