ambience  That portion of the background sound in a music hall, auditorium, or listening environment other than that resulting from reflected program sound. Ambience includes intrusion of natural environmental noise inherent in the location, air-conditioning noise, audience noise, etc.

Ambisonics  Method of recording the spatial information of the entire sound field as defined by absolute sound pressure and three pressure gradients of the cardinal directions (left/right, fore/aft, up/down).

amplifier  Most commonly an electronic circuit or other device that increases the signal level by increasing voltage or current. Buffer amplifiers isolate one part of a circuit from another. Impedance matching amplifiers actually could decrease level. Preamplifiers are designed to handle small voltages, such as microphone signals. Power amplifiers are designed to drive loudspeakers.

amplitude  Another term for signal level. Measured often in decibels above or below a standard reference level.

analysis of sound  A process of determining the energy distribution of a signal with respect to frequency. A narrow filter is swept throughout the frequency range of interest to determine the spectral distribution.

anthropomorphic manikin (dummy head)  A manikin having head, shoulders, and upper torso with average human dimensions and with surface characteristics and pinna convolutions conforming closely to those of humans. High-quality microphones, commonly located in the concha region, record sounds encoded with directional information. A two-channel binaural signal results (German, kunstkopf).

attenuator  A device for reducing or adjusting signal level. A resistance with a sliding contact.

audio frequency  The range of human hearing is commonly accepted as 20 Hz to 20 kHz in audio work. Psychologists would use slightly different figures. (Hz = cycles per second)

auditory canal (auditory meatus)  The canal extending from the concha to the eardrum.

auditory filters   (See critical bands.)

auditory perspective  Early experiments in stereophonics at Bell Laboratories were considered analogous to sight and the term auditory perspective was used because of the spatial dimension added to sound.

auditory system  The hearing system composed of the outer ear (pinna, concha, auditory meatus, and tympanic membrane), the middle ear (ossicles: malleus, incus, stapes), the inner ear (cochlea), and the brain.

axial modes  Acoustical resonance effects between two spaced, parallel surfaces, such as side walls of a room, end walls, and floor/ceiling.


baffle  Barrier to provide some acoustical isolation between microphones assigned to specific instruments in a musical group (aka gobo).

balance control  A potentiometer on a stereo set by which the relative level of the left and right loudspeakers can be adjusted.

basilar membrane  One of the cochlear partitions whose vibrations are involved in the analysis of sound in the auditory system.

bass  The low-frequency portion of the audible spectrum.

bi-directional  A bi-directional microphone pattern, also known as figure eight, provides lobes with equal sensitivity (although with opposite polarity) on opposite sides of the diaphragm and steep nulls at right angles to the diaphragm.

binaural  Having or involving the use of two ears.

biphonic  A listening system providing a separate microphone/amplifier channel for each earphone.

Blumlein technique  A stereophonic microphone arrangement in which two bi-directional, coincident microphones are arranged so that their principal axes are oriented ±45° to the source of sound, thus 90° to one another. Named for Alan Dower Blumlein.

BMLD (Binaural Masking Level Difference)  The measured improvement in audibility resulting from the unmasking effect of incoherence between the two ears.


cardioid  A heart-shaped microphone directivity pattern that is sensitive primarily to sound from the forward direction and rather insensitive to the rear. The sensitivity to the sides is approximately half that of the forward direction.

channel  The single signal path from microphone, through initial amplifiers via the transmission medium (radio or record, etc.), to reproduction amplifiers and loudspeakers. Mono uses a single channel, stereo uses two or more channels.

cochlea  The inner ear; the sound analyzing part of the auditory system connected to the brain by the auditory nerves.

Cocktail Party Effect  A name given to the ability of the human auditory system to hear a desired sound by suppressing the noise interfering with it. This is a binaural function requiring both ears.

coherence  The degree of similarity between two sounds.

coincident  Two microphones are said to be coincident if their diaphragms are as close together as physically possible — usually accomplished by vertical alignment of the diaphragms.

coloration of sound  A deterioration in the spectrum of an audio signal. If the spectrum of a sound is altered, it is colored, analogous to light, which is colored as its spectrum is changed.

comb filter, combing  Alteration of the frequency response of a system as a result of constructive and destructive interference between a signal and a delayed version of the signal. Plotted on a linear frequency scale, a comb-filter response looks like the teeth of a comb.

compatibility  Mono compatibility is the quality of a stereo signal that allows the summed reduction of the two channels to mono without serious comb-filter distortion.

compression  A sound wave traveling in air is made up of alternating cycles of crowding air particles together, compression, and spreading of air particles apart, rarefaction.

conceptual  A mental impression or image.

concha  The resonant cavity of the outer ear between the pinna and the ear canal.

condenser (capacitor) microphone  A microphone in which the diaphragm is one plate of a capacitor (condenser);  vibrations in air cause changes in the spacing between the two plates, thus transducing these air pressure fluctuations of the sound wave to a usable signal voltage. A polarizing voltage is required.

correlation coefficient  A coefficient that expresses the degree of similarity between two functions.

critical bands  The frequency resolving power of the auditory system can be considered as the result of bandpass filters. Such filters have been measured extensively by masking techniques and have become known as critical bands. Critical bands can be centered on any frequency, and their width varies with frequency.

critical distance  The distance from a source of sound at which the total energy of the direct sound level equals the total energy of the reverberant sound field.

crosstalk  The presence of an unwanted signal “leaking” into another. Listening to binaural (dummy head) signals over stereo loudspeakers is unsatisfactory because of the unwanted leakage of sound from the right loudspeaker to the left ear, and the unwanted sound from the left loudspeaker to the right ear. These unwanted components are called crosstalk.


dead acoustics  A term applied to a space with little reverberation, or a space with too much sound absorbing material in it. Music quality suffers more than speech quality in such a space.

Decca Tree  A configuration of three omnidirectional microphones, positioned in a T-shaped frame, noted for its warm and enveloping sound. Developed by Decca Records in England.

decibel, dB  A unit of measurement defined as the logarithm of a ratio of two powers. Although defined in terms of a power ratio, it can also be computed from voltage and current ratios because of their relationship to power. One element of the ratio is usually a standard reference value, such as 1 mW for power, 1 V or 20 micropascals for sound pressure level. The extremely wide ranges of sound pressure encountered, for example in audio work, are conveniently expressed in dB.

decoding  1) Directionally encoded sounds arriving at the human ear are decoded by the brain. Directionally encoded sounds picked up by a microphone yield voltage fluctuations that must be decoded before they are used to drive loudspeakers. 2) An electronic process of recovering encoded information from a stereo signal and reconstructing a surround soundfield. 3) The reproduction stage of a complementary process for reducing noise in transmission systems (see encoding).

definition  The definition of a musical sound or an audio system, for example, is a measure of the clarity and purity by which small changes and nuances are conveyed.

delay  A measure of the time separation between two events. Examples from the field of audio include the time separation between a direct signal and a reflection of it, and the delay of a signal in electronic circuits or in loudspeakers. Audio signals are often purposely delayed by digital delay devices to achieve certain effects.

diaphragm  There are diaphragms in ears (eardrums), microphones, loudspeakers, etc. They either excite the air or are excited by it. Their purpose is to increase the efficiency of sound energy transfer to and from a tenuous medium, such as air.

diffraction  The scattering of sound. (See diffraction grating.)

diffraction grating  Diffraction gratings in optics scatter light into its spectral (frequency) components. In acoustics, theoretically similar devices are used to scatter or diffuse sound. In acoustical treatment of spaces, such gratings add diffusion to previously available tools of absorption and reflection.

diffuse field  A sound field composed of sound arriving from random directions.

digital delay  An electronic circuit capable of delaying an audio signal a controllable amount with high fidelity.

distance factor  A comparison of the sensitivity of a directional microphone to that of an omnidirectional microphone in any given direction.

distortion  Any unwanted difference between the input and output signal as it passes through a device (amplifier, loudspeaker, microphone) is distortion attributed to the device.

dummy head  (See anthropomorphic manikin.)

dynamic microphone  The vibration of the diaphragm in a dynamic microphone causes an electrical conductor to move in a magnetic field. A signal voltage corresponding to the air pressure variations acting on the diaphragm is generated in the conductor by magnetic induction.

dynamic range  The extremes between loud and soft. All audio systems are limited on the low side by circuit and other noise, and on the high side by distortion or mechanical failure. The usable range between is called the dynamic range of the system.


ear canal  (See auditory canal.)
eardrum  (See tympanic membrane.)
early reflections  Those reflections reaching a listener after the arrival of the direct sound, but before the arrival of reverberation sound resulting from late reflections. The early reflections give rise to a feeling of spaciousness in the music hall, but in the typical listening room they tend to confuse the stereo image, giving rise to coloration of sound due to combing.
echoes  If a reflected or delayed sound arrives at the listener 50 or more milliseconds after the arrival of the direct sound, a discrete echo can be heard.

electret microphone  A condenser or capacitor microphone with the element permanently charged so no external polarizing voltage is necessary.

encoding  1) As related to auditory perception, the process by which sound arriving at a listener’s ears is encoded with directional information through the action of pinna reflections, head diffraction, and reflections from shoulders and torso. 2) An electronic process for reducing the several channels of a surround sound system into a compatible stereo transmission medium. Encoding processes in surround sound include the 4:2:4 matrix and Dolby Surround, etc. 3) A process for reducing noise in transmission systems, employing processing at the input and complementary processing at the output, e.g. Dolby or dbx noise reduction.

ensemble  1) A group of musicians performing together. 2) The ability of musicians in a group to hear each other.

equalization (EQ)  The use of filters to alter the frequency response of a system. Commonly arranged with fixed filter spacings of one octave, 1/3 octave, etc., or variable characteristics (see also filter and parametric equalizer).

Eustachian tube  The tube between the middle ear and the pharynx that serves to equalize air pressure on both sides of the eardrum.


filter  A device that attenuates or increases certain portions of the audio frequency spectrum. A band-pass filter passes signal energy only in a specified band and attenuates all others. A high-pass filter attenuates energy below a certain cutoff frequency. A low-pass filter attenuates signal energy above a certain frequency.

flanging or phasing  The utilization of comb-filters for generating certain special sound effects.

frequency  The number of cycles in a periodic wave per second; measured in hertz (Hz).


gobo  (See baffle.)


Haas effect  (See precedence effect.)

Helmholtz resonator  A reactive, tuned, sound absorber usually employing slats or perforated facing over a cavity. (Named for Hermann Helmholtz.)

hertz  The unit of frequency replacing the term cycles per second, abbreviated Hz. (Named for Heinrich Hertz.)

hole in the middle  Stereo systems sometimes fail to yield a strong phantom image between the two loudspeakers. This is called a hole in the middle. Commonly the result of excessive spacing between the microphones or loudspeakers.

hypercardioid  A combination of omnidirectional and bidirectional microphone polar response produces a family of directional patterns, one of which is the hypercardioid that has appreciable sensitivity in the rear direction, although less than the forward direction.


impedance  The opposition to the flow of electrical or acoustical energy, measured in ohms.

incoherence  The degree of difference between two signals.

incus  One of the three ossicles that convey eardrum vibrations to the cochlea (see ossicles), sometimes called the “anvil.”

inner ear  The cochlea; the sound analyzing portion of the ear.

intensity  Commonly implied as the amplitude of an acoustical signal. In acoustics, intensity is the sound energy flux per unit area.

intensity stereo  Stereo dependent only on the relative amplitude (intensity) of signals in the two channels.

interaural  The response of the two ears to a given stimulus.

interference  The combining of two or more signals resulting in constructive or destructive interaction as in the comb-filter effect. The word is also used to denote undesired signals resulting from sources outside the signal path.

inverse square law  The intensity of sound diverging spherically from a point in free space decreases inversely as the square of the distance. Sound pressure, the commonly measured parameter, varies as the first power of the distance. Thus, sound pressure decreases 6 dB with each doubling of the distance in free space.


kunstkopf (German) Dummy head binaural recording device.


Law of the First Wavefront  The first sound to strike the ears determines the perception of direction.
live acoustics  A space is considered live if there is little absorption so that reverberation effects dominate hearing.

loudness  A subjective term for the sensation of the intensity of a sound. While intensity is a primary factor in the perception of loudness, it also is affected by frequency and duration of the stimulus.


magnitude  (See amplitude.)
malleus  One of the three ossicle bones that conduct the vibrations of the eardrum to the oval window of the cochlea (see ossicles), sometimes called the “hammer.”

masking  1) The process by which the threshold of audibility of one sound is raised by the presence of another sound. 2) The interference of one sound by another;  the interfering sound is called the masking sound. Masking is considered to be undesirable if it interferes with the audibility of desired sounds, or it may be used to beneficial effect in some forms of environmental noise control and in noise reduction or perceptual audio coding systems.

matrix  1) An array of elements comprising a multidimensional subject. 2) An electronic circuit designed to accomplish a specific task, such as encoding or decoding spatial information from a multi-channel sound system.

meatus, auditory  (See auditory canal.)

median plane  The vertical plane equidistant from the two ears bisecting the head.

medium  1) Sound is transmitted through media, such as air, liquids, and/or solids. 2) A transmission format, such as recording (tape or disc), broadcast, film, etc.

microphone  A device that transduces air vibrations into corresponding electrical signals.

microphone distance factor  (See distance factor.)

middle ear  The portion of the ear between the eardrum and the cochlea.

mid-side  A stereo microphone arrangement comprised of two microphones by which one directional microphone contributes the principal pickup of an ensemble and the other, a bidirectional, contributes the lateral information.

mixdown  The process by which a multichannel recording is combined into one, two, or more channels.

modes  The low-frequency resonances (modes) prominent in small rooms.

monaural  A single channel recording or reproducing system arranged for one-eared listening.

mono  A contraction of monophonic (or, less frequently, monaural).

mono compatibility  The capability of a stereo recording to be reduced to a monophonic recording with low distortion.

monophonic  A single-channel recording or reproducing system arranged for two-eared listening.


noise floor  The inherent noise of a circuit or system that establishes the lowest usable signal level.

noise, pink  Random noise having equal energy per octave, commonly used for evaluating and calibrating sound reproduction systems.

noise, random  An undefined blend of all audio frequencies, heard as a hiss.

noise, white  Random noise with a uniform distribution of energy throughout the entire audible spectrum, analogous to the electromagnetic spectrum of white light.

N.O.S. technique  An arrangement of two cardioid microphones set at an angle of 90° with a capsule spacing of 30 cm (developed by Netherlands Radio).


objective  Without bias or prejudice; detached; impersonal. An analytical approach based on observation of data.

oblique modes  The normal modal resonances of a room that involve all six surfaces of the room.

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

omnidirectional  A microphone with uniform sensitivity to sound arriving from all directions is said to have an omnidirectional pattern.

O.R.T.F. technique  An arrangement of two cardioid microphones whose principal axes are angled 110° and with capsules separated 17 cm (developed by French Radio).

ossicles  The three bones of the middle ear (malleus, incus, stapes) providing a mechanical linkage between the eardrum and the oval window of the cochlea.

OSS technique (Optimum Stereo Signal)  Utilizes a pair of omnidirectional microphones separated 16.5 cm with an acoustically opaque baffle between them. (Jecklin Disk)

outer ear  (See pinna.)

oval window  A membranous window of the cochlea to which the stapes ossicle is attached.


panoramic potentiometer (panpot)  A variable resistance control by which a mono signal might be placed anywhere between the two channels of the stereo field.

parametric equalizer  A filter having a variable bandwidth and adjustable center-frequency. A commercial parametric equalizer usually will have three or four filters per channel.

perception, percept  The faculty of consciously perceiving. A recognizable sensation or impression received through the senses.

perceptual  Involving perception.

periodic  A signal having a regularly repeating pattern, i.e., a tone as opposed to speech, noise, or music signals.

phantom image  The image appearing between two stereo loudspeakers driven with a stereo signal.

phase  The time relationship between two signals.

phase coherency  Two coincident microphones are in phase coherency if they yield primarily intensity cues and minimize all phase (time) cues. Phase integrity might be a better phrase to avoid confusion with interaural coherency.

phasiness  Audible combing distortion.

phasing  (See flanging.)

pinna  The outer ear. Directional cues result from the reflection of sound from the folds of the pinna.

pitch  The subjective perception of frequency. Although based primarily on frequency, the perception of pitch also can be affected by other factors such as intensity and spectrum.

polar pattern  A graphical representation of the sensitivity of a microphone or loudspeaker, commonly shown as a diagram of the horizontal plane through 360°. Vertical polar patterns also are important, but are not as commonly represented in sales literature.

polarity, absolute  A sound wave compression moving outward from the source can be considered as positive pressure. The concept of absolute polarity can be maintained by making a positive electric wave conform to positive acoustic waves (and vice versa). This can be done by adjustment of polarity of electrical equipment and loudspeakers.

potentiometer  A variable voltage divider or a resistance with an adjustable slider by which voltage can be varied between two outputs.

precedence effect (Haas effect)  If two loudspeakers in normal stereo arrangement are energized with the same signal and a small amount of signal delay is introduced in one of them, the sound from the earlier one will determine the localization of the sound. This is the Haas or precedence effect. Helmut Haas found that delays within a certain range could be compensated by intensity changes. A 10 ms delay requires a 10 dB adjustment to return the phantom image to the center. This fusion or integration zone persists up to about 30 or 35 ms delay. Delays greater than this result in discrete echoes.

presence  Speech can be given presence and made to stand out from a music background by an equalization boost of 2–4 dB at a frequency of 2–3 kHz. Sounds in this region tend to be coded for arrival from the front.

pressure gradient  The ribbon microphone is called a pressure gradient microphone because the force acting on the diaphragm is proportional to the differences between the pressures on both sides of the diaphragm. This type of microphone is also called a velocity microphone because its output depends on the air particle velocity at the diaphragm or ribbon. A purely pressure gradient response has a bidirectional polar pattern.

pressure microphone  An microphone that responds only to the fluctuations of air pressure at the diaphragm. A purely pressure microphone has an omnidirectional polar response.

pressure, sound  Small fluctuations of air pressure above and below the static atmospheric pressure carry acoustic information. Sound pressure can be measured by common sound level meters.

psychoacoustics  The study of the interrelationships between the human hearing mechanism and acoustics.


quadratic residue diffuser  A sound diffuser based on a sequence of numbers derived from number theory.


random energy efficiency  A measure of the degree to which a microphone responds to the desired sound relative to the total sonic environment surrounding it.

random noise  (See noise, random.)

rarefaction  The negative portion of a sound wave in which the air particles are spread apart.

reflection, early  (See early reflections.)

refraction  The bending of sound rays usually resulting from stratification of the medium.

Reissner’s Membrane  With the basilar membrane, Reissner’s Membrane divides the cochlea into three longitudinal chambers.

resonance  An increase in the acoustical pressure in an enclosed (or partially enclosed) space as a result of the dimension of the space interacting with the wavelength of the signal. Acoustical resonance in a room results from the combination of a forward-going wave with a backward-going wave resulting from reflections between two opposite, plane, parallel surfaces.

reverberation  A temporal extension of acoustical events in a space generated by multiple sound reflections.

ribbon microphone  A microphone whose diaphragm is a metallic ribbon stretched in a magnetic field so that its movement by sound waves induces a signal voltage between its ends. It operates on the differential of pressure between its two sides, which results in a figure-eight pattern.

room tone  Similar to ambience.

round window  A window of the cochlea opening into the middle ear that acts as a pressure release on the fluid of the inner ear.


S.A.S.S.™, (Stereo Ambient Sampling System)  A mono-compatible, near-coincident array of microphones designed to give highly localized stereo imaging for loudspeaker reproduction.

self noise  (See noise floor.) The inherent noise floor of a microphone or electronic circuit.

sensation  The feeling resulting from a stimulus.

shotgun microphone  A microphone utilizing differential phase interference to achieve its highly directional pattern. Used for speech pickup at a distance from the subject.

simple harmonic motion  A vibratory motion in which the moving object sweeps back and forth over the same path.

sine wave  A combination of simple harmonic motion and uniform linear motion. Called a sine wave because the displacement is proportional to the sine of the angle.

sonic  Pertaining to sound.

SoundField™ microphone  A stereo microphone system composed of four subcardioid capsules mounted on the faces of a tetrahedron. After electronic combination, they yield the four components of the sound field:  absolute pressure and the fore/aft, left/right, and vertical pressure gradients.

sound pressure level  A sound pressure referred to a standard level; expressed in dB; abbreviated SPL.

spaciousness  The sense of envelopment that is created in music halls by early lateral reflections from a variety of different horizontal directions.

spatial  Having to do with space.

spectrum  The spectrum of a signal is the distribution of its energy throughout the audible range:  20 Hz to 20 kHz.

speed of sound (velocity of sound)  Approximately 1,130 ft/sec (344 m/sec) at normal air temperature and normal atmospheric pressure.

spherical divergence  (See inverse square law.)

spot microphone  A microphone placed close to a performer to augment that performer’s sound in the overall recording.

standing wave  A room resonance in the low-frequency region that results in uneven distribution of sound energy in the room.

stapes  (See ossicles) Sometimes called the “stirrup.”

stereo seat  That position giving the optimum listening conditions in a stereo reproduction environment; often considered to be at a point forming an equilateral triangle with the two loudspeakers.

stereo  The Greek word carries the meaning of solid, i.e., with depth, breadth, and height implied.

stereoscopic  Pertaining to the visual ability to perceive depth. Judgments of direction to a sound source utilize information from two ears.

stimulus  Something that incites a sensation.

supercardioid  A microphone polar pattern midway between cardioid and bi-directional. (See hypercardioid.)

super stereo  A phrase applicable to a recording/reproducing system that would accurately convey all spatial information with true fidelity, via only two loudspeakers.

sweet spot  (See stereo seat.)

synthesis of sound  The act of artificially building up a sound from component parts.


tangential mode  The room resonance that involves four of the six surfaces of a room.
Three-to-One Rule  A practical rule for avoiding comb-filter distortion; keep adjacent microphones at least three times as far apart as the distance between any microphone and its sound subject.

threshold  The point at which a stimulus is just strong enough to be perceived or produce a response.

timbre  The perceived tonal quality of a sound based on the pitch and the relative mix of fundamental and harmonic frequencies;  the perceptual counterpart of spectrum.

tone  A sound that is distinct and identifiable by its constant pitch.

transducer  A device that converts one form of energy into another. In audio the most common examples are the microphone and loudspeaker.

transfer function  Description of a function in terms of its frequency and phase (time) response.

treble  Tones of higher frequency.

Two-to-One Ratio  For the same signal level and direct-to-reverberant sound ratio, a cardioid microphone can be used generally at twice the distance from the sound source as an omnidirectional microphone.

tympanic membrane  The eardrum. The tympanic membrane is actuated by the vibration of air in the ear canal. These vibrations are transferred to the chain of ossicles in the middle ear, which in turn mechanically transmit them to the oval window of the cochlea.


unidirectional  A microphone whose sensitivity is dominant in one direction.

unmasking  The application of certain interaural incoherences to achieve a release from masking.


velocity of sound  (See speed of sound.)


wall of sound  A theoretical concept employed by early workers in stereo at Bell Laboratories; a two-dimensional wavefront.

wave form  The shape of a signal wave.

wavelength  The distance between successive similar points on a sound wave. Easily computed by dividing speed of sound by the frequency.


XY stereo  The arrangement of two directional microphones in a coincident stereo pickup.