Audio Glossary

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Acoustic Suspension
A method of enclosing a low-frequency driver (see woofer) in a totally sealed box, such that the air trapped inside the airtight cabinet provides a linear restoring force on the woofer’s diaphragm. Acoustic suspension designs are characterized by their clean, articulate bass response.

A circuit that strengthens an electrical signal (increases its amplitude). The power of an amplifier is usually specified in watts.

To reduce or lower, as in the reduction of the strength or level of a signal: “The tweeter level control attenuates the tweeter’s output.”


Back Surround Speaker
The single speaker or pair of speakers placed behind the listening position in a 6.1 or 7.1 channel home theater system.

A continuous range of the frequency spectrum. For example, the mid-range band of frequencies runs from approximately 200-3000 Hertz.

The lowest audible band of frequencies, running from about 20-200 Hertz.

Bass Reflex
A method of bass loading whereby a woofer is installed in a cabinet and the cabinet has a vent or port that allows the woofer’s backwave to exit the cabinet and reinforce the sound wave coming off the front of the woofer. See also port and vent.

Bi-amp or bi-amplification
The use of multiple amplifiers to power the high- and low-­frequency sections of a loudspeaker separately.

Binding Post
A high-quality hookup terminal on the rear of a loudspeaker that can accept several different types of wires and connectors. Binding posts are generally regarded as being superior to other kinds of loudspeaker connections.

A type of speaker that radiates sound in phase in two directions. See also dipole, monopole, and direct-radiating speaker.

A wiring configuration that uses two separate pairs of wires from the amplifier to the separate woofer and midrange/tweeter input terminals on the rear of the speaker.

An informal term used to describe the sound of a speaker that has an accentuated or elevated midrange or treble character to its sound. “That speaker is very bright sounding.”


Center Channel
The channel of information in a multi-channel home theater system that is primarily responsible for conveying the action and dialog that occurs on-screen. Depending on the way in which a particular movie or television show was recorded, anywhere from 50-80% of the sound comes from the center channel.

Center Channel Speaker
The middle front speaker in a multi-channel home theater system that reproduces the center channel information. Typically, this speaker is mounted on, just above or just below the television, or directly behind an acoustically-transparent pull-down movie screen.

An informal term describing the undesirable alteration of the original signal as a result of inaccurate sound reproduction. For example, a poorly-designed speaker is said to “color” the sound.

Crossover Frequency
In a crossover network, the frequency at which the audio signal is directed to the appropriate driver (low frequencies to the woofer, high frequencies to the tweeter).

Crossover (or Crossover Network)
A circuit that directs high, mid, and low frequencies to a specific driver. Loudspeakers that have multiple drivers employ an internal crossover to send the highs and lows to the correct driver.

Powered subwoofers also have a crossover that allows only the low frequencies to be reproduced by the subwoofer (see low-pass filter).


The application of any material that inhibits the ringing or resonance of a surface or area. Absorptive foam, for example, can damp out the audible resonances of a cabinet where a speaker is mounted.

D’Appolito array
A vertical midrange-tweeter orientation where one tweeter is placed midway between two midrange drivers so as to control their vertical dispersion. See also M-T-M array.

 dB (decibel)
The most common unit of measurement in audio. Although it is technically the ratio of two powers, voltages, currents, or sound-pressure levels (SPLs), it is the latter with which the term dB is most commonly associated. Logarithmic in scale, an SPL that is 10dB louder than another is perceived to be twice as loud.

The moving surface of a driver. The diaphragm is the part of the driver that actually moves the air, producing airwaves that we perceive as sound.

A type of speaker that radiates sound in opposite polarity (out of phase) in two directions, which causes a cancellation or null where the two sound fronts meet. This type of speaker is often used as a surround speaker in a home theater, because it’s very difficult to pinpoint its location by ear (“localize”), and therefore it very effectively mimics the sound of multiple direct-radiating speakers in a commercial movie theater. See also bipole, monopole, and direct-radiating speaker.

Direct-radiating speaker
A speaker whose drivers are pointed straight ahead—not angled off to the sides or rear. This is the kind of speaker most people are familiar with. See also monopole.

The characteristic angle of a speaker’s sound coverage, usually expressed in dB and degrees relative to its on-axis response. For example, a typical speaker might have a frequency response that is down 3dB at 5000 Hertz 60 degrees off axis relative to its on-axis response. Because of the laws of physics and acoustics, speakers tend to have narrowing dispersion with increasing frequency.

Any unintended or unwanted alteration of the original signal. Some forms of distortion are more audibly objectionable than others. The human ear/brain system can tolerate surprisingly high total distortion if the distortion is mathematically related to the original signal; the human ear is incredibly sensitive to minute amounts of distortion if the distortion components are random and unrelated to the original signal.

An individual speaker element (woofer, tweeter, midrange, etc.) contained in a complete speaker.


Early reflections
The first sounds in a listening room that reach the listener’s ears after the direct sound from the speakers. Typically, in a normal room, the early reflections are the sounds that bounce off the sidewalls nearest the front speakers.

Equalization (EQ)
Deliberate electronic alteration of frequency response with the goal of attaining a better-sounding final result. A powered subwoofer, for example, usually uses some form of equalization in its amplifier to achieve a more satisfying sound than if it was not equalized.


An oil with suspended magnetic particles used in the voice coil gap of tweeters and some midrange drivers to conduct heat away from the voice coil and improve its power handling capacity. Because ferrofluid is infused with magnetically-attracted metal (iron, hence “ferro”) particles, the driver’s magnet holds the oil in place in the voice coil gap.

A term most often applied to frequency response, meaning that a graph of the output of a given device (whether it be a speaker or an amplifier or other audio device) shows virtually no deviation from the input to that device. The resulting graph is said to be “flat,” i.e., the output shows no deviation on a graph, either up or down, from the input. “Flat” is a highly desirable result, indicating virtually perfect accuracy of signal reproduction.

The rate of vibration or oscillation of sound waves in the air, measured in Cycles per Second (Cycles per Second is commonly abbreviated “Hertz,” or Hz.). The band of frequencies audible to humans spans from 20 Hz in the bass to 20,000 Hz (20kHz) in the treble.

Frequency Response
The range or band of frequencies that an audio device such as a speaker or amplifier can reproduce, usually expressed with a specified limit of deviation of decibels (dB) from flat. For example, a speaker may be said to have a frequency response of 40Hz-20kHz, ± 3dB.


H-PAS (Hybrid Pressure Acceleration System)
A patented cabinet bass loading system that combines elements of several speaker technologies–bass reflex, acoustic suspension, inverse horn, and transmission line–in a unique configuration to deliver very powerful, extended low-frequency response. The combined speaker technologies are cascaded one to another in such a manner as to pressurize and accelerate the woofer’s low-frequency back waves as they travel through the cabinet. The dimensional relationships of the chambers, the internal partitions, and the drivers are precisely calculated to maximize and optimize the system’s bass response, resulting in deep bass from a compact cabinet with no loss of sensitivity.

Harmonic Distortion
Unintended signal products generated by an audio device such as a speaker or amplifier that are whole number multiples of the original signal. For example, if an audio device is tasked with trying to reproduce a 40 Hertz signal, and instead produces 40 Hz and a small amount of 80 Hz, the 80 Hz product is called harmonic distortion. Small amounts close in multiples to the original signal are barely audible; larger amounts of distortion in greater multiples away from the original signal are grossly objectionable to the human ear. The sum total of all harmonic distortion products is usually expressed as a percentage of the original signal, or % Total Harmonic Distortion (% THD).

Hertz (Hz)
The standard measuring unit of frequency, measured in Cycles per Second. Cycles per Second is commonly abbreviated “Hertz,” or Hz. The band of frequencies audible to humans spans from 20 Hz in the bass to 20,000 Hz (20kHz) in the treble.

High-pass filter
A circuit, such as in a speaker’s crossover network, that progressively attenuates signals below a pre-determined frequency. Thus, the higher frequencies are allowed to “pass through,” while the lower frequencies are filtered out. See also Low pass filter.

Home Theater System
A collection of audio and video equipment designed to closely replicate the commercial theater experience in a home setting. A typical home theater system will include a large-screen television, a high-quality program-delivery system such as a DVD player or HD satellite TV or HD cable TV, and a multi-channel surround-sound audio system.

Home Theater in a Box (HTiB)
An all-in-one home theater audio system that is purchased as a single item, as opposed to a home theater system comprised of separate components (stand-alone DVD player, speakers, a powered subwoofer, an A/V receiver, etc.). The audio performance of an HTiB system is almost always markedly inferior to that of even a modestly-priced component system, although an HTiB can be more convenient to buy and is generally less expensive than separate components.


The ability of a set of speakers (or an entire audio system) to reproduce sound in a spacially convincing, realistic manner. For example, a pair of high-quality speakers may be said to have good “imaging” if the listener is able to easily identify the locations of the musicians within the boundaries of the musical landscape.

The resistance of a component or complete circuit to the flow of alternating current (AC), expressed in ohms. This is most relevant to the end user when determining the compatibility of speakers and amplifiers. A speaker’s impedance is stated in ohms; it must then be ascertained whether the amplifier is capable of “driving” (being safely used with) that speaker. Only better quality amplifiers are able to drive low-impedance (4 ohms or less) loads; cheaper amplifiers need higher resistance loads (8 ohms or more) or the amplifier may malfunction—sometimes catastrophically.

Infinite baffle
A type of speaker design in which the woofer’s front radiation is isolated from its rear radiation, so as to minimize or eliminate any unwanted interference or cancellation between the two.

Intermodulation distortion (IM)
A form of audio distortion where the distortion products occur at frequencies that are sums and differences of the input signal. For example, if the input is 500 Hz and 2200 Hz, then the IM distortion products will occur at 1700 Hz and 2700 Hz. IM distortion is particularly objectionable, since the distortion is not related harmonically in any way to the original signal, unlike THD.


LFE (Low Frequency Effects) Channel
This is the “.1”-channel in a 5.1 or 7.1 Home Theater System. The LFE channel contains only very low bass effects, such as explosions and collisions. The .1 channel is typically reproduced by a powered subwoofer.

A circuit that prevents a signal from exceeding a pre-determined level. A limiter is most often used as a distortion-prevention device, to prevent an associated circuit from being driven too loud. For example, the amplifier in a powered subwoofer will often have a limiter to prevent it from being driven into distortion.

The tendency or ability to detect the location of a sound source by ear. For example, if a subwoofer plays too far up into the upper bass/lower midrange, it becomes easier to localize on its position.

See Speaker.

Low-pass filter
A circuit, such as in a speaker’s crossover network, that progressively attenuates signals above a pre-determined frequency. Thus, the lower frequencies are allowed to “pass through,” while the higher frequencies are filtered out. The crossover in a powered subwoofer is a low-pass filter. See also High-pass filter.


The frequency band that occurs just above the low bass region, but below the midrange. This band runs from approximately 75-150 Hertz. A speaker with excessive output in this range will tend to sound “heavy” or “muffled.” A speaker with inadequate output in this range will tend to sound “thin.”

The broad frequency band that runs between the bass and the treble. The midrange band runs from approximately 200-3000 Hertz. Most vocal and instrumental sounds lie in this region. An audio device must have an accurate flat frequency response with low distortion throughout the midrange in order to sound convincingly realistic.

Midrange driver
A speaker driver designed specifically to reproduce the midrange.

A type of speaker that radiates sound in one direction, usually straight ahead. See also direct-radiating speaker.

M-T-M array
A driver orientation whereby one tweeter is centered between two midrange drivers. See also D’Appolito array.


Random, unwanted signals that are not correlated to the original input signal. Typical audio examples are hiss, hum, and static; a video example is “snow” on a television screen.

The absence of sound in the physical area where the out-of-phase sound fronts from a dipolespeaker overlap and cancel each other out. Also called a null field or null zone.


A musical term indicating a change in frequency in a ratio of 1:2 or 2:1. Middle C on a piano keyboard is approximately 260 Hertz. The C one octave higher is 520 Hertz; the C one octave lower is 130 Hertz. There are approximately 10 octaves that are audible to the human ear, running in frequency from 20-20,000 Hertz. In the commonly-known notes “doe-ray-me-fah-so-lah-ti-doe,” the “does” are one octave apart.

The basic measurement unit of electrical resistance. See also impedance.

On-axis response
The frequency response of a speaker as measured directly in front of its cabinet. The on-axis response of a speaker is generally the best (or flattest) response that a speaker produces.


Parametric equalizer
An equalizer that has variable adjustment characteristics. Conventional equalizers adjust the frequency response through fixed-width frequency bands (usually an octave wide). In a parametric equalizer, the parameters of the individual bands—such as center frequency and octave width— are user-adjustable.

Passive radiator
An unpowered diaphragm in a speaker, driven by the pressure from the back wave from the woofer. A passive radiator serves to reinforce the bass response of a speaker. See also Pressure driver.

The timing relationship between two electrical or acoustic waveforms. When two speakers are connected “in phase,” they play together, and their output reinforces one another’s. When two speakers are connected “out of phase,” then one speaker’s waveform is essentially at “minus” when the other speaker’s is at “plus.” Out-of-phase signals cancel each other out. Dipole speakers employ two sets of drivers angled away from each other connected out of phase, which causes a cancellation or null where the two sound fronts meet.

Port (or Ported Cabinet)
An opening in a speaker’s cabinet that allows the woofer’s back wave to exit the cabinet and reinforce the sound wave coming off the front of the woofer. See also bass reflex and vent.

Power amplifier
A component (or section of a larger component) that strengthens (amplifies) the audio signal to a level that can drive a speaker to make sound. Power amplifiers can be stand-alone components, as in very expensive home theater systems, or they can be part of another product, such as the power amplifier section of a powered subwoofer.

Power handling
The ability of a speaker to effectively dissipate heat away from its drivers’ voice coils before permanent thermal damage occurs. Power handling is typically specified in watts.

Power Response
The sum of the total radiated acoustic output of a loudspeaker as measured at several points on- and off-axis in the far (reverberant) field. This measurement essentially captures the total sound emitted by a loudspeaker at all frequencies, in all directions, and is therefore more representative of how a speaker will sound in an acoustically well-balanced listening environment than what can be inferred from a simple on-axis anechoic frequency response measurement.

Powered Subwoofer
A speaker designed to reproduce only the low bass frequencies. A powered subwoofer comes with its own amplifier; and that amplifier is usually equalized to produce the best results with that specific woofer/cabinet combination.

Pressure driver
An unpowered diaphragm in a speaker, driven by the pressure from the back wave from the woofer. A pressure driver serves to reinforce the bass response of a speaker. See also Passive radiator.


Receiver (also A/V receiver or Home Theater receiver)
An audio component that processes and decodes audio signals (such as from a DVD player, satellite TV receiver, video recorder, etc.), receives radio broadcasts, and then amplifies those signals with a power amplifier to drive speakers. A receiver often forms the heart of a good-quality home theater system.

The tendency of a mechanical surface to vibrate when excited by an external force, and to keep vibrating even after the exciting force has been removed. A good example is a bell that is hit with the bell striker. The bell’s ringing is the resonance. Resonances produce undesirable coloration in audio equipment. A typical example of audio resonance is the vibration in the panels of a speaker’s cabinet when the cabinet is not internally braced. Vibrating cabinet panels cause the cabinet itself to “speak,” coloring the speaker’s sound.

A gradual attenuation (either intentional or unintentional) of a signal above or below a given frequency.


Signal-to-noise ratio (S/N)
As measured in decibels (dB), S/N is the difference in signal level and the residual noise of the component or circuit. The higher, the better.

The rate at which a high-pass filter, low-pass filter, or crossover network attenuates out of band frequencies, measured in decibels per octave (dB/oct). Typical slopes are 6, 12, 18, and 24dB/octave. These are also referred to as first-, second-, third-, and fourth-order slopes.

Sound Pressure Level (SPL)
The measurement of loudness, in decibels (dB). Normal room conversation is around 60 dB SPL; a loud rock concert is about 100 dB SPL; a jet airliner taking off is in excess of 130 dB SPL.

Speaker (sometimes called speaker system or loudspeaker)
A system comprised of drivers and a crossover network that accepts an audio signal from an amplifier and transforms that electrical signal into acoustic sound. A speaker system can be freestanding (totally enclosed in its own cabinet) for placement on a bookshelf or standing on the floor, or it can be open-backed, for installation with a companion frame in the wall or ceiling. The best in-wall speakers have their own totally-enclosed cabinets, like freestanding speakers.

A speaker designed to reproduce only the low bass frequencies. Most often, such speakers have their own amplifier; they are called powered subwoofers.

Surround sound system
General term to describe any system that reproduces sound in a three-dimensional manner, with sound occurring not only in front of the listener, but also to the sides and rear as well. A home theater system is a type of surround sound system.

Surround speaker (or surround channel speaker)
The speakers used to reproduce the side and rear channel information in a home theater system. Typically, the surround speakers are mounted high on the sidewalls or in the ceiling, alongside or slightly behind the listening/viewing location.


An independent organization that certifies the performance of audio and video equipment as meeting the stringent standards and specifications as set forth by Lucasfilm THX. Equipment that carries the THX certification can reproduce the film (video) and its soundtrack (audio) with an extremely high degree of fidelity relative to what the producer and director intended during the program material’s original production.

Total Harmonic Distortion (THD or % THD)
The sum total of all harmonic distortion products present in the output of an audio device, measured as a percentage of the original signal. See also harmonic distortion.

A rapid, short-duration change in the waveform of an audio signal. Most often referred to as “transient response,” which is the ability of an audio device—especially a speaker—to accurately follow and cleanly reproduce the rapidly changing signal. An example of a transient would be a sharp snare drum CRACK! coming out of a quiet background.

The upper or highest band of audio frequencies, running from about 3000 Hertz to 20,000 Hertz. See also bass and midrange.

A speaker driver designed to reproduce the treble region of the audio spectrum. See also woofer and midrange driver.

Two-way, three-way, etc.
Refers to the number of frequency bands into which a speaker’s output is divided. A two-way speaker uses two drivers (a woofer and a tweeter) to reproduce the audio spectrum; a three-way speaker uses three drivers—a woofer, a midrange driver, and a tweeter.


Vent (or Vented Cabinet)
An opening in a speaker’s cabinet that allows the woofer’s back wave to exit the cabinet and reinforce the sound wave coming off the front of the woofer. See also bass reflex and port.

Voice coil
A cylindrical tube wound with wire that is attached at one end to the apex or periphery of a speaker’s diaphragm. The other end of the wound coil of wire is immersed in a magnetic field and the coil moves back and forth within the magnetic field depending on changes in the signal it receives from the amplifier to which it is connected.

Voice coil gap
The columnar airspace between the voice coil and the driver’s magnet. In smaller drivers, such as midranges and tweeters, the voice coil gap is usually filled with ferrofluid to draw heat away from the voice coil and improve its power handling and damping.


The primary measuring unit of electrical audio power. Used to specify a power amplifier’s output or a speaker’s power handling ability.

A driver specially designed to reproduce the bass frequencies.