Glossary

This is a glossary of audio/visual terms mainly relating to sciences. It does not incorporate many terms relating to technologies.

Absorption: In acoustics, the changing of sound energy to heat.

Absolute pitch: The rare ability of an individual to identify a particular pitch as a particular note. For example, a person who has absolute pitch can hear a tone and say that it is a B-flat. (Also see Relative pitch)

Absorption Coefficient: The fraction of the incident sound power that is absorbed by a material on a scale from 0 to 1 and varies with the frequency and angle of incidence of the sound. (Also see Sound Absorption, Sabin)

Accelerometer: A transducer for measuring acceleration levels associated with low frequency vibration.

Acoustic Impedance: Z=R+jX ; [ML-4T - 1] ; mks acoustic ohm *(Pa*s/m3) ---of a surface for a given frequency, the complex quotient obtained when the sound pressure averaged over the surface is divided by the volume velocity through the surface. The real and imaginary components are called  acoustic resistance and acoustic reactance, respectively.

Acoustic material: Of course any material has acoustical properties, but the term is commonly used to describe materials designed to absorb sound.

Acoustics: (1) The science of sound, including its production, transmission and reception. (2) The physical qualities that determine how a room sounds.

Acuity: The capacity to discriminate fine details of objects.

AES: Audio Engineering Society.

Afterimage: A visual effect occurring after a light stimulus has been removed. This effect can either be positive (when the brightness or color is the same as the stimulus) or negative (when the brightness or color of the image is the opposite of the stimulus).

Airborne Sound: Sound that arrives at the point of interest, such as one side of a partition, by propagation through air.

Airflow Resistance, R: [ML-4T-1]; mks acoustic ohm *(Pa*s/m3) ---the quotient of the air pressure difference across a specimen divided by the volume velocity of airflow through the specimen. The pressure difference and the volume velocity may be either steady or alternating.

Airflow Resistivity: [ML-3T-1]; mks rayl/m *(Pa*s/m3) ---of a homogeneous material, the quotient of its specific airflow resistance divided by its thickness.

ALcons: The measured percentage of Articulation Loss of Consonants by a listener. % ALcons of 0 indicates perfect clarity and intelligibility with no loss of consonant understanding, while 10% and beyond is growing toward bad intelligibility, and 15% typically is the maximum loss acceptable.

Ambience: The distinctive acoustical characteristics of a given space.

Ambient Noise: The background noise, including sounds from many sources near and far, associated with a given environment. It is the composite of airborne sound in which no particular sound is singled out for interest.

Amplitude: The instantaneous magnitude of an oscillating quantity such as sound pressure. The peak amplitude is the maximum value. It is one of three measurements of the vibration of a sound wave. The others are frequency and wavelength.

Amplitude Distortion: A distortion of the wave shape of a signal.

Amusia: The loss through brain damage of one or more musical skills. This could mean skills required for just listening to music, such as the ability to hear harmonic intervals, or skills required for making music, such as performing or reading music.

Analog: An electrical signal whose frequency and level vary continuously in direct relationship to the original electrical or acoustic signal.

Angle of Incidence: The angle at which a light ray or acoustic wave strikes a surface, measured between the incoming wave and the line perpendicular to the surface.

Angle of Reflection: The angle at which a light ray or acoustic wave is reflected from a surface, measured between the ray and the line perpendicular to the surface.

Anechoic: Without echo.

Anechoic Chamber: A laboratory test room where all surfaces are covered by sound absorbing material in an attempt to eliminate sound reflections in order to correctly measure sound energies.

Antinode: A place of minimum sound pressure level.

Aqueous Humor: A watery liquid between the lens and the cornea which supplies nutriment to the cornea.

Arithmetic Mean Sound Pressure Level: Of several related sound pressure levels measured at different positions or sound pressure levels measured at different positions or different times, or both in a specified frequency band, the sum of the sound pressure level divided by the number of levels.

Articulation: A quantitative measure of the intelligibility of speech; the percentage of speech items correctly perceived and recorded.

Articulation Index: A calculated coefficient used for rating the intelligibility of speech.

Artificial Reverberation: Reverberation generated by electrical or acoustical means to simulate that concert halls, etc. added to a signal to make it sound more lifelike.

ASA: Acoustical Society of America.

ASHRAE: American Society of Heating, Refrigerating, and Air-Conditioning Engineers.

Astigmatism: A vision defect usually caused by uneven curvature of the cornea.

Attack: The beginning of a sound; the initial transient of a musical note.

Attenuate: To reduce the level of an electrical or acoustical signal. Reduction in sound level.

Attenuator: A device, usually a variable resistance, used to control the level of amplitude of an electrical signal.

Audible Frequency Range: The range of sound normally heard by the human ear, typically 20Hz - 20kHz.

Audiology: The study and measurement of deafness.

Audiophile: Someone who is interested in sound reproduction.

Audio Frequency: An acoustical or electrical signal of a frequency that falls within the audible range of the human ear, usually taken as 20 Hz to 20 kHz.

Auditory Area: The sensory area lying between the threshold of hearing (close to the standard reference level of sound pressure, 20 micropascal) and the threshold of feeling or pain (about 120 dB above the threshold of hearing).

Auditory Cortex: The region of the brain receiving nerve impulses from the ear. Generally speaking, it consists of the primary auditory cortex for identifying individual sounds, and the secondary auditory cortex for detecting relations among sounds.

Auditory System: The human hearing system made up of the external ear, the middle ear, the inner ear, the nerve pathways, and the brain.

Auditory Area: The sensory area lying between the threshold of hearing and the threshold of feeling pain.

Aural: Having to do with the auditory system.

Auricle: Part of the outer ear, also called the pinna.

Axial Mode: The room resonances associated with each pair of parallel walls.

Average Room absorption Coefficient: Total room absorption in sabins or metric sabins, dividing by the total room surface area in constant units of square feet or square meters.

Average Sound Pressure Level (Leq): The average sound pressure level occurring in a specified period (e.g., 1 hour), or number of positions, or both, in a specified frequency band, ten times the common logarithm of the arithmetic mean of the squared pressure ratios from which the individual level were derived.

A-Weighting: A standard frequency response adjustment of a sound-level meter that de-emphasizes low-frequency sound to make its reading conform similar to the average human hearing response at a loudness level of 40 phons, and approximates loudness and annoyances of noise. A-weighted sound levels are frequently reported as dBA.

Axial Mode: The room resonances associated with each major pair of parallel surfaces i.e.,  front/back walls,  side walls and floor/ceiling.

Background Noise: Noise from all sources unrelated to a particular sound that is the object of interest. Background noise may include airborne, structureborne and instrument noise.

Baffle: (1) A movable barrier used in the recording studio to achieve separation of signals from different sources to be recorded. (2) The surface or board upon which a loudspeaker is mounted.

Bandpass Filter: A filter that attenuates signals both below and above the desired passband.

Bandwidth: A specific range of frequencies.

Basal ganglia: Bundles of neurons that extend beneath the cerebral cortex, which processes inputs from many parts of the brain. The basal ganglia play an important role in executing complex motions.

Basilar Membrane: A membrane inside the cochlea that vibrates in response to vibrations of the fluid of the inner ear, exciting the hair cells. It helps translate sound waves into nerve impulses.

Bass: The lower range of audible frequencies. Generally referred to as frequencies below 300 Hz.

Beaming: The phenomenon of sound being emitted within a comparatively small solid angle. This characteristic becomes more acute as frequency increases.

Beats: Periodic fluctuations that are heard when sounds of slightly different frequencies are superimposed.

Bell: 10 dB, named after Alexander Graham Bell.

Binaural: Auditory reception associated with two ears.

Bipolar Cells: A layer of nerve cells in the retina which receive stimulation from the photoreceptors and pass it on to the next layer, the ganglion cells.

Blind Spot: The point of exit of the optic nerve from the retina. There are no rods or cones at this point and therefore no response to light.

Boomy: Listening term. Expression for excessive bass response in a recording, playback, or sound reinforcing system.

Brain stem: A bulging segment at the top of the spinal cord, filled with bundles of neurons that perform basic tasks. Information from the ears is processed in this part of the brain before it moves on to the cortex.

Break: A physical break in the assembly or construction which acts to decouple sound vibrations from traveling through the structure.

Brightness: (1) Visually, a subjective description of light intensity. (2) Audibly, excessive high frequency content.

Broadband: Spectrum consisting of a large number of frequency components, non of which is individually dominant.

Capacitor: An electrical component that passes alternating current but blocks direct currents. Also called a condenser, it is capable of storing electrical energy.

Characteristic impedance of the medium: [ML-2T-1]; mks rayl *(Pa*s/m) - the specific normal acoustic impedance at a point in a plane wave in a free field. It is a pure specific resistance since the sound pressure and the particle velocity are in phase and it is equal in magnitude to the product of the density of the medium, p, and the speed of sound in the medium, c. Its value when the medium is air at 20 degrees Celsius and 101.25 kPa is 413 mks rayl (Pa-s/m).

Choroid: The middle membrane of the eye, between the retina and outer coating, which supplies blood to the eye.

Ciliary Body: The group of muscles which regulates the shape of the lens for near- and far-vision.

Clipping: An electrical signal is clipped if the signal level exceeds the capabilities of the amplifier. It is the distortion of the signal. Usually containing excessive high-frequency energy.

Cochlea: The spiral shaped cavity within the inner ear that changes the mechanical vibrations of the cochlear fluid into electrical signals, or nerve impulses.

Consonance: Harmony in which simultaneous sounds "go well" together with minimal tension.

Color Blindness: An inexact term covering all major deviations from normal color vision. The most common form is dichromatism, sensitivity to only two distinct colors rather than the three needed to match all colors in the spectrum.

Color Constancy: The tendency to perceive a familiar object as the same color under different light conditions.

Coherence: Listening term. Refers to how well integrated the sound system is.

Coloration: Listening term. The distortion of a signal detectible by the auditory system.

Comb Filter: A distortion produced by combining an electrical or acoustical signal with a delayed replica of itself. This is often experienced in playback systems where the direct sound is heard along with a reflection from a nearby wall or ceiling. The result is constructive and destructive interference that results in peaks and nulls being introduced into the frequency response. When plotted to a linear frequency scale, the response resembles a comb, hence the name.

Complementary Colors: Two spectral colors which add together to make white or gray.

Compression: In audio, to reduce the dynamic range of a signal actively with electrical circuits that reduce the level of loud passages. Passively, compression can occur in rooms that are overly absorptive because there is no re-enforcement of the sound waves. It is also the portion of a sound wave in which molecules are pushed together, forming a region with higher-than-normal atmospheric pressure.

Condenser: See capacitor.

Cones: Plump photoreceptor cells (about 7 million of them) in the retina which provide color sensitivity and bright light detail.

Cornea: The transparent coating of the eyeball over the iris and pupil. The cornea is part of the eye's focusing system.

Cortex: See Auditory Cortex.

Coefficient: A unitless quantity generally scaling from 0 to 10.

Critical Band: In human hearing, only those frequency components within a narrow band, called critical band, will mask a given tone. Critical bandwidth varies with frequency but is usually between 1/6 and 1/3 octave.

Critical Distance: The distance from a sound source at which the direct sound and reverberant sound are the same level.

Crossover Frequency: In a loudspeaker with multiple radiators, the crossover frequency is the -3 dB point of the network dividing the signal energy.

Crosstalk: The signal of one channel, track, or circuit interfering with another.

Cycles Per Second (cps): The frequency that an electrical signal or sound wave repeats itself during a period of one second. Measured in Hertz (Hz).

Damping: The dissipation of sound or vibration. Damping reduces the velocity of vibrating molecules in an elastic medium at resonant frequencies, resulting in the more rapid dissipation of energy.

dB (A): See A-weighting. A sound-level meter reading with an A-weighting network simulating the human ear response at a loudness level of 40 phons.

dB (B): A sound-level meter reading with an B-weighting network simulating the human ear response at a loudness level of 70 phons.

dB (C): A sound-level meter reading with no weighting network in the circuit, i.e., flat. The reference level is 20 uPa.

Decade: Ten times any quantity or frequency range. The range of the human ear is about 3 decades.

Decay: The decrease of sound or vibration with time. Decays are most readily observable subsequent to an impulse.

Decay Rate: d, [T-1]; dB/s - for airborne sound, the rate of decrease of vibratory acceleration, velocity, or displacement level after excitation has stopped.

Decibel (dB): The measurement unit used in acoustics for expressing the relative logarithmic ratio of two like quantities proportional to power or energy. Thus, one decibel corresponds to a power of 100:1.  Typically used to describe the magnitude of a sound with respect to a reference level equal to the threshold of human hearing. One decibel unit is approximately equal to the smallest change in loudness that the human ear can detect.

Dark and Light Adaptation: The eye's adjustment in sensitivity to changing light conditions. This adjustment is accompanied not only by changes in the size of the pupil, but also by chemical changes to the visual pigments. Complete adaptation going from daylight to a dark theater may take 30 minutes.

Diaphragm (also diaphragmatic): 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.

Diffraction: The distortion of a light or sound wavefront due to the presence of an obstacle or aperture. A change in direction of propagation of sound energy in the neighborhood of a boundary discontinuity, such as the edge of a reflective or absorptive surface.

Diffuse Sound Field: A sound field where the sound pressure level is the same in all directions and the flow of sound energy is equally probable in all directions. Also used in room acoustics to describe the property of being "immersed" in a uniform reverberant sound field.

Dipole Speaker: A loudspeaker system in a single enclosure that radiates from both the front and back. Often used in home theater to provide a diffuse sound field.

Direct Sound: A wave radiated directly from a sound source that reaches a receiving point without reflections.

Driver: A device that supplies energy to another system or circuit (such as a loudspeaker).

Eardrum: A conically shaped membrane stretched across the inner end of the external auditory canal. Vibrations in the air, picked up by the eardrum, are passed on to the bones of the middle ear. Also called the tympanic membrane.

Echo: A discrete sound reflection arriving at least 50 milliseconds after the direct sound and significantly above the level of the reverberation at the time.

Echolocation: The technique of locating objects by emitting sound pulses and interpreting the echoes; used by such animals as bats and porpoises, and in man made sonar.

Electroacoustics: The conversion of acoustic energy into electrical energy, or vise versa.

Electromagnetic Spectrum: Waves of electric and magnetic energy traveling at the speed of light. This energy takes many forms, including light, heat, X-rays and radio waves.

Equal Loudness Contours (Fletcher-Munson Curves): A set of frequency curves that indicates the loudness levels of pure tones heard monaurally.

Equalizer: A device that adjusts the frequency response characteristics of an electronic signal.

Eustachian Tube: The passage connecting the middle ear with the throat. It serves to equalize pressure differences between the middle ear and the atmosphere.

Far Field: The portion of the acoustic radiation field of a noise source where sound pressure level decreases at a rate of 6 dB per doubling of distance.

Feedback: A part of the output of a system (as an electronic circuit) that is returned to the input.

Filter: A device (usually electronic) for separating specific frequency components of a signal.

Flanking: The transmission of airborne sound from a source room to an adjacent receiver room by a path other than the common partition.

Flutter: Distortion that occurs in sound reproduction as a result of undesirable speed variations during the recording, duplicating, or reproducing process.

Flutter Echo: Echoes occurring repetitively between a pair of surfaces.

Formant: A range of frequencies that an instrument or voice amplifies through resonance.

Fourier Analysis: Frequency and phase analysis of a signal based on the addition of sine waves.

Fovea: A small depression in the central part of the retina which contains only cones and is responsible for acuity.

Free Field: The sound field or portion of a sound field unaffected by reverberation or reflected sound.

Frequency: A descriptor for a periodic phenomenon. The frequency is equal to the number of items that the pressure wave repeats in a specified period of time. In the case of sound vibration, frequency is measured in units of Hertz (Hz), which correspond to one cycle per second (cps). Along with amplitude and wavelength, one of three measurements of a sound wave.

Frontal lobe: Parts of the cortex behind the forehead or above the eyes. The frontal lobes modify the function of other parts of the brain and are concerned with planning, directing attention, and sustaining short-term memories.

Fundamental Frequency: The lowest common factor in a series of harmonic partials. The fundamental frequency of a periodic waveform is the reciprocal of its period. The "first" harmonic or resonance within a sound spectrum often having the greatest amplitude.

Gain: The ratio of increase in output compared to the input in an amplifier. Frequently measured in units of decibels referenced to voltage.

Ganglion Cells: The third important layer of the retina, which receives stimulation from the bipolar cells and transmits impulses to the optic nerve.

Hair cells: A special kind of neuron found in the cochlea that projects small "hairs" that are vibrated by sound to stimulate the neuron.

Hass Effect: See Precedence Effect.

Harmonic: A single component of a sound's spectrum that has a frequency that is an integer multiple of the fundamental frequency.

Hertz (Hz): A unit of measure for describing the frequency of a wave phenomenon, such as sound.

Hippocampus: A part of the brain situated within each temporal lobe and concerned with attending and remembering.

Hue: The distinctive color by which each narrow band of light wavelengths is perceived.

Hyperopia: Farsightedness, or ability to focus only on objects at a distance.

Impact Insulation Class (IIC): A single number rating derived from one-third octave band values of impact noise levels measured through a floor/ceiling system using a standard tapping machine. The IIC describes the impact noise insulating properties of a floor/ceiling assembly and approximates footfalls on hard-surfaced floors. ASTM E989 and E1007 govern the measurement and calculation of IIC.

Impulsive Noise: Usually a brief noise characterized by an instantaneous sound pressure that significantly exceeds the ambient noise level.

Incus: One of the three ossicles of the middle ear.

Inertia Base: A concrete slab which serves as a base for mechanical equipment such as fans and pumps. The base is supported on vibration isolators to reduce the transmission of vibration to the building structure.

Inner Ear: A fluid-filled chamber which contains the cochlea and semicircular canals. It is connected to the brain by the auditory nerve.

Insertion Loss: The difference in sound levels before and after a noise barrier or sound attenuator is constructed or installed.

Intensity: The strength of a sound, usually measured by the amplitude of its wave. Intensity, which the brain perceives as loudness, is also measured in decibels.

Inverse Square Law: The sound attenuation due to distance from a source based on the reciprocal of the square of the distance. For a point source in the far field, this is 6 dB per doubling of distance.

Iris: Contractile eye tissue behind the cornea which regulates the amount of light entering the eye.

Lambert's Law: The angle of a reflected sound "ray" from a surface is equal to its angle of incidence.

Lens: A transparent oval body behind the iris which adjusts the eye's focus for near- and far- vision.

Limbic system: A collection of primitive structures deep in the brain that are important in emotion, attention and memory.

Lobe: A large section of cerebral cortex loosely defined by the overall form of the brain. (See frontal lobe, parietal lobe and temporal lobe.

Light: A small band in the electromagnetic spectrum, ranging from about 400 to 700 millimicrons, which contain all of the visible colors.

Linear Perspective: The depth cue of parallel straight lines seeming to converge at a distant vanishing point.

Localization: The process of identifying the direction a sound arrives from, and possibly its distance. Also, the topography of functions in cerebral cortex.

Loudness: The magnitude of the physiological sensation produced by sound, which varies directly with sound intensity and is also dependent on both frequency and time.

Macula: The pigmented central area, or "yellow spot", of the retina.

Malleus: One of the three ossicles of the middle ear.

Masking: The process by which sensitivity to a sound is decreased by the presence of another (masking) sound. Masking noise can be used to reduce the intelligibility or distraction of an intruding sound, such as speech.

Middle Ear: The air-filled cavity containing the three minuscule bones (the ossicles), which conduct sound vibration from the eardrum to the inner ear.

Mode: Any of various stationary vibration patterns of which an oscillatory system is capable; for example, a standing wave in a room or a fundamental on a guitar string.

Monaural: A single channel of sound reproduced through one or more loudspeakers.

Movement Parallax: The relative motion of near and far objects perceived when either the object or the observer is moving.

Munsell Color System: A classification of color in terms if three attributes, hue, value and chroma. By this system, any color can be specified by numbers and letters.

Myopia: Nearsightedness, or an ability to focus only on objects near the eye.

Near Field: The acoustic field that is near a sound source, where the sound level drops off less than 6 dB per doubling of distance.

Neuron: A nerve cell consisting of a body and fibers projecting to other nerve cells, fibers that in some instances span the breadth of the brain or length of the spinal cord. What we normally call "nerves" are merely such fibers containing messages between neuron bodies.

Newton: Unit of force that will impart acceleration of one meter per second squared to a kilogram mass.

Node: A place where minimal motion takes place.

Noise: (1) Any undesired sound. (2) Any statistically random wave.

Noise Criteria (NC) Curves: A set of spectral curves used to obtain a single number rating describing the "noisiness" of environments for a variety of uses. NC is typically used to rate the relative loudness of ventilation systems.

Noise Isolation Class (NIC): A single number rating representing the sound isolation between two enclosed spaces separated by a partition. The sound paths include the partition and the flanking paths. (Also see Sound Transmission Class)

Noise Reduction (NR): The difference (reduction) in sound pressure level of sound transmitted through a building partition, usually measured in octave or one-third octave frequency.

Nucleus: A large bundle of neurons interconnected to perform a specific function.

Octave: A separation in frequency constituting doubling or halving of frequency.

Octave Band: A frequency band with the highest frequency being twice the lowest frequency defining the frequency range.

Optic Nerve: The bundle of nerves which carries light-generated pulses from the eye to the brain.

Optical Illusion: A visual perception in which there is an unusual discrepancy between the stimulus and the object perceived.

Organ of Corti: The microscopic structure housing the hair cells that trigger the auditory nerve impulses. It is located on the basilar membrane.

Ossicles: Three tiny bones in the middle ear-the malleus, incus and stapes, also known as the hammer, anvil and stirrup-which conduct sound to the inner ear. The malleus is connected to the eardrum and the stapes to the oval window of the middle ear. The incus joins the other two.

Otology: The branch of medicine concerned with the ear.

Outer Ear: The visible ear, consisting of the pinna, which picks up sound waves, and the external canal, which channels them as far as the eardrum.

Parietal lobe: Cerebral cortex, situated above and behind each ear, that is concerned in the left brain with grammatical and analytical reasoning, and in the right brain with spatial reasoning.

Pascal (Pa): Unit of pressure equal to one Newton per square meter. Twenty micro pascals is the reference pressure used in determining sound pressure level.

Perceptual present: The span of events that our brains can perceive directly, without recourse to recalled imagery.

Perceived pitch: The perceived sensation of frequency. A sound wave whose frequency is 440 cycles per second is perceived as the pitch we call "middle A." A change in frequency is not always accompanied by an equal change in perceived pitch, so the two terms are not equivalent.

Persistence of Vision: A brief continuation of a visual effect beyond stimulus, representing a kind of inertia of the visual system.

Phase: A time point reference in a periodic wave, usually measured in degrees (angular), with 360 degrees representing one complete cycle. Phase is used to measure the amount a wave leads or lags another.

Phon: A unit of loudness of a sound that relates sound pressure at different frequencies to the loudness of a reference sound pressure level of a 1,000 Hz tone.

Phoneme: A member of a set of the smallest units of speech that serve to distinguish one utterance from another in a language or dialect.

Photon: The smallest unit by which the energy of light can be measured.

Photopigments: Molecules which selectively absorb and reflect different wavelengths of light.

Photoreceptors: Cells at the back of the retina containing light-sensitive pigments. When light is absorbed, these pigments undergo reaction which triggers the nerve impulses resulting in vision. (Also see Cones and Rods)

Pink Noise: Noise whose spectrum is filtered so that there is equal power in each octave band. For example, the sound power in the 63 Hz octave band (45 to 89) is equal to the sound power in the 1,000 Hz octave band (707 to 1,414 Hz).

Pinna: The part of the outer ear that projects from the side of the head and collects sound waves. Also called the auricle. (Plural: Pinnae)

Pitch: The perceived frequency (highness and lowness) of a sound determined primarily by the fundamental frequency of the sound and also upon its harmonics and their relative sound pressure levels (e.g., spectrum).

Plasma Display: A video display in which the image is created by the glow of electricity excited neon and xenon gases arranged in a flat panel matrix.

Polarized Light: Light waves vibrating in a single plane rather than in all directions.

Preamplifier: An amplifier designed to amplify extremely low level signals from a device (as a microphone) before the signals are fed to additional amplifier circuits.

Precedence Effect: A form of sensory inhibition. The ability of the human auditory system to suppress perception of echoes, primarily up to 40 msec after the direct sound. Also known as the Haas effect, it is also means the ability of two ears to localize the earliest sound as the source and "dismiss" a repeated signal arriving from a different location (as in a reflection) even when the repeated signal is as much as 10 dB louder then the first.

Primary auditory cortex: Cerebral cortex that receives input from the ears via the brain stem and pieces together individual sounds. It is located on the top-center of the temporal lobes.

Primary Colors: Any set of three colors whose mixture in various proportions will produce white and all colors in the visible spectrum. The most common primaries are red, green and blue.

Psychoacoustics: The scientific study of human auditory perception.

Psychophysics: The study of the relationship between physical stimuli and the brain's interpretation of them, or psychological or behavioral response.

Pupil: The hole in the center of the iris that governs the amount of light passing through to the interior of the eye. This aperture controlled by the iris.

Pure Tone: A sound made up of a single frequency, with no harmonics or overtones. A tuning fork produces a pure tone.

Psychophysics: The study of the relationship between physical stimuli and the brain's interpretation of them.

Rarefaction: The instantaneous, local reduction in density of a gas resulting from passage of a sound wave. For example, this occurs in the environment when sound travels through the atmosphere which changes temperature with elevation above the ground.

Reflection: The phenomenon by which a sound wave is re-radiated ("bounced") from a surface. The surface must be as large as the frequency to be reflected. The angle of incidence equals the angle of reflection.

Refraction: (1) For sound, the phenomenon by which the direction of a sound wave is changed when it travels through layered media of different sound velocities. For example, this occurs when sound travels through an atmosphere in which temperature changes with the elevation above the ground. (2) For vision, the bending of a light ray when passing from one transparent medium into another of different density.

Relative pitch: The ability to identify the interval between two sounds. Most of us can identify the notes of melodies and cords as distances between pitches rather than as particular frequencies. (Also see absolute pitch)

Resonance: (1) The large sympathetic amplitude vibration in a mechanical system caused by a relatively small stimulus having the same period as the natural vibration period of the system. (2) An increase in the sound pressure of one more harmonics of sound.

Retina: The innermost layer of the eye where the light-sensitive rods and cones are located.

Reversed Perspective: A change in the perception of the dimensions of an object, bringing background to foreground and vice-versa.

Reverberation Time: The time (in seconds) required for the sound pressure level to decrease 60 dB in a room after a noise source is abruptly stopped. Reverberation time relates to a room's volume and sound absorption properties.

Reverberance: The subjective impression produced by reflected sound.

Reverberation: Collection of time-delayed sounds following a direct sound that results from reflections indoors.

Reverberant Field: The sound field in a room that is dominated by reverberation. Usually the sound level is no longer reduced by moving further from the sound source.

Rhodopsin: Also called visual purple, the light sensitive pigment found in the rods of the vertebrate retina. It bleaches in the light and regenerates the dark. Light falling on the rod causes a chemical change in rhodopsin, which initiates the transmission of a nerve impulse to the brain. The great sensitivity of rhodopsin allows us to see in dim light (night vision).

Rods: Straight, thin photoreceptors (about 125 million of them) which respond to low levels of illumination and give black and white response.

Room Constant (Absorption): The total acoustic absorption provided by objects and surfaces, including air, in a specific room. For a given frequency, the single number value equal to the product of the average absorption coefficient of the room and the total internal area of the room.

Room Modes: Room modes are resonances that occur when frequencies reinforce and cancel each other as they reflect back and forth between the hard boundaries of the room.  They are often referred to as "standing waves" because they represent areas of high and low pressure zones which "stand" within a room. 

Saturation: Refers to the purity or richness of a color. One measure of how much white or gray is in the mixture.

Sabin: A unit of acoustic absorption equal to the absorption provided by a unit area of a perfect absorber. For example, one square foot of open window equals an absorption of one sabin. Named after Wallace Clement Sabin, a Harvard professor of physics in the early 1900s who defined reverberation time, developed means to measure and predict it.

Secondary auditory cortex: A collection of cortical modules adjacent to primary auditory cortex and chiefly concerned with detecting relations among multiple sounds, whether simultaneous or successive.

Scattering: For light this is the deflection of light particles. This effect is most pronounced for high frequencies such as blue light, giving the sky a blue hue.

Sclera: The "white", thick outer coating of the eyeball.

Semicircular Canals: Three fluid-filled loops in the inner ear, sensitive to movements of the head. They serve no hearing functions, but are an essential part of the body's balancing mechanism.

Sine Wave: A wave whose amplitudes vary sinusoidally as a function of time. The simplest possible wave without harmonics (pure tone).

Size Constancy: The perception of familiar objects as being unchanged in size regardless of the distance at which they are seen.

Slap Echo: See Flutter Echo

Somatic: Referring to sensation arising from the body, including sensations from viscera, from the skin, and from joints and muscles.

Somatosensory cortex: A strip of cerebral cortex arching from ear to ear that interprets sensations from the skin, joints and muscles.

Sone: A subjective unit of loudness for an average listener equal to a 1,000Hz tone that is 40 dB above the listener's threshold of hearing. The scale is subjectively linear where a doubling of the sone value represents a doubling of loudness.

Sound: (1) An oscillation in pressure, resulting from molecular motion, in a viscous or elastic medium such as air, water, wood, steel, etc. (2) Sound is an auditory sensation evoked by air molecules vibrating in a frequency range between 20-20 kHz.

Sound Absorption: The property of absorbing sound energy possessed by objects and surfaces, including air.

Sound Insulation: (1) The capacity of a structure to prevent sound from being transmitted from one space to another. (2) Insulation used in a wall, floor, or ceiling cavity to add damping and decrease transmitted sound. Also see Sound Transmission Loss)

Sound Intensity: The rate of flow of energy per unit area in a specified direction.

Sound Isolation: Sound attenuation through a material or assembly.

Sound Masking System: An electronic system that generates noise to help render speech less intelligible.

Sound Power (W): The acoustical energy of a sound independent of distance.

Sound Power Level (PWL): Sound power radiated by a sound source reported as a level in decibels referenced to 1 pico-watt (without regard for distance from the source).

Sound Pressure: The time varying pressure exerted in an elastic medium such as air, generally in the 20-20 kHz frequency range.

Sound Transmission Class (STC): A single-number rating derived from laboratory measurement of sound transmission loss. STC is calculated in accordance with ASTM E413. "Classification for Rating Sound Insulation". The STC describes the sound-insulating properties in the 125Hz-4kHz center frequency range, primarily for assessing speech transmission through a structure, such as a partition. (Also see Noise Isolation Class)

Sound Transmission Loss (TL): A laboratory measure of sound insulation indicative of the sound-intensity flow transmitted through a partition without regard to the partition size, usually measured in one-third octave bands.

Sound Wave: A pressure area produced by mechanical vibration, which moves through a medium such as air or water, causing the reaction in the ear which the brain interprets as sound.

Spatial Induction: Also called simultaneous contrast. The contrast of one part of the visual field with an adjacent area, affecting the perception of black, white and colors.

Spectrophotometer: A device to measure the amount of each color, or wavelength, in the light reflected by an object.

Spectrum: The relative magnitude of different frequency components that comprise a single wave.

Speech Interference Level (SIL): A single-number rating used to evaluate interference based on the background noise level and voice level. The SIL is the arithmetic mean of the noise level at 500, 1,000, and 2,000 Hz.

Speech Privacy Potential: A single-number rating used to evaluate the degree of speech privacy between two adjacent work stations.

Speech Reinforcement System: An electronically amplified audio system designed to reproduce speech at a sufficient level for intelligibility to overcome distance and other acoustical limitations.

Standing Wave: See Room Modes

Stapes: One of the three ossicles of the middle ear.

Stereoscopic Vision: Perception of the visual field in three dimensions through the mixture of images from each eye.

Structure-borne Noise: Noise propagated through a structure and re-radiated as airborne noise.

Subsonic or Infrasonic: Pertaining to signals involving frequencies below the range of human hearing (20 Hz).

Subtractive Color Mixture: The absorption and reflection of light by two or more superimposed pigments or dyes, creating a new color.

Sympathetic Vibration: A vibration produced in one object by a vibration of the same frequency in another object.

Synapse: The point of connection between adjacent nerve cells.

Temporal lobe: Cerebral cortex spanning each side of the head. It performs a number of functions, with auditory processing on top, long-term memory and categorization at center and bottom, and emotional functions toward the front.

Textural Perspective: The crowding and blending of details- like the pebbles in a field- as they are seen from increasingly greater distances.

Timbre: Perceived auditory sensation of "tone color" mainly dependent on sound spectrum. Timbre varies by the attacks and decays of overtones and their relative intensities.

Tone: A pure tone is a singular frequency produced by a sine wave.

Transducer: A device that is actuated by energy from one system and supplies energy, usually in another form, to a second system (as a microphone or loudspeaker).

Tripole Speaker: A loudspeaker system in a single enclosure that radiates from the front, back, and side.

Tweeter: a high-frequency transducer component of a loudspeaker system often used in conjunction with a woofer (low-frequency transducer).

Tympanic Membrane: The eardrum.

Ultrasonic: Pertaining to signals involving frequencies above the range of human hearing (20 kHz).

Vibration: A periodic motion of molecules in an elastic medium with respect to equilibrium. Vibration by mechanical equipment can be a factor in structure-borne noise radiation.

Vibration Isolation: The methods used to reduce vibration in a structure caused by vibrating equipment, including the use of springs and elastomeric materials.

Visual Cortex: That part of cerebral cortex in the brain primarily responsible for interpreting signals from the eye.

Visibility Curve: A graph illustrating the eye's sensitivity to light at each wavelength in the visible spectrum.

Vitreous Humor: A thick, transparent jelly which fills the cavity between the lens and retina, giving the eye its shape.

Wavelength: In a periodic wave, the distance between two points having the same phase in consecutive cycles along a line in the direction of propagation (e.g., the distance between two adjacent peaks of a sine wave). For sound, one of three measurements, with amplitude and frequency, which describe a sound wave. The longer the wavelength, the lower its frequency.

Weighting: A prescribed frequency response commonly provided by an electronic filter in a sound level meter.

White Noise: Noise having a frequency spectrum with equal power at each frequency. For example, the sound power between 100 and 200 Hz is equal to a sound power between 1,000 and 1,200 Hz.