Acoustic Glossary — 150 Essential Terms

A-weighting is a frequency-dependent filter applied to sound measurements to approximate the human ear’s sensitivity, which is less responsive to very low and very high frequencies.

Acoustic impedance is the product of the medium’s density and the speed of sound within it, representing the opposition a material presents to the passage of sound waves.

An axial mode is a room resonance that occurs between two parallel surfaces, involving only one dimension of the room.

The absorption coefficient (α) is the fraction of incident sound energy absorbed by a surface, ranging from 0 (perfect reflection) to 1 (perfect absorption).

An acoustic ceiling tile is a factory-made panel designed to be installed in a suspended ceiling grid to provide sound absorption and reduce reverberation.

Acoustic foam is an open-cell polyurethane or melamine foam used primarily for mid-to-high frequency sound absorption in recording studios, broadcast rooms, and home offices.

An acoustic baffle is a sound-absorbing panel suspended vertically from the ceiling, typically used in spaces with exposed soffits where conventional ceiling tiles cannot be installed.

An acoustic cloud is a horizontal sound-absorbing panel suspended below the structural ceiling, designed to absorb sound energy and control reverberation in the area directly below it.

Acoustic plaster is a spray-applied or trowel-applied surface treatment that provides sound absorption while maintaining a seamless, smooth-plaster appearance.

An anechoic chamber is a specially designed room whose walls, ceiling, and floor are lined with deep wedge-shaped absorbers to eliminate virtually all sound reflections at frequencies above a design cutoff (typically 50–100 Hz).

Auralization is the process of rendering audible sound fields based on acoustic simulation data, allowing listeners to "hear" a room design before it is built.

The Articulation Index (AI) is a measure of speech intelligibility expressed as a value from 0.

Absorption area (A) is the total equivalent absorption in a room, measured in square metres of perfect absorption (metric sabins).

Acoustic treatment is the application of absorptive, diffusive, and reflective materials to room surfaces to control reverberation, improve speech intelligibility, reduce noise levels, and optimize the listening environment.

Airborne sound insulation is the ability of a building element or construction to reduce the transmission of sound generated by sources that radiate into the air, such as speech, music, television, and traffic.

Basotect is BASF’s brand name for open-cell melamine resin foam, widely used as a premium acoustic absorber.

A bass trap is an acoustic treatment device specifically designed to absorb low-frequency sound energy, typically below 300 Hz.

Binaural recording is an audio capture technique that uses two microphones placed at the ear positions of a dummy head or in a human’s ear canals to preserve the three-dimensional spatial cues that the listener would experience.

Background noise measurement quantifies the ambient sound level in a room with all building services operating normally but without the intended activity (speech, music) taking place.

The Boundary Element Method (BEM) is a numerical wave-based simulation technique that solves acoustic problems by discretizing only the boundary surfaces rather than the entire volume.

Calibration in acoustic measurement is the process of adjusting a sound level meter or microphone system to ensure that its readings accurately correspond to known sound pressure levels.

C80 measurement involves extracting the Clarity Index for music from a measured room impulse response.

Critical distance is the distance from a sound source at which the direct sound level equals the reverberant sound level.

The cocktail party effect is the human auditory system’s ability to focus on a single conversation amid a background of multiple simultaneous talkers and noise.

Convolution reverb is a digital signal processing technique that applies the acoustic characteristics of a real or simulated room to an audio signal by mathematically convolving it with the room’s impulse response.

C50 is the Clarity Index for speech, defined as the logarithmic ratio of early sound energy (arriving within the first 50 milliseconds) to late sound energy (arriving after 50 ms), expressed in decibels.

C80 is the Clarity Index for music, defined as the logarithmic ratio of early sound energy (first 80 milliseconds) to late sound energy (after 80 ms), expressed in decibels.

The coincidence frequency (also called the critical frequency) is the frequency at which the wavelength of bending waves in a panel equals the wavelength of the incident airborne sound wave projected onto the panel surface.

Diffraction is the bending of sound waves around obstacles and through openings.

The decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, most commonly sound pressure or sound power.

A damping compound is a viscoelastic material applied to rigid panels (plasterboard, metal, plywood) to reduce resonant vibrations and improve sound insulation.

A decay curve shows how sound energy in a room decreases over time after the source is stopped, plotted as level (dB) versus time (seconds).

D50 measurement involves extracting the Definition (Deutlichkeit) parameter from a measured room impulse response.

The direct-to-reverberant ratio (D/R or DRR) is the level difference between the direct sound from a source and the reverberant sound field at a receiver position, expressed in decibels.

The diffusion coefficient (d) measures how uniformly a surface scatters reflected sound across different directions, ranging from 0 (all energy reflected in one direction) to 1 (energy equally distributed in all directions).

D50 (Deutlichkeit or Definition) is the ratio of early sound energy (first 50 ms) to total sound energy, expressed as a decimal or percentage.

D80 is the Definition parameter for music, calculated as the ratio of sound energy arriving within the first 80 milliseconds to the total sound energy, expressed as a decimal or percentage.

DnT,w is the weighted standardized level difference between two rooms, the primary field measurement parameter for airborne sound insulation between rooms in a building.

Directivity factor (Q) is the ratio of the sound intensity produced by a source in a specific direction to the intensity that would be produced by an omnidirectional source of equal power at the same distance.

Directivity Index (DI) is the logarithmic expression of the directivity factor, expressed in decibels: DI = 10×log₁₀(Q).

Early Decay Time (EDT) is the reverberation time derived from the initial 10 dB of the sound decay curve, extrapolated to a 60 dB range.

The Eyring equation (also called the Eyring-Norris equation) is a reverberation time prediction formula that accounts for the reduction of sound energy at each reflection, making it more accurate than the Sabine equation for rooms with higher absorption.

Equivalent absorption area is the area of a perfectly absorptive surface (α = 1.

Frequency is the number of complete vibration cycles per second, measured in hertz (Hz).

A fabric-wrapped panel is a wall-mounted or ceiling-mounted acoustic absorber consisting of a rigid fiberglass or mineral wool core wrapped in acoustically transparent fabric.

The Fitzroy equation is a reverberation time prediction formula designed for rooms where absorption is concentrated on one or two pairs of surfaces rather than uniformly distributed.

A flutter echo is a rapid series of discrete reflections heard as a buzzing or ringing sound, caused by sound bouncing repeatedly between two parallel reflective surfaces.

Finite-Difference Time-Domain (FDTD) is a numerical wave-based simulation method that solves the acoustic wave equation by discretizing space and time into a grid.

The Finite Element Method (FEM) is a numerical simulation technique that divides the acoustic domain into small elements (typically tetrahedral or hexahedral) and solves the Helmholtz equation at each element.

Flanking transmission is the transmission of sound between rooms via paths other than directly through the separating element, including structure-borne paths through connected floors, walls, and junctions.

Flow resistivity is the resistance a porous material offers to airflow per unit thickness, measured in Pa·s/m².

Glass wool is a fibrous insulation material manufactured from molten glass spun into fine fibers, widely used for acoustic absorption and thermal insulation.

A green wall (living wall or vertical garden) is a wall system incorporating living plants that provides moderate sound absorption alongside thermal insulation, air quality, and biophilic design benefits.

A Helmholtz resonator is an acoustic absorber consisting of an enclosed air volume connected to the room through a narrow neck or perforated opening.

A Head-Related Transfer Function (HRTF) describes how the human head, pinnae, and torso modify incoming sound as a function of frequency and direction of arrival.

Acoustic impedance is the ratio of sound pressure to particle velocity at a point in a medium, expressed in Pa·s/m (rayl).

Acoustic interference occurs when two or more sound waves overlap, resulting in a combined wave whose amplitude is the sum of the individual amplitudes.

The inverse square law states that the intensity of sound from a point source in free field decreases proportionally to the square of the distance from the source.

An impulse response is the complete time-domain record of how a room responds to an ideal impulsive excitation (a Dirac delta function).

ISO 354:2003 specifies the method for measuring the sound absorption coefficient of acoustic materials in a reverberation chamber.

ISO 3382 is the international standard series for measuring room acoustic parameters.

ISO 140 was the former international standard series for measuring sound insulation of building elements and between rooms.

ISO 717 defines the procedures for calculating single-number ratings from frequency-dependent sound insulation measurement data.

The image source method is a computational technique for calculating early reflections in room acoustics by creating virtual "image" sources mirrored across each reflecting surface.

Inter-Aural Cross-Correlation Coefficient (IACC) measures the similarity between the sound signals arriving at the left and right ears of a listener or dummy head, quantifying the spatial impression and envelopment in a room.

Impact Insulation Class (IIC) is a single-number rating of a floor assembly’s ability to reduce impact sound transmission (footsteps, dropped objects, furniture movement) to the room below.

Impact sound insulation is the ability of a floor construction to reduce the transmission of sound caused by direct physical impact on the surface, such as footsteps, dropped objects, and furniture movement.

The Lombard effect is the involuntary tendency of speakers to increase their vocal effort in response to increasing background noise.

Lateral Fraction (LF or LF80) is the ratio of laterally arriving early sound energy (0–80 ms) to total early energy, measured using a figure-of-eight microphone oriented to reject frontal sound and an omnidirectional microphone.

L'nT,w is the weighted standardized impact sound pressure level, the primary field measurement parameter for impact sound insulation of floor assemblies.

The mean free path is the average distance a sound wave travels between successive reflections in an enclosed space.

A membrane absorber (also called a panel absorber or diaphragmatic absorber) is a non-rigid, impervious panel mounted over an enclosed air cavity.

Mineral wool is a fibrous insulation material made from molten rock (stone wool) or slag, used extensively in acoustic applications for sound absorption and sound insulation.

A micro-perforated panel (MPP) is a thin sheet with sub-millimetre holes (typically 0.

Mass loaded vinyl (MLV) is a thin, dense, flexible sheet material (typically 1–6 mm thick, 2–5 kg/m²) used to increase the mass of lightweight partitions for improved sound insulation.

Measurement uncertainty in room acoustics quantifies the range within which the true value of a measured parameter lies, accounting for spatial variation, equipment precision, and environmental factors.

A Maximum Length Sequence (MLS) is a deterministic pseudo-random binary signal used to measure impulse responses in room acoustics.

The Millington-Sette equation is a reverberation time prediction formula that sums the absorption contribution of each surface independently using natural logarithms, rather than averaging the absorption coefficient across all surfaces as in the Eyring equation.

The mass law is a fundamental principle in sound insulation stating that the transmission loss of a single homogeneous panel increases by approximately 6 dB for each doubling of either its surface mass or the frequency.

Mass-air-mass resonance is the fundamental resonant frequency of a double-leaf partition system, where the two panel leaves act as masses and the enclosed air cavity acts as a spring.

The Noise Reduction Coefficient (NRC) is a single-number rating of a material’s sound absorption, calculated as the arithmetic average of the absorption coefficients at 250, 500, 1000, and 2000 Hz, rounded to the nearest 0.

Noise criteria are standardized octave-band spectral curves used to rate and specify acceptable background noise levels in occupied spaces.

A Noise Rating (NR) curve is a standardized octave-band spectral contour used to rate and specify background noise levels in buildings.

A Noise Criteria (NC) curve is a standardized octave-band spectral contour for rating background noise levels in occupied rooms, developed by Beranek in 1957 and widely used in North American practice.

Balanced Noise Criteria (NCB) is a background noise rating method defined in ANSI S12.

Noise Rating (NR) is a single-number descriptor derived from the NR curve system, representing the maximum NR curve exceeded by a measured octave-band noise spectrum.

Noise Isolation Class (NIC) is a single-number field rating of the overall airborne sound isolation between two enclosed rooms, including all transmission paths (direct and flanking).

An octave band is a frequency range where the upper frequency is exactly twice the lower frequency.

An oblique mode is a room resonance involving reflections from all six surfaces of a rectangular room.

A porous absorber is any material containing interconnected air-filled pores through which sound energy is converted to heat by viscous friction and thermal exchange.

A perforated panel is a rigid sheet (metal, wood, plasterboard, or MDF) with regularly spaced holes that acts as an acoustic absorber when mounted over a porous absorber or air cavity.

Pink noise is a random signal with equal energy per octave band, meaning its spectral density decreases at 3 dB per octave (inversely proportional to frequency).

The Privacy Index (PI) quantifies the degree of speech privacy between two spaces as a percentage, calculated as PI = (1 − AI) × 100, where AI is the Articulation Index.

Preferred Noise Criteria (PNC) curves were developed as an improved alternative to NC curves, extending the frequency range to include 31.

A Quadratic Residue Diffuser (QRD) is a sound-scattering device consisting of a series of wells of different depths, calculated using quadratic residue number theory.

RT60 is the time in seconds for sound to decay by 60 decibels after the source stops.

Reverberation time is the duration required for sound energy in a room to decrease by 60 dB after the sound source is turned off.

Acoustic reflection occurs when a sound wave encounters a surface and bounces back into the room rather than being absorbed or transmitted.

Acoustic refraction is the bending of sound waves as they pass through regions of varying propagation speed.

Resonance is the phenomenon where a system vibrates with maximum amplitude at specific frequencies determined by its physical properties.

A room mode is a resonance of the enclosed air volume at a frequency determined by the room’s dimensions.

A resonant absorber is any absorption device that works by resonating at specific frequencies, converting acoustic energy to heat through the vibration of a membrane, the oscillation of air in a neck (Helmholtz), or a combination of both.

A reverberation chamber is a laboratory room designed to create a highly diffuse sound field with long reverberation times, typically 5–15 seconds.

RT60 measurement is the process of determining the reverberation time of a room according to ISO 3382-2:2008.

Acoustic ray tracing is a computational method that models sound propagation by tracking individual sound rays as they travel from sources, reflect off surfaces, and arrive at receivers.

Room acoustic simulation is the computational prediction of how sound behaves in an enclosed space, using mathematical models of the room geometry, surface materials, and sound sources.

The room constant (R) is a measure of a room’s total effective absorption that accounts for multiple reflections, used in calculating sound pressure levels in enclosed spaces.

Room correction (also called room equalization) is the use of electronic signal processing to compensate for acoustic imperfections in a room’s frequency response at the listening position.

RASTI (Rapid Speech Transmission Index) was a simplified two-band speech intelligibility measurement method using octave bands at 500 Hz and 2000 Hz with a total of 9 modulation frequencies.

Rw is the internationally standardized single-number rating for airborne sound insulation of a building element, defined in ISO 717-1:2020.

Room Criteria (RC) is a noise rating method developed by Blazier (1981) for ASHRAE that evaluates both the level and spectral quality of background noise in buildings.

RC Mark II is an updated version of the Room Criteria method published by Blazier (1997) and recommended by ASHRAE for evaluating HVAC-related background noise in occupied spaces.

Sound Pressure Level (SPL) is the logarithmic measure of the effective pressure of a sound relative to a reference value of 20 micropascals, expressed in decibels (dB).

Sound Power Level (SWL or Lw) is the total acoustic energy radiated by a source per unit time, expressed in decibels relative to a reference power of 1 picowatt.

The speed of sound is the rate at which acoustic waves propagate through a medium.

A standing wave is a stationary pattern of nodes (minimum pressure) and antinodes (maximum pressure) formed when sound reflects between parallel surfaces at resonant frequencies.

The Schroeder frequency (also called the crossover frequency or large-room frequency) is the transition point above which a room’s acoustic behavior changes from individual resonant modes to a statistically diffuse sound field.

Sound intensity is the acoustic power passing through a unit area perpendicular to the direction of propagation, measured in watts per square metre (W/m²).

Sound Absorption Average (SAA) is a single-number rating calculated as the average of twelve third-octave band absorption coefficients from 200 Hz to 2500 Hz, rounded to the nearest 0.

Schroeder integration (backward integration) is a mathematical method for converting an impulse response into a smooth decay curve by integrating the squared impulse response backwards from the end of the recording.

Signal-to-noise ratio (SNR) is the difference in decibels between the peak level of the measurement signal and the steady background noise level.

A sound level meter (SLM) is an instrument designed to measure sound pressure level in accordance with IEC 61672-1.

A swept sine (also called exponential sine sweep or ESS) is a test signal that continuously sweeps through frequencies from low to high at an exponentially increasing rate.

Speech transmission measurement evaluates the degree to which a room supports clear speech communication, quantified by the Speech Transmission Index (STI) per IEC 60268-16:2020.

STIPA (Speech Transmission Index for Public Address systems) is a simplified, single-channel measurement method for STI defined in IEC 60268-16:2020 §5.

The Sabine equation is the most widely used formula for predicting reverberation time, expressing RT60 as a function of room volume and total absorption area.

Speech intelligibility is the degree to which speech can be understood by listeners in a given acoustic environment.

Sound focusing is the concentration of reflected sound energy into a localized area by concave surfaces, creating hot spots of high intensity and dead zones elsewhere.

Sound masking is the deliberate introduction of a controlled, uniform background sound into a space to reduce the intelligibility of unwanted speech and improve acoustic privacy.

Spatial decay rate (D₂,S) is the rate of decrease in A-weighted sound pressure level per doubling of distance from a sound source in an open-plan office, measured at desk height (1.

Speech privacy is the degree to which speech from one person is unintelligible to unintended listeners in an adjacent space.

Speech Interference Level (SIL) is the arithmetic average of sound pressure levels at the octave band centre frequencies of 500, 1000, 2000, and 4000 Hz.

Sabine absorption refers to the total equivalent absorption area in a room expressed in sabins (imperial: ft²) or metric sabins (m²).

Sound diffusion is the scattering of reflected sound energy in many directions rather than in a single specular direction.

The scattering coefficient (s) quantifies the fraction of reflected sound energy that is scattered in non-specular directions by a surface, ranging from 0 (perfect specular reflection) to 1 (complete diffuse scattering).

The Speech Transmission Index (STI) is the primary objective measure of speech intelligibility in a room, quantifying how faithfully the temporal modulation patterns of speech are preserved from talker to listener.

STIPA (Speech Transmission Index for Public Address systems) is a simplified measurement method for speech intelligibility defined in IEC 60268-16:2020 §5 that approximates the full STI using a single test signal.

Sound Transmission Class (STC) is a single-number rating of the airborne sound insulation performance of a building element (wall, floor, door, window), defined in ASTM E413.

Structure-borne sound is vibration energy propagating through solid building elements (walls, floors, columns, beams) that radiates as airborne sound into occupied spaces.

Sound absorption class is a letter rating (A through E) assigned to acoustic products based on their weighted absorption coefficient (αw), as defined in ISO 11654:1997.

T20 is a reverberation time measurement derived from the portion of the decay curve between −5 dB and −25 dB below the initial level, then extrapolated to a 60 dB decay.

T30 is a reverberation time measurement evaluated from the decay curve between −5 dB and −35 dB below the initial level, then extrapolated to a 60 dB range.

A third-octave band divides each octave into three sub-bands, providing higher frequency resolution than octave band analysis.

A tangential mode is a room resonance involving reflections between four surfaces (two pairs of parallel walls).

Centre Time (Ts) is the time at which the centre of gravity of the squared impulse response occurs, representing the balance point between early and late energy in the room’s temporal response.

Transmission loss (TL), also called sound reduction index (R) in international standards, is the ratio in decibels of the sound power incident on a building element to the sound power transmitted through it.

Viscoelastic damping is the energy dissipation mechanism in materials that exhibit both viscous (fluid-like) and elastic (spring-like) behavior.

Wavelength is the physical distance between two consecutive points of the same phase in a sound wave, such as two successive compressions.

The weighted absorption coefficient (αw) is a single-number rating defined in ISO 11654:1997, derived by fitting the measured absorption curve to a reference curve and reporting the value of the reference curve at 500 Hz.

White noise is a random signal with constant spectral density across all frequencies, meaning equal energy per hertz.