Glossary

The guides each open with the standard they implement, so a quantity is always defined where it is used. This page is the other direction: you have a symbol, from a report, a specification or a colleague’s email, and you want to know what it is, what it is measured in, which document defines it and where in this documentation it is computed. A quantity that is not here is still defined where it is computed: every guide states its quantities before its code.

Two conventions govern the line that says where a quantity is defined, and both matter more than they seem to. Where a clause, formula or table number appears, it is the one the implementation cites, taken from the standard the guide implements. Where only a designation appears, the standard is established but the defining clause is not stated anywhere in this documentation, and inventing a plausible one would be worse than leaving it out. A handful of quantities have no governing standard at all; their source is the paper or book the model comes from, named as such.

A third convention governs the unit, and it is the one most often lost between a measurement and a report. A decibel is always a ratio, so an entry whose unit reads re something — dB re 20 µPa in air, dB re 1 µPa in water, dB re 1 pW for power, dB re 1 pW/m² for intensity, dB re 50 nm/s for a velocity level — is an absolute level, and the reference quantity is part of the unit. An entry whose unit is a bare dB is a level difference or a rating: $D$ , $R$ , $R_{w}$ , $C$ , $C_{t r}$ , $Δ L_{w}$ , $K_{ij}$ , where the reference cancels and no reference applies. Two numbers can only be added or subtracted when they are of the same kind, and the commonest error in a report is subtracting two levels referred to different quantities, or adding a rating to a level.

Symbols that collide

Symbols collide across domains, and the glossary does not pretend otherwise. Some collisions are harmless because the two quantities never meet in one calculation; others change a number by tens of decibels, or by a whole rating step, and those are the ones below. Where two entries share a symbol, the qualifier in brackets after the notation says which one you are reading, and on the site a symbol index above the cards lists every meaning of a letter side by side — the answer for a reader who has the symbol and not the domain, which is the one case the grouping by domain cannot serve.

Symbol	Meanings that are not the same quantity
$C$	The decay curvature $100 (T_{30} / T_{20} - 1)$ in per cent, and the ISO 717-1 spectrum adaptation term in decibels. Both are defined on one page, Rooms and buildings, a couple of sections apart.
TL	A partition’s transmission loss and a duct element’s transmission loss: a different measurement, with an anechoic termination in the definition. The underwater quantity the sonar equation carries is not one of them, and is written PL: ISO 18405:2017 deprecates the two names as synonyms (3.4.1.3 Note 6, 3.4.1.4 Note 7).
NR	The noise rating curve family of a room criterion, and the noise reduction $L_{p 1} - L_{p 2}$ between two rooms in noise control.
$α$	The absorption coefficient of a surface, the atmospheric attenuation coefficient per unit length (dB/m here, dB/km in ISO 9613-2), the volume absorption coefficient of sea water (dB/km), and the tortuosity $α_{\infty}$ of a porous model, which is not an absorption at all.
$A$	An equivalent absorption area in m², an accelerance in 1/kg, and the total attenuation of ISO 9613-2 in dB.
$D$	A level difference, the definition $D_{50}$ , the withdrawn D-weighting, the directivity correction $D_{c}$ and barrier screening $D_{z}$ of ISO 9613-2, the spatial decay rate $D_{2, S}$ of an open-plan office, and a microphone’s directivity index $D = 20 lo g_{10} (M_{0} / M_{diff})$ .
$H$	The receptance of ISO 7626-1, the frequency response estimators $H_{1}$ and $H_{2}$ , and the harmonic transfer functions $H_{n}$ of a swept-sine measurement — where $H_{2}$ is the second harmonic, not the noise-on-input estimator.
$η$	The loss factor of a resilient element, the coupling loss factor $η_{ij}$ of a junction, and the internal and total loss factors of a building element.
$τ$	The transmission factor of a partition, the time constant of a detector, the lag of a correlation, and the air temperature in CNOSSOS-EU.
$T_{s}$	The ISO 3382-1 centre time of a room impulse response, typically tens of milliseconds; the structural reverberation time of a plate, which is seconds; and the number of months per year that studded tyres are fitted in CNOSSOS-EU.
$R$	The sound reduction index in building acoustics, the airflow resistance in materials, the roughness in psychoacoustics, and the cumulative stress variable of ISO 2631-5.
$σ$	The airflow resistivity of a porous material, the radiation efficiency of a plate, and the standard deviation $σ_{R}$ of a building-acoustics measurement situation.
$L_{N}$ , $L_{n}$	$L_{N}$ is a percentile level in environmental noise, a loudness level in phon, and the level equivalent to a microphone’s inherent noise; $L_{n}$ , with a lower-case subscript, is the normalized impact sound pressure level of ISO 10140-3. In building acoustics the case of the subscript is the whole difference.
$v_{0}$	A reference velocity of 1 nm/s in EN 15657 and EN 12354-5, and of 50 nm/s in ISO/TS 7849 and ISO 9611 — 34 dB apart, in one section of one area.
$F_{0}$	A reference force of 1 N in ISO 16283-2, and of 1 µN in EN 15657 — 120 dB apart.
$K_{1}$	The ISO 3744 background noise correction in dB, and a CNOSSOS-EU temperature coefficient in dB/°C.
$F$	Fluctuation strength in vacil, the Fast time weighting, and the modulation frequency in $m (F)$ — all three on this page.

Where two meanings meet inside one navigation section, the guide says so at first use; this table is the site-wide list.

For the source of each definition rather than the definition itself, the bibliography lists every cited work with a DOI or publisher link, and the conformance report shows the numerical check that pins each quantity to its standard’s own expected value.

Symbol index: every meaning of a letter, side by side

AA (octave-band attenuation)Adiv Aatm Agr Abar A, C, Z AU A Absorption class A aw A(8)ANP database AUD INJ onset (PTS onset)

CCmet Cepstrum and quefrency Crest factor C50 C80 C, Ctr CI Critical band CN (the cfl argument)

DDc D D50 D2,S D DnT Dn Dn,e Dn,e,w D2m,nT Dls,2m,nT,w DnT,A, D2m,nT,Atr DI,n,e d dd,n (difference frequency)DIM dBTP dp DI (receiving array)DT

EE ENBW EDT ESTOI ERBN (Cam scale)EPNL

FF, S, I fc F FOM

GG Gxx, Gxy

HHTLAN H1, H2 (FRF estimators)H (receptance)Hn (harmonic order $n$)

IITU-R 468 weighting IL IMD, dm,n (modulation)

KK1 K2 KI Kt, Kf, Ki K Kij k21

LLp Leq LAeq LAE, SEL LCpeak LN (L10, L50, L90)LW, SWL LI Lp - LI LWAd, KWA LWA (apparent, wind turbine)Lden Ldn Lr LAr,T LEX,8h, LEP,d Lp,A,eqT LKeq,T Lp,A,S,4m Ln L'nT Ln,w, L'nT,w Lk Lv LWmathrms LN LK, LUFS LRA LAmax Lp (underwater)LRN Ls

Mm(F)MTVV

NNIPTS N (Fresnel number)NC NR (curve family)NR (level drop)N N'N5 NPD NL, LN Numerical dispersion

PPerformance class (0, 1, 2)PR PA PNL PNLT PNLTM PL, NPL (underwater)PML (perfectly matched layer)

RRC rD rP R R'Rw, R'w, DnT,w R'45°RA, RA,tr RI R Rs R R

SSIL STI STIPA SII STOI s s'S Short-term and long-term loudness SINAD SEL (underwater)SL, LS SE, Δ LSE SELcum

TT20 T30 T60, RT Ts (centre time)TL (panel)TL (duct element)T TNR THD THD+N TDFD TS, NTS TTS onset

Uu(y)U

VVDV

WWb, Wc, Wd, We, Wf, Wj, Wk, Wm (whole-body)Wh (hand-arm)W(f)

ZZ Zc, k Z z (critical-band rate)

αα (atmospheric)α αs αp αw α∞ (tortuosity)

γγ2iy (ordinary coherence)γ2y:x (multiple coherence)γ2iy(i-1)! (partial coherence)

ΔΔ Lw Δ Rw Δ L

εε

ηη (resilient element)ηint (internal)ηtot (total, in situ)ηij

ΛΛ, Λ'

σσ σ σR

ττ τij (junction)

φφ (porosity)

Sound pressure, power and intensity levels

$L_p$

dB re 20 µPa

Sound pressure level: twenty times the base-10 logarithm of the r.m.s. sound pressure over the reference pressure.

$L_{eq}$

dB re 20 µPa

Equivalent continuous sound pressure level: the level of the steady sound carrying the same mean-square pressure over the interval.

$L_{Aeq}$

dB re 20 µPa

The same integral applied to the A-weighted signal, the default descriptor of environmental and occupational noise.

$L_{AE}$ , SEL

dB re (20 µPa)²·s

Sound exposure level: the whole A-weighted energy of a single event normalised to one second.

$L_{Cpeak}$

dB re 20 µPa

C-weighted peak sound level: the absolute maximum of the C-weighted pressure, not a time-weighted maximum.

$L_N$ ( $L_{10}$ , $L_{50}$ , $L_{90}$ )

dB re 20 µPa

Percentile level: the level exceeded $N$ % of the measurement time, read off the time-weighted level distribution.

$L_W$ , SWL

dB re 1 pW

Sound power level: the power a source radiates, referred to 1 pW.

$L_I$

dB re 1 pW/m²

Sound intensity level: the magnitude of the intensity vector referred to 1 pW/m², with the flow direction reported separately as a sign.

$L_p - L_I$

Pressure-intensity index: the difference between the pressure and intensity levels at a position, the field indicator that qualifies an intensity measurement.

$K_1$

Background noise correction: what is subtracted from the surface level to remove the background's own contribution, $-10\log_{10}(1 - 10^{-0,1\Delta L_p})$ from the source-on minus source-off margin. It is a cliff rather than a slope: above a 15 dB margin it is taken as zero, between 6 and 15 dB it is computed, and below 6 dB the standard caps it at 1,3 dB and warns that the result has lost accuracy.

$K_2$

Environmental correction: what is subtracted to remove the energy the test room reflects back onto the measurement surface, $10\log_{10}(1 + 4S/A)$ from the surface area and the room's equivalent absorption area. Its ceiling is the grade of accuracy rather than a preference: an engineering-grade result is only valid where $K_{2A} \le 4$ dB, the survey method allows 7 dB, and a qualified hemi-anechoic room gives zero.

$L_{WAd}$ , $K_{WA}$

dB re 1 pW

The declared noise emission of a machine: either the dual-number form, the measured $L_{WA}$ and its uncertainty $K_{WA}$ stated separately, or the single-number form $L_{WAd} = L_{WA} + K_{WA}$ , both rounded to the nearest whole decibel. The declared value is an upper limit a verification measurement is unlikely to exceed, not a best estimate, so it is never the number to feed into a propagation calculation.

$L_{WA}$ (apparent, wind turbine)

dB re 1 pW

Apparent sound power level of a wind turbine: the A-weighted level of a point source at the rotor centre that would radiate the same downwind emission as the machine measured. It is written like a sound power level but is not one in the usual sense: the ground-board measurement builds a downwind reflection into it, so feeding it to a propagation model that adds a ground effect counts that reflection twice.

Environmental and occupational descriptors

$L_{den}$

dB re 20 µPa

Day-evening-night level: the energy mean of the three periods with 5 dB added to the evening and 10 dB to the night.

$L_{dn}$

dB re 20 µPa

Day-night level: the same construction with the 10 dB night penalty only.

$L_r$

dB re 20 µPa

Rating level: the whole-day composite level after the source-character and time-of-day adjustments.

$L_{Ar,T}$

dB re 20 µPa

Rating level of an impulsive source over a reference interval, $L_{Aeq}$ plus the graduated impulse adjustment.

$K_I$

Impulse adjustment added to $L_{Aeq}$ , graduated by the predicted prominence of the impulses.

$E$

Pa²h

Sound exposure: the time integral of the squared A-weighted sound pressure over the exposure period.

$L_{EX,8h}$ , $L_{EP,d}$

dB re 20 µPa

Daily noise exposure level: the steady level that, sustained over a nominal 8 h day, carries the same A-weighted sound exposure as the measured one.

$L_{p,A,eqT}$

dB re 20 µPa

A-weighted equivalent continuous level of a task, a job sample or a full day, the building block $L_{EX,8h}$ is assembled from.

NIPTS

Noise-induced permanent threshold shift: the median hearing loss attributable to a stated exposure level, duration and audiometric frequency.

HTLAN

Hearing threshold level associated with age and noise: the NIPTS combined with the age component.

$L_{Keq,T}$

dB re 20 µPa

Corrected equivalent level: the A-weighted equivalent level of the interval plus the three penalties for tonal, low-frequency and impulsive character, $L_{Aeq,T} + K_t + K_f + K_i$ . It is the quantity the Spanish immission limits are written against, so an activity is judged on it and not on the bare $L_{Aeq}$ .

$K_t$ , $K_f$ , $K_i$

The three character penalties added to $L_{Aeq,T}$ : $K_t$ for emergent tonal components, read from an unweighted one-third-octave spectrum against the arithmetic mean of the two adjacent bands; $K_f$ for low-frequency content, from $L_{Ceq} - L_{Aeq}$ ; and $K_i$ for impulsive content, from $L_{AIeq} - L_{Aeq}$ . They are stepped, not continuous, so a spectrum just short of a threshold scores nothing.

Outdoor propagation

$A$ (octave-band attenuation)

Total octave-band attenuation between an outdoor point source and a downwind receiver: the sum $A_{div} + A_{atm} + A_{gr} + A_{bar} + A_{misc}$ subtracted from the sound power level and the directivity correction. Everything the method knows about the path between source and receiver is in this one term.

$A_{div}$

Geometrical divergence: $20\log_{10}(d/d_0) + 11$ dB, the spreading of a point source radiating into free space with $d_0 = 1$ m. The 11 dB constant is what refers the level to a sound power level rather than to a level measured at one metre.

$A_{atm}$

Atmospheric absorption: the attenuation coefficient of the air times the path length. It is the term that removes the high bands over long distances, and it depends strongly on frequency, temperature and humidity — which is why a long-range prediction has to state the weather it assumed.

$\alpha$ (atmospheric)

dB/m (ISO 9613-2 tabulates dB/km)

Atmospheric attenuation coefficient: the excess loss per unit path length from classical absorption and the nitrogen and oxygen relaxation processes, a function of frequency, temperature, humidity and pressure. Watch the length unit: the library returns decibels per metre while ISO 9613-2 Table 2 tabulates decibels per kilometre, a factor of a thousand.

$A_{gr}$

Ground effect: the interference between the direct path and the path reflected off the ground, split into a source region, a receiver region and the middle between them. Over hard ground it comes out negative — a net gain, not a loss — which is why it cannot be treated as an attenuation that is merely optional.

$A_{bar}$

Barrier attenuation: the screening $D_z$ of the diffracting edge minus the ground effect the barrier removes, floored at zero. The subtraction is the point: a barrier over soft ground buys much less than its raw screening, because the ground was already doing part of the work.

$C_{met}$

Meteorological correction: what is subtracted from the downwind level to obtain a long-term average over many wind directions, driven by the local factor $C_0$ and by the source and receiver heights against the distance. It is zero close to the source and grows only where the path is long compared with the heights.

$D_c$

Directivity correction: how far the level from the point source in the chosen direction departs from that of an omnidirectional source of the same sound power. It is the directivity index of the source plus an index for radiation into less than the full sphere, and it is 0 dB for an omnidirectional source in free space.

$N$ (Fresnel number)

dimensionless

Fresnel number of a screen: twice the extra path length the sound has to travel over the edge, divided by the wavelength. It is the single geometric parameter of the Kurze-Anderson insertion loss, which is why a barrier that works at 1 kHz can be worth almost nothing two octaves lower for the same geometry.

Frequency and time weighting

A, C, Z

The normative frequency weightings: the ear-response curves applied before integration, Z being the flat reference.

G

Infrasound weighting, defined by its poles and zeros for the 0.25 Hz to 315 Hz range.

B

Historical mid-level weighting, withdrawn from the current meter standard.

D

Historical aircraft-noise weighting, derived from the 40-noy perceived-noisiness contour.

AU

Weighting for audible sound measured in the presence of ultrasound.

F, S, I

s (time constant)

Fast, Slow and Impulse exponential time weightings: the detector ballistics that produce a displayed level.

Performance class (0, 1, 2)

dB (tolerance)

Performance class of a filter or an instrument: the width of the tolerance corridor its response has to stay inside, band by band. Class 1 and class 2 share the same design goals and differ in the acceptance limits and in the operating temperature range, class 2 being the looser; class 0, the laboratory reference grade, comes from the withdrawn IEC 61260:1995 and ANSI S1.11-2004 and the current edition no longer defines it. A class is a property of the response, not of the result: a class 2 bank does not make a measurement wrong, it makes its band levels less certain.

K

K-weighting: the programme-loudness curve, a two-stage pre-filter that models the high-frequency boost a spherical head gives and then applies the revised low-frequency B-curve high-pass. It is applied per channel before the channel mean squares are summed and gated, and the LKFS designation records it.

ITU-R 468 weighting

The broadcast noise weighting: zero at 1 kHz, peaking at $+12{,}2$ dB at 6,3 kHz and falling to $-29{,}9$ dB at 31,5 Hz, shaped to how audible a noise is rather than how loud a tone is. The Recommendation pairs it with a quasi-peak detector and quotes results as dB(468); AES17 reuses the same curve with an r.m.s. detector, which is a different number from the same filter.

$W_b$ , $W_c$ , $W_d$ , $W_e$ , $W_f$ , $W_j$ , $W_k$ , $W_m$ (whole-body)

dimensionless

The whole-body frequency weightings, one parameter row each of the same four-stage filter: $W_k$ for the vertical axis and $W_d$ for the two horizontal ones in health and comfort, with $W_b$ , $W_c$ , $W_e$ , $W_f$ , $W_j$ and $W_m$ for ride comfort, the seat back, rotational axes, motion sickness and the head. The weighted acceleration $a_w$ is what comes out of them; the standard that names the curve is not the one that says where to apply it.

$W_h$ (hand-arm)

dimensionless

The hand-arm frequency weighting: one curve, band-limited from 8 Hz to 1 kHz, applied to each of the three axes before they are combined into the vibration total value. Unlike the whole-body case there is no axis multiplier, so the three weighted values enter the total on equal terms.

Spectral and system analysis

$G_{xx}$ , $G_{xy}$

(unit of $x$)²/Hz

One-sided auto- and cross-spectral density: mean-square content per hertz, so the power in a band is the integral over it and not the height of a line. Everything else in this group is a ratio of these: the coherences, the two frequency-response estimators and the coherent output spectrum.

$\gamma^2_{iy}$ (ordinary coherence)

dimensionless

Ordinary coherence: the fraction of the output autospectrum, at each frequency, that a linear time-invariant path from one input accounts for, $|G_{xy}|^2/(G_{xx}G_{yy})$ . It is one where the pair is noiselessly linearly related, and with additive output noise it settles at $\mathrm{SNR}/(1+\mathrm{SNR})$ — so it reads as a quality figure, not as a cause.

$\gamma^2_{y:x}$ (multiple coherence)

dimensionless

Multiple coherence: the fraction of the output that all the measured inputs together account for, one minus the residual spectrum over the total. It is the ceiling the partial coherences are apportioned under, and what is left of it is the part of the output no measured input explains.

$\gamma^2_{iy\cdot(i-1)!}$ (partial coherence)

dimensionless

Partial coherence: the coherence of one input with the output after the inputs ranked before it have been conditioned out. It is what separates a genuine source from one that merely correlates with a genuine source, and it depends on the conditioning order, so the order has to be reported with the number.

$H_1$ , $H_2$ (FRF estimators)

output per input

The two averaged estimates of a frequency response function: $H_1 = G_{xy}/G_{xx}$ , unbiased when the noise is on the output, and $H_2 = G_{yy}/G_{yx}$ , unbiased when it is on the input. Their ratio is exactly the ordinary coherence, so they agree only where the measurement is clean, and the gap between them is a measure of how far it is not.

ENBW

bins (or Hz)

Equivalent noise bandwidth of an analysis window: the width of the ideal rectangular filter that would pass the same broadband noise power. It is exactly 1 bin for a rectangular window and 1,5 for a Hann, and it is the factor that turns a windowed line spectrum into a density — a broadband level read off the lines sits $10\log_{10}(\mathrm{ENBW})$ dB high without it.

Cepstrum and quefrency

quefrency in s

Cepstrum: the inverse transform of the logarithmic spectrum, in which the periodic ripple an echo or a harmonic family leaves across the spectrum collapses onto a single peak. Quefrency is its independent variable, a time in seconds, at which that peak stands at the echo's own delay — which is why a bearing report quotes a quefrency and not a frequency.

Crest factor

dimensionless

Crest factor: the modulus of the ratio of the peak to the r.m.s. value over the measurement period. It decides whether an r.m.s. description is honest — above 9 the standard says the basic evaluation method is not sufficient and dose measures have to be reported beside it — and for a test signal it decides whether a device clips before the signal has delivered its energy.

Room acoustics

$T_{20}$

Reverberation time extrapolated to a 60 dB decay from a least-squares fit over −5 dB to −25 dB of the Schroeder curve.

$T_{30}$

The same extrapolation from a fit over −5 dB to −35 dB, the usual choice when the decay range allows it.

$T_{60}$ , RT

Reverberation time as such: the time for the sound energy to fall by 60 dB. Measured in practice as $T_{20}$ or $T_{30}$ .

EDT

Early decay time: the same slope taken over the first 10 dB of decay, which tracks perceived reverberance rather than the tail.

$C_{50}$

Clarity for speech: the energy ratio between the first 50 ms of the impulse response and everything after it.

$C_{80}$

Clarity for music: the same ratio with the boundary at 80 ms.

$D_{50}$

dimensionless

Definition, or Deutlichkeit: the fraction of the total energy arriving in the first 50 ms.

$T_s$ (centre time)

Centre time: the centre of gravity of the squared impulse response in time, a boundary-free alternative to the clarity indices. It runs to tens of milliseconds in a room; the building-prediction guides write $T_s$ for something else entirely, the structural reverberation time of a plate, which is seconds.

$A$

m²

Equivalent sound absorption area of a room: the area of a perfectly absorbing surface that would give the same reverberation time.

NC

dB (index)

Noise criteria rating of a background spectrum: the speech interference level selects the curve, and the tangency method rates the spectrum when a band exceeds it.

SIL

Speech interference level: the average of the 500, 1000, 2000 and 4000 Hz octave-band levels.

RC

dB (index)

Room criteria Mark II rating: the average of the 500, 1000 and 2000 Hz levels, with a rumble, hiss or neutral spectral tag.

NR(curve family)

dB (index)

Noise rating, the European counterpart curve family of NC. Discussed for comparison and deliberately not implemented. Not the NR of the noise-control guides, which is a level drop.

$D_{2,S}$

Spatial decay rate of speech: the drop in A-weighted speech level per doubling of distance along a line of workstations, taken from the regression of level against the logarithm of distance over positions between 2 m and 16 m. It is a slope only — it says how fast speech dies away, not how loud it starts.

$L_{p,A,S,4m}$

A-weighted speech level at 4 m: the nominal level of normal speech four metres from the talker, read off the same regression line rather than measured at that distance. It fixes the absolute height of the decay curve that $D_{2,S}$ only gives the slope of, which is why the two are always reported together.

$r_D$

Distraction distance: the distance from the talker at which the speech transmission index falls below 0,50, beyond which concentration and privacy start to improve rapidly. It is the single number an open-plan office is usually specified on, and the only rating in the corpus that is a distance rather than a level.

$r_P$

Privacy distance: the distance at which the speech transmission index falls below 0,20, beyond which speech is as private as it would be between separate rooms. In offices with small volume or poor privacy it can be out of reach entirely.

Speech and intelligibility

$m(F)$

dimensionless

Modulation transfer function: the fraction of the speech envelope modulation depth at modulation frequency $F$ that survives the transmission path.

STI

dimensionless

Speech transmission index: the modulation transfer matrix converted to effective signal-to-noise ratios and weighted into a single value on 0 to 1.

STIPA

dimensionless

The direct STI measurement, made by playing a standardised two-modulation-per-band test signal through the real chain.

SII

dimensionless

Speech intelligibility index: the band-importance-weighted audibility of the speech spectrum against noise and the listener's threshold.

STOI

dimensionless

Short-time objective intelligibility: the clipped per-band envelope correlation between clean and degraded speech.

ESTOI

dimensionless

The extended measure, row- and column-normalised so that it tracks modulated maskers.

Sound insulation

NR(level drop)

Noise reduction: the sound pressure level in the source room minus the level in the receiving room, $L_{p1} - L_{p2}$ . It is not the transmission loss of the partition: the two differ by a term set by the partition area against the receiving room's absorption, so a small partition into a well-absorbing room delivers more noise reduction than its transmission loss, and a large one into a hard room delivers less.

$D$

Level difference: the energy-averaged source-room level minus the receiving-room level, with no normalisation.

$D_{nT}$

Standardized level difference: the level difference referred to a reference reverberation time, 0.5 s for dwellings.

$D_n$

Normalized level difference: the level difference referred to a reference absorption area of 10 m².

$D_{n,e}$

Element-normalized level difference of a small element or air path, referred to a reference area of 10 m².

$R$

Sound reduction index: the level difference corrected by the partition area over the receiving-room absorption area, measured in the laboratory with flanking suppressed.

$R'$

Apparent sound reduction index: the same construction measured in the building, so it includes every flanking path. The prime is the lab-versus-field marker.

TL(panel)

Transmission loss: the airborne insulation of a partition predicted from its physical properties, ten times the base-10 logarithm of the reciprocal transmission factor, the same quantity as $R$ in a prediction context.

TL(duct element)

Transmission loss of a duct element: ten times the base-10 logarithm of the incident plane-wave power over the power transmitted into an anechoic termination, computed from the four-pole transfer matrix and the two port impedances. The anechoic termination is part of the definition, which is why this transmission loss describes the element alone and is not the noise reduction the same silencer delivers once it is installed between a real source and a real outlet.

IL

Insertion loss: the level at a receiver before an element is inserted minus the level after, for a silencer the drop in radiated sound power level when a length of duct is replaced by it. Unlike a transmission loss it depends on the source and the termination as well as on the element, which is what makes it the number a client can hear and the transmission loss the number a catalogue can print.

$\tau$

dimensionless

Transmission factor, or transmission coefficient: the fraction of the incident sound power a partition passes on. It is the quantity the whole group is a logarithm of, since $R = -10\log_{10}\tau$ and the transmission loss is the same logarithm; a $\tau$ of $10^{-5}$ is a 50 dB partition. Transmission factors of parallel paths add, which is why a prediction sums the direct, flanking and indirect airborne factors and only then takes the logarithm.

$R_w$ , $R'_w$ , $D_{nT,w}$

The weighted single-number ratings: a fixed reference curve is shifted toward the measured spectrum until the unfavourable deviations reach their allowed sum, and the shifted curve is read at 500 Hz.

$D_{n,e,w}$

The same reference-curve rating applied to the element-normalized level difference.

$C$ , $C_{tr}$

Spectrum adaptation terms: the corrections that re-rate the measured curve against A-weighted pink noise ( $C$ ) and against A-weighted urban road traffic ( $C_{tr}$ ).

$L_n$

dB re 20 µPa

Normalized impact sound pressure level: the receiving-room level under the standard tapping machine, referred to a 10 m² absorption area.

$L'_{nT}$

dB re 20 µPa

Standardized impact sound pressure level, referred to a reference reverberation time. Note the sign: more reverberation lowers it, the opposite of $D_{nT}$ .

$L_{n,w}$ , $L'_{nT,w}$

The weighted impact ratings. The reference curve is shifted the same way, but an unfavourable deviation is now one where the measurement exceeds the reference.

$C_I$

Impact spectrum adaptation term, from the energetic sum over 100 Hz to 2500 Hz. The enlarged-range $C_{I,50\text{–}2500}$ extends it down to 50 Hz.

$\Delta L_w$

Weighted reduction of impact sound pressure level given by a floor covering, measured as the improvement over the bare reference floor.

$\Delta R_w$

Weighted improvement of airborne insulation contributed by a lining or additional layer, added to the element rating in the prediction.

$K_{ij}$

Vibration reduction index of a junction: the direction-averaged velocity level difference corrected by the junction length and the equivalent absorption lengths.

$f_c$

Critical frequency: the frequency at which the bending wavelength of a panel equals the wavelength in air, where the coincidence dip appears.

$\sigma$

dimensionless

Radiation efficiency of a plate: the airborne power radiated per unit mean-square surface velocity, normalised by the plane-wave value.

$D_{2m,nT}$

Standardized facade level difference: the level 2 m in front of the facade minus the indoor level, standardized to a reference reverberation time of 0,5 s for dwellings. The 2 m position sits in the field the facade itself reflects, so it is not a free-field level, and the notation records the source — $D_{tr,2m,nT}$ for road traffic, $D_{ls,2m,nT}$ for a loudspeaker.

$D_{ls,2m,nT,w}$

The weighted facade rating: $D_{2m,nT}$ measured with a loudspeaker put through the ISO 717-1 reference-curve procedure. The `ls` subscript is not decoration — it records that the source was a loudspeaker at 45 degrees rather than real road traffic, and a facade rated with one source is not interchangeable with the same facade rated with the other.

$R'_{45°}$

Apparent sound reduction index of a facade element under loudspeaker sound at 45 degrees: the level difference corrected by the specimen area over the receiving-room absorption area, with a further $-1{,}5$ dB that carries the single-angle geometry. It is apparent, so flanking and any other component of the facade are inside the number.

$R_A$ , $R_{A,tr}$

dBA

Global airborne index: the A-weighted level a partition transmits when it is excited by a normalised spectrum — pink noise for $R_A$ , road traffic for $R_{A,tr}$ — summed energetically over eighteen one-third-octave bands from 100 Hz to 5 kHz. It is a close relative of $R_w + C$ and $R_w + C_{tr}$ but is computed directly, not by shifting a reference curve, and it uses two bands the ISO 717-1 range does not.

$D_{nT,A}$ , $D_{2m,nT,Atr}$

dBA

The same global index applied to a standardized level difference: between two rooms ( $D_{nT,A}$ , pink noise) and between the outside and a protected room ( $D_{2m,nT,Atr}$ , road traffic or aircraft). These are the quantities the DB-HR requirement tables are written against, so a project is checked in them and not in the ISO 717-1 ratings.

$R_I$

Intensity sound reduction index: the source-room level minus the intensity level scanned over the radiating face, so the transmitted power is measured directly instead of inferred from the receiving room. It is the method of choice where flanking is strong; add the $K_c$ adaptation to get $R_{I,M}$ , the value the ISO 10140 pressure method would have produced.

$D_{I,n,e}$

Intensity element-normalized level difference: the small-element counterpart of $D_{n,e}$ , measured by scanning the element and normalised to a reference absorption area of 10 m². It is rated through the same ISO 717-1 procedure, as $D_{I,n,e,w}$ , so a ventilator or a transit sealing system can be compared with a wall on one scale.

Materials and surfaces

$\alpha$

dimensionless

Sound absorption coefficient at normal incidence: the fraction of incident energy not returned by the surface, obtained in the impedance tube from the reflection factor.

$\alpha_s$

dimensionless

Random-incidence sound absorption coefficient measured in a reverberation room, from the change in equivalent absorption area with and without the specimen.

$\alpha_p$

dimensionless

Practical sound absorption coefficient: the one-third-octave data grouped into octave bands and rounded to steps of 0.05.

$\alpha_w$

dimensionless

Weighted sound absorption coefficient: the fixed reference curve shifted toward the practical values and read at 500 Hz.

Absorption class

class letter

The A to E letter class the weighted coefficient maps to, or "not classified".

$R$

Pa·s/m³

Airflow resistance: the pressure difference across a specimen divided by the volumetric airflow rate through it.

$R_s$

Pa·s/m

Specific airflow resistance: the airflow resistance referred to the specimen face area.

$\sigma$

Pa·s/m²

Airflow resistivity: the specific airflow resistance per unit thickness, the primary input to every empirical porous model.

$Z$

Pa·s/m

Surface impedance: the complex ratio of sound pressure to particle velocity at the face of the sample, usually reported normalised by the characteristic impedance of air.

$s$

dimensionless

Scattering coefficient: the fraction of reflected energy that is not returned specularly, measured at random incidence on a turntable in a reverberation room.

$d$

dimensionless

Diffusion coefficient: the uniformity of the polar response of a surface, from the autocorrelation of the free-field goniometer measurement.

$s'$

MN/m³

Dynamic stiffness per unit area of a resilient layer: a dynamic force per unit area divided by the resulting change in thickness.

$\phi$ (porosity)

dimensionless

Open porosity: the fraction of the material's volume that is air connected to the outside, $\phi = V_a/V_T$ . A closed bubble counts as frame, not as pore, because sound cannot enter it; for most fibrous materials and foams $\phi$ lies very close to one, which is why a one-parameter model that assumes so can still work.

$\alpha_\infty$ (tortuosity)

dimensionless

Tortuosity: how much longer the winding path through the pores is than the straight line, squared — exactly $1/\cos^2\varphi$ for pores inclined at an angle $\varphi$ , and one for straight cylindrical pores. It sets the high-frequency limit of the effective density, and despite the $\alpha$ it is not an absorption coefficient of any kind.

$\Lambda$ , $\Lambda'$

The two pore sizes the Johnson-Champoux-Allard model needs: $\Lambda$ weights the pore surface by the squared flow velocity, so it is set by the narrow constrictions where the viscous losses happen, and $\Lambda'$ is the plain surface-to-volume length that governs the thermal exchange with the frame. $\Lambda'$ is normally the larger, and the two are equal only for identical straight cylindrical pores.

$Z_c$ , $k$

Pa·s/m and 1/m

Characteristic impedance and complex wavenumber: the pair that describes a porous medium as an equivalent fluid — the ratio of pressure to particle velocity in a travelling wave inside it, and the wavenumber whose imaginary part is the attenuation per metre. Every empirical or semi-phenomenological model produces this pair, and it is what the transfer-matrix method stacks layer by layer.

Vibration and structure-borne sound

$Y$

m/(N·s)

Mobility: the complex ratio of a velocity response to the force that produces it.

$Z$

N·s/m

Mechanical impedance: the reciprocal of mobility, force per unit velocity.

$H$ (receptance)

m/N

Receptance, or dynamic compliance: displacement response per unit force, the pivot the whole family converts through.

$A$

1/kg

Accelerance, or inertance: acceleration response per unit force. Its reciprocal is the apparent mass.

$k_{21}$

N/m

Dynamic transfer stiffness of a resilient element: the blocking force on the output side divided by the displacement on the input side.

$L_k$

dB re 1 N/m

Level of the dynamic transfer stiffness, referred to 1 N/m.

$\eta$ (resilient element)

dimensionless

Loss factor of a resilient element: the tangent of the phase angle of its dynamic transfer stiffness.

$\eta_{int}$ (internal)

dimensionless

Internal loss factor of a building element: the fraction of its vibrational energy dissipated per radian inside the material itself, before anything is radiated or leaves through the junctions. It is an input to the prediction, not a measured output: Table B.3 of the same standard gives 0,005 for concrete and about 0,01 for most masonry.

$\eta_{tot}$ (total, in situ)

dimensionless

Total loss factor of an element as built in: the internal loss factor plus the losses radiated into the air and absorbed at the perimeter junctions. It is what damps the resonant transmission above the critical frequency, and it fixes the structural reverberation time through $T_s = 2{,}2/(f\,\eta_{tot})$ — so a laboratory value measured in a heavy test frame does not transfer to the building unchanged.

$a_w$

m/s²

Frequency-weighted acceleration: the root sum of squares of the band accelerations after the human-response weightings.

$A(8)$

m/s²

Daily vibration exposure: the exposure magnitude normalised to a reference 8 h day, combined over the operations of the day.

VDV

m/s^1.75

Vibration dose value: the fourth-power time integral of the weighted acceleration, which weights shocks far more heavily than an r.m.s. does.

MTVV

m/s²

Maximum transient vibration value: the largest 1 s running r.m.s. of the weighted acceleration.

$R$

dimensionless

Cumulative stress variable of the multiple-shock model: the daily compressive stresses accumulated over the years of exposure, which the lumbar injury probability is read from.

$L_v$

dB re 50 nm/s

Velocity level: twenty times the base-10 logarithm of the surface velocity over the reference velocity.

$\varepsilon$

dimensionless

Radiation factor, or radiation efficiency, of a vibrating machine surface: the airborne power radiated per unit mean-square velocity and area.

$L_{Wmathrm{s}}$

dB re 1 pW

Structure-borne sound power level injected by equipment into a reception plate.

$\eta_{ij}$

dimensionless

Coupling loss factor: the fraction of energy per radian that a statistical energy analysis subsystem loses into a neighbouring one across a junction.

$\tau_{ij}$ (junction)

dimensionless

Junction transmission coefficient: the fraction of the bending-wave power arriving at a junction from plate $i$ that continues into plate $j$ , angle by angle and then averaged over a diffuse field. Both the coupling loss factor $\eta_{ij}$ and the vibration reduction index $K_{ij}$ are derived from it, so it is the wave-approach quantity the junction family bottoms out in.

Psychoacoustics

$N$

sone

Loudness: the perceived magnitude of a sound, anchored so that a 1 kHz tone at 40 dB SPL is exactly 1 sone.

$N'$

sone/Bark

Specific loudness: the loudness density along the critical-band scale, whose integral is $N$ .

$L_N$

phon

Loudness level: the level of the 1 kHz free-field tone judged equally loud as the sound.

$S$

acum

Sharpness: the position of the centre of gravity of the specific loudness on the critical-band scale, normalised so that the reference narrow-band noise is exactly 1 acum.

$R$

asper

Roughness: the perceived harshness of fast amplitude modulation, around 70 Hz, normalised so that the reference modulated tone is 1 asper.

$F$

vacil

Fluctuation strength: the perceived slow amplitude modulation, around 4 Hz, normalised so that the reference modulated tone is 1 vacil.

$T$

Tonality: the perceived tonal content of a sound, derived from the autocorrelation of the band envelopes.

TNR

Tone-to-noise ratio: the level of a discrete tone above the masking noise in the critical band around it.

PR

Prominence ratio: the level of the critical band containing the tone above the mean of the two adjacent bands.

$\Delta L$

Audibility of a tone in noise: the tone level minus the critical-band masking level minus the masking index.

PA

dimensionless

Psychoacoustic annoyance: the percentile loudness scaled by sharpness and by a fluctuation-plus-roughness term.

Critical band

Critical band: the frequency span within which the ear sums energy as one event, about 100 Hz wide below 500 Hz and roughly a fifth of the centre frequency above it. Masking, sharpness and the two tone-prominence ratios are all computed band by band on this scale, which is why none of them can be read off a fixed fractional-octave spectrum.

$z$ (critical-band rate)

Bark

Critical-band rate: the auditory frequency scale on which one unit is one critical band, running 0 to 24 Bark over the audible range. Specific loudness is a density along it, which is why its unit is sone/Bark and why a loudness pattern is plotted against $z$ rather than against frequency.

$ERB_N$ (Cam scale)

Hz (scale in Cam)

Equivalent rectangular bandwidth of the auditory filter, and the Cam scale built from it by counting one unit per $ERB_N$ : about 132 Hz at 1 kHz, so the step from 934 Hz to 1066 Hz is one Cam. It is narrower than the Bark band at low frequencies, and it is the scale ISO 532-2 and ISO 532-3 compute specific loudness on, so a sone/Cam density is not numerically a sone/Bark one.

$N_5$

sone

Percentile loudness: the loudness exceeded 5 % of the time, read off the time-varying loudness of the ISO 532-1 model. It is the stationary stand-in for a fluctuating sound that psychoacoustic annoyance is built on, and it is a loudness in sone, not a loudness level in phon.

Short-term and long-term loudness

sone

The two smoothed loudness time series of the Moore-Glasberg-Schlittenlacher model: short-term loudness is the loudness of a brief segment — a syllable, a single note, typically up to 500 ms — and long-term loudness that of a longer one, a whole sentence or musical phrase, typically up to 5 s. The loudness of a sound lasting two or three seconds is predicted by the maximum of the long-term series, not by its average.

Electroacoustics and programme loudness

THD

% or dB

Total harmonic distortion: the harmonic content of the output relative to the fundamental ( $\mathrm{THD}_F$ ) or to the total signal ( $\mathrm{THD}_R$ ).

THD+N

% or dB

Total harmonic distortion plus noise: everything left after notching out the fundamental, within the standard measurement bandwidth.

SINAD

Signal to noise and distortion ratio, the reciprocal expression of THD+N.

IMD, $d_{m,n}$ (modulation)

Modulation intermodulation distortion: the sidebands a strong low-frequency tone produces around a weak high-frequency one. IEC 60268-3 defines several intermodulation families with different test signals, so a bare "IMD" has to say which one it is, and the single number an SMPTE-type analyzer prints combines the modulation sidebands in r.m.s. and is none of them.

$d_{d,n}$ (difference frequency)

Difference-frequency intermodulation distortion: the products two closely spaced high-frequency tones of equal amplitude create at their difference and its multiples, referred to the fundamentals. Its test signal and its products are both different from the modulation form, so the two numbers are not comparable.

TDFD

Total difference-frequency distortion: the second- and third-order difference products of the two-tone test combined into one figure and referred to the sum of the two fundamentals, the single number the difference-frequency family reports.

$H_n$ (harmonic order $n$)

output per input

Harmonic transfer function of order $n$ : the impulse response that an exponential-sweep deconvolution places $L\ln n$ seconds *before* the linear one, so a single sweep separates the linear response and every harmonic order into its own window. The distortion of order $n$ at excitation frequency $f$ is then read as $|H_n(nf)|/|H_1(f)|$ .

DIM

Dynamic intermodulation distortion, measured with a 15 kHz sine against a filtered 3.15 kHz square wave.

$L_K$ , LUFS

LUFS

Programme loudness: the channel-weighted sum of K-weighted mean-square powers, gated in 400 ms blocks. LUFS and LKFS name the same unit.

LRA

Loudness range: the spread between the 10th and 95th percentiles of the gated short-term loudness distribution.

dBTP

True peak level: the peak of the signal reconstructed by oversampling, which catches the inter-sample peaks a sample-domain maximum misses.

Aircraft and airport noise

PNL

PNdB

Perceived noise level: the 24 one-third-octave band levels converted to noisiness in noys and recombined.

PNLT

PNdB

Tone-corrected perceived noise level: PNL plus the penalty for spectral irregularities such as fan and turbine tones.

EPNL

EPNdB

Effective perceived noise level: the maximum PNLT plus the duration correction over the 10 dB-down window, the noise-certification metric.

PNLTM

PNdB

Maximum tone-corrected perceived noise level: the largest PNLT of the half-second time history, after the bandsharing adjustment. It is the peak the certification metric is built on, since $\mathrm{EPNL} = \mathrm{PNLTM} + D$ and the 10 dB-down integration window is measured down from it.

NPD

Noise-power-distance table: the event level of one aircraft — $L_{Amax}$ or SEL — tabulated against distance for a handful of engine power settings, measured in steady flight along a notionally infinite straight path at a reference speed. A calculation interpolates it linearly in power and logarithmically in distance, then corrects that baseline segment by segment.

ANP database

Aircraft Noise and Performance database: the international collection of NPD tables, aircraft and engine performance coefficients and default departure and approach profiles, supplied mostly by the manufacturers, that an airport-noise calculation is normally run from.

$d_p$

Slant distance: the perpendicular distance from the receiver to the flight-path segment, which is the abscissa of every NPD table. It is not the distance along the ground and not the aircraft's altitude, and to the side of the track it is the minimum distance to the segment rather than to the whole path.

$L_{Amax}$

dB re 20 µPa

Maximum A-weighted level of a single event: the largest instantaneous value $L_A(t)$ reaches while the aircraft passes, the simpler of the two event metrics an NPD table carries. Doc 29 reads $L_A$ on the Slow sound-level-meter scale, so the time weighting is part of the quantity; it is not the peak level, which has no time weighting at all.

Underwater acoustics

$L_p$ (underwater)

dB re 1 µPa

Underwater sound pressure level, referred to 1 µPa rather than 20 µPa. An airborne level never converts to it by subtraction alone.

SEL(underwater)

dB re 1 µPa²·s

Underwater sound exposure level, the time integral of squared pressure referred to 1 µPa²·s.

$L_{RN}$

dB re 1 µPa·m

Radiated noise level of a ship: the level of the product of the far-field r.m.s. pressure and the source distance.

$L_s$

dB re 1 µPa·m

Equivalent monopole source level: the radiated noise level after the Lloyd's-mirror surface correction, so that one number describes the source itself.

PL, $N_{PL}$ (underwater)

Propagation loss, the quantity the sonar equation uses: the difference between the source level and the mean-square sound pressure level at the receiver, $N_{PL}(x) = L_S - L_p(x)$ , in practice a geometrical spreading law plus the volume absorption $\alpha R$ . Clause 3.4.1.3 keeps the name transmission loss for the reduction in level between two stated points, and both entries deprecate using one name as a synonym of the other, which is why so much of the literature calls this one a transmission loss.

SL, $L_S$

dB re 1 µPa²m²

Source level: the level of the source factor, equal to the level 1 m from a hypothetical point source radiating into an infinite lossless medium, so it is back-projected from a measurement made much further away and never measured at 1 m. The reference value carries a squared metre, which is why the widespread "dB re 1 µPa at 1 m" is the same number written loosely.

NL, $L_N$

dB re 1 µPa

Sonar noise level: the background the signal has to be detected against, in the band of interest, from wind, thermal agitation, distant shipping and the platform's own self-noise. It has to be quoted over the same bandwidth as the source level, since an ambient spectrum level and a broadband level differ by $10\log_{10}B$ .

DI(receiving array)

Directivity index of a receiving array: the array gain in the special case the sonar equation assumes, a plane-wave signal in isotropic background noise. It is the signal-to-noise ratio the beam buys over a single omnidirectional hydrophone, and it enters the equation as a credit against the noise level.

DT

Detection threshold: ten times the base-10 logarithm of the signal-to-noise ratio at which a signal counts as just detectable, for a stated probability of detection — often 0,5 — and probability of false alarm. It is where the processing gain and the operator's tolerance for false alarms enter the sonar equation.

TS, $N_{TS}$

dB re 1 m²/sr

Target strength: the level of the target's differential scattering cross section, the free-field ratio of what it scatters back to what is incident on it. It is the one term that separates the active sonar equation from the passive one, and it depends on both the incidence and the scattering direction, so a backscattering value is the monostatic special case.

SE, $\Delta L_{SE}$

Signal excess: the amount by which the signal-to-noise ratio at the processor output exceeds the detection threshold. It is what the sonar equation returns; the target is detectable where it is positive, and the range at which it crosses zero is the detection range.

FOM

dB (re 1 m² as a propagation factor)

Figure of merit: the propagation loss a passive system can afford before the signal excess reaches zero, that is, the loss at which SE = 0. It is read as the loss at which the probability of detection falls to 50 % only under the usual convention that the detection threshold is itself referred to that probability; SE = 0 fixes no probability on its own. Inverting any loss law at $PL = \mathrm{FOM}$ gives the detection range directly, which is why it is quoted instead of the whole curve.

$W(f)$

Auditory weighting function: a generic band-pass filter shaped to one hearing group's susceptibility to noise-induced hearing loss, with its gain chosen so the flat central part sits at 0 dB. It is applied to the spectrum before an exposure is summed, so the same physical sound weighs differently for a porpoise and for a baleen whale.

TTS onset

Onset of temporary threshold shift: the exposure at which a recoverable loss of hearing sensitivity begins, published per hearing group as a weighted sound exposure level and, for impulsive sound, together with an unweighted peak level. Both metrics have to be tested; whichever is reached first decides.

AUD INJ onset (PTS onset)

Onset of auditory injury, the 2024 guidance's name for what earlier versions called permanent threshold shift onset: the exposure above which the shift no longer recovers. For non-impulsive sound it is the TTS onset plus 20 dB for every group; for impulsive sound the guidance sets it 15 dB above in exposure and 6 dB above in peak level.

$SEL_{cum}$

dB re 1 µPa²·s

Cumulative sound exposure level: the weighted sound exposure of every event of an activity — every strike of a piling campaign, say — summed over its whole duration. The onset criteria are written against this accumulated quantity, not against a single event, so halving the strike energy and doubling the strike count changes nothing.

Numerical simulation

$C_N$ (the cfl argument)

dimensionless

Courant number: how far a wavefront travels in one time step, measured in grid cells, $c\,\Delta t\sqrt{1/\Delta x^2 + 1/\Delta y^2}$ . An explicit scheme is stable only up to one and meaningless above it, so it is the number the time step is chosen from rather than the other way round; the library defaults to 0,6.

Numerical dispersion

% (speed error)

Numerical dispersion: the error the discrete scheme makes in the propagation speed, which grows with frequency and depends on the direction of travel — largest along a coordinate axis, and zero along the diagonal of square cells at a Courant number of one. It is why ten cells per shortest wavelength is the working rule, and why an FDTD arrival time drifts if the grid is too coarse.

PML (perfectly matched layer)

Perfectly matched layer: a boundary region whose absorption is graded so that, in principle, it reflects nothing at any angle or frequency, which is what lets a finite grid stand in for open space. What this library ships is the graded sponge layer, its simple precursor, so a grazing-incidence residue at the edge is expected rather than a bug.

Measurement uncertainty

$u(y)$

unit of the result

Combined standard uncertainty of a result, propagated from the standard uncertainties of its inputs by the law of propagation of uncertainty.

$U$

unit of the result

Expanded uncertainty: the combined standard uncertainty multiplied by a coverage factor, which defines a coverage interval.

$\sigma_R$

Reproducibility standard deviation of a sound-insulation quantity: the standard uncertainty ISO 12999-1 assigns to situation A, the widest of the three. The same clause assigns $\sigma_{situ}$ to situation B and the repeatability $\sigma_r$ to situation C — for $R'_w$ , 1,2 dB against 0,9 and 0,4 — so the situation has to be stated with the number.

Created and maintained by José Manuel Requena Plens ORCID All projects