Here are a few terms you might hear in the AV world and their explanations


In acoustics, the changing of sound energy to heat.


The science of sound propagation. It can also refer to the effect a given environment has on sound.


Acoustical qualities of a room and its contents.


The acoustical reverberation characteristics of a given space.

Aspect Ratio

This describes the width of a picture to the height. The NTSC standard is 4:3. The current HDTV standard is 16:9. Modern movies range from 1.66:1 to 2.4:1.


A frequency-response adjustment of a sound-level meter that makes its reading conform, very roughly, to human response. Attenuates the lower and very high frequency ranges.


Any barrier to a sound sources. The surface upon which a loudspeaker is mounted.


Low-frequency sounds of around 160 Hz and below.

Bipole Speakers

One type of surround speaker. In this instance two or more drivers are facing different directions, and their cones vibrate in phase.

CD (Compact Disc)

A 4.5-inch plastic disc containing a digital audio recording that is played optically on a laser-equipped player. Has a 16-bit quantization rate to produce audio with high-fidelity sound.

CRT Projector

A type of front projector. It consists of three tubes each putting out one color: red, green, and blue.


A frequency-response adjustment of a sound-level meter that measures wideband energy uniformly. Attenuates very high frequency ranges.


A representation of ratio between two any values. Annotated as dB. The deciBel ratio of A and B = 20logA/B. deciBels are often used in audio since the human ear responds logarithmically to sound pressure. dB SPL (Sound Pressure Level) is the ratio of a sound pressure (in Pascal units) compared to 20 ÁPA, the threshold of hearing. Dynamic range is often quoted in dB and represents the ratio between the loudest signal available (amplifier overload level) and the lowest (electrical noise floor).


Any surface that vibrates in response to sound or is vibrated to emit sound, such as in microphones and loudspeakers. Also applied to wall and floor surfaces vibrating in response to sound or in transmitting sound.


A proprietary device that diffuses impinging sounds (instead of reflecting them) through reflection-phase-grating means.

Digital Light Processor (DLP)

Used to control Digital Micro-mirror Devices (DMD) in order to make an extremely bright sharp pictures.

Digital Micro-mirror Device (DMD)

A very small mirror (hence, micro-mirror) that is turned on and off at various rates per second to achieve different levels of brightness. Commonly used together to form micro-mirror "wafers" that are controlled by a Digital Light Processor (DLP).

Digital Theater Systems (DTS)

An 8-channel sound format used in commercial movie theaters.

Digital Versatile Disc (DVD)

Previously known as Digital Video Disc, this represents the latest in home theater. This format is has the ability to have multiple aspect ratios, several different versions of a movie with several different captions as well as Dolby Digital sound.

Dipole Speakers

A type of surround speaker that has two or more drivers facing different directions (usually 180 degrees).

Dolby AC-3

Refers to the 5.1-channel home theater sound system (also called Dolby Digital). Consists of front left/right speakers, a center speaker, left/right surrounds, and a subwoofer.

Dolby Digital

A newer term for Dolby AC-3. Consists of front left/right speakers, a center speaker, left/right surrounds, and a subwoofer.

Dolby Digital Surround EX

Dolby's latest surround format. It is essentially Dolby Digital with an added center rear channel. It premiered with Star Wars Episode I: The Phantom Menace.

Dolby Pro-Logic

Most common surround format. It consists of four channels of sound: left/right front channels, a center channel, and one surround channel.

Dolby Surround

The encoding process used to make material compatible with Dolby Pro-Logic.


Any sound-producing device.


A box that holds a loudspeaker.


A device for adjusting the frequency response of a signal or system.


As applied to sound quality, the faithful representation of the original signal.


The art of recreating incidental sound effects, such as footsteps or rustling clothes, synchronized with a moving picture. Named after one of its first practitioners.


The measure of the rapidity of alterations of a periodic signal, expressed in cycles per second or Hz. An alternating sound pressure of a specific frequency results in an auditory sensation of pitch.

Frequency response

The changes in the sensitivity of a circuit or system with frequency.

Front Projector

This is a separate unit (projector) that projects the video image onto a separate movie screen.

Harmonic distortion

Distortion of a signal by adding content that is harmonically related to the original signal. Clipping overload of an amplifier adds odd-order harmonics to the signal.


Integral multiples of the fundamental frequency. The first harmonic is the fundamental, and the second in twice the frequency of the fundamental, etc.


The unit of frequency, abbreviated Hz. The same as cycles per second. Named after German physicist Heinrich Rudolph Hertz (1857-1894).


The opposition to the flow of electricity or acoustic energy. In electronics is measured in ohms. Named after German physicist Georg Simon Ohm (1789-1854).

Initial time-delay gap

The time gap between the arrival of the direct sound and the first sound reflected from the surfaces of the room.


1,000 Hz.

LCD Projector

The smallest front projector available today. One major benefit is that it is easy to set up.


Low-frequency diffusion.

Light-Valve Projector

A type of front projector. It combines the technologies of LCD projectors and CRT projectors. They offer exceptional detail and brightness.

Line Doubler/Tripler/Quadrupler

Doubles, triples or quadruples the number of lines that make up a picture, therefore increasing detail by eliminating scan lines.


A device or circuit with linear characteristics means that a signal passing through it is not distorted.

Monopole Speakers

A speaker with all drivers facing one direction. Used for precise placement of sounds. Usually used in front and center speakers. Also termed "front-firing".


Interference of an electrical or acoustical nature. Random noise is sometimes used as a test signal in acoustical measurements. Pink noise is random noise whose spectrum falls 3 dB per octave: it is useful for use with sound analyzers with constant percentage bandwidths.


The standard by which TV is broadcast in the US.


A low or minimum point on a graph. A minimum pressure region in a room.


The interval between two frequencies having a ratio of 2:1.

Out of phase

Two related alternating signals with opposing polarity.


The standard by which TV is broadcast in Europe.

Pink noise

A noise signal whose spectrum level decreases at a 3-dB-per-octave rate. This gives the noise equal energy per octave.

Rear Projector

Another name for a "big screen TV".


The heart and brain of many home theaters. It has a decoder, audio/video switcher, AM/FM tuner, and an amplifier.

Reference level

The standardized level at which a signal is measured for calibration purposes. In Home Theatres and cinemas the reference level is 85dB SPL, when the electrical signal is at a level 20dB below the clip point of the medium (and all the meters in the studio read 0). Note that a sound pressure level in dB means that it is calculated with respect to the standard reference level of 20 ÁPa.


For surfaces large compared to the wavelength of impinging sound, sound is reflected much as light is reflected, with the angle of incidence equaling the angle of reflection.


The tailing off of sound in an enclosed area after multiple reflections from the boundaries.

Room Mode

An acoustic wave room resonance. Axial modes are associated with pairs of parallel walls. Tangential modes involve four room surfaces and oblique modes all six surfaces. Their effect is greatest in the range of 25Hz to 150HZ in typical residential rooms.


The material and frame the picture is projected onto using a front projector.

Sine wave

A periodic wave related to simply harmonic motion.

Sound pressure level

A sound pressure expression in dB above the sound pressure of 20 microPascals (ÁPA). Named after French mathematician, physicist, and philosopher Blaise Pascal (1623-1662).

Standing wave

A response condition in an enclosed space in which sound waves traveling in one direction interact with reflected waves traveling in the opposite direction, resulting in a condition of resonance, with peaks and nulls along the length of the travel path. A synonym of Room Modes.


This is a separate speaker used to handle the bass of movie soundtracks.

Surround Sound

The popular term used to describe an experience where the sound envelops the listener. This is done using surround-encoded material, a receiver, and surround speakers.

Surround Speaker

This type of speaker diffuses the sound so as to make it harder to discern where the sound is coming from.

Threshold of hearing

The lowest level sound that can be perceived by the human auditory system. This is close to the standard reference level of sound pressure, 20 ÁPA.


A trademark licensed to movie theaters and manufacturers of home theater products, identifying compliance with the performance parameters of Lucas film Ltd. for commercial and home theater sound systems. Unlike Dolby, which focuses on soundtrack formats, THX develops standards for the playback environment, including theater and room acoustics.


A device for changing electrical signals to acoustical or vice versa, such as a microphone or loudspeaker.

Tube Traps

Proprietary low frequency sound-absorbing units.


The distance a sound wave travels in the time it takes to complete one cycle of alternation.


Adjustment of signal frequency response to achieve a desired measurement. Often used in sound-level meters to match the human ears response to sounds.

White noise

Random noise having uniform amplitude at all frequencies.