Reference
Acoustics glossary
The terms that appear on an acoustic panel's test report and in a specification — defined in plain English.
A-weighting and dB(A)
A standard frequency weighting applied to a measured sound level to approximate the ear's reduced sensitivity to low and very high frequencies, with the result expressed in dB(A). Most environmental, workplace and building noise limits, including background-noise targets in guidance such as BS 8233 and BB93, are stated in dB(A).
Absorption class (A–E)
A BS EN ISO 11654 rating band: Class A (αw ≥ 0.90) is the most absorptive, down through B, C and D to Class E. A quick way to compare products without reading every frequency.
Absorption coefficient (α)
The fraction of sound energy a surface absorbs rather than reflects, from 0 (fully reflective) to 1 (fully absorptive). It is measured separately in each frequency band, because most materials absorb high frequencies far better than low ones.
Acoustic slat panel
Timber slats mounted over an acoustic felt or backing board. Sound is absorbed in the gaps between the slats and by the backer, while the face presents a continuous wood finish.
Air gap / cavity depth
The depth of air, often with a porous infill, left behind an acoustic panel or ceiling. Increasing the cavity depth generally improves low-frequency absorption, because porous materials absorb best where the air is moving fastest, which is some distance from the solid backing wall. Since the same panel performs differently at different cavity depths, any absorption figure must state the mounting and gap it was tested with, taken from a test report.
Airborne sound
Sound that travels as pressure waves through the air, such as speech, music or a television. It is distinct from structure-borne sound, and controlling it between rooms is a matter of sound insulation (mass and construction), whereas controlling it within a room is a matter of absorption.
Ambient (background) noise level
The steady underlying noise present in a space from building services such as ventilation, from outside, and from general activity, usually measured in dB(A) as an equivalent continuous or background level. Documents such as BB93 for schools and BS 8233 for other buildings set recommended maximum ambient levels by room type, because a quiet background is needed for good speech clarity.
Approved Document B
The statutory guidance titled 'Fire safety' supporting Part B of the Building Regulations in England, covering means of escape, internal fire spread, external fire spread and access for firefighting. It includes limits on the reaction-to-fire classification permitted for wall and ceiling linings, which is where a decorative panel's Euroclass rating becomes relevant to its use in a building.
Approved Document E
The statutory guidance titled 'Resistance to the passage of sound' supporting Part E of the Building Regulations in England, covering airborne and impact sound insulation between dwellings and rooms for residential purposes, reverberation in the common internal parts of blocks of flats, and (by reference to BB93) schools. Its insulation requirements are satisfied by mass and construction, so absorptive wood panels address only the reverberation aspect, such as corridors and stairwells, not insulation between rooms.
Baffle
A vertical acoustic element suspended from the ceiling, absorbing sound on both faces. Used to add absorption where covering the walls or the full ceiling is impractical.
Bass trap
An absorber optimised for low frequencies, which are otherwise difficult to control, typically made from thick porous material or a resonant panel or Helmholtz device. Bass traps are often placed in the corners and wall-ceiling edges of a room, where low-frequency energy tends to accumulate.
BB93
Building Bulletin 93, the acoustic design standard for schools in England. It sets reverberation-time and noise limits by room type, and is a statutory reference under the Building Regulations for school buildings.
BREEAM Hea 05
The 'Acoustic performance' credit within the BREEAM building sustainability assessment scheme, which awards credits where a project meets defined criteria for indoor ambient noise, sound insulation and room acoustics, typically confirmed by testing or acoustic modelling. The exact thresholds depend on the scheme version and building type, so the relevant BREEAM manual should be consulted.
BS 8233
The British Standard 'Guidance on sound insulation and noise reduction for buildings', which gives recommended internal ambient noise levels and general design guidance for dwellings and other building types. It is guidance rather than statutory law; mandatory requirements come through the Building Regulations. Its specific numeric criteria should be read from the current published standard.
BS EN 13501-1
The European standard that classifies construction products for their reaction to fire, assigning a Euroclass (A1 to F) together with smoke (s1 to s3) and flaming-droplet (d0 to d2) sub-classes on the basis of standardised fire tests. Reaction to fire describes how a product surface contributes to the early growth of a fire; it is a different property from fire resistance.
BS EN ISO 11654
The standard that converts ISO 354 measured values into the single-number weighted coefficient αw and a sound-absorption class from A to E.
Coincidence dip and critical frequency
A frequency region where the bending wavelength within a panel matches the wavelength of sound in the surrounding air, causing a marked dip in the panel's sound insulation, known as the coincidence dip. The lowest frequency at which this occurs is the critical frequency, and it is governed by the panel's stiffness, thickness and material rather than its mass alone.
Critical distance
The distance from a source at which the direct sound and the reverberant sound are equal in level. Beyond it, turning a source up mostly raises reverberation rather than clarity, so in a large or hard room the effective way to improve intelligibility is to increase absorption, not volume.
Decibel (dB)
A logarithmic unit of sound level. Because it is logarithmic, a 10 dB increase is perceived as roughly twice as loud, and small dB changes can matter.
Declaration of Performance (DoP)
The document in which a manufacturer declares a construction product's tested performance under the UK/EU construction products regime — the formal record behind a marketing figure.
Diffusion and diffusers
Diffusion is the scattering of reflected sound in many directions so that acoustic energy is spread evenly through a space, softening strong specular reflections and echoes while retaining a sense of liveliness. A diffuser is a surface or device with a shaped, stepped or irregular profile designed to achieve this; unlike an absorber, it redirects sound energy rather than removing it. Slatted timber over an uneven backing can provide a degree of scattering as well as absorption.
Direct sound
The sound that travels in a straight line from a source to a listener without reflecting off any surface. It carries the clearest information and dominates close to the source, before reflected and reverberant energy build up further away.
Euroclass (reaction to fire)
The BS EN 13501-1 classification of how a product reacts to fire — A1, A2, B, C, D, E or F — with sub-classes for smoke (s1–s3) and flaming droplets (d0–d2). A fire-rated panel is typically specified as B-s1,d0.
Fire resistance (vs reaction to fire)
Fire resistance is the ability of a building element such as a wall, floor or door to contain or withstand a fire for a stated period, expressed in minutes against criteria of load-bearing capacity (R), integrity (E) and insulation (I), tested to standards such as BS EN 1363 and classified to BS EN 13501-2. It is distinct from reaction to fire (the Euroclass rating under BS EN 13501-1), which describes how a surface material contributes to fire growth. A decorative acoustic panel is normally rated for reaction to fire, not fire resistance.
First reflection point
The point on a wall, ceiling or floor where sound from a source reflects once before reaching the listener. Because these early reflections arrive soon after the direct sound and can blur clarity, treating the first reflection points with absorption or diffusion is a common way to improve intelligibility and stereo imaging.
Flanking transmission
Sound that passes between two rooms by indirect paths, such as continuous floors, ceilings, junctions or shared structure, rather than directly through the separating wall. It can undermine even a heavy, well-built partition, which is why sound insulation depends on careful junction detailing as well as the wall itself. Surface absorption does not address it.
Flutter echo
A rapid, repetitive echo that occurs when sound bounces back and forth between two parallel, hard, reflective surfaces, heard as a buzzy ringing after a hand clap or transient. It is treated by introducing absorption or diffusion on one of the surfaces, or by breaking up the parallel geometry.
Frequency (Hz)
The pitch of a sound, in hertz. Acoustic performance varies with frequency, so absorption is measured band by band — usually 125 Hz to 4000 Hz — not as a single figure.
FSC (Forest Stewardship Council)
A chain-of-custody certification that timber comes from responsibly managed forests. It is a documented claim, published when the certificate is held — not an assumption.
Helmholtz resonator (resonant absorber)
An absorber consisting of an enclosed air cavity connected to the room through a neck, slot or perforations, which absorbs sound strongly in a narrow band around its resonant frequency. That frequency is tuned by the cavity volume and the size of the opening. Slatted acoustic panels backed by a cavity behave partly as resonant absorbers as well as porous ones.
Impact (structure-borne) sound
Sound created when an object physically strikes or vibrates a structure, for example footsteps, dropped items or machinery, which then travels through the building fabric and radiates as audible sound elsewhere. Between dwellings it is rated by the weighted standardised impact sound pressure level (L'nT,w, per BS EN ISO 717-2), where a lower value indicates better performance. It is reduced by resilient layers, floating floors and isolation, not by adding wall absorption.
ISO 354
The reverberation-room test method that measures how much sound a material absorbs across frequency bands. It is the source measurement behind every αw and NRC figure.
ISO 354 mounting type (A/D/E-mount)
A standardised way of mounting a test specimen during reverberation-room absorption testing, since the same product absorbs differently depending on how it is fixed. Type A denotes a specimen placed directly against the solid test surface, while other designations (such as D or E, defined in the test standards and ASTM E795) introduce a specified air gap or suspension distance behind it. Because low-frequency absorption in particular depends strongly on this gap, a stated absorption result is only meaningful when its mounting is quoted alongside it.
Mass law
The principle that the airborne sound insulation of a single-leaf partition rises predictably as its mass per unit area increases (broadly of the order of 6 dB for each doubling of mass, and per octave of frequency). It explains why blocking sound between rooms requires heavy, sealed construction, and why lightweight absorptive panels cannot 'soundproof' a wall.
NRC (Noise Reduction Coefficient)
A single number that averages the absorption coefficients at 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05 (ASTM C423). The North-American counterpart to αw; the two are similar but not identical.
Octave band and one-third-octave band
An octave band is a frequency range whose upper limit is twice its lower limit, centred on standard frequencies such as 125, 250, 500, 1000, 2000 and 4000 Hz. A one-third-octave band is a third of that width, so three bands fill each octave and give finer resolution; absorption results to ISO 354 are typically reported in one-third-octave bands from 125 Hz to 4000 Hz.
Panel (membrane) absorber
A resonant absorber formed by a solid, airtight panel mounted over a sealed air cavity; the panel flexes in sympathy with incoming sound and absorbs energy most effectively around its resonant frequency. Because that frequency is typically low, panel absorbers target bass, complementing porous absorbers that work mainly at mid and high frequencies.
PET (polyester) acoustic felt
A porous absorber made from compressed polyester fibres, frequently containing recycled PET (drinks-bottle) content, used as the board or backing behind timber slats and as free-standing panels or baffles. As a porous absorber it works mainly at mid and high frequencies and reduces reverberation within a room; it does not provide sound insulation between rooms.
Porous absorber
A material — mineral wool, acoustic felt, open foam — that absorbs sound by turning the energy of air moving through it into a tiny amount of heat. Most effective at mid and high frequencies.
Reaction to fire
How a product itself contributes to the early growth of a fire — ignitability, flame spread, heat release and smoke. It is a different property from fire resistance, which is about holding back an established fire.
Reverberant field
The region of a room, away from the source, where reflected sound energy dominates over the direct sound. Within it the sound level is roughly uniform throughout the space and is determined largely by the room's total absorption, which is why adding absorption lowers the reverberant level.
Reverberation time (RT60)
The time it takes for sound to decay by 60 decibels after the source stops. The primary measure of how 'live' or 'echoey' a room is, and the number most acoustic treatment is designed to control.
Room mode (standing wave)
A resonance of a room at a frequency where sound reflecting between opposite surfaces reinforces itself, producing an uneven distribution of level with loud peaks and quiet nulls. Modes depend on the room's dimensions and are most audible at low frequencies in small rooms, giving 'boomy' or inconsistent bass.
SAA (Sound Absorption Average)
The ASTM successor to NRC — the average of twelve one-third-octave bands from 200 to 2500 Hz, giving a more detailed single-number rating.
Sabine's equation
RT60 = 0.161 · V / A, relating reverberation time to a room's volume V and its total sound absorption A. Adding absorptive panels increases A, which lowers RT60.
Scattering coefficient
A number between 0 and 1 describing the proportion of reflected sound energy that a surface scatters away from the mirror-like (specular) direction, measured to ISO 17497-1. It quantifies how diffusing a surface is and is a separate property from the absorption coefficient, which describes energy removed rather than redirected.
Slat open area
The proportion of a slatted panel's face that is open (the gaps between the timber slats) rather than covered by solid slat. It influences how much sound reaches the absorptive backing and shapes the panel's frequency response, but it is a geometric property, not a measured acoustic rating in itself.
Sound insulation (airborne)
The reduction of airborne sound as it passes from one space to another through a separating element such as a wall, floor or door. It is a function of the element's mass, stiffness, air-tightness and construction, not of surface absorption, so decorative acoustic wood panels do not provide it. In England it is regulated for dwellings by Approved Document E.
Sound masking
The deliberate introduction of a low-level, engineered background sound, usually broadband and shaped to resemble gentle airflow, to reduce the intelligibility and distraction of intruding speech in a space such as an open-plan office. It works by raising the noise floor so that other sounds are less noticeable, rather than by removing or blocking any noise.
Sound pressure level (SPL)
A measure of the magnitude of sound in decibels, referenced to the standard threshold pressure of 20 micropascals. Because the decibel scale is logarithmic, a 10 dB increase in level is generally perceived as roughly twice as loud.
Sound Transmission Class (STC)
A North American single-number rating of a partition's airborne sound insulation, classified to ASTM E413 from laboratory measurements to ASTM E90. It is broadly analogous to the weighted sound reduction index (Rw) but calculated by a different method, so the two are not directly interchangeable.
Speech intelligibility
How clearly speech can be understood in a space. It falls when reverberation is too long or background noise is too high — the core reason classrooms and offices are treated.
Speech Transmission Index (STI)
An objective measure of how intelligible speech is in a space, expressed on a scale from 0 (unintelligible) to 1 (excellent) and measured or predicted to IEC 60268-16. It is degraded by excessive reverberation and by high background noise, so a higher STI indicates clearer speech.
αw (weighted absorption coefficient)
A single-number rating to BS EN ISO 11654, derived by fitting a reference curve to a material's measured absorption values. Reported in 0.05 steps from 0.00 to 1.00 — the headline figure most acoustic products quote.
Weighted sound reduction index (Rw)
A single-number laboratory rating of a building element's airborne sound insulation, derived to BS EN ISO 717-1 from measurements to BS EN ISO 10140, where a higher figure means more sound is blocked. It is a property of the element's mass and construction; absorptive wood panels fixed to a surface do not meaningfully change it. Rw describes how much sound passes through a partition, not how reverberant a room sounds.
WELL Building Standard
A voluntary building certification scheme whose 'Sound' concept sets criteria for background noise, reverberation, sound insulation and speech privacy in support of occupant comfort. Compliance is demonstrated against the standard's own published thresholds and verified by on-site measurement.