Acoustic Glossary
Industry-standard terminology for acoustics, soundproofing, and acoustic treatment — compiled by Enoshmink Technology.
A
15 termsA-Weighted Sound Level
A measure of sound pressure level designed to reflect the response of the human ear, which does not respond equally to all frequencies. To describe sound in a manner representative of the human ear’s response it is necessary to reduce the effects of the low and high frequencies with respect to the medium frequencies. The resultant sound level is said to be A-weighted, and the units are dBA. The A-weighted sound level is also called the noise level. Sound level meters have an A-weighting network for measuring A-weighted sound levels.
AATCC 8
Determines the amount of color transferred from the surface of colored textile material to other surfaces by rubbing.
AATCC 16 Option 3
AATCC 16 Option 3 is a test method of the American Association of Textile Chemists and Colorists (AATCC). With option 3, a Xenon-Arc lamp is used as a light source. A strip of fabric (part of which is protected by a special card) is placed in a fadometer and exposed to 40 hours of accelerated fading units (AFU). After exposure, the difference in color between the exposed and protected parts of the fabric are compared to the AATCC grey scale, and the degree of fading is rated. Grade 5 = no fading. Grade 4 = slight fading. Grade 1 = high degree of fading.
AATCC 107
Test method to determine colorfastness to water.
Absorption
The properties of a material composition to convert sound energy into heat thereby reducing the amount of sound energy that can be reflected.
Acoustical
The properties of a material to absorb or reflect sound (adjective) acoustically, (adverb).
Acoustical Analysis
A review of a space to determine the level or reverberation or reflected sound in the space (in seconds) as influenced by the building materials used to construct the space. Also, a study of the amount of acoustical absorption required to reduce reverberation and noise.
Acoustical Environment
The acoustical characteristics of a space or room influenced by the amount of acoustical absorption, or lack of it, in the space.
Acoustics
Acoustics is the science of sound, including its production, transmission and effects. The acoustics of a room are those qualities that together determine its character with respect to the perception of sound.
Ambient Noise
Ambient noise encompasses all sound present in a given environment, being usually a composite of sounds from many sources near and far.
Architectural Acoustics
The control of noise in a building space to adequately support the communications function within the space and its effect on the occupants. The qualities of the building materials used to determine its character with respect to distinct hearing.
Area Effect
Acoustical materials spaced apart can have greater absorption than the same amount of material butted together. The increase in efficiency is due to absorption by soft exposed edges and also to diffraction of sound energy around panel perimeters.
ASTM
Acronym for American Society of Testing and Materials.
Attenuation
The reduction of sound energy as a function of distance traveled.
Audiometer
An instrument for measuring hearing acuity.
B
2 termsBaffle
A free hanging acoustical sound absorbing unit. Normally suspended vertically in a variety of patterns to introduce absorption into a space to reduce reverberation and noise levels.
Boominess
Low frequency reflections. In small rooms, acoustical panels with air space behind can better help control low frequency reflectivity.
C
4 termsCloud
In acoustical industry terms, an acoustical panel suspended in a horizontal position from ceiling/roof structure. Similar to a baffle but in a horizontal position.
Cocktail Party Effect
Sound in a noisy crowded room generated mostly by conversation. Levels rise and fall as people compete with one another to be heard. Perception of speech can be nearly impossible in high levels of noise.
Cycle
In acoustics, the cycle is the complete oscillation of pressure above and below the atmospheric static pressure.
Cycles Per Second
The number of oscillations that occur in the time frame on one second. (See Frequency). Low frequency sounds have fewer and longer oscillations.
D
4 termsDamping
Damping is the dissipation of vibratory energy in solid media and structures with time or distance. It is analogous to the absorption of sound in air.
Decibel (dB)
A dimensionless unit which denotes the ratio between two quantities that are proportional to power, energy or intensity. One of these quantities is a designated reference by which all other quantities of identical units are divided. The sound pressure level in decibels is equal to 10 times the logarithm (to the base 10) of the ratio between the pressure squared divided by the reference pressure squared. The reference pressure used in acoustics is 20 micro Pascals.
Deflection
Deflection is the distance an elastic body or spring moves when subjected to a static or dynamic force. Typical units are inches or mm.
Diffusion
The scattering or random reflection of a sound wave from a surface. The directions of reflected sound is changed so that listeners may have sensation of sound coming from all directions at equal levels.
E
2 termsEcho
Reflected sound producing a distinct repetition of the original sound. Echo in mountains is distinct by reason of travel after original signal has ceased.
Elastomeric
Flexible, low modulus material capable of expanding and contracting and returning to original dimensions without fatigue.
F
8 termsFG
Abbreviation for Fiberglass Board.
Firestop System
An assembly consisting of 1) the penetrant or penetrants, 2) the opening through a particular type of wall or floor, and 3) the firestop materials and design that are used to seal the opening or protect the penetrants.
Flame Spread
Classification indicating propagation of flame across a sample compared to flame propagation across concrete panels and red oak. Results are obtained through an ASTM E84 or UL723 test.
Flanking
The transmission of sound around the 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 path above partitions; ductwork, piping, and electrical conduit penetrations through partitions; back-to-back electrical boxes within partitions, window mullions, etc.
Flutter Echo
Short echoes in small reverberative spaces that produce a clicking, ringing, or hissing sound after the original sound signal has ceased. Flutter echoes may be present in long narrow spaces with parallel walls.
Free Field
Sound waves from a source outdoors where there are no obstructions.
Frequency
The number of oscillations or cycles per unit of time. Acoustical frequency is usually expressed in units of Hertz (Hz) where one Hz is equal to one cycle per second.
Frequency Analysis
An analysis of sound to determine the character of the sound by determining the amount of sound at various frequencies that make up the overall sound spectrum. For example, higher frequency sound or pitch vs. low frequency.
H
5 termsHearing Impairment
A degree of hearing loss, temporary or permanent, due to many causes. Hearing loss can be caused by illness, disease, or exposure to excessively high noise levels. Affects 25–50 million people in USA of all ages. Hearing impairment as generally used means a hearing loss of mild, moderate or severe degree as opposed to “deafness” which is generally described as little or no residual hearing with or without the aid of an assisting listening device. Hearing impaired persons are particularly adversely affected by long reverberation times.
Hearing Range
16–2000 Hz (Speech Intelligibility) / 600–4800 Hz (Speech Privacy) / 250–2500 Hz (Typical small table radio).
Hertz (Hz)
Frequency of sound expressed by cycles per second.
HPC Coated
HPC is a high-performance coating that is applied to an acoustical melamine foam base. Our exclusive water-based HPC formulation improves durability and protects the acoustical foam from dust and dirt, allowing the surface to be cleaned using a damp cloth. The HPC coating is especially appropriate for use in high-traffic areas.
I
2 termsIIC
Impact Insulation Class (IIC) is a number rating of how well a building material or construction assembly attenuates impact sounds, such as footsteps. A larger number means more attenuation.
Inverse Square Law
Sound levels fall off with distance traveled. Sound level drops off 6 dB from the source point for every doubling of distance.
L
3 termsLEED
Leadership in Energy and Environmental Design (LEED) is a green building certification program that uses a set of systems to rate the efficiency for the design, construction, operation and maintenance of buildings.
Live End / Dead End
An acoustical treatment plan for rooms in which one end is highly absorbent and the other end is reflective and diffusive.
Loudness
A listener’s auditory impression of the strength of a sound. The average deviation above and below the static value due to a sound wave is called sound pressure. The energy expended during the sound wave vibration is called intensity and is measured in intensity units. Loudness is the physical resonance to sound pressure and intensity.
M
2 termsMasking
The process by which the threshold of hearing of one sound is raised due to the presence of another.
Mass
Mass is the fundamental property of a material relevant to sound transmission loss through that material. Generally, the more massive the material, the greater the sound transmission loss.
N
4 termsNFPA-265
This test method evaluates flammability characteristics of interior textile wall coverings, measures certain fire performance characteristics, and determines the potential contribution to fire growth in a room and for fire spread beyond the room.
Noise
Unwanted sound that is annoying or interferes with listening. Not all noise needs to be excessively loud to represent an annoyance or interference.
Noise Criteria (NC)
Noise criteria curves used to evaluate existing listening conditions at ear level by measuring sound levels at the loudest locations in a room. NC criteria can be referred to equivalent dBA levels.
NRC
The Noise Reduction Coefficient (commonly abbreviated NRC) is a scalar representation of the amount of sound energy absorbed upon striking a particular surface. An NRC of 0 indicates perfect reflection. An NRC of 1 indicates perfect absorption. Higher Numbers = Higher Performance.
O
3 termsOctave
A pitch interval of 2 to 1. The tone whose frequency is twice that of the given tone.
Octave Bands
Sounds that contain energy over a wide range of frequencies are divided into sections called bands. A common standard division is in 10 octave bands identified by their center frequencies 31.5, 63, 250, 500, 1000, 2000, and 4000 Hz.
Open-cell Melamine-based Foam
Open-cell melamine foam is the base material for many acoustical foam products. It is manufactured from a melamine resin and is Class 1 fire rated per ASTM E84. This structurally-stable, open celled acoustic foam provides excellent sound absorption ratings, high fire resistance, low thermal conductivity and is mold resistant.
P
3 termsPCF
Unit of measure for density: Pounds per cubic foot.
Pitch
The perceived auditory sensation of sounds expressed in terms of high or low frequency stimulus of the sound.
PSI
Pounds per square inch.
R
4 termsReflection
The amount of sound wave energy (sound) that is reflected off a surface. Hard, non-porous surfaces reflect more sound than soft porous surfaces. Some sound reflection can enhance the quality of the signal of speech and music.
Resonance
The emphasis of sound at a particular frequency.
Resonant Frequency
A frequency at which resonance exists.
Reverberation Time
The reverberation time of a room is the time it takes for sound to decay by 60 dB once the source of sound has stopped. Reverberation time is the basic acoustical property of a room which depends only on its dimensions and the absorptive properties of its surfaces and contents. Reverberation has an important impact on speech intelligibility.
S
12 termsSabin
A unit of sound absorption based on one square foot of material. Baffles are frequently described as providing X number of sabins of absorption based on the size of the panel tested through the standard range of 125–4000 Hz.
Sabine Formula
A formula developed by Wallace Clement Sabine that allows designers to plan reverberation time in a room in advance of construction and occupancy. The Sabine Formula is T = 0.049(V/A) where T = Reverberation time in seconds, V = Volume of room in cubic feet, A = total square footage of absorption in sabins.
Septum
A thin layer of material between 2 layers of absorptive material, such as foil, lead, steel, etc. that prevents sound wave from passing through absorptive material.
Signal to Noise Ratio
The sound level at the listener’s ear of a speaker above the background noise level. The inverse square law impacts the S/N ratio.
Smoke Developed Index
Classification that relates to a comparison of smoke development of a particular material compared to concrete panels and red oak. Results are obtained through an ASTM E84 or UL723 test.
Sound
Sound is an oscillation in pressure, stress particle displacement, particle velocity in a medium. Sound produces an auditory sensation caused by the oscillation.
Sound Absorption
The ability of a material to convert sound energy, thereby reducing the amount of sound energy that can be reflected back.
Sound Barrier
A material that when placed around a source of noise inhibits the transmission of that noise beyond the barrier. Blocking sound requires materials that are dense and heavy.
Sound Pressure Level
The sound pressure level, in decibels, of a sound is 20 times the logarithm to the base of 10 of the ratio of the sound pressure to the reference pressure. The reference pressure shall be explicitly stated and is defined by standard.
Speech Intelligibility
The ability of a listener to hear and correctly interpret verbal messages. In a classroom with high ceilings and hard parallel surfaces such as glass and tile, speech intelligibility is a particular problem.
Speech Privacy
The degree to which speech is unintelligible between offices. Three ratings are used: Confidential, Normal (Non-Obtrusive) and Minimal.
STC
Sound Transmission Class: Quantifies the performance of a material or partition to attenuate airborne sound. This roughly reflects the decibel reduction of a building material or partition. Higher Numbers = Higher Performance.
T
2 termsTime Weighted Average (TWA)
The yardstick used by the Occupational Safety and Health Administration (OSHA) to measure noise levels in the workplace. It is equal to a constant sound level lasting eight hours that would cause the same hearing damage as the variable noises that a worker is actually exposed to.
TPI
Measurement for screws and bolts indicating the amount of threads per an inch.
U
1 termUltrasounds
Sounds of a frequency higher than 20,000 Hz. The frequency region containing these frequencies is called the ultrasonic region.
V
3 termsVibration
Vibration is a force which 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 (rpm) and strokes per minute (spm).
Vibration Isolator
A resilient support that tends to isolate a mechanical system from steady state excitation.
Volume
The cubic space of a room bounded by walls, floors, and ceilings determined by the formula Volume = Length x Width x Height of space. Volume influences reverberation time.
G
8 termsGain
The amount by which an audio or electrical signal is increased, usually expressed in decibels. Used in microphones, preamplifiers, mixers, DSP chains, and loudspeaker systems.
Gaussian Noise
Random noise whose amplitude distribution follows a normal probability curve. Commonly used for excitation, testing, simulation, and controlled acoustic measurements.
Geometric Acoustics
A method that models sound propagation as rays reflecting, scattering, and transmitting through spaces. Useful for room-acoustic design, auditoria, studios, and large-enclosure prediction.
Ground Effect
The change in sound level caused by interference between direct sound and sound reflected from the ground. Important in outdoor noise prediction, environmental acoustics, and transportation noise.
Group Delay
The time delay experienced by different frequency components as they pass through a system. Used to evaluate filters, loudspeakers, crossovers, and signal-processing latency.
Guided Wave
A wave constrained to propagate along a structure, boundary, duct, plate, or layered medium. Relevant to ducts, waveguides, bone conduction, structural acoustics, and ultrasonic testing.
Grating Lobes
Unwanted directional lobes produced when array spacing is too large relative to wavelength. Critical in microphone arrays, beamforming, sonar, ultrasound, and spatial audio systems.
J
7 termsJND – Just Noticeable Difference
The smallest detectable change in a sound attribute such as level, frequency, or time. A core psychoacoustic concept used in perception, coding, and auditory experiments.
Jitter
Small, irregular time variations in a periodic signal or digital sampling process. Important in voice analysis, digital audio clocks, speech quality, and measurement accuracy.
Jet Noise
Broadband aerodynamic noise generated by high-speed turbulent jet flow. Studied in aeroacoustics, aircraft engines, nozzles, ventilation, and industrial exhausts.
Joint Time-Frequency Analysis
A signal-analysis approach that examines how spectral content changes over time. Includes STFT, wavelet analysis, spectrograms, and nonstationary acoustic-signal analysis.
Junction Loss
The reduction of vibration or sound transmission across a structural junction. Used in building acoustics, flanking transmission, floating floors, and wall-ceiling connections.
Just Intonation
A tuning system based on simple whole-number frequency ratios between musical tones. Relevant to musical acoustics, consonance, pitch perception, and instrument tuning.
K
6 termskHz – Kilohertz
A unit of frequency equal to one thousand cycles per second. Used to describe audio bandwidth, hearing range, sampling-related frequencies, and filters.
k – Wave Number
A spatial frequency that represents the number of radians of phase change per unit distance. Defined as k = 2π/λ and used in wave propagation, modal analysis, and diffraction.
Kirchhoff-Helmholtz Integral
A boundary-integral formulation used to describe acoustic fields from pressure and velocity over a surface. Applied in acoustic radiation, boundary element methods, sound-field reconstruction, and holography.
K-weighting
A standardized frequency weighting used in loudness and broadcast audio measurements. Used in LUFS loudness metering and modern broadcast/streaming level control.
Kundt’s Tube
A classic tube apparatus used to demonstrate standing waves and estimate the speed of sound. Useful in education, material testing, and basic acoustic-resonance demonstrations.
Kurtosis
A statistical measure describing how impulsive or peak-dominated a signal distribution is. Used in vibration analysis, fault detection, impulsive noise assessment, and signal classification.
Q
6 termsQ Factor
A dimensionless measure of resonance sharpness or selectivity. High Q indicates narrowband resonance; low Q indicates broader damping.
Quality Factor
Another name for Q factor, often used in resonance, filters, and vibrating systems. Important for resonators, absorbers, loudspeaker enclosures, and structural modes.
Quantization Noise
Noise introduced when a continuous-amplitude signal is represented by discrete digital values. Relevant to ADCs, digital recording, bit depth, dithering, and signal-to-noise ratio.
Quasi-Anechoic Measurement
A measurement method that reduces or gates out room reflections without requiring a full anechoic chamber. Common in loudspeaker testing, impulse-response analysis, and practical laboratory measurements.
Quarter-Wavelength Resonator
A resonator whose effective length is approximately one quarter of the target wavelength. Used for narrowband absorption, mufflers, ducts, and low-frequency resonance control.
Quiet Zone
A region where sound pressure is intentionally reduced by design, control, or wave interference. Used in active noise control, acoustic comfort design, and measurement environments.
W
1 termWavelength
Sound that passes through air produces a wavelike motion of compression and refraction. Wavelength is the distance between two identical positions in the cycle or wave. Similar to ripples or waves produced by dropping a stone in water. Length of sound wave varies with frequency. Low frequency equals longer wavelengths.
X
7 termsX-axis Response
The measured acoustic, vibration, or sensor response along the X spatial axis. Used in 3D vibration measurement, directional sensing, arrays, and coordinate-based analysis.
XLR Connector
A professional balanced audio connector commonly used for microphones and line-level signals. Provides noise rejection and robust connection in studios, live sound, and measurement systems.
X-over / Crossover
A filter network that divides an audio signal into frequency bands for different drivers. Used in loudspeakers to route lows to woofers, mids to midrange drivers, and highs to tweeters.
Xtalk / Crosstalk
Unwanted leakage of a signal from one channel, path, or circuit into another. Important in stereo imaging, multichannel audio, headphones, amplifiers, and measurement chains.
X-correlation / Cross-Correlation
A method for comparing two signals as one is shifted in time relative to the other. Used for time-delay estimation, localization, impulse-response analysis, and pattern matching.
X-curve
A cinema sound calibration curve defining a target high-frequency roll-off in large rooms. Relevant to film dubbing stages, cinema playback calibration, and large-room monitoring.
Y
7 termsY-axis Response
The measured acoustic, vibration, or sensor response along the Y spatial axis. Used in spatial measurements, accelerometers, modal testing, and 3D acoustic mapping.
Young’s Modulus
A material property that describes stiffness under tensile or compressive stress. Affects wave speed, structural vibration, panel radiation, and sound transmission through materials.
Yule-Walker Method
A mathematical method for estimating autoregressive model parameters from signal autocorrelation. Used in spectral estimation, speech analysis, formant analysis, and acoustic signal modeling.
Yaw Noise
Noise or acoustic variation associated with rotation or misalignment around a vertical axis. Relevant to drone acoustics, aircraft orientation, rotating machines, and aeroacoustic testing.
Yaw Angle
The angular orientation of a body around its vertical axis relative to a reference direction. Can affect directional radiation, rotor noise, array measurements, and aerodynamic sound.
Y – Acoustic Admittance
The reciprocal of acoustic impedance, representing how easily a medium or boundary accepts volume velocity. Used in impedance tubes, absorber design, boundary conditions, and room-acoustic modeling.
Z
8 termsZ-weighting
A nearly flat frequency weighting used for unweighted sound-pressure-level measurements. Used when no A-, C-, or other perceptual weighting is intended.
Zero Crossing Rate – ZCR
The rate at which an audio waveform crosses the zero-amplitude axis. Used in speech processing, noise detection, onset detection, and simple audio classification.
Zone of Silence
A region where direct sound is weak or absent due to refraction, barriers, shadowing, or interference. Important in outdoor propagation, atmospheric acoustics, and sound-shadow prediction.
Z-transform
A mathematical transform used to analyze discrete-time signals and digital filters. Fundamental in DSP, filter design, stability analysis, and digital acoustic processing.
Zwicker Loudness
A psychoacoustic loudness model based on critical-band analysis of human hearing. Used in objective loudness assessment, product sound quality, and auditory modeling.
Z – Acoustic Impedance
The ratio of acoustic pressure to volume velocity or particle velocity, depending on context. Central to reflection, absorption, transmission, boundary conditions, and material characterization.
Z-axis Response
The measured acoustic, vibration, or sensor response along the Z spatial axis. Used in 3D accelerometry, structure-borne sound, spatial audio, and modal analysis.