Glossary

By soundlevelmeter.review editors, practising acousticians. Published 31 October 2026. Neutral by policy: no brand pays for placement.

82 terms used on datasheets, in measurement standards and on this site, each defined in a few sentences. Terms are grouped by subject; every entry has its own anchor, so a definition can be linked directly. Standards referred to are explained under Standards, and Understanding sound level meter specifications shows how the terms appear on a datasheet.

Levels and descriptors

DecibeldB: A logarithmic ratio used to express sound pressure, power and intensity relative to a reference. For sound pressure level the reference is 20 µPa, the nominal threshold of hearing at 1 kHz. A doubling of sound pressure is +6 dB; a doubling of sound energy is +3 dB; a tenfold increase in energy is +10 dB. Levels in decibels cannot be added arithmetically; they are combined on an energy basis.
Sound pressure levelLp: Twenty times the base-10 logarithm of the ratio of the root-mean-square sound pressure to the reference pressure of 20 µPa, expressed in decibels. It is the quantity every sound level meter measures before weightings and averaging are applied. A frequency weighting (A, C or Z) and a time weighting (F, S) are written as subscripts, as in LAF or LCS.See also: Decibel, A-weighting
Equivalent continuous sound levelLeq, LAeq: The level of a steady sound that would contain the same acoustic energy as the actual fluctuating sound over the measurement period. It is an energy average, not an arithmetic mean, so loud events dominate it. LAeq is the A-weighted version and is the basis of nearly every environmental and occupational noise criterion. A meter that measures Leq is called integrating-averaging.See also: Sound exposure level, A-weighting
Maximum time-weighted levelLAFmax, LASmax: The highest A-weighted sound level reached during the measurement period using a time weighting, usually F (fast). It characterises the loudest moment rather than the average and is used for night-time criteria, sleep disturbance assessments and construction noise limits. The time weighting must be stated; LASmax is lower than LAFmax for the same event because the slow detector smooths the peak.See also: Time weighting, Peak sound pressure level
Peak sound pressure levelLpeak, LCpeak: The level of the highest instantaneous sound pressure during the measurement, with no time weighting applied. It is normally C-weighted and is the quantity that occupational noise regulations limit, typically at 135, 137 or 140 dB(C). It is not the same as LAFmax, which is time-weighted and can be 20 dB or more lower for an impulsive sound such as a hammer blow.See also: Maximum time-weighted level, C-weighting, Peak range
Percentile (statistical) levelsLN, L10, L90: The A-weighted level exceeded for N percent of the measurement period, computed from a time history of short samples. L90 is widely used as a measure of background noise; L10 characterises the noisier periods and was the basis of older road traffic noise indices. Values depend on the time weighting and sampling rate used, which a datasheet should state. LA90,15min means L90 of A-weighted fast levels over 15 minutes.See also: Time weighting
Day-evening-night levelLden: A 24-hour A-weighted equivalent level in which evening levels are penalised by 5 dB and night levels by 10 dB before energy averaging, as defined in ISO 1996-1 and the EU Environmental Noise Directive. Default periods are day 07:00 to 19:00, evening 19:00 to 23:00 and night 23:00 to 07:00, although member states may shift them. Lden is the indicator used for strategic noise maps and action plans.See also: Day-night level, Equivalent continuous sound level
Day-night levelLdn, DNL: A 24-hour A-weighted equivalent level with a 10 dB penalty applied to the night period, conventionally 22:00 to 07:00, and no separate evening period. It is the predecessor of Lden and remains the primary metric for aircraft and transportation noise assessment in the United States, where it is written DNL.See also: Day-evening-night level
Sound exposure levelSEL, LAE: The level of a one-second steady sound that would contain the same acoustic energy as a given event. It normalises events of different duration so they can be compared and added: the LAeq of a period is the energy sum of the SELs of its events minus ten times the logarithm of the duration in seconds. Aircraft and railway pass-bys are commonly described by their SEL.See also: Equivalent continuous sound level
Sound exposureE, Pa²h: The time integral of the squared A-weighted sound pressure over a period, expressed in pascal-squared seconds or, for occupational work, pascal-squared hours. It is the quantity a personal sound exposure meter to IEC 61252 fundamentally measures; dose, TWA and LEX,8h are derived from it. An exposure of 1 Pa²h corresponds to an LEX,8h of about 85 dB(A).See also: Daily noise exposure level, Noise dosimeter (personal sound exposure meter)
Rating levelLr, LAr: An equivalent continuous level to which adjustments have been added for character, such as tonality, impulsiveness or intermittency, and sometimes for the time of day, so that different sources can be compared against a single limit. ISO 1996-1 defines the concept; national standards set the adjustment values, which commonly range from 3 to 6 dB for tonal or impulsive sound.See also: Tonality, Equivalent continuous sound level

Occupational exposure

Noise dosimeter (personal sound exposure meter): A small instrument worn by a worker, with the microphone on the shoulder, that integrates A-weighted sound exposure over the shift and reports dose, TWA or LEX,8h together with peak level. Specified by IEC 61252 and, in the United States, ANSI S1.25. Badge types are self-contained; cable types carry the microphone on a lead to a body-worn unit.See also: Sound exposure, Exchange rate
Daily noise exposure levelLEX,8h, LAeq,8h: The A-weighted equivalent continuous level normalised to an eight-hour working day, as defined in ISO 1996-1 and ISO 9612 and used by the EU Noise Directive. A worker exposed to 88 dB(A) for four hours has an LEX,8h of 85 dB(A) using the 3 dB exchange rate. It is the European and international counterpart of the American TWA.See also: Time-weighted average, Exchange rate
Time-weighted averageTWA: The constant A-weighted level that, over eight hours, would give the same noise dose as the actual exposure, computed with the exchange rate of the regulation in use. Under US OSHA rules the TWA uses a 5 dB exchange rate and so is not an energy average; a TWA and an LEX,8h for the same shift differ unless the exposure is steady.See also: Daily noise exposure level, Noise dose, Exchange rate
Noise dose: The exposure received expressed as a percentage of the allowed daily exposure, where 100 percent corresponds to the criterion level for eight hours. A dose of 50 percent at a 3 dB exchange rate is 3 dB below the criterion; at a 5 dB exchange rate it is 5 dB below. Dosimeters often report a projected dose extrapolated from the measured part of the shift.See also: Criterion level, Exchange rate
Exchange rate: The increase in sound level that halves the permissible exposure time. A 3 dB exchange rate follows the equal-energy principle and is used by ISO 9612, the EU directive and NIOSH. A 5 dB exchange rate, retained in the US OSHA permissible exposure limit, allows longer exposure at higher levels on the assumption that intermittency gives the ear time to recover. Dosimeters must be set to the rate the regulation requires.See also: Time-weighted average, Noise dose
Criterion level: The A-weighted level that, maintained for eight hours, produces a 100 percent noise dose. It is 90 dB(A) for the US OSHA permissible exposure limit and 85 dB(A) for NIOSH and for most other jurisdictions, where the term action value or exposure limit value is used instead. It is a dosimeter setting, not a measured quantity.See also: Noise dose, Threshold level
Threshold level: A dosimeter setting below which sound is excluded from the dose calculation. It is a feature of US OSHA practice, where 80 dB(A) applies to hearing conservation measurements. ISO 9612 and European practice integrate all sound, so the threshold is switched off. A threshold makes the reported dose smaller and is one reason OSHA and ISO results for the same shift differ.See also: Criterion level
Action values and exposure limit value: The thresholds in the EU Noise Directive 2003/10/EC and its national transpositions. The lower action value is LEX,8h 80 dB(A) or LCpeak 135 dB(C); the upper action value is 85 dB(A) or 137 dB(C); the exposure limit value, which takes hearing protection into account, is 87 dB(A) or 140 dB(C). Each triggers specific employer duties.See also: Daily noise exposure level, Peak sound pressure level

Classes, approvals and calibration

Class 1 and class 2: The two performance classes of IEC 61672-1 for sound level meters. Both share the same design goals; class 1 has tighter tolerances (about ±1.1 dB at 1 kHz against about ±1.4 dB for class 2), a wider frequency range over which the tolerances apply and a wider operating temperature range (−10 °C to +50 °C against 0 °C to +40 °C). The class applies to the complete instrument with its microphone.See also: Type 1 and type 2, Group X and group Z
Type 1 and type 2: The grades of the superseded IEC 60651 and IEC 60804 standards and of ANSI S1.4-1983. IEC 61672-1 replaced them with class 1 and class 2 and tightened several requirements, so a type 1 claim is not evidence of class 1 performance. The terms survive on older datasheets and in American regulations that still cite ANSI S1.4.See also: Class 1 and class 2
Group X and group Z: Designations introduced in IEC 61672-1:2013 that describe how an instrument's electrical and acoustical tests are to be performed during pattern evaluation and periodic testing, according to how the microphone and electrical input are arranged. They inform the calibration laboratory's procedure. A buyer rarely needs to act on them, but the designation should appear in the manual.See also: Class 1 and class 2, Periodic test (IEC 61672-3)
Pattern approval (type approval): Certification by a national metrology institute, such as PTB in Germany, LNE in France, METAS in Switzerland or NMi in the Netherlands, that a sample of an instrument design has passed the full pattern-evaluation tests of IEC 61672-2. It is a property of the design, not of the individual instrument, and is required in several countries for measurements with legal effect, usually together with periodic verification of each unit.See also: Periodic test (IEC 61672-3), Class 1 and class 2
Periodic test (IEC 61672-3): The laboratory test of an individual sound level meter against the class limits of IEC 61672-1 using the reduced test set of IEC 61672-3, which covers the aspects that can drift or fail in service: calibration level, frequency weightings, linearity, time weightings, peak response, overload and self-noise. The result is a certificate stating measured values, uncertainties and conformance. Filters are tested to IEC 61260-3.See also: Calibration, ISO/IEC 17025
Calibration: The set of operations that establishes, under specified conditions, the relationship between the values indicated by an instrument and the corresponding values realised by standards, with a stated uncertainty. In acoustics the word is also used loosely for the field check with an acoustic calibrator, which is a single-point function check or adjustment rather than a calibration in this sense.See also: Acoustic calibrator, Verification
Verification: Confirmation, by examination and evidence, that an instrument meets specified requirements, for example the class 1 limits of IEC 61672-1. A calibration certificate often includes a verification statement. In countries with legal metrology schemes, verification is the periodic official test of a pattern-approved instrument, after which it is sealed or marked for a fixed period.See also: Calibration, Pattern approval (type approval)
Acoustic calibrator: A portable sound source that produces a known sound pressure level, usually 94 dB or 114 dB at 1 kHz, in a small cavity that fits over the microphone. Specified by IEC 60942 in classes LS, 1 and 2, and used to check a meter before and after each measurement. Some models correct for atmospheric pressure automatically; others require a correction from a barometer. The calibrator itself needs periodic laboratory calibration.See also: Calibration
Traceability: The property of a measurement result whereby it can be related to a national or international reference through a documented, unbroken chain of calibrations, each with a stated uncertainty. A traceable certificate names the reference standards used and the laboratory's own calibration source, ultimately a national metrology institute. Accreditation to ISO/IEC 17025 is the usual evidence that a laboratory's traceability claims have been assessed.See also: ISO/IEC 17025
ISO/IEC 17025: The international standard for the competence of testing and calibration laboratories. Accreditation to it is granted by a national accreditation body for a defined scope and is recognised across borders through the ILAC mutual recognition arrangement. A calibration certificate bearing the accreditation mark is accepted by regulators and auditors without further justification; a certificate from a non-accredited laboratory may be sound but will be questioned.See also: Traceability, Measurement uncertainty
Measurement uncertainty: A parameter characterising the dispersion of values that could reasonably be attributed to the quantity measured, usually stated as an expanded uncertainty with a coverage factor of 2 for about 95 percent confidence. Every value on a calibration certificate carries one. Measurement standards such as ISO 1996-2 and ISO 9612 also require the user to estimate the uncertainty of the field measurement, to which instrument uncertainty is only one contribution.See also: ISO/IEC 17025

Microphones and hardware

Condenser (capacitor) measurement microphone: The microphone type used in all professional sound level meters. A thin metal diaphragm forms one plate of a capacitor with a fixed backplate; sound moves the diaphragm and changes the capacitance, which is converted to voltage by a preamplifier. Measurement microphones are made to IEC 61094 in standard diameters, most commonly ½ inch, and are characterised by sensitivity in millivolts per pascal and by their free-field or pressure response.See also: Prepolarised (electret) microphone, Externally polarised microphone, Microphone sensitivity
Free-field microphone: A microphone whose frequency response is flat for sound arriving from directly in front in a free field, that is, with the effect of the microphone's own presence in the sound field compensated. Sound level meters for environmental, occupational and most building acoustics use free-field microphones pointed at the source. IEC 61672-1 specifies the meter's response in these terms.See also: Pressure-field microphone, Diffuse-field (random-incidence) microphone
Pressure-field microphone: A microphone whose response is flat to the sound pressure actually present at its diaphragm, without compensation for diffraction. It is used in couplers, small enclosures, flush mounting in walls or ducts and for measurements where the microphone is part of a boundary. Fitted to a sound level meter in a free field, it reads high at high frequencies unless a correction is applied.See also: Free-field microphone
Diffuse-field (random-incidence) microphone: A microphone whose response is flat for sound arriving equally from all directions, as in a reverberant room. ANSI S1.4 historically specified sound level meters for random incidence while IEC standards specify free field, which is one reason instruments for the American market were supplied with different microphones or correction filters. Most current meters offer a switchable correction.See also: Free-field microphone
Prepolarised (electret) microphone: A condenser microphone in which the polarising charge is permanently stored in an electret layer on the backplate, so no external polarisation voltage is needed. It works with simple constant-current preamplifiers, tolerates humidity well and is the usual choice for handheld meters, dosimeters and outdoor stations. Early electrets were less stable than externally polarised types; current measurement-grade electrets meet class 1.See also: Externally polarised microphone, Condenser (capacitor) measurement microphone
Externally polarised microphone: A condenser microphone that requires a polarisation voltage, conventionally 200 V, supplied by the preamplifier. The design has the longest track record for stability and is preferred in laboratories and for high-temperature or high-level work. The disadvantages are the need for a preamplifier that provides the voltage and greater sensitivity to condensation, which can cause crackling and sensitivity loss outdoors.See also: Prepolarised (electret) microphone
Phantom power and constant-current (IEPE, CCP) supply: Methods of powering a microphone preamplifier over the signal cable. Phantom power, 48 V on balanced audio lines, is the professional-audio convention and is found on some audio-oriented analysers. Constant-current supply, known by trade terms such as IEPE or CCP, feeds a few milliamps down a coaxial cable to a prepolarised microphone's integral preamplifier and is common on multichannel analysers and some monitoring stations. Meter and microphone must use the same scheme.See also: Prepolarised (electret) microphone, Preamplifier
Preamplifier: The electronics immediately behind the microphone capsule that convert its very high impedance to a low impedance capable of driving a cable, and that supply polarisation voltage where needed. On many meters the preamplifier is detachable, so the microphone can be placed on an extension cable several metres from the operator or inside an outdoor kit. The class rating applies to the meter with its specified preamplifier.See also: Condenser (capacitor) measurement microphone, Phantom power and constant-current (IEPE, CCP) supply
Microphone sensitivitymV/Pa: The open-circuit output voltage per pascal of sound pressure at 1 kHz, typically 50 mV/Pa for a general-purpose ½-inch microphone and 12.5 mV/Pa for high-level types. Higher sensitivity gives a lower noise floor but a lower overload limit. The value is stated on the microphone's calibration chart and is entered into the meter at laboratory calibration; the field check with a calibrator confirms it.See also: Noise floor (self-generated noise), Measurement range
Windscreen: A ball of open-cell foam fitted over the microphone to reduce wind-generated noise and protect the diaphragm from dust and light rain. It alters the frequency response slightly, chiefly above a few kilohertz, and IEC 61672-1 requires the manufacturer to publish the correction; the class rating is usually stated with the windscreen fitted. Outdoor kits use larger screens, often with a rain cover and bird spike.See also: Outdoor microphone kit
Outdoor microphone kit: An assembly that allows a sound level meter microphone to be left outdoors: a large windscreen, a rain cover, a bird spike, a weatherproof preamplifier housing, often a dehumidifier or heater, and a cable to the meter in an enclosure. The frequency response differs from the bare microphone, so the manufacturer should state the class with the kit fitted and publish the corrections.See also: Windscreen, Electrostatic actuator, Charge injection calibration
Electrostatic actuator: A perforated metal plate placed close to the microphone diaphragm to which a high alternating voltage is applied, producing a known electrostatic force on the diaphragm. It is used in laboratories to measure the pressure frequency response and in some outdoor monitoring systems to check the microphone remotely at intervals, since it tests the diaphragm itself and not only the electronics.See also: Charge injection calibration, Outdoor microphone kit
Charge injection calibrationCIC: A remote check in which a known electrical charge is injected at the microphone terminals and the response of the complete chain is compared with a reference recorded at installation. A change beyond a set tolerance indicates a fault in microphone, preamplifier or cable. It is a stability check, not a traceable calibration, and complements rather than replaces the acoustic calibrator.See also: Electrostatic actuator, Acoustic calibrator

Ranges and performance

Measurement range: The span of levels, from the lowest to the highest the instrument will display before overload, as stated by the manufacturer. It is a headline figure and is not defined by IEC 61672-1. The lower end is set by the noise floor and the upper by microphone or electronic overload; the linearity range, which is defined by the standard, is usually narrower at the bottom.See also: Linearity range (level linearity), Noise floor (self-generated noise)
Linearity range (level linearity): The IEC 61672-1 term for the range of levels over which the indicated level tracks the input within the class tolerance, tested at 1 kHz by increasing and decreasing the signal in steps. A class 1 instrument must hold its linearity within about ±1.1 dB across the whole range. This is the range to rely on; a measurement range extends below it into territory where readings are not guaranteed.See also: Measurement range, Class 1 and class 2
Peak range: The span of levels over which the C-weighted peak sound level is within tolerance. Its upper limit is normally higher than that of the time-averaged range because the peak detector must accommodate crest factors of 20 dB or more without overload, and its lower limit is higher because peak values near the noise floor are meaningless. A separate figure should appear on the datasheet.See also: Peak sound pressure level, Crest factor
Noise floor (self-generated noise): The level indicated by the meter when no sound reaches the microphone, arising from thermal noise in the microphone and electronics. It should be stated A-weighted with the microphone fitted; an electrical figure measured with a dummy microphone is lower and not comparable. Signals should be at least 10 dB above the floor, where the floor adds about 0.4 dB, for a credible reading.See also: Measurement range, Microphone sensitivity
Overload: The condition in which the instantaneous signal exceeds what the microphone, preamplifier or converter can handle, so that the indicated level is wrong. IEC 61672-1 requires an overload indicator that latches for the measurement period, so that a brief event during an unattended run is not missed. The under-range condition, where the signal is below the linearity range, must also be indicated.See also: Measurement range, Crest factor
Crest factor: The ratio of the peak value of a signal to its root-mean-square value, expressed in decibels. A sine wave has a crest factor of 3 dB; impulsive sounds such as hammering or gunfire can exceed 30 dB. An instrument must have enough headroom above its RMS range to capture peaks without overload, which is why the peak range extends above the measurement range.See also: Peak range, Overload
Frequency range: The span of frequencies over which the instrument meets its class tolerances. IEC 61672-1 specifies tolerances from 10 Hz to 20 kHz, with class 2 limits widening markedly at both ends. A datasheet may also quote the raw microphone response, which can extend to 40 kHz or beyond; that figure says nothing about tolerance. Infrasound work below 20 Hz needs a microphone and meter specified for it.See also: Class 1 and class 2, G-weighting

Frequency and time weightings

A-weightingdB(A), LA: A frequency weighting defined in IEC 61672-1 that attenuates low and very high frequencies roughly as human hearing does at moderate levels; it is −39 dB at 31.5 Hz, 0 dB at 1 kHz and about +1 dB between 2 and 4 kHz. Nearly every environmental and occupational limit is expressed in A-weighted levels. It underestimates the annoyance of strong low-frequency sound, which is why C- and Z-weighted values are also recorded.See also: C-weighting, Z-weighting
C-weightingdB(C), LC: A frequency weighting that is nearly flat between about 31.5 Hz and 8 kHz, rolling off outside that range. It is used for peak sound pressure level in occupational regulations, for assessing low-frequency content by the difference LCeq minus LAeq, and in the HML method of hearing protector selection. Defined in IEC 61672-1 alongside A and Z.See also: A-weighting, Peak sound pressure level
Z-weightingdB(Z), LZ: Zero weighting: a flat response from 10 Hz to 20 kHz within the class tolerances, introduced in IEC 61672-1 to replace the loosely defined linear or flat settings of earlier instruments. Z-weighted levels are used for octave-band analysis, for low-frequency assessments and whenever the unweighted spectrum is wanted. Some older standards refer to the same quantity as Lin.See also: A-weighting, C-weighting
G-weightingdB(G), LG: A frequency weighting for infrasound defined in ISO 7196, centred on the range 1 to 20 Hz with 0 dB at 10 Hz and steep roll-off above 20 Hz. It is used to assess infrasound from wind turbines, industrial plant and ventilation. Only a minority of sound level meters implement it, and it requires a microphone and preamplifier specified below 10 Hz.See also: Frequency range
Time weightingF, S, I: The exponential averaging applied to the squared sound pressure before the level is displayed. F (fast) has a time constant of 125 ms and follows fluctuations closely; S (slow) has 1 s and smooths them. I (impulse) has a 35 ms rise and a slow decay and is retained in IEC 61672-1 only as an option; it is unsuitable for assessing impulsive sound and most current standards do not use it.See also: Maximum time-weighted level, Equivalent continuous sound level

Spectral analysis

Octave band (1/1 octave): A frequency band whose upper limit is twice its lower limit, identified by its centre frequency: 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000 and 16000 Hz in the standard series of IEC 61260-1. Octave-band spectra are used for hearing protector selection, noise control engineering and room acoustics. The filters have their own class 1 and class 2 tolerances, independent of the meter class.See also: One-third octave band, IEC 61260-1 filter class
One-third octave band: A band one third of an octave wide, about 23 percent of its centre frequency, giving three bands per octave: 100, 125, 160, 200, 250 Hz and so on. Third-octave analysis is the standard resolution for environmental tonality assessment, building acoustics to ISO 16283 and ISO 717, and most product noise standards. Real-time analysers compute all bands in parallel; a few low-cost instruments step through them sequentially.See also: Octave band (1/1 octave), Fast Fourier transform analysis
IEC 61260-1 filter class: The standard for octave and fractional-octave filters sets tolerance masks for the filter shape and defines class 1 and class 2. A sound level meter's class under IEC 61672-1 says nothing about its filters; a datasheet should state the IEC 61260-1 class separately. Building acoustics standards require class 1 filters. Older datasheets cite the superseded IEC 61260:1995, whose class 0 was tighter still.See also: Octave band (1/1 octave), One-third octave band
Fast Fourier transform analysisFFT: Narrow-band spectral analysis with constant absolute bandwidth, for example 1 Hz across a 20 kHz span, which resolves individual tones and harmonics that octave bands blur together. Specified by number of lines, span, window function and averaging; no instrument standard covers it. Used for identifying tonal sources, machinery diagnostics and tonality assessment to ISO 1996-2 and ISO/PAS 20065.See also: One-third octave band, Tonality
Tonality: The presence of a prominent discrete tone in a noise, which increases annoyance and attracts a penalty in rating-level assessments. ISO 1996-2 offers a simplified one-third-octave method, comparing a band with its neighbours, and a narrow-band engineering method based on FFT and critical bands; ISO/PAS 20065 and national standards such as DIN 45681 give related procedures. Instruments that implement these need stored spectra for the whole measurement.See also: Rating level, Fast Fourier transform analysis
Impulsiveness: The presence of short, sharp sound events such as hammering, dropping metal or gunfire, which are judged more annoying than steady sound at the same LAeq and attract a penalty in rating levels. ISO 1996-1 distinguishes regular, highly impulsive and high-energy impulsive sources; ISO/PAS 1996-3 and national methods give objective tests based on onset rate or the difference between LAFmax or LAImax and LAeq.See also: Rating level, Time weighting

Building and room acoustics

Reverberation timeRT60, T20, T30: The time for the sound level in a room to decay by 60 dB after the source stops, measured to ISO 3382. Because the full 60 dB is rarely available above background, the decay is measured over 20 or 30 dB and extrapolated, giving T20 or T30. Measured in octave or third-octave bands from an interrupted noise or an impulse, it describes room acoustics and is needed to correct sound insulation measurements.See also: Standardised level difference, Standardised impact sound pressure level
Speech transmission indexSTI: A single number between 0 and 1 expressing how well a transmission path preserves the intelligibility of speech, defined in IEC 60268-16. It is derived from the modulation transfer function in seven octave bands and fourteen modulation frequencies, accounting for reverberation, background noise, echoes and distortion. Full STI measurement takes minutes per point; STIPA is the faster practical method.See also: STIPA
STIPA: Speech transmission index for public address systems: a reduced measurement of STI using a special test signal with two modulation frequencies per octave band, taking about 15 to 20 seconds per point. It is specified in IEC 60268-16 and used for voice alarm and public address commissioning. Instruments must state which edition of the standard they implement, since revision 5 (2020) changed some corrections.See also: Speech transmission index
Level differenceD: The difference, in each frequency band, between the average sound pressure level in a source room and that in a receiving room, as measured in field sound insulation tests to ISO 16283-1. It is the raw quantity from which the standardised and normalised level differences and the apparent sound reduction index are derived by correcting for the receiving room's reverberation time or absorption.See also: Standardised level difference, Apparent sound reduction index
Standardised level differenceDnT, DnT,w: The level difference between rooms corrected to a reference reverberation time of 0.5 s in the receiving room, so that results are comparable between rooms of different furnishing. DnT,w is the single-number rating obtained by fitting the ISO 717-1 reference curve. It is the quantity most residential building regulations specify for airborne sound insulation between dwellings.See also: Level difference, Weighted sound reduction index, Spectrum adaptation terms
Apparent sound reduction indexR': The field measure of airborne sound insulation of a building element, calculated from the level difference, the partition area and the receiving room's equivalent absorption area. The prime indicates a field value including flanking transmission, as against the laboratory sound reduction index R. R'w is the weighted single-number rating; many commercial and institutional specifications are written in R'w.See also: Weighted sound reduction index, Level difference
Weighted sound reduction indexRw: The single-number rating of a building element's laboratory airborne sound insulation, obtained by shifting the ISO 717-1 reference curve against the measured one-third-octave R values from 100 to 3150 Hz. It is the figure quoted on partition and door datasheets. The American counterpart is the sound transmission class, STC, which uses a similar but not identical procedure under ASTM E413.See also: Apparent sound reduction index, Spectrum adaptation terms
Spectrum adaptation termsC, Ctr: Corrections defined in ISO 717-1 that are added to Rw or DnT,w to account for a particular source spectrum. C uses a pink-noise spectrum, representing speech and general activity; Ctr uses a spectrum weighted towards low frequencies, representing urban road traffic. Ctr is usually negative and larger for lightweight constructions, so Rw + Ctr is the rating specified for facades and for separating walls near traffic.See also: Weighted sound reduction index, Standardised level difference
Standardised impact sound pressure levelL'nT, L'nT,w: The sound pressure level in a receiving room produced by the ISO tapping machine operating on the floor above, corrected to a reference reverberation time of 0.5 s. Lower values indicate better impact insulation. L'nT,w is the single-number rating from ISO 717-2. The alternative normalised level L'n corrects for absorption area instead and is used in some national regulations.See also: Reverberation time, Standardised level difference

Hazardous areas and environmental protection

Intrinsically safeEx ia, Ex ib: A protection concept in which the electrical and thermal energy in a circuit is limited so that it cannot ignite a specified explosive atmosphere, even with faults present. Level ia tolerates two faults and is suitable for zone 0; ib tolerates one fault, for zone 1. Intrinsic safety is a property of the certified assembly: only the specified battery, cables and accessories may be used.See also: ATEX, IECEx, Gas group and temperature class
ATEX: The European Union directive 2014/34/EU on equipment for use in potentially explosive atmospheres, and the certification scheme that implements it. An ATEX certificate number and the Ex marking on an instrument identify the notified body, the equipment group (I mining, II surface industry), the category and the protection concept. ATEX approval is required for dosimeters and meters used in zoned areas in the EU and is widely recognised elsewhere.See also: IECEx, Intrinsically safe
IECEx: The international certification scheme of the IEC for equipment in explosive atmospheres, based on the IEC 60079 series of standards. An IECEx certificate is accepted directly or with minor formalities in many countries outside the EU, including Australia and much of Asia and the Middle East, and underpins national schemes such as UKEX. Instruments for international use commonly carry both ATEX and IECEx certification.See also: ATEX, Intrinsically safe
UL 913, CSA and MSHA approvals: North American routes to hazardous-area approval. UL 913 and the corresponding CSA standard cover intrinsically safe apparatus for Class I (gas), Division 1 locations under the NEC and CEC classification system, which differs from the IEC zone system. MSHA approval is required for equipment taken underground in United States mines. A dosimeter sold worldwide may carry ATEX, IECEx and one or more of these.See also: ATEX, IECEx
Gas group and temperature classIIC T4: Parts of the Ex marking. The gas group states the most easily ignited gases the equipment is safe for: IIA (propane), IIB (ethylene) or IIC (hydrogen and acetylene), with IIC covering all. The temperature class is the maximum surface temperature: T4 is 135 °C, T6 is 85 °C. Group I is reserved for mining. A marking such as Ex ia IIC T4 Ga therefore permits use with any gas in zone 0.See also: Intrinsically safe, ATEX
IP ratingIP65, IP67: The ingress protection code of IEC 60529. The first digit rates protection against solid objects and dust (6 is dust-tight); the second against water (4 splashing, 5 jets, 6 powerful jets, 7 temporary immersion). A handheld meter is often IP54 or unrated; an outdoor station enclosure should be IP65 or better. The rating applies to the housing, not to the microphone, which is protected by the outdoor kit.See also: Outdoor microphone kit, Operating temperature range
Operating temperature range: The range of ambient temperature within which the manufacturer states the instrument meets its specification. IEC 61672-1 requires class 1 meters to meet their tolerances from −10 °C to +50 °C and class 2 meters from 0 °C to +40 °C. Battery capacity and display response fall sharply in the cold, so a datasheet may quote separate ranges for operation and for storage.See also: Class 1 and class 2, IP rating

Logging and monitoring

Logging interval: The period over which a meter computes and stores each set of values in a time history, commonly selectable from 100 ms or less to one hour. One-second logging suits most environmental surveys; shorter intervals are needed for impulsive sources and for percentile statistics. The datasheet should say which parameters, including spectra, are stored at each interval, since memory and battery life depend on it.See also: Percentile (statistical) levels, Audio event recording
Audio event recording: Storage of the microphone signal as an audio file, continuously or when a trigger level is exceeded, so that the source of a reading can be identified afterwards by listening. Recordings are typically compressed and are for identification, not measurement. Recording in public places may engage privacy law, which is why many systems limit clips to a short, pre-set duration and offer to record only triggered events.See also: Logging interval, Markers
Markers: Flags inserted into the time history by the operator, by a trigger or by a schedule, to label events such as an aircraft pass-by, a pause or a period of extraneous noise. Marked periods can be excluded from or isolated in later analysis without editing the data. The number of marker types and whether they can be added during unattended runs vary between instruments.See also: Logging interval
Autonomy: The number of days a monitoring station operates unattended on its own power before a battery change or recharge, as stated by the manufacturer for a particular configuration. It depends on the meter's consumption, modem duty cycle, heating, audio and logging settings, battery capacity and temperature. Solar or mains supply makes autonomy nominally unlimited; the datasheet should state the conditions behind any figure.See also: Remote monitoring platform
Remote monitoring platform: A web service, usually run by the instrument manufacturer, that receives data from monitoring stations over a cellular or network link and presents levels, spectra and audio, with alerts when a threshold is exceeded and tools for reporting. It is normally a subscription. Buyers should establish who owns the data, how raw data can be exported, what remains accessible after the subscription ends and where the servers are located.See also: Autonomy, Audio event recording

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