A-B Test
Comparing the performance of two or more models of equipment such as amplifiers or speaker systems by listening and switching quickly from one to the other.

Absorption
The ability of a room to take up or absorb the acoustic energy radiated within it. There are many types of absorption since it can be frequency dependent. There are certain materials such as acoustical ceilings that may absorb more high frequencies than lows. Diaphragmatic absorptions (caused by loose wall panels or cavities behind the panels) cause certain low frequencies to be absorbed.

AC Mains
110-120 Volts alternating current (60 Hz)

Acoustic
Relating to the production, effects and transmission of sound waves; the transmission of sound waves through various mediums. Pertaining to the act or sense of hearing, the science of sound, or the sound heard.

Acoustical
Sound or properties of sound; the acoustical response of a room has to do with the way that room responds to sound.

Acoustic Treatment
The application of acoustic or sound absorbing material to a room or enclosure to obtain the desired acoustic characteristics.

Active
A type of electronic circuitry that can increase the gain or amplitude of a signal. Active gain controls. Active Equalization. Active Direct Boxes. Active Crossover.

ADA
The Americans with Disabilities Act. A federal law enacted in 1990 designed to end discrimination against persons with handicaps and to provide full accessibility for persons with handicaps. With respect to sound systems, the ADA requires all large public facilities to provide for the needs of hearing impaired individuals by offering assistive listening devices. [see ALS]

ALS
Assistive Listening System. This term refers to systems used to augment the regular sound system to allow hearing impaired persons to more clearly hear. These can be either hardwired (earphones) or wireless ( inductive loop, FM, infrared). The FM and infrared wireless systems are most common and allow the user to have individual control of the listening volume.

Ampere
Named after Andre Ampere (1775-1836), French scientist. A unit of measurement of electrical current (I). One amp of current represents 6.2818 x 10 electrons flowing past a given point in one second, and is equal to one coulomb.

Amplification
An increase in signal magnitude from one point to another, or the process causing this increase.

Amplifier
A device capable of increasing the gain (magnitude) or power level of a voltage or current that is varying with time (frequency), without distorting the wave form of the signal. An amplifier is used to increase weak signals (such as those from a program source) to a level sufficient to drive loudspeakers.

Analog
A physical variable which remains similar to another variable insofar as the proportional relationships are the same over some specified range. The electrical signal produced by a microphone is an electrical analog of the acoustic sound that the microphone is reproducing. The continuous electrical signal that the microphone produces varies in voltage and frequency as a direct correlation to the nonelectrical acoustic information impressed on the transducer. The electrical signal is analogous to the acoustical sound that the microphone reproduces, i.e., the voltage that the microphone produces is the electrical analog of the acoustic sound source.

Anechoic
Refers to a room in which all surfaces are lined with acoustic absorption material to such an extent that the room absorbs sound energy instead of reflecting it around the room (no echo). A room that offers nearly total absorption is called an Anechoic Chamber and must be quite large in order to accommodate low frequencies.

Attenuation
A decrease in signal magnitude from one point to another, or the process causing this decrease.

Attenuator
An adjustable passive network which reduces the power level of a signal without introducing any appreciable distortion.

Audio Chain
The order of sequence for connecting audio components, i.e., microphone, preamplifier (mixer), effects device, graphic equalizer, crossover, amplifier, and speaker.

Audio Description
A service to allow vision impaired individuals to better enjoy live and televised performances by providing narration of visual aspects of the performance. In live performances, the audio description is delivered to the listeners via wireless receivers similar to those used for hearing impaired individuals. [see ALS]

Audio Frequency
Any frequency corresponding to a normally audible sound wave.

Audio Range
20 Hz to 20,000 Hz. (Twenty cycles per second to twenty thousand cycles per second). The frequency response spectrum of human auditory perception.

Automatic Gain Control (AGC)
A process by which gain is automatically adjusted as a function of the input level or other parameter. A device used to automatically control the level of a sound system based on the input level.

Aux Input / Aux Output
An auxiliary input or output that is generally a direct connection to the device's internal signal bus. These are used to interconnect devices which have the same signal level. That is, where gain is not required such as when connecting a microphone to a mixer. Typical devices that operate at "aux level" are tape decks, CD players, equalizers, signal processors, etc.

Baffle
The panel on which the speaker is mounted within the speaker enclosure. The term derives from its original use in preventing or baffling the speaker's rear sound waves from interfering with its front waves.

Balanced Cable
A cable consisting of a pair of wires surrounded by either a braided shield or foil wrap with a drain wire.

Balanced Line
A transmission line consisting of two conductors plus a shield, capable of being operated so that the voltages of the two conductors are equal in magnitude (voltage) and opposite in polarity with respect to ground. A balanced line offers common mode rejection, or cancellation by attenuation, of signals electromagnetically induced into the signal lines. [see Common Mode Rejection]

Bandpass
Refers to a type of filter that passes a certain band of frequencies uniformly and attenuates or reduces the level of frequencies below and above the specified bandpass.

Bandwidth
Response characteristic in which a definite band of frequencies having a low frequency and high frequency limit are transmitted or amplified uniformly.

Bass Reflex
A type of speaker enclosure in which the speaker's rear sound wave emerges from a critically dimensioned auxiliary opening, or port, to reinforce the bass tones.

Biamp
Separating the audio spectrum into two bands, i.e., high frequencies (high pass) and low frequencies (low pass) by means of an electronic crossover and using two separate amplifiers or channels of an amplifier. One amplifier or channel passes the high pass signals (high frequencies) to drive the high frequency component or horn of the speaker system. The other amplifier or channel passes the low pass signals (low frequencies) and drives the woofer or low frequency component of the speaker system. The advantage to biamp operation of a sound system is increased headroom and dynamic range.

Boost
A term used to indicate an increase in gain of a frequency or band of frequencies when equalizing an audio signal. Opposite of cut.

Bridge Mode (Mono)
Operating a stereo amplifier in mono via the bridge mode switch, which typically makes Channel A output the positive power rail and Channel B output the negative power rail. Since the signal swings between A and B Channels, the output of the amplifier is twice that of single channel operation.

Bridging
Connecting one electrical circuit in parallel with another. Typically used to describe the process or connecting the output of a single device to the inputs of multiple other devices.

Bus
A conductor that serves as a common connector to several signal sources, most often associated with a mixer. A separate signal routing to a specified output.

Capacitor
A device which consists essentially of two conductors (such as parallel metal plates) insulated from each other by a dielectric (a material in which an electric field can be sustained with a minimum dissipation in power) and which introduces capacitance into a circuit, stores electrical energy, blocks the flow of direct current (DC), and permits the flow of alternating current (AC), to a degree dependent on the capacitor's capacitance and the current frequency.

Cardioid
A type of microphone having a heart shape pickup pattern that picks up sound better from the front (on axis) than back (off axis). [See Unidirectional]

Clipping
Amplifier overload causing a squaring off or undesirable change in the wave form resulting in distortion or perceptible mutilation of audio signals.

Cluster
An array of loudspeakers or horns designed to act as a single or point source of sound.

Coincident
Two signals are said to be coincident when they correspond exactly, fall upon or meet at the same point. Coinciding or occurring in space or time in exact agreement.

Comb Filter
When two combining sound waves have different amplitudes, phases, and frequencies, the resultant soundwave develops many nulls or spaces where the energy has cancelled. When viewed on a graphic recorder the resultant frequency response resembles a comb due to the nulls or notches of information that have cancelled.

Combining Unit
Typically used with microphones to combine two or more mics into a single input of a mixer. These devices are usually passive transformer or resistive circuits, but active versions are available which provide a higher degree of isolation between the multiple inputs..

Common Mode Rejection
The ability of an amplifier to cancel a common mode signal (such as interference) that is applied equally to both input terminals of a balanced amplifier, while responding to a signal from the source that is constantly changing direction (alternating current) so it is out of phase with respect to the two balanced signal lines; therefore it is not common mode and will be passed and not rejected.

Compander
A combination of a compressor at one point of a signal path for reducing the level of the signal, followed by an expander at another point for a complimentary increase in signal level.

Compression
Reduction of the effective gain of an amplifier at one level of signal with respect to the gain at a lower signal level.

Conductor
A wire, cable or other material (metal, liquids, gases, or plasma) that is suitable for carrying electric current

Continuous Power
This power rating represents the most conservative statement of the capability of an amplifier. It is also called "RMS" power. It denotes the amount of power an amplifier can deliver when amplifying a constant steady tone. It is usually measured at a signal frequency of 1kHz for a specific distortion. Continuous power in watts is expressed as: W = V2/R Power in watts equals the voltage squared divided by the resistance of the load.

Continuous Program Material
A signal, such as speech or music, that contains voltages continuously changing in both frequency and voltage (time and amplitude).

Contour
A circuit which adds a bass boost to attain equal loudness at lower volumes. Also known as Loudness.

CPS
Abbreviation for "Cycles per second", the units for expressing frequency. The term "CPS" has been obsoleted and replaced by "Hertz". Hertz = Cycles per second. 1 kHz = 1 Kc.

Critical Distance
The point within a room where the sound level of the direct field radiating from the loudspeaker and the reverberant field within the room become equal in intensity or level.

Crossover (X-OVER)
An electronic device that is used to separate an audio signal into two or more bands of frequencies or component signals above and below a certain frequency, said to be the crossover frequency or crossover point. Crossovers can be active or passive.

Crossover, Active
Electronic or active crossovers are used in biamplified sound systems where two amplifier channels are used to individually operate the woofer(s) and horn(s) of the speaker system. The active crossover is placed in the audio chain just ahead of the amplifiers, and separates the audio signal into low and high frequency groups. Some active crossovers separate the signal into low, mid and high frequency bands for 3-way or triamplified speaker systems. The advantages of active crossovers and biamplification are low distortion, increased headroom and better control over the relative levels of the low and high frequency sections of the speaker system. Disadvantages include the need for additional amplifier channels, higher cost of the crossover compared to passive units and multiple wire runs from the amplifiers to the speakers.

Crossover, Passive
A passive crossover is built into most speaker cabinets in order to separate the full range signal from the output of a power amplifier into low frequency and high frequency bands to operate the woofer and horn (or tweeter) respectivly. Some speakers utilize a 3-way crossover which separates the signal into low, mid and high frequency bands. The advantages of passive crossovers are that a single amplifier channel can be used and they can be relatively inexpensive. Disadvantages include higher potential for low frequency distortion caused by saturation of the inductors used in the low frequency section and excess amplifier power consumption due to heat losses within the passive components.

Crosstalk
Interaction of adjoining channels or circuits. Crosstalk can occur by being induced electromagnetically or electrostatically. Crosstalk is a common specification for mixing consoles.

Current
(I) The rate of flow (measured in amperes) of electricity in a conductor or circuit. The amount of current that flows is determined by the voltage or electrical pressure applied and the conductivity of the substance or material (which also determines the resistance or opposition to current flow).

Cut
A term used to indicate the reduction in gain or attenuation of a frequency or band of frequencies when equalizing an audio signal.

Cycle or Hertz
A unit of motion referenced to a time period of one second. The frequency of a vibration or oscillation in units per second. 100 Hertz or 100 c.p.s. (cycles per second) refers to the number of times a second (100) a string is vibrated or an amplifier is swinging between its positive and negative supply voltage.

Damping Factor
The ratio of the speaker impedance to the amplifier's internal output impedance. Damping factor is a measure of how well an amplifier can actually control the movement of a speaker cone or diaphragm by preventing it from moving farther than it is supposed to. Damping factor is arrived at by dividing the speaker impedance by the amplifier's internal output impedance. The internal output impedance of any amplifier is determined by the transconductivity (internal resistance) of the output devices. Everything connected in the speaker line (including the speaker cable itself or a crossover) looks to the speaker like an increase in the output impedance of the amplifier, thus lowering the effective damping factor. Because any speaker is a mechanical device, it will have its own resonant frequencies, which will cause the cone to continue in motion after the electrical signal has stopped. [see transient distortion]. An amplifier with a high damping factor will damp out the unwanted speaker cone excursion.

dB (Decibel)-
A unit for describing the ratio of two voltages, currents, or powers. The decibel is based on a logarithmic scale; when measuring differences in sound pressure level (SPL), the amount of change in sound pressure level perceivable is directly proportional to the amount of stimulus (the more sound present, the greater the change must be, to be perceived).

The Decibel
The origin of the decibel is the "bel" which was named in honor of Alexander Graham Bell. It was a logarithmic term called the "transmission unit" which was used to express the transmission losses of long telephone lines. The "bel", being too large for practical use, was later changed to "decibel". The decibel has no actual numerical value, but is used only to express a ratio between two voltages, currents, powers, or impedances. The decibel uses logarithms to the base 10 called LOG. This should not be confused with the natural logarithm to the base "e" called LN used in many electronic formulas. Examples of decibel calculation: To calculate for voltage, current, SPL, distance: 20 Log X1/X2. To calculate for power = 10 Log P1/P2.

Decay
The gradual reduction in sound energy once the sound source is turned off.

Diaphragm
A thin flexible sheet that can be moved by sound waves as in a microphone, or can produce sound waves when moved as in a loudspeaker or compression driver.

Differential Amplifier
An amplifier whose output is proportional to the difference between the voltages applied to its two inputs. Used to balance or offer common mode rejection of interference signals.

Diffraction
The bending or redistribution of acoustic sound waves in a room caused by some obstacle, such as a column or divider. Only low frequency wave forms can be diffracted.

Diffusion
The scattering of sound waves by a solid object.

Digital
Refers to the processing of audio signals as having discrete values as opposed to a continuous analog audio signal. In digital audio the continuous analog signal is converted to an encoded discrete value or digital word.

Dip
A reduction (attenuation or cut) in gain at a certain frequency also called a notch.

Directivity
Area of coverage of a speaker or microphone.

Dispersion
The spread or distribution or coverage of sound generated from a horn or loudspeaker. For any given frequency, the area of dispersion is defined as that area between the -6 dB down points of that frequency plotted against amplitude. It is measured in degrees related to an imaginary line descending from the center of the speaker cone. As you move away from the imaginary line, up or down, right or left, the loudness level of the sound decreases. When the sound level decreases rapidly on either side of the imaginary line, the dispersion in degrees is relatively small and the speaker is said to be highly directional.

Distortion
Any undesired change in the wave form of an electrical signal passing through a circuit or transducer. Any distortion can be defined as deviation from the original sound, the discrepancy between what the amplifier should do and what it actually does. All distortion is undesirable. Distortion occurs when the amplifier alters the original sound in the process of amplification so that what comes out of an amplifier is no longer a true replica of what went in. Performers, however, will sometimes desire the application of electronically induced distortion for extra-musical effect in the production of their "sound". The undesirability of inherent distortion is associated with high fidelity and should not be confused with the desirability of distortion as it is expected to be produced through circuitry. When reproducing sound, distortion is unwanted.

Driver
The motor structure portion of a horn loaded loudspeaker system that converts electrical energy into acoustical energy and feeds that acoustical energy into the entry of a horn throat or the narrow end of the horn. Most often used when referring to a high frequency compression driver, called a driver for short. The definition also includes the loudspeaker in a horn loaded woofer or mid bass horn.

Dynamic Range
In a musical instrument, the dynamic range is the difference in decibels between the loudest and softest level of notes that can be played on that instrument. In electronic equipment, dynamic range is the difference in decibels between the highest (overload level) and lowest (minimum acceptable) level compatible with that piece of equipment.

Echo
A wave which has been reflected or otherwise returned with sufficient magnitude and delay to be perceived as a distinct wave, separate from the original wave. A delay in sound of more than 50 milliseconds resulting in a distinct repeat or number of repeats of the original sound.

Efficiency
The ratio, usually expressed as a percentage, of the useful power output to the power input of a device. The efficiency of a speaker system is the SPL the unit produces at a 1 W RMS input power level measured 1 meter from the unit. Doubling the input power raises the SPL 3 dB. Doubling the number of enclosures raises the SPL 3 dB. Doubling the input power and the number of enclosures raises the SPL 6 dB. Doubling the distance (near field) lowers the SPL 6 dB.

Electronically Balanced Input
A differentially balanced amplifier; an amplifier whose output is proportional to the difference between the voltages applied to its two inputs. Offers common mode rejection or attenuation of interference signal that was introduced electromagnetically in the signal carrying conductors.

Enclosure
An acoustically designed housing or structure for a speaker.

Equalization
The act of obtaining a desired overall frequency response through the implementation of graphic equalizers or tone controls.

Equalizer
A device designed to compensate for an undesired amplitude-frequency characteristic of a system or speaker.

Excursion
Movement of the cone of a loudspeaker or the diaphragm of a compression driver. The higher the voltage or amplitude of the signal applied, the greater the movement or excursion of the loudspeaker or diaphragm.

Exponential Horn
A speaker designed to reproduce the high frequencies. An exponential horn has a flare rate that increases with the square of the distance from the entry to the horn throat.

Far Field
That portion of the direct field that is at least twice the distance of a frequency's wavelength.

Feedback
(Electronic) The return of a portion of the output of a circuit to its input. (Acoustic) A squeal of a sound system caused by the regeneration of a signal from the output of a sound system into a microphone input.

Filter
An electrical or electronic device that permits certain frequencies to pass while obstructing others. Examples include loudspeaker crossovers, equalizers, feedback reducers and even simple bass and treble controls.

Flutter Echo
A multiple echo in which the reflections occur in rapid succession caused by large surfaces being acoustically parallel to each other.

Frequency
The number of vibrations or oscillations in units per second. Measured in cycles or Hertz per second. The rate of repetition in cycles per second (Hertz) of musical pitch as well as of electrical signals. For example, the number of waves per second a vibrating device such as a piano or violin string moves back and forth each second of time to produce a musical tone.

Frequency Response
A measure of the effectiveness with which a circuit, device or system transmits the different frequencies applied to it. The way in which an electronic device (mic, amp or speaker) responds to signals having a varying frequency. This is a measurement of how well an amplifier reproduces and amplifies a specified audible range with equal amplitude or intensity, for example, 30 to 16,000 Hz.

Full Range
The entire audio spectrum, 20 Hz - 20 kHz.

Gain
An increase in strength or amplitude (voltage) in a signal. The increase in signal power that is produced by an amplifier; usually given as the ratio of output to input voltage, current, or power expressed in decibels.

Ground
A heavy cable connected to earth via a metal copper stake for the purpose of grounding electrical equipment. In the U.S. a third wire in our electrical system is connected to this earth ground to provide a means of connecting the chassis of electrical equipment to the earth ground and thus provide protection against hazardous electrical shock.

Ground Loop
Hum caused by return currents or magnetic fields from relatively high-powered circuits or components which generate unwanted, noisy signals in the common return of relatively low-level signal circuits. A potentially detrimental loop formed when two or more points in an audio system that are nominally at ground potential are connected by a conducting path.

Haas Effect
Refers to the condition of the human auditory system that permits a listener to merge all the information arriving in the first 20 milliseconds as a single event. This is sometimes called the precedent effect.

Harmonic
One of a series of sounds, each of which has a frequency which is an integral multiple of some fundamental frequency.

Headroom
The difference between the average operating power level of an amplifier circuit and the point at which clipping or severe distortion occurs.

Hearing
The human hearing system is very well designed. It has a dynamic range of over 120 dB. Contemporary digital recording techniques can only achieve a dynamic range of about 90 dB. The typical threshold of pain is around 140 dB, with discomfort starting around a sound level of 118 dB. The normal hearing range is considered to be 15 Hz to 20 kHz. The typical Hi-Fi specification range is 20 Hz to 20 kHz. Typically, however, the average person cannot hear 20Hz. Sound reinforcement specifications reflect 50 Hz to 15 kHz (sometimes 40 Hz). Interestingly enough, this just happens to be the FCC limits on FM radio. The typical telephone has a frequency response of 400 Hz to 4 kHz. The human ear does not hear all frequencies at the same intensity. It's less sensitive at both the lower and upper ends of the frequency spectrum, and this characteristic varies with both age and sex. The amount of sensitivity is also a function of sound pressure level. The greatest intensity variations occur at very low sound pressure levels. The curve is relatively flat at sound pressures of 90 dB or so (Fletcher-Munson). The decibel is used in acoustic measurements because the human ear responds to the intensity of sound in approximately a logarithmic manner.

• Sensitivity: Only 5% of people can hear a 1 dB difference in level (60 dB, 1 kHz); about 50% of people can hear a 2 dB change; and almost everyone can hear a 3 dB change. This means that when looking at equipment specifications, 1 dB frequency response specs are good; 3 dB specs are fair.
  
  Relative level: 50% of people say about a 7.5 dB increase in level seems twice as loud . . . some as low as 5 dB, and some as high as 10 dB. This test is very level and frequency sensitive. Higher sound levels produce lower numbers and frequencies below 1 kHz and above 5 kHz yield higher numbers. The rule of thumb is 10dB. Applied to sound systems, to achieve output levels that appear to be twice as loud the system must produce over eight times more output power!

High Pass
All signals above a given crossover frequency.

High Z or High Impedance
Any resistance to AC voltage or current generally greater than 2,000 Ohms.

Hiss
Audio-frequency noise having subjective characteristics analogous to prolonged sibilant sounds.

Hum
An electrical disturbance that can occur in sound equipment due to the frequency of the power distribution system or any number of its harmonics. Our power line frequency in the U.S. is 60 Hz. Hum can occur at 60 Hz, 120 Hz, 180 Hz, 240 Hz, etc.

IM (Intermodulation Distortion)
Nonlinear distortion characterized by the appearance of output frequencies equal to the sums and differences of integral multiples of the input frequency components. For instance, an amplifier with high IM distortion amplifying two frequencies of 100 Hz and 2,000 Hz would also generate distortion components of 1,900 Hz and 2,100 Hz.

Impedance
The total opposition to alternating current flow presented by a circuit. The resistance to the flow of alternating current in an electrical circuit, generally categorized as either "high" or "low", but always expressed in ohms. Commonly used to rate electrical input and output characteristics of components so that a proper "match" can be made when interconnecting two or more devices, such as a microphone, loudspeaker or amplifier.

Impedance Match
The condition in which the external impedance of a connected load is equal to the internal impedance of the source, thereby giving maximum transfer of energy from source to load, minimum reflection, and minimum distortion.

Impulse
A type of signal that switches on and off as opposed to remaining in a steady state like a continuous sine wave. Music is more impulsive in nature than it is steady state.

Inductance
That property of an electric circuit or of two neighboring circuits whereby an electromotive force is generated (by the process of electromagnetic induction) in one circuit by a change in itself or in the other.

Inductor
A coil of wire used to create an impedance whose reactive component is low, therefore offering low resistance at low frequencies and high resistance at high frequencies. An inductor passes low frequencies and attenuates or rolls off high frequencies.

Infinite Baffle
A baffle that effectively prevents all of the loudspeaker's rear sound waves from interfering with its front waves.

Input Overload
Distortion produced by too strong a signal from the output of a microphone or other signal source connected to the input of a preamplifier.

Insertion Loss
A loss in gain of a system after a component has been added or inserted in the system, usually expressed in decibels.

Jack
A receptacle on a receiver, tape recorder, amplifier or other component into which a mating connector can be plugged.

Kilohertz
A frequency of one thousand cycles per second (1 kHz).

LED
Light emitting diode, a semiconductor device that produces visible light when a voltage of a certain polarity and potential is applied to it.

Limiter
An electronic circuit used to prevent the amplitude of an electronic waveform from exceeding a specified preset level while maintaining the shape of the waveform at amplitudes less than the preset level.

Linear
Having an output that varies in direct proportion to the input.

Linear Frequency Scale
A scale on which each equal length division represents an equal number of Hertz. If you add the equal length division in Hertz to the last division frequency, you get the next frequency in a series.

Line Out
An output connection found on mixers, preamps, tape decks, etc. providing an output at a level sufficient to drive the input of a power amplifier.

Load Impedance
The actual impedance of the load or speaker that a power amplifier is connected to, thus driving a load of a certain impedance.

Logarithmic Frequency Scale
A scale on which each division represent an exponential constant, i.e., each division from a reference point is proportional to its logarithms.

Loudness Control
A volume control with special circuitry added to compensate for the normal decreased hearing ability of the human ear at the extreme ends of the audio range when listening to lower sound levels. A typical loudness control boosts the bass frequencies and to a lesser extent, the high frequencies. Sometimes this control is called contour.

Loudspeaker Efficiency
The ratio, expressed in percentage, of signal output to signal input used to state the power needed to drive a loudspeaker. An example: Power output 2 watts; Power input 10 watts; Ratio 2/10=20% efficiency. Efficiency can vary from 2% to as high as 25%.

Low Pass
All of the frequencies below a given crossover frequency.

Low Z or Low Impedance
Any resistance to AC voltage or current flow generally less than 2000 Ohms.

Master
Main level or gain control for a device, bus or mix.

Microphone
A microphone is a transducer that changes acoustical energy (sound) into electrical energy.

Mixer
A device in sound reinforcement that has two or more signal inputs and a common signal output. Used to combine separate audio signals in desired proportions to produce an output audio signal.

Mode
Another word for room resonance. When sound energy is restricted by boundaries (such as walls, floor, and ceiling) waves are developed at certain frequencies or wavelengths that are integers of the distance between the room boundaries. Room modes or resonances cause standing waves because once the wave is generated it stands there, i.e., the positive pressure peaks (anti-nodes) and negative pressure troughs (nodes) stay stationary within the boundaries.

Monitor
A loudspeaker or system of loudspeakers that permits the performer to evaluate or monitor his sound alone or in conjunction with other sounds that may be desired and is mixed to the listeners preference by means of a separate monitor or reference mix.

Mono
Monophonic Sound - Sound produced by a system in which one or more microphones feed a single signal to an amplifier(s) whose output is coupled to one or more loudspeakers.

Multimeter
Also called a Volt-Ohm-Meter (VOM). A measuring instrument that can measure different ranges of voltage, current, and resistance. A multimeter can have an analog needle indicator or a digital read out.

Music Power
This is a power rating generally applied to high fidelity amplifiers for tones of short duration. It takes into account the fact that most amplifiers can produce a greater amount of power in short bursts than they can continuously. The rationale is that music is made up of such bursts rather than sustained single frequencies. It is higher than continuous power ratings for the same amplifiers. It is measured at a signal frequency of 1000 Hz for a specified distortion.

Noise
Any extraneous sound or signal that intrudes into the original as a result of environmental noise, distortion, hum, or defective parts in the equipment.

Notch Filter
A band rejection filter that produces a sharp notch in the frequency response of a system, thus reducing the gain or amplitude of a narrow band of frequencies centered on a given frequency.

Octave
The interval between any two frequencies having a ratio of 2 to 1. Example: "A 440" is an octave above "A 220".

Ohm
The unit of electrical resistance, equal to the resistance through which a current of one ampere will flow when there is a potential difference of one volt across it. Ohm is the unit of measure used to express opposition to current flow. Every wire or part through which electricity passes has some resistance to that passage.

Omnidirectional
A term usually applied to microphones to refer to uniform pickup of sound from all directions.

Oscilloscope
A test instrument that shows a picture of electrical waveforms by means of a cathode ray tube. An oscilloscope is calibrated to allow measurement of the instantaneous values and waveforms of electrical signals that are changing rapidly or varying as a function of voltage or time. Also known as a Scope.

Output Impedance
The internal output impedance of an amplifier presented by the amplifier to the load. (not to be confused with load impedance.)

Output Power
The power delivered by a system to its load. (i.e. an amplifier driving a loudspeaker system)

Overtone Harmonic
Multiples of frequency of a fundamental waveform.

Pad
A fixed passive network which reduces the electrical level of a signal. An attenuator.

Parallel
An electric circuit in which the elements or components are connected between two points with one of the two ends of each component connected to each point.

Parametric
A type of equalization circuit that has three variable parameters; frequency, cut or boost bandwidth, and Q (the width of the filter).

Passive
An electronic circuit composed of passive elements, such as resistors, inductors, or capacitors, without any active elements, such as vacuum tubes or transistors generally resulting in a signal loss.

Peak
The maximum instantaneous value of a signal amplitude.

Peaking
A term used to indicate an increase in gain of a frequency or band of frequencies when equalizing an audio signal.

Peak Limiter
A device which automatically limits the level of its output signal to approximate a preset maximum value by reducing its gain when the signal exceeds a preset value.

Peak-to-Peak
Amplitude of an alternating voltage measured from negative peak to positive peak.

PFL
Pre Fade Listen. This describes a button on a mixing console that permits a channel or sub to be monitored before that channel or sub's level control or fader. This allows an operator to listen to a channel without that channel being heard by the audience.

Phase
Phase is the time interval between two related events. Two signals are in phase when they reproduce the same sound or signal simultaneously; they are out of phase to the extent that one leads or lags behind the other in time. A signal is said to be in phase with another when the voltage and current amplitudes begin at the same time and move in the same direction.

Phase Cancellation
Signals that are out of phase will cancel one another according to the difference in phase in degree. A transducer (speaker or microphone) wired out of phase with another will result in reduced output from both; instead of their combined outputs adding, they will subtract due to phase cancellation.

Piezo-Electric
Having the ability to generate a voltage when mechanical force is applied; or to produce a mechanical force when a voltage is applied, as in a piezo-electric crystal.

Point Source
A source of acoustic sound waves having definite position but no extension into space. A point source is an ideal; an imaginary single point in space. This imaginary point source provides a good approximation for distances from the point source that are much larger compared to the actual dimensions of the source. A cluster or array of horns and loudspeakers is positioned using this imaginary point in space as a reference for the actual source of the sound. Properly configured the array will perform as a single or point source of sound.

Polarity
The quality of having opposite poles. In electro-magnetic-mechanical systems, some form of potential is referenced to one of two poles with different (usually opposite) characteristics, such as one which has opposite charges or electrical potentials, or opposite magnetic poles.

Post
(after) on a mixer; post indicates that the function is derived after the channel slider or gain control.

Power
Electrical energy, measured in watts, such as the current from an amplifier used to drive a loudspeaker. Power in watts is expressed as W = V2/R

Power Amplifier
The final active stage of the audio chain, designed to deliver maximum power to the load or speaker impedance for a given percent of distortion.

PRE
(before) On a mixer, pre fade listen (PFL) indicates that the function is derived before the channel slider or gain control pre monitor send, the monitor send is before and independent of the channel slider or gain control. A pre monitor send is usually pre channel EQ also.

Preamp (preamplifier)
An amplifier whose primary function is boosting or amplifying the output of a low level audio-frequency source, (such as a microphone), so that the signal may be further processed without appreciable degradation of the signal-to-noise ratio of the system. An amplifier which increases electrical signals from a microphone or other instrument to a level usable by a power amplifier. Preamp levels are approximately .1 volt.

Preamp Out
A means of obtaining an output signal from the preamplifier of a channel of a mixer or musical instrument amplifier. The preamp out is actually a line level signal or 1 volt.

Program
This usually refers to a signal source composed of music and/or speech as opposed to sine wave or noise.

Program Level
The level of program material in an audio system expressed in VU.

Proximity Effect
The boost in low-frequency response when a unidirectional or proximity effect microphone is used close to a sound source.

Q
The directivity factor of a transducer (loudspeaker) used for sound emission. The higher the value of Q the more directional the speaker.

Reactance
A resistive like property that offers opposition to electron flow in an alternating current (AC) circuit. There are two types of reactance; capacitive reactance (XC) and inductive reactance (XL). Reactance varies with frequency.

Real Time Analyzer (RTA)
An electronic instrument used to measure the combined response of an audio system and the room in which the system is operating.

Reference Levels

• O dB
  In the measurement of SPL or Sound Pressure Level, 0 dB is referenced to the threshold of hearing or auditory perception of a tone of 1000 cycles (hertz) per second (1 kHz). 0 dB must always be referenced to some base of measurement. In gain functions 0 dB is unity gain (1).
• 3 dB
  The amount of SPL gained by doubling the power to a speaker. Also the amount gained by doubling the number of speakers.
• +/- 3 dB
  Plus or minus 3 dB as used to express a measurement of frequency response indicates that the response will be no more than +3 dB and no less than -3 dB below a given reference. It is actually a 6 dB window. The Response of 60 Hz to 14 kHz +/-3 dB means that within the bandwidth of sixty cycles per second to fourteen thousand cycles per second, no frequency is +3 dB more nor -3 dB less than a specified reference frequency.
• 3 dB DOWN (-3 dB)
  The point at which a measured power level is 3 dB below the specified level. In an electronic crossover, the point (frequency) at which the high pass signal is -3 dB down in response or power level is considered the crossover point (frequency).
• -6 dB
  The amount of loss in SPL as you double the distance away from a sound source.
• dBm
  A decibel scale referenced to 0 dBm = 1 milliwatt of power into 600 Ohms or .775 volts RMS across 600 ohms.
• dBu
  Primarily a British term for gain referenced to 0 dBu = .775 volts RMS.
• dBV
  A decibel scale referenced to 1 volt RMS; 0 dBV = 1 volt.
• dBW
  A term for power gain referenced to 0 dBW = 1 Watt.

Reflection
The bouncing back or return of sound waves from walls or other obstacles which they strike.

Refraction
A change in direction or bending of the propagation of a sound wave when it passes from one medium to another in which the velocity of sound is different.

Regeneration
In audio, regeneration is another word for feedback; when something regenerates it continues or sustains itself as an oscillation. When an electronics engineer designs an oscillator, he takes the output of a gain stage and feeds it back into the input through a tank circuit (an RC, resistor and capacitor or an LC, inductor and capacitor combination); when the circuit is turned on it begins to regenerate or oscillate at a specific frequency determined by the value of the RC or LC combination. When the output of a loudspeaker in a sound reinforcement system is able to get back into a microphone or sound system input, at some level and resonant frequency, the system is going to go into regeneration or feedback oscillation (squeal).

Resistance
Opposition to the flow of electrical current. Measured in ohms.

Resistor
An electronic component designed to have a definite amount of resistance; used in circuits to limit current flow or to provide a voltage drop.

Resonance
A tendency of mechanical parts, loudspeaker cone, enclosure panels or electrical circuits to vibrate at or emphasize one particular frequency, every time that frequency, or one near it, occurs.

Response
The range of frequencies to which an amplifier or speaker will respond, and the relative amplitude or intensity with which these frequencies are reproduced.

Return
An input, typically found on a mixing console, used to patch a signal returning to a particular BUS after having been further processed, such as an echo or effects return.

Reverb (Reverberation, acoustical)
The prolongation of sound at a given point after direct sound from the source has ceased, due to such causes as reflection from physical boundaries. (Electro-mechanical) An electro-mechanical device usually employing springs which randomly reflect as great amount of sound as possible, therefore simulating natural reverberation. (Digital Reverb) An electronic reverberation effects processor that uses digital electronics to introduce the multiple delay paths.

Reverberation
The sustaining of acoustical energy in a room after the reception of the direct field (the sound coming directly from the source) ceases in producing sound. Reverberation is caused by the reflections and scattering of sound energy from the boundary surfaces of the room.

RIAA
Stands for Recording Institute Association of America. A type of preamplifier used for turntables. It is necessary to use an RIAA preamp when using a magnetic cartridge.

Ring Mode
A tone or frequency sounded in a room with a live sound reinforcement system prior to the system breaking into feedback. A ring mode lies just below the threshold of feedback.

RMS (root mean square value)
The square root of the time average of the square of a quantity; for a periodic quantity the average is taken over one complete cycle. RMS voltage is .707 times the peak voltage of a sine wave.

Roll Off
A signal is rolled off when it is attenuated or reduced in level above (high pass roll off) or below (low pass roll off) a certain frequency. The amount of roll off is rated at so many decibels per octave. A signal that is rolled off below 100 Hz at a rate of 18 dB/octave would be reduced in level or attenuated -18 dB at 50 Hz, -36 dB at 25 Hz, etc.

RT60 (Reverberation Time)
The time required for sound to drop to - 60 dB in level once the source of sound has been stopped. The -60 dB is below that of the measured level while the sound system was on and after any initial transients or fluctuations settled.

Send
An output used to patch a signal from a channel or bus of a mixer to an external signal processor such as an echo or digital delay.

Sensitivity
The minimum input signal required to produce a specified level of output. In an amplifier, the input sensitivity is the amount of voltage at the input necessary to drive the amplifier to its rated power output. Loudspeaker sensitivity is the power level necessary to produce a stated SPL at a given distance from the loudspeaker, usually rated at 1 watt 1 meter.

Series
An arrangement of circuit components, end-to-end, to form a single path for current.

Series-Parallel
A circuit in which some of the components or elements are connected in parallel, and one or more of these parallel combinations are in series with other components of the circuit.

Shelving
A type of equalization circuit that has a shelf-like characteristic at the upper or lower ends of the spectrum. A shelving EQ at 15 kHz, in the boost position, would increase the high frequencies up to 15 kHz where it would shelf.

Signal-to-Noise Ratio (S/N )
The ratio of the amplitude or level of a desired signal at any point to the amplitude or level of noise at the same point.

Sine Wave
A wave whose amplitude varies as the mathematical sine of a linear function of time, also known as the sinusoidal wave.

Slapback Echo
A echo caused from reflections off the rear wall in many auditoriums. Such echo can reduce the intelligibility of a sound reinforcement system, as well as distract speakers and performers on stage. Severe slapback echo is very distracting for a musician, as it can cause confusion that makes following that beat in time extremely difficult. A professional high quality stage monitor system can mask some of the slapback echo to a certain extent by providing more direct sound from the monitor speaker in the first 25 milliseconds which the performer perceives through auditory fusion as more direct field, thus increasing the ratio of the level of the direct field to that of the slapback echo.

Slew Limiting
The failure of the amplifier's output to move as fast, voltage-wise, as the input would have it move.

Slew Rate
Refers to the ability of an amplifier's output to accurately reflect the input waveforms' rise time transients. An amplifier is said to have a slew rate of so many volts per microsecond. A slew rate of 20 volts per microsecond (20 V/U sec.) means that the amplifier is capable of swinging 20 volts positive or negative in the period of one microsecond.

Snake
A multiconductor shielded input cable which allows a single run between the microphones and the mixing console. Snakes often provide "return" wire pairs to permit the mixer outputs to be sent to amplifiers located on stage.

Sound
A pressure wave motion propagated in an elastic medium (air) producing an auditory sensation in the ear by the change of pressure at the ear. Sound waves are produced by a vibrating body in contact with air.

Sound Level Meter
The instrument used to measure noise and sound pressure levels (SPL), calibrated in decibels.

Sound Fields

• Direct Field
  The sound that emanates directly from a sound source or loudspeaker.
• Indirect Field
  That sound perceived from behind a speaker system, i.e. when no direct field is present.
• Near Direct Field
  The sound field that is generated close to the source or loudspeaker. Generally considered that sound field that is within a distance from the source of less than two wavelengths.
• Far Direct Field
  The sound field that is perceived at a distance from the source loudspeaker that is greater than 2 wave lengths.
• Free Field
  That portion of the direct field of a sound source or loudspeaker that is reflection free or not yet affected by boundaries such as walls or ceiling (such as may be encountered with an outdoor sound system).
• Reverberant Field
  That sound field beyond critical distance where most of the energy arriving at the listener is in the form of reflected energy off the room's boundaries.

Spectrum Analyzer
[See Real Time Analyzer (RTA)]

SPL (sound pressure level)
The level or intensity at a point in a sound field (loudness). The deviation above and below normal atmospheric pressure. The unit of measurement of Sound Pressure... the microbar. One microbar is equal to the sound pressure of 1 dyne per square centimeter, which is a sound level of 74 dB above the threshold of hearing (0.0002 microbar.) It is also equal to approximately one-millionth of normal atmospheric pressure. Sound pressure levels are stated in decibels as follows: Where P is the RMS sound pressure in microbars, and the reference is the threshold of hearing of 0.0002 microbars (50% of young men, 1 to 4 kHz).

Splitter
A box which has one microphone or signal input and has two or more individual outputs available for that signal. Used to connect one signal source to multiple other devices.

Standing Wave
Standing waves occur in rooms because of the boundaries. A standing wave is a soundwave that once excited, stands there, i.e., the positive air pressure peaks (antinodes) and negative air pressure troughs (nodes) remain in the same position within the room's boundaries. Also known as a stationary wave. [see Modes]

Stereo
In a sound reproducing system, stereo refers to the use of two separate signal processing channels driving two separate power amplifiers, which in turn power two separate speaker systems. However, most times in sound reinforcement, a stereo mixer is employed to drive a mono (single channel) system in order to allow separate mixes of the program (such as speaking mics, instrurnents, vocal mics, etc).

Sub-Master
A separate mixing bus assigned to a group of mixing channels that enables the sound mixer to regulate the level of that group with one control called the Sub-Master.

Sub-Mix
A level control preceding the main (master) level control, that regulates the level of an individual sub-mix.

Switching Jack
An input or output jack that performs some switching function in addition to providing an input or output for a signal. These jacks disconnect the normal flow of signal and allow for additional patching capabilities such as inserting equalizers and feedback reducers. Most typically these are 1/4" phone jacks, but some equipment manufacturers are now using switching RCA type phono connectors.

THD (Total Harmonic Distortion)
When a single frequency of specified level is applied to the input of a system, the ratio of the voltage of the fundamental frequency to the voltage of all harmonics observed at the output of the system because of the nonlinearities of the system. THD is expressed in percent.

Timbre
Timbre is a word that relates to the musical quality of sound. It is the relation of the fundamental frequency to the level and number of the associated harmonics. The human ear can perceive differences in timbre. Two different instruments, such as a saxophone and a flute playing the same note or fundamental at the same loudness, sound different to the listener due to the two instruments different number and level of related harmonics also produced at the same time as the fundamental. The two instruments are said to have a difference in timbre.

Transducer
Any device or element which converts an input signal into an output signal of a different form. A transducer changes energy from one form to another. A microphone is a transducer that changes acoustical energy (sound) into electrical energy (voltage). A loudspeaker is a transducer that changes electrical energy into mechanical energy, producing sound or acoustical energy.

Transformer
An electrical component consisting of multiple turns of wire placed in a common magnetic field (medium) which will transfer electrical energy from one electrical circuit to the next. A transformer will only pass alternating currents (AC) and will not pass direct current (DC). By adjusting turn ratios, a step up or down condition of voltage can be achieved.

Transformer Balanced
An input or output that is coupled by means of a transformer in a configuration that makes it balanced or capable of being operated so that the voltages of the two conductors at any transverse plane are equal in voltage and opposite in polarity with respect to ground. A transformer balanced input or output will offer common-mode rejection, which means any common-mode interference signal will not pass through the transformer because it will be cancelled out.

Transient
Rapidly changing peaks of short duration in the level of sound such as would be produced by a cymbal crash or a rim shot on a snare drum. A wave having a very short or no sustain time.

Transient Distortion
Transient distortion interferes with the ability of an amplifier to accurately follow abrupt changes in volume, such as the sudden burst of sound when an instrument is first played. Minimum transient distortion is vital to clean and crisp overall sound.

Transient Response
Ability of an amplifier or loudspeaker to accurately follow abrupt changes, such as the sudden burst of sound generated by an instrument. Good transient response is vital to "clear" or "crisp" overall sound.

Tri-Amp
Separating the audio spectrum into three bands, i.e., high frequencies, mid-band frequencies and low frequencies by means of an electronic crossover and using three separate power amplifiers to amplify the three outputs of the crossover (high pass, mid pass, low pass outputs) driving three separate components of a speaker system; resulting in increased headroom and dynamic range.

Unbalanced Cable or Line
A single conductor cable with a surrounding shield that connects to ground. Such a system is called unbalanced because it cannot be balanced or offer common mode rejection.

Unbalanced Input
An input in which one of the two terminals is at ground potential or connected to the chassis ground.

Unidirectional
This term commonly refers to microphones which pickup sounds predominately from one direction as opposed to all directions (omnidirectional). Unidirectional microphone types include cardioid, super-cardioid and hyper-cardioid. Unidirectional microphones are most often used in sound reinforcement applications because they are generally less prone to feedback than omnidirectional types.

Velocity
In audio the velocity or speed of sound is approximately 1130 feet per second. The speed of sound changes slightly with changes in temperature, humidity, and altitude.

Voltage
Voltage is a measurement of electrical pressure or the potential to do work. Voltage is sometimes called EMF or Electro Motive Force. The familiar 120 volts at the wall socket is an example of available electrical pressure. If the prefix "m" is used (as in mv) it stands for millivolts, or thousandths of volts. Microvolts, abbreviated "µV", are millionths of volts.

Volt (voltage)
Potential difference or electromotive force (EMF).

Volume
The intensity or loudness of sound.

VU (volume unit)
A unit for expressing the audio frequency power level of a complex electronic waveform such as that corresponding to speech or music. 0 VU is referenced to 1 milliwatt of power.

VU Meter
A meter than indicates the audio frequency power level or volume units of a complex electronic waveform.

Watt
A unit of measure of power. The electrical wattage of an amplifier describes the power it can develop to drive a speaker. The greater the voltage capability, the higher the wattage. Amplifier wattage requirements are greatly dependent upon the speakers that will be used, the size of the listening room and average loudness that will be played through the speakers. Wattage is expressed as W =V2/R

Wavelength
In audio the wavelength of sound is the actual physical size that one complete cycle of sound energy requires in air for a given frequency. The wavelength is found by dividing the velocity of sound, 1130 ft/sec, by the frequency of interest. The wavelength of 1000 Hz would be calculated as 1130/1000, or 1.13 ft.

Woofer
A speaker designed to reproduce bass or low frequencies.

XLR
A connector (sometimes called a Cannon connector after ITT-Cannon, the original manufacturer) used in interfacing audio components. The connector on a low impedance microphone is an XLR connector.

Y are there no audio terms that begin with the letter "y"?

Z
The abbreviation for impedance, as in low-Z.