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.

Two conventions govern the fourth column, and they matter more than they look. 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.

Symbols collide across domains, and the table does not pretend otherwise. R is the sound reduction index in building acoustics, the airflow resistance in materials, the roughness in psychoacoustics and the cumulative stress variable in ISO 2631-5. Sigma is the airflow resistivity of a porous material and the radiation efficiency of a plate. A is an equivalent absorption area and an accelerance. L_N is a percentile level here and a loudness level in phon there. Read the symbol together with its domain, which is what the grouping below is for.

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	Quantity	Unit	Defined in	Guide
Lp	Sound pressure level: twenty times the base-10 logarithm of the r.m.s. sound pressure over the reference pressure.	dB re 20 µPa	IEC 61672-1:2013	Calibration
Leq	Equivalent continuous sound pressure level: the level of the steady sound carrying the same mean-square pressure over the interval.	dB	IEC 61672-1:2013	Levels
LAeq	The same integral applied to the A-weighted signal, the default descriptor of environmental and occupational noise.	dB	IEC 61672-1:2013	Levels
LAE, SEL	Sound exposure level: the whole A-weighted energy of a single event normalised to one second.	dB	IEC 61672-1:2013, Equation 8 (Table 4)	Levels
LCpeak	C-weighted peak sound level: the absolute maximum of the C-weighted pressure, not a time-weighted maximum.	dB	IEC 61672-1:2013, subclause 5.13	Levels
LN (L10, L50, L90)	Percentile level: the level exceeded N % of the measurement time, read off the time-weighted level distribution.	dB	ISO 1996-2:2017 (Annex I uses L90 as the residual level)	Levels
LW, SWL	Sound power level: the power a source radiates, referred to 1 pW.	dB re 1 pW	ISO 3745:2012, Clause 8	Sound power
LI	Sound intensity level: the magnitude of the intensity vector referred to 1 pW/m², with the flow direction reported separately as a sign.	dB re 1 pW/m²	IEC 61043:1993	Intensity
Lp − LI	Pressure-intensity index: the difference between the pressure and intensity levels at a position, the field indicator that qualifies an intensity measurement.	dB	ISO 9614-1:1993, Equation (A.3)	Intensity

Symbol	Quantity	Unit	Defined in	Guide
Lden	Day-evening-night level: the energy mean of the three periods with 5 dB added to the evening and 10 dB to the night.	dB	ISO 1996-1:2016, 3.6.4	Levels
Ldn	Day-night level: the same construction with the 10 dB night penalty only.	dB	ISO 1996-1:2016, 3.6.5	Levels
Lr	Rating level: the whole-day composite level after the source-character and time-of-day adjustments.	dB	ISO 1996-1:2016, clause 6.5 (Formulae 5 and 6)	Levels
LAr,T	Rating level of an impulsive source over a reference interval, LAeq plus the graduated impulse adjustment.	dB	NT ACOU 112:2002, clause 8	Impulse prominence
KI	Impulse adjustment added to LAeq, graduated by the predicted prominence of the impulses.	dB	NT ACOU 112:2002, clause 8	Impulse prominence
E	Sound exposure: the time integral of the squared A-weighted sound pressure over the exposure period.	Pa²h	IEC 61252:1993, 3.1	Levels
LEX,8h, LEP,d	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.	dB	IEC 61252:1993, 3.3	Occupational exposure
Lp,A,eqT	A-weighted equivalent continuous level of a task, a job sample or a full day, the building block LEX,8h is assembled from.	dB	ISO 9612:2009, clauses 9 to 11	Occupational exposure
NIPTS	Noise-induced permanent threshold shift: the median hearing loss attributable to a stated exposure level, duration and audiometric frequency.	dB	ISO 1999:2013	Noise-induced hearing loss
HTLAN	Hearing threshold level associated with age and noise: the NIPTS combined with the age component.	dB	ISO 1999:2013	Noise-induced hearing loss

Symbol	Quantity	Unit	Defined in	Guide
A, C, Z	The normative frequency weightings: the ear-response curves applied before integration, Z being the flat reference.	dB	IEC 61672-1:2013, Annex E (acceptance limits in Table 3)	Weighting
G	Infrasound weighting, defined by its poles and zeros for the 0.25 Hz to 315 Hz range.	dB	ISO 7196:1995, Table 1 (nominal responses in Table 2)	Weighting
B	Historical mid-level weighting, withdrawn from the current meter standard.	dB	ANSI S1.4-1983, Appendix C (Formula C2)	Weighting
D	Historical aircraft-noise weighting, derived from the 40-noy perceived-noisiness contour.	dB	IEC 537:1976 (withdrawn)	Weighting
AU	Weighting for audible sound measured in the presence of ultrasound.	dB	IEC 61012:1990, subclause 2.2 (Tables 1 and 2)	Weighting
F, S, I	Fast, Slow and Impulse exponential time weightings: the detector ballistics that produce a displayed level.	s (time constant)	IEC 61672-1:2013	Time weighting

Symbol	Quantity	Unit	Defined in	Guide
T20	Reverberation time extrapolated to a 60 dB decay from a least-squares fit over −5 dB to −25 dB of the Schroeder curve.	s	ISO 3382-2:2008, Clause 6 and Annex C	Room acoustics
T30	The same extrapolation from a fit over −5 dB to −35 dB, the usual choice when the decay range allows it.	s	ISO 3382-2:2008, Clause 6 and Annex C	Room acoustics
T60, RT	Reverberation time as such: the time for the sound energy to fall by 60 dB. Measured in practice as T20 or T30.	s	ISO 3382-1:2009	Room acoustics
EDT	Early decay time: the same slope taken over the first 10 dB of decay, which tracks perceived reverberance rather than the tail.	s	ISO 3382-1:2009 (just-noticeable difference in Table A.1)	Room acoustics
C50	Clarity for speech: the energy ratio between the first 50 ms of the impulse response and everything after it.	dB	ISO 3382-1:2009	Room acoustics
C80	Clarity for music: the same ratio with the boundary at 80 ms.	dB	ISO 3382-1:2009 (just-noticeable difference in Table A.1)	Room acoustics
D50	Definition, or Deutlichkeit: the fraction of the total energy arriving in the first 50 ms.	dimensionless	ISO 3382-1:2009 (just-noticeable difference in Table A.1)	Room acoustics
Ts	Centre time: the centre of gravity of the squared impulse response in time, a boundary-free alternative to the clarity indices.	s	ISO 3382-1:2009, Equation (A.13)	Room acoustics
A	Equivalent sound absorption area of a room: the area of a perfectly absorbing surface that would give the same reverberation time.	m²	ISO 354:2003, Equations (5) and (7)	Materials
NC	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.	dB (index)	ANSI/ASA S12.2-2019, 5.2.2 and 5.2.3 (curves in Table 1)	Room noise
SIL	Speech interference level: the average of the 500, 1000, 2000 and 4000 Hz octave-band levels.	dB	ANSI/ASA S12.2-2019, clause 3.2	Room noise
RC	Room criteria Mark II rating: the average of the 500, 1000 and 2000 Hz levels, with a rumble, hiss or neutral spectral tag.	dB (index)	ANSI/ASA S12.2-2019, Annex D (clauses D.3 and D.4)	Room noise
NR	Noise rating, the European counterpart curve family of NC. Discussed for comparison and deliberately not implemented.	dB (index)	Kosten and van Os (1962); no governing standard	Room noise

Symbol	Quantity	Unit	Defined in	Guide
m(F)	Modulation transfer function: the fraction of the speech envelope modulation depth at modulation frequency F that survives the transmission path.	dimensionless	IEC 60268-16:2020	Speech transmission
STI	Speech transmission index: the modulation transfer matrix converted to effective signal-to-noise ratios and weighted into a single value on 0 to 1.	dimensionless	IEC 60268-16:2020, A.5.2 to A.5.6	Speech transmission
STIPA	The direct STI measurement, made by playing a standardised two-modulation-per-band test signal through the real chain.	dimensionless	IEC 60268-16:2020, clause 6.3 and Table 3 (direct method, Annex B)	Speech transmission
SII	Speech intelligibility index: the band-importance-weighted audibility of the speech spectrum against noise and the listener’s threshold.	dimensionless	ANSI S3.5-1997, clause 6 (procedure in clause 5, importance function in Table 3)	Speech intelligibility
STOI	Short-time objective intelligibility: the clipped per-band envelope correlation between clean and degraded speech.	dimensionless	Taal et al. (2011), Equations 5 and 6; no governing standard	Objective intelligibility
ESTOI	The extended measure, row- and column-normalised so that it tracks modulated maskers.	dimensionless	Jensen and Taal (2016), Equation 8; no governing standard	Objective intelligibility

Symbol	Quantity	Unit	Defined in	Guide
D	Level difference: the energy-averaged source-room level minus the receiving-room level, with no normalisation.	dB	ISO 16283-1:2014, 3.12 to 3.15	Field insulation
DnT	Standardized level difference: the level difference referred to a reference reverberation time, 0.5 s for dwellings.	dB	ISO 16283-1:2014, 3.12 to 3.15	Field insulation
Dn	Normalized level difference: the level difference referred to a reference absorption area of 10 m².	dB	ISO 10052:2021	Field insulation
Dn,e	Element-normalized level difference of a small element or air path, referred to a reference area of 10 m².	dB	EN 12354-3:2000	Insulation prediction
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.	dB	ISO 10140-2:2010	Laboratory insulation
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.	dB	ISO 16283-1:2014, 3.12 to 3.15	Field insulation
TL	Transmission loss: the airborne insulation of a panel predicted from its physical properties, the same quantity as R in a prediction context.	dB	Bies, Hansen and Howard (2017), Section 7.2; no governing standard	Panel insulation
Rw, R′w, DnT,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.	dB	ISO 717-1:2020	Field insulation
Dn,e,w	The same reference-curve rating applied to the element-normalized level difference.	dB	ISO 717-1:2020	Insulation prediction
C, Ctr	Spectrum adaptation terms: the corrections that re-rate the measured curve against A-weighted pink noise (C) and against A-weighted urban road traffic (Ctr).	dB	ISO 717-1:2020, Annex A	Field insulation
Ln	Normalized impact sound pressure level: the receiving-room level under the standard tapping machine, referred to a 10 m² absorption area.	dB	ISO 10140-3:2010	Laboratory insulation
L′nT	Standardized impact sound pressure level, referred to a reference reverberation time. Note the sign: more reverberation lowers it, the opposite of DnT.	dB	ISO 16283-2:2015	Field insulation
Ln,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.	dB	ISO 717-2:2020	Field insulation
CI	Impact spectrum adaptation term, from the energetic sum over 100 Hz to 2500 Hz. The enlarged-range CI,50-2500 extends it down to 50 Hz.	dB	ISO 717-2:2020 (enlarged range in A.2.1 NOTE)	Field insulation
ΔLw	Weighted reduction of impact sound pressure level given by a floor covering, measured as the improvement over the bare reference floor.	dB	ISO 717-2:2020 (measurement in ISO 16251-1:2014, Formulae (3) and (4))	Laboratory insulation
ΔRw	Weighted improvement of airborne insulation contributed by a lining or additional layer, added to the element rating in the prediction.	dB	EN 12354-1:2000, Formulae 27 and 28a	Insulation prediction
Kij	Vibration reduction index of a junction: the direction-averaged velocity level difference corrected by the junction length and the equivalent absorption lengths.	dB	ISO 10848-1:2006, Formula (13)	Laboratory insulation
fc	Critical frequency: the frequency at which the bending wavelength of a panel equals the wavelength in air, where the coincidence dip appears.	Hz	Bies, Hansen and Howard (2017), Equation 7.3; no governing standard	Panel insulation
σ	Radiation efficiency of a plate: the airborne power radiated per unit mean-square surface velocity, normalised by the plane-wave value.	dimensionless	Hopkins (2007), Equations 2.227 to 2.230; no governing standard	Panel insulation

Symbol	Quantity	Unit	Defined in	Guide
α	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.	dimensionless	ISO 10534-2:1998, Equations (17) to (19)	Materials
αs	Random-incidence sound absorption coefficient measured in a reverberation room, from the change in equivalent absorption area with and without the specimen.	dimensionless	ISO 354:2003, Equations (8) and (9)	Materials
αp	Practical sound absorption coefficient: the one-third-octave data grouped into octave bands and rounded to steps of 0.05.	dimensionless	ISO 11654:1997, Clause 4.1	Materials
αw	Weighted sound absorption coefficient: the fixed reference curve shifted toward the practical values and read at 500 Hz.	dimensionless	ISO 11654:1997, Clause 4.2	Materials
Absorption class	The A to E letter class the weighted coefficient maps to, or “not classified”.	class letter	ISO 11654:1997, Table B.1	Materials
R	Airflow resistance: the pressure difference across a specimen divided by the volumetric airflow rate through it.	Pa·s/m³	ISO 9053-1:2018, Clause 3	Materials
Rs	Specific airflow resistance: the airflow resistance referred to the specimen face area.	Pa·s/m	ISO 9053-1:2018, Clause 3	Materials
σ	Airflow resistivity: the specific airflow resistance per unit thickness, the primary input to every empirical porous model.	Pa·s/m²	ISO 9053-1:2018, Clause 3	Materials
Z	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.	Pa·s/m	ISO 10534-2:1998, Equations (17) to (19)	Materials
s	Scattering coefficient: the fraction of reflected energy that is not returned specularly, measured at random incidence on a turntable in a reverberation room.	dimensionless	ISO 17497-1:2004+A1:2014, Formula (5)	Surface scattering
d	Diffusion coefficient: the uniformity of the polar response of a surface, from the autocorrelation of the free-field goniometer measurement.	dimensionless	ISO 17497-2:2012, Formula (5) (normalised form in Formula (7))	Surface scattering
s′	Dynamic stiffness per unit area of a resilient layer: a dynamic force per unit area divided by the resulting change in thickness.	MN/m³	EN 29052-1:1992 (ISO 9052-1:1989), Formula 1	Dynamic stiffness

Symbol	Quantity	Unit	Defined in	Guide
Y	Mobility: the complex ratio of a velocity response to the force that produces it.	m/(N·s)	ISO 7626-1:2011, 3.1.2 and Table 1	Mechanical mobility
Z	Mechanical impedance: the reciprocal of mobility, force per unit velocity.	N·s/m	ISO 7626-1:2011, Table 1	Mechanical mobility
H	Receptance, or dynamic compliance: displacement response per unit force, the pivot the whole family converts through.	m/N	ISO 7626-1:2011, Table 1	Mechanical mobility
A	Accelerance, or inertance: acceleration response per unit force. Its reciprocal is the apparent mass.	1/kg	ISO 7626-1:2011, Table 1	Mechanical mobility
k21	Dynamic transfer stiffness of a resilient element: the blocking force on the output side divided by the displacement on the input side.	N/m	ISO 10846-1:2008, 3.7	Transfer stiffness
Lk	Level of the dynamic transfer stiffness, referred to 1 N/m.	dB re 1 N/m	ISO 10846-2:2008 and ISO 10846-3:2002, 3.17	Transfer stiffness
η	Loss factor of a resilient element: the tangent of the phase angle of its dynamic transfer stiffness.	dimensionless	ISO 10846-1:2008, 3.8	Transfer stiffness
aw	Frequency-weighted acceleration: the root sum of squares of the band accelerations after the human-response weightings.	m/s²	ISO 2631-1:1997, Equation (9)	Human vibration
A(8)	Daily vibration exposure: the exposure magnitude normalised to a reference 8 h day, combined over the operations of the day.	m/s²	ISO 5349-1:2001, Equations (2) and (3)	Human vibration
VDV	Vibration dose value: the fourth-power time integral of the weighted acceleration, which weights shocks far more heavily than an r.m.s. does.	m/s^1.75	ISO 2631-1:1997, Equation (5)	Human vibration
MTVV	Maximum transient vibration value: the largest 1 s running r.m.s. of the weighted acceleration.	m/s²	ISO 2631-1:1997, Equation (4)	Human vibration
R	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.	dimensionless	ISO 2631-5:2018, Annex C (Formulae C.1 and C.3 to C.5)	Multiple-shock vibration
Lv	Velocity level: twenty times the base-10 logarithm of the surface velocity over the reference velocity.	dB	ISO/TS 7849-1:2009, Formula 3	Vibration sound power
ε	Radiation factor, or radiation efficiency, of a vibrating machine surface: the airborne power radiated per unit mean-square velocity and area.	dimensionless	ISO/TS 7849-1:2009 and ISO/TS 7849-2:2009	Vibration sound power
LWs	Structure-borne sound power level injected by equipment into a reception plate.	dB re 1 pW	EN 15657:2018, Formula 14	Structure-borne power
ηij	Coupling loss factor: the fraction of energy per radian that a statistical energy analysis subsystem loses into a neighbouring one across a junction.	dimensionless	Hopkins (2007), Equation 2.154; no governing standard	Junction transmission

Symbol	Quantity	Unit	Defined in	Guide
N	Loudness: the perceived magnitude of a sound, anchored so that a 1 kHz tone at 40 dB SPL is exactly 1 sone.	sone	ISO 532-1:2017, clause 5 (stationary) and clause 6 (time-varying)	Loudness
N′	Specific loudness: the loudness density along the critical-band scale, whose integral is N.	sone/Bark	ISO 532-1:2017 (sone/Cam form in ISO 532-2:2017, Formula 7)	Loudness
LN	Loudness level: the level of the 1 kHz free-field tone judged equally loud as the sound.	phon	ISO 226:2023, Formula (2) (contours in Formula (1))	Loudness
S	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.	acum	DIN 45692:2009, clause 6	Sound quality
R	Roughness: the perceived harshness of fast amplitude modulation, around 70 Hz, normalised so that the reference modulated tone is 1 asper.	asper	ECMA-418-2:2025, clause 7 (Formula 104)	Sound quality
F	Fluctuation strength: the perceived slow amplitude modulation, around 4 Hz, normalised so that the reference modulated tone is 1 vacil.	vacil	ECMA-418-2:2025, clause 9 (Formula 163)	Sound quality
T	Tonality: the perceived tonal content of a sound, derived from the autocorrelation of the band envelopes.	tu	ECMA-418-2:2025, clause 6	Sound quality
TNR	Tone-to-noise ratio: the level of a discrete tone above the masking noise in the critical band around it.	dB	ECMA-418-1:2024, clause 11 (Formulae 9 to 11)	Tone prominence
PR	Prominence ratio: the level of the critical band containing the tone above the mean of the two adjacent bands.	dB	ECMA-418-1:2024, clause 12 (Formula 23)	Tone prominence
ΔL	Audibility of a tone in noise: the tone level minus the critical-band masking level minus the masking index.	dB	ISO/PAS 20065:2016, Formula 14	Tone audibility
PA	Psychoacoustic annoyance: the percentile loudness scaled by sharpness and by a fluctuation-plus-roughness term.	dimensionless	Fastl and Zwicker (2007), Equation 16.2; no governing standard	Psychoacoustic annoyance

Symbol	Quantity	Unit	Defined in	Guide
THD	Total harmonic distortion: the harmonic content of the output relative to the fundamental (THD_F) or to the total signal (THD_R).	% or dB	IEC 60268-3:2013, 14.12.2 to 14.12.11 (the R form in 14.12.3.2)	Electroacoustics
THD+N	Total harmonic distortion plus noise: everything left after notching out the fundamental, within the standard measurement bandwidth.	% or dB	AES17-2015, clause 6.3.1 (notch and bandwidth in 5.2.5 and 5.2.8)	Electroacoustics
SINAD	Signal to noise and distortion ratio, the reciprocal expression of THD+N.	dB	AES17-2015, clause 6.3.1	Electroacoustics
IMD	Modulation intermodulation distortion: the sidebands a low-frequency tone produces around a high-frequency one.	%	IEC 60268-3:2013, 14.12.7	Electroacoustics
DIM	Dynamic intermodulation distortion, measured with a 15 kHz sine against a filtered 3.15 kHz square wave.	%	IEC 60268-3:2013, 14.12.9	Electroacoustics
LK, 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.	LUFS	ITU-R BS.1770-5, Formula 2 (gating in Formulae 3 to 7)	Programme loudness
LRA	Loudness range: the spread between the 10th and 95th percentiles of the gated short-term loudness distribution.	LU	EBU Tech 3342	Programme loudness
dBTP	True peak level: the peak of the signal reconstructed by oversampling, which catches the inter-sample peaks a sample-domain maximum misses.	dBTP	ITU-R BS.1770-5, Annex 2	Programme loudness

Symbol	Quantity	Unit	Defined in	Guide
PNL	Perceived noise level: the 24 one-third-octave band levels converted to noisiness in noys and recombined.	PNdB	ICAO Annex 16, Vol. I, Appendix 2 (noisiness law in Table A2-3)	Aircraft noise
PNLT	Tone-corrected perceived noise level: PNL plus the penalty for spectral irregularities such as fan and turbine tones.	PNdB	ICAO Annex 16, Vol. I, Appendix 2	Aircraft noise
EPNL	Effective perceived noise level: the maximum PNLT plus the duration correction over the 10 dB-down window, the noise-certification metric.	EPNdB	ICAO Annex 16, Vol. I, Appendix 2	Aircraft noise
Lp (underwater)	Underwater sound pressure level, referred to 1 µPa rather than 20 µPa. An airborne level never converts to it by subtraction alone.	dB re 1 µPa	ISO 18405:2017 (mean-square level in ISO 18406:2017, Formula 7)	Underwater acoustics
SEL (underwater)	Underwater sound exposure level, the time integral of squared pressure referred to 1 µPa²·s.	dB re 1 µPa²·s	ISO 18405:2017	Underwater acoustics
LRN	Radiated noise level of a ship: the level of the product of the far-field r.m.s. pressure and the source distance.	dB re 1 µPa·m	ISO 17208-1:2016	Underwater acoustics
Ls	Equivalent monopole source level: the radiated noise level after the Lloyd’s-mirror surface correction, so that one number describes the source itself.	dB re 1 µPa·m	ISO 17208-2:2019, Formula 3	Underwater acoustics

Symbol	Quantity	Unit	Defined in	Guide
u(y)	Combined standard uncertainty of a result, propagated from the standard uncertainties of its inputs by the law of propagation of uncertainty.	unit of the result	ISO/IEC Guide 98-3:2008 (JCGM 100:2008), clause 5	GUM uncertainty
U	Expanded uncertainty: the combined standard uncertainty multiplied by a coverage factor, which defines a coverage interval.	unit of the result	ISO/IEC Guide 98-3:2008 (JCGM 100:2008), clause 6 and Annex G	GUM uncertainty
σR	Standard uncertainty of a building-acoustics measurement situation, tabulated per quantity and situation.	dB	ISO 12999-1:2020, Clause 5.2 (coverage factors in Table 8)	Field insulation