Glossary of Acoustic Terms
To help you with the acoustic language we have set out a glossary of terms for you. But if you have life! then just give us a call with your acoustic problem and we will recommend a solution.
Absorption – The opposite of reflection. Sound absorption results from the conversion of sound energy into another form, usually heat or motion, when passing through an acoustical medium. When a sound wave encounters resistance, absorption occurs. Absorption is measured in sabins (after Wallace Clement Sabine). One sabin is the amount of absorption offered by one square foot of open air.
Absorption coefficient – Ratio of sound-absorbing effectiveness, at a specific frequency, of a unit area of acoustical absorbent to a unit area of perfectly absorptive material. The portion of energy absorbed when a sound wave strikes a material. The absorption coefficient of a material is dependent on the frequency of the sound wave. An absorption coefficient of 1.0 = total absorption, 0.0 = total reflection.
Acoustical tile – A porous architectural material, usually constructed from fiberglass or pressed board, which is most absorptive at the high frequencies.
Acoustics – The sound characteristics of a room. The science of the production, control, transmission, reception and effects of sound and the phenomenon of hearing.
Ambience – The residual “room sound” of a listening environment.
Ambient noise – All pervasive noise associated with a given environment.
Analog – Analog representations of sound replicate its waveform, while transferring it through different media. All sound is analog. Audio may be analog or digital.
Anechoic – Literally “without echo”.
Area Effect – Due to exposed edges and diffraction of sound energy around perimeters, acoustical materials spaced apart can exhibit greater absorption than same amount of material with no gaps. The surface of an anechoic wedge has a total surface area greater than the flat surface it replaces.
Articulation – A measure of the intelligibility of speech.
Attenuate – To reduce the level (volume, loudness, energy) of an acoustical (or electrical) signal.
Audio frequency – A frequency that falls within the range of the human hearing, usually taken as 20 Hz to 20 kHz.
Axial mode – The room resonances associated with each pair of parallel walls (including ceiling and floor.
Background noise – The ambient noise level above which signals must be presented or noise sources measured.
Baffle – A device used to inhibit the propagation of sound waves. Baffles are usually suspended vertically from the ceiling to reduce reverberation time.
Barrier – Heavy, dense and massive material used to block sound.
Bass Trap – A low-frequency absorber. Low frequencies are particularly difficult to absorb due to their long wavelengths. Bass traps are designed and constructed to absorb these longer waves and control unwanted room resonances. Broadband absorbers extending to lower frequencies are often called Bass Traps, imprecisely. The term “bass trap” is counterintuitive since these devices eliminate low-frequency room cancellations, allowing the bass to be heard.
Block – To reduce airborne sound transmission.
Boomy / Boominess – Listening term that usually refers to an excessive amount of low frequency (bass) energy.
Break – A physical gap in the assembly or construction, which acts to decouple sound vibrations from traveling through a structure.
Bright – Listening term that usually refers to excessive upper frequency (treble) energy.
Cancellation – The destructive interference of two or more sound waves. Waves of similar frequencies and amplitude, but of opposite phase (180°F) produce mutual cancellation effects.
Ceiling Cloud – An acoustical panel suspended in a horizontal position from ceiling or roof structure.
Comb filter – A distortion produced by combining an acoustical (or electrical) signal with a delayed replica of itself (offset in time). The result is constructive and destructive interference that results in peaks and nulls being introduced into the frequency response. This response, when plotted to a linear frequency scale, resembles a comb (teeth) rather than a smooth curve.
Cocktail Party Effect – The ability of a listener to focus attention on a single talker among a mixture of crowd conversations and background noise while, at the same time, ignoring speech from other locations. Understanding is possible due to the ear / brain discrimination of unwanted sound as well as lip reading and body language. A single microphone recording of the same conversation, absent these additional factors, may be totally unintelligible and unusable as evidence in court.
Coloration – A term used to indicate audible alterations to sound due to its environment. Coloration can be a result of standing waves or room resonances.
Constructive Interference – The addition of two waveforms of similar phase. Constructive interference is responsible for the production of standing waves in which a signal and its successive reflections are continually added to one another. The opposite is destructive interference.
Critical distance – The distance from a sound source at which the direct energy (energy radiated directly from the source) is equal to the reverberant energy (radiated from walls, floor and ceiling).
Cutoff frequency – The lowest frequency above which the normal incidence sound absorption coefficient is at least .990 for an anechoic wedge or set of wedges,
Cycle – A complete positive (forward) and negative (backward) movement of a vibration corresponding to a high and low-pressure wave.
Cycles per second – The frequency of an electrical signal or sound wave, measured in Hertz (Hz).
Damping – The loss of energy in a vibration system, usually through friction.
Dead – An acoustical condition in which reverberation is absent, as in a room whose surfaces are covered with highly absorptive materials.
Decay time – The length of time taken for a signal to drop in strength to a specific portion of its initial value. The decay time is often frequency-dependent. The decay time of a room at a specific frequency is the time necessary for a sound of that frequency to decay 60 dB. (RT60)
Decibel (dB) – The measuring unit of sound pressure, and hence loudness. The decibel is a numerical ratio between the sound pressure of a given sound and the sound pressure of a reference sound (usually .0002 microbar). Common decibel levels encountered vary from the rustling of grass (15 dB) to conversation (50 dB), to live rock groups (110 dB) to jet plane engines at close range (130 dB). See Threshold of Hearing.
De-Coupling – See Isolation.
Diffraction – The bending of a sound wave around an obstacle, or through an opening, such as slats. The scattering of sound waves at an object smaller than one wavelength, and the subsequent interference of the scattered wavefronts.
Diffuse sound field – A sound field in which the sound pressure level is the same everywhere and the flow of energy is equally probable in all directions.
Diffuse sound – Sound that is completely random in phase. Sound which appears to have no single source/
Diffuser (also: Diffusor) – A device for the complex scattering of sound energy in all directions. Traditional spatial diffusers, such as the poly cylindrical (barrel) shapes may also double as low-frequency traps. Temporal diffusers, such as binary arrays and quadratics, scatter sound in a manner similar to diffraction of light, where the timing of reflections from an uneven surface of varying depths causes interference which spreads the sound.
Diffusion – The scattering or random distribution of a sound wave after striking a surface.
Diffusor – See Diffuser.
Direct sound – Sound waves arriving at the listening location directly from the source. Differing from reflected sound, which arrives at the listening location after bouncing off the surrounding surfaces.
Doppler effect – The apparent shift in frequency when the sound source, or the observer, is in motion.
Drywall – A dense architectural wall construction material applied to wood or metal studs, A/K/A Sheet Rock or Gypsum Wall Board (GWB), useful primarily as a sound barrier but is low frequency absorber in some applications.
Early reflection – Reflected energy that occurs in close proximity to the source but is slightly out of synchronization (time / phase) with the source information..
Echo – A distinctly discernible reflection, or repetition of a source signal. Note: The term is often used incorrectly to refer to reverberation which consists of densely spaced, indistinguishable reflections.
Equal loudness contours – A set of curves of equivalent loudness, which model the ear’s frequency response throughout the audible spectrum. The curves, obtained from actual testing, show how much more sound power is required at one frequency than another to obtain a sound of equal loudness. The results show that the human ear is less sensitive to sound at the extreme high and low frequencies.
Equalization – The adjustment of timbre, or tone quality, achieved by changing the amplitude of a signal at different frequencies. (Abbreviated: EQ.) Tone controls are simple forms of equalization.
Far-field – Distribution of acoustic energy at a very much greater distance from a source than the linear dimensions of the source itself.
Flanking – The transmission of sound around a perimeter or through holes within partitions (or barriers) that reduces the otherwise obtainable sound transmission loss of a partition. Examples of flanking paths within buildings are ceiling plena above partitions; ductwork, piping, and electrical conduit penetrations through partitions; back-to-back electrical boxes within partitions, window mullions, etc. Flanking occurs when a free-standing partition size is less than the wavelength of sound to be blocked.
Flat – The term used to describe an even frequency response in which no frequency is accentuated.
Fletcher-Munson curves – The equal loudness contours plotted by the researchers Fletcher and Munson. Human ears are most sensitive to sound between 1,000 Hz and 4,000 Hz. Above and below those approximate frequencies, a tone must be several dB greater in order to be perceived as equally loud as a tone in the 1,000 Hz to 4,000 Hz range. See Weighting network.
Flutter – A repetitive echo set up by parallel reflecting surfaces.
Free field – An environment in which there are no reflective surfaces within the frequency region of interest.
Frequency – The speed of vibration of a sound wave, measured in cycles per second, or Hertz. Frequency determines pitch; the faster the frequency, the higher the pitch.
Front-to-back imaging – The placement of vocal or musical information ahead of (closer to) or behind (farther from) center position, front-to-back.
GoBo – A freestanding device used to inhibit the propagation of sound waves. Gobos are usually employed to prevent microphone leakage between two instruments being recorded simultaneously. Typically a set of portable dividers covered with acoustical treatments.
Grating, diffraction – The principle now used to achieve diffraction of acoustical waves, analogous to optical grating by which light is broken into its component colors as when passing through a prism.
Grating, reflection phase – An acoustical diffraction grating to produce diffusion of sound.
Grazing Effect – The way in which sound is absorbed by the audience; stepping or raking the seating reduces the absorption, and improves sightlines.
Haas Effect – Also called the precedence effect. Delayed sounds are integrated if they fall on the ear within 20 to 40 msec of the direct sound. The level of the delayed components contributes to the apparent level of the sound.
Hertz – The measuring unit of frequency or the speed of vibration of a sound wave. Synonymous with “cycles per second” (CPS).
Image shift – Sound dislocated from its correct position, to be more left and / or right of center.
Impact noise – The noise heard as a result of vibrations transferred through the structure of a room. Foot thumps are impact noise.
Impulse – A very short, transient, acoustical (or electrical) signal.
In phase – Two periodic waves reaching peaks and going through zero at the same instant are said to be “in phase”.
Initial time-delay gap (signal delay) – The time interval between the arrival of a direct sound and its first reflection from the surfaces of the room.
Intensity – The amount of sound energy radiated per unit area, measured in watts per square centimeter.
Isolation – Resistance to the transmission of sound by materials and structures. The separation of airborne or mechanically transmitted energy.
Kilohertz / kHz – 1,000 Hz increments. See Hertz.
Late reflection – Reflected energy that occurs a greater distance away from the source than an early reflection. Sometimes referred to as “slap-back” or echo.
Law of the first wavefront – The first wavefront falling on the ear determines the perceived direction of the sound.
Leakage – Any unwanted sound picked up by (or “leaking” into) a microphone from another instrument or loudspeaker. Sound from one room that is heard in another.
LEDE – Live end, dead end. An optimal acoustical treatment plan for rooms in which one end is highly absorbent and the other end reflective and diffusive.
Live – A reverberant acoustical condition, usually used in reference to a room whose many reflective surfaces encourage a lengthy reverberation time.
Longitudinal wave – A wave in which vibrations are in the direction of propagation of sound as are sound waves in air.
Loudness – Subjective impression of the intensity of a sound.
Masking – The process by which one sound is used to obscure the presence of another.
Mass law – The law of physics that states that a material’s ability to reduce the transmission of sound is proportional to its weight. According to the mass law, to increase a wall’s transmission loss by 6 dB it is necessary to double the thickness (weight) of the wall. See Inverse Square Law.
Mass Loaded Vinyl – A high density vinyl material that acts as a barrier to sound transmission. Directly applied to surfaces, suspended or used in wall, floor and ceiling construction. See Sound Transmission Class.
Mechanical Coupling – To rigidly connect two isolated objects. (Also called a mechanical “short”.) Example: Two isolated wall partitions are mechanically coupled if rigid electrical conduit is fastened to both walls. Air ducts and plumbing are prime candidates for causing mechanical shorts. When an acoustically isolated room “leaks”, it is often a mechanical short created during construction.
Mechanically Decoupled – The elimination of mechanical shorts. See Mechanical Coupling. Typically accomplished by inserting a flexible loop between rigid components. Flexible metallic conduit (Greenfield) is used for decoupling electrical conduit, accordion-shaped canvas collars decouple rigid air-ducts and flexible tubing does the same for plumbing. For structural decoupling, resilient materials are used to separate rigid components.
Mineral Wool – A non-rigid acoustical substrate (A/K/A “rock wool” or “slag wool”) made from molten rock.
Mode (Room Mode) – A 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 at low frequencies and in small rooms.
Mono – Common abbreviation for “monaural”, meaning from a single source.
Near field – That part of a sound field usually within about two wavelengths from a noise source, where there is no simple relationship between sound level and distance. The area in a room which is in the immediate vicinity of the sound source.
Node (Dead Spot) – A point or line where minimal air motion takes place.
Noise – Unwanted sound. Interference of an electrical or acoustical nature. Random noise is a desirable signal used in acoustical measurements. Pink noise is random noise whose spectrum falls at 3 dB per octave. It is useful for use with sound analyzers with constant percentage bandwidths.
Noise Criteria (NC) – Standard spectrum curves by which a given measured room’s ambient noise may be described by a single NC number.
Noise isolation class, NIC – A single-number rating calculated in accordance with Classification E 413 using measured values of noise reduction. It provides an estimate of the sound isolation between two enclosed spaces that are acoustically connected by one or more paths.
Noise Reduction Coefficient (NRC) – The arithmetic average of the Sound Absorption Coefficients of a material at 250 Hz, 500 Hz, 1000 Hz and 2000 Hz. This is the range having the most impact upon speech intelligibility.
Notch filter – A filter of extremely narrow bandwidth used to eliminate discrete frequencies. Notch filters are usually tunable, and can be used to eliminate specific room or instrument resonances.
Oblique mode – See Mode.
Octave – The musical spacing between a frequency and its double. For example, the distance between “A” (440Hz) and “high A” (880Hz) is an octave. The audible range is about ten and one-half octaves.
Octave band – A frequency spectrum which is one octave wide (i.e. all frequencies from 125 Hz to 250 Hz). In recording and audio testing, the octave itself is divided into thirds for increased accuracy.
Out of phase – Two related signals offset in time. See In Phase.
Passive absorber – A sound absorber that dissipates sound energy as heat.
Path length difference – The difference in time/distance of source energy from reflected energy.
Period – The length of time (measured in seconds) it takes for a wave to complete a cycle. t = 1/f
Phase – The time relationship between two signals.
Phase interference – The addition and/or subtraction of two waves of similar or multiple frequencies, causing peaks and dips in the overall response curve.
Phase shift – The time or angular difference between two signals.
Phon – The empirical unit of loudness. Since the ear has different sensitivities at various frequencies (Fletcher-Munson), it does not hear equivalent sound pressure levels as being equally loud.
Pink noise – Broadband noise whose energy content is inversely proportional to frequency.(-3dB per octave) This gives the noise equal energy per octave.
Pitch – The human perception of frequency. In general, the higher the frequency, the higher the pitch.
Privacy Index (PI) – A measure for rating the speech privacy performance of an architectural space (or lack of speech intelligibility), where the PI is calculated from the Articulation Index (AI). A privacy level of PI above 95% represents confidential speech privacy whereas a PI of less than 80% is poor privacy.
Polarity – The positive (forward) or negative (backward) direction of an acoustical, electrical, or magnetic force. Two identical signals in opposite polarity are 180°F apart at all frequencies. Polarity, unlike phase, is not frequency dependent.
Polar Plot – The graphic representation of diffusion or scattering, over all incident angles at a rated frequency.
Pressure zone – As sound waves strike a solid surface, the particle velocity is zero at the surface and the pressure is high, thus creating a high-pressure layer near the surface.
Psychoacoustics – The study of the perception of sound.
Random noise – Noise whose instantaneous amplitude is not specified at any instant of time.
Rarefaction – A decrease in density and pressure in a medium, such as air, caused by the passage of a sound wave. Opposite of compression.
Reactive absorber – A sound absorber, such as the Helmholtz resonator which involves the effects of mass and compliance as well as resistance.
Reactive silencer – A silencer in air-conditioning systems that uses reflection effects for its action.
Reflected sound – Sound arriving at the listening location after bouncing off one or more of the surrounding surfaces. The sum total of all reflected waves determines the room’s reverberation time and acoustical character.
Reflection – The bouncing of a sound wave off of a surface. Sound is reflected much as light is reflected, with the angle of incidence equaling the angle of reflection.
Reflection Phase Grating – See Grating.
Refraction – The bending of sound waves traveling through layered media with different sound velocities.
Resilient – Free from rigid contact, such as a spring-mounted floating floor. Resiliency reduces the transfer of noise and vibration from one structure to another.
Resonance – The sympathetic vibration of an object (or air column) at a specific frequency when it is excited into motion by a sound wave of similar frequency in the immediate vicinity.
Resonant frequency dip – The degradation of transmission loss of a barrier at a specific frequency due to inner resonance. The exact frequency at which this phenomenon occurs is a function of the mass and stiffness of the barrier. Laminated “safety” glass has lower resonance, passing less sound than plain glass.
Reveal – The exposed edge at the side of ceiling tile that is at right angles to the general face of the ceiling, visible (revealed) below the supporting grid.
Reverberation – The persistence of sound in an enclosure after a sound source has been stopped. This is a result of the multiple reflections of sound waves throughout the room arriving at the ear so closely spaced that they are indistinguishable from one another and are heard as a gradual decay of sound. The area in which this occurs is the Reverberation Field.
Reverberation Room – A test chamber so designed that the reverberant sound field within the room has an intensity that is approximately the same in all directions and at every point. It is commonly used to measure sound absorption, ASTM C-423 and transmission loss, ASTM E-90.
Reverberation time – The time, in seconds, required for sound pressure at a specific frequency to decay 60 dB after the source is stopped. 60 dB of decay is equal to one millionth of their original level. The reverberation time of a room varies with frequency and is a function of the room volume as well as the total number of absorption units in the room. It can be determined by the Sabine equation.
Reverberant field – The area in a room in which the multitude of decaying reflections has created a reverberant and diffuse condition.
RT60 – See Reverberation time and Decay Time.
Room Modes – See Mode.
Rumble – Low-frequency vibration.
Sabin – A measure of sound absorption of a surface. One sabin is equal to 1 square foot of open window. Sabins are calculated by multiplying the absorption coefficient of a material multiplied by its area.
Scrim – A sheer, loosely woven fabric used as “backing” for acoustical panels.
Septum – A thin layer of material between two layers of absorptive material, such as foil, vinyl, lead, gypsum, steel, etc., that prevents sound wave from passing through absorptive material.
Signal-to-noise ratio – The difference between nominal or maximum operating level and the noise floor expressed in dB.
Shifting center – An apparent shift of the position of an instrument or voice in the stereo image due to a discrepancy in the phase relationships of the signals from either side. See Image Shift.
Sound – Energy that is transmitted by pressure waves in air (as well as water or solids) and is the objective cause of the sensation of hearing. The phenomenon caused by the vibration of the eardrum. The drum itself is set into motion by pressure waves traveling through the air, originating at the sound source.
Sound isolation – The degree of acoustical separation between two locations, especially adjacent rooms.
Sound level – The intensity of sound measured with a sound level meter and one of its weighting networks.
Sound level meter – A pressure-sensitive device that measures loudness.
Sound power – The total sound energy radiated by a source per unit of time.
Sound pressure – A dynamic variation in atmospheric pressure. The pressure at a point in space minus the static pressure at that point.
Sound Pressure Level (SPL) – The fundamental measure of sound pressure. The measurement of what sound we hear expressed in decibels in comparison to a reference level.
Sound Stage – A room or studio that is usually soundproof, used for the production of movies. Or: The psycho acoustic phenomena where a two-dimensional image (left-to-right and front-to-back) is created in the mind suggesting the physical relationship of the listener to the individual performers. A well designed listening space will create the impression of a much larger sound stage than the physical placement of the speakers, or the size of the room would otherwise allow.
Sound transmission (airborne) – The conduction of a sound wave through air. The speed of airborne sound transmission varies with temperature and humidity, and is 1130 feet/second in air at 70°F.
Sound transmission (structure-borne) – The conducting of a sound wave through a physical structure (such as a wall, floor, ceiling or door). Because of the increased speeds of sound through common building materials as well as the physical connection of such materials in the structural framework of a building, structure-borne sound transmission is much more difficult to stop than airborne sound transmission and thus requires special measures to be dealt with effectively.
Sound transmission loss – Ratio of sound energy emitted by an acoustical material or structure to the energy incident upon the opposite side.
Speech intelligibility – A measure of sound clarity that indicates the ease of understanding speech. It is a complex function of psychoacoustics, signal-to-noise ratio of the sound source, and direct-to-reverberant energy within the listening environment. See Articulation.
Speech Privacy – See Privacy Index (PI). The degree to which speech is unintelligible between offices. Three ratings are used: Confidential, Normal (Non-Obtrusive), and Minimal.
Splaying – Walls are splayed when they are constructed at angles of varying degrees from normal rectangular form.
Standing Wave – A sound wave continuously reinforced by its own reflections, influencing the character of all sound within a room. Since the standing waves are a direct result of the size and geometry of the space itself, each room has a unique set of standing waves. The presence of these waves can easily be determined by a combination of mathematical calculation and audio analysis.
Substrate – The underlying material to which a covering is applied, or by which it is supported. A substrate (sometimes referred to as “core”) can also have important functional characteristics such as acoustical performance, impact resistance, and trackability.
Symmetrical room design – A basic acoustical design to create a desirable balanced listening environment
Threshold of hearing – The minimum sound pressure level of a pure tone that can be perceived by a person with good hearing.
Threshold of pain – The minimum sound pressure level of a pure tone which causes a sensation of pain in the ear. (At approximately 140 db
Timbre – The subjective tonal quality of a sound. The timbre of any musical or non-musical sound is determined largely by the harmonic structure of the sound wave. Rich sounding musical tones tend to have a great number of inner harmonics which contribute to their lush timbre, while thin sounding musical tones tend to be lacking in the presence of harmonics.
Transmission – The propagation of sound through a medium or barrier. (See Sound Transmission.
Transmission co-efficient – The portion of sound energy transmitted through a material.
Transmission loss (TL) – The number of dB by which a barrier reduces the transmission of sound. Transmission loss varies significantly with frequency. For an accurate representation of soundproofing ability, Transmission Loss should be indicated at several frequencies for any given barrier.
Vibration – A force that oscillates about some specified reference point. Vibration is commonly expressed in terms of frequency such as cycles per second (cps), Hertz (Hz), cycles per minute (cpm) or revolutions per minute (rpm), and strokes per minute (spm). This is the number of oscillations that occur in that time period. The amplitude is the magnitude or distance of travel of the force.
Vibration Isolator – A resilient support that tends to isolate a mechanical system from steady-state excitation.
Volume – The cubic space capacity of a room bounded by walls, floors, and ceilings determined by the formula: Volume = Length x Width x Height. Volume influences reverberation time. Also: Colloquial for loudness.
VU – A visual meter indicating the RMS value of a signal. Since the human perception of loudness corresponds to the RMS value of the signal, VU meters indicate volume (VU stands for Volume Units). Zero VU is considered to be standard operating level.
Warmth – A listening term. In frequency, it is generally considered to be the range from approximately 150Hz – 400Hz. A system with the “proper” warmth will sound natural within this range.
Watt – The unit of acoustical (or electrical) power.
Wavelength – The distance measured perpendicular to the wavefront in the direction of propagation between two successive points in the wave, which are separated by one period. The distance between the beginning and end of a wave or cycle. Wavelength is determined by the formula:
White noise – Broadband noise having a constant energy per unit of frequency. Random noise having a uniform distribution of energy with frequency.