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Oil and gas industry noise monitoring
Acoustic Glossary

Acoustic Glossary

This acoustic glossary covers the most essential noise terminology, as well as listing all of the parameters that you may see displayed by the Optimus Sound Level Meters, Trojan Noise Nuisance Recorders and the doseBadge Noise Dosimeter. A brief explanation of each parameter is provided along with additional information where appropriate.

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Guide to Common Sound Measurement Terms

Understand the meaning behind the most commonly used noise and sound measurement terms with the Cirrus Research acoustic glossary.

This guide explains key acoustic definitions such as Decibels (dB), SPL (Sound Pressure Level), Leq (Equivalent Continuous Sound Level), Cpeak, Lmin, Lmax, and many more terms used in noise measurement and analysis.

We’ve compiled a clear, easy-to-use reference covering widely used acoustic terms, weightings and statistical measurements, helping you better interpret noise data and measurement results.

Simply click on a letter below to get started.

A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
  • A Weighting: A-weighting is the standard way of weighting the audible frequencies, which is designed to reflect the response of the human ear to noise.
    The ‘A’ frequency weighting network is the most widely used, and is used to represent the response of the human ear to loudness. Measurements made with this frequency weighting will be displayed as dB(A) or dBA.
    For example, as LAeq, LAFmax, LAE etc where the A shows the use of ‘A’ Weighting.
  • ACGIH: American Conference of Governmental Industrial Hygienists – this is a measurement standard.
  • Acoustic Calibrator: An instrument that provides a reference noise source that is used to calibrate and check the performance of a noise measurement instrument.
  • Acoustic Environment: The combination of sound sources, reflections, absorption, and background noise present in a location.
  • Acoustic Measurement: The process of measuring sound pressure levels using instruments such as sound level meters or noise dosimeters.
  • Acoustic Weighting: A frequency weighting applied to sound measurements to reflect how the human ear responds to different frequencies (e.g. A-weighting, C-weighting, Z-weighting).
  • Action Value: A noise exposure level defined in regulations that triggers specific employer actions, such as providing hearing protection or health surveillance.
  • Broadband: Noise measurements using parameters, which include all audible noise, such as dB(A) and dB(C).
  • Background Noise: The ambient sound present in an environment when the sound of interest is not occurring.
  • Bandwidth: The range of frequencies within a measurement band, such as octave or one-third octave bands.
  • Broadband Noise: Noise containing a wide range of frequencies, such as traffic noise or industrial machinery.
  • Class 1: Precision grade meters for laboratory and field use as defined in IEC 61672. This may also be referred to as Type 1, although the IEC 61672 standard uses the term Class rather than Type.
  • Class 2: General grade meters for field use as defined in IEC 61672. This may also be referred to as Type 2, although the IEC 61672 standard uses the term Class rather than Type.
  • C Weighting: ‘C’ weighting gives much more emphasis to low frequency sounds than the ‘A’ weighting response and is essentially flat or linear between 31,5Hz and 8kHz, the two ‑3dB or ‘half power’ points. In addition, Peak Sound Pressure measurements are made using the ‘C’ Frequency Weighting.
    Measurements made with this frequency weighting will be displayed as dB(C) or dBC.
    For example, as LCeq, LCPeak, LCE etc where the C shows the use of ‘C’ Weighting.
  • C-A: This is the LCeq-LAeq value over a measurement period. Commonly used in determining the most appropriate hearing protection using the HML method.
  • Criterion Level or CL: This is the maximum Leq sound level allowed for an 8-hour period and corresponds to the 100% noise dose value.
    Used for calculating % Dose and Estimated % Dose.
    In the UK, this is set to 85dB.
  • Criterion Time or CT: This is the time over which instruments calculate exposure and dose values.
  • Calibrated to: The level to which a noise measurement instrument has been calibrated. This will usually be 93.7dB in the case of the Optimus/Optimus+ sound level meters and 114dB in the
    case of the doseBadge noise dosimeters.
  • Calibration: The process of determining the accuracy of your measurement chain.
  • Calibration Offset: The difference between the expected calibration level set in the instrument and the level measured by the instrument during calibration.
  • CE Mark: A label used to show that an instruments conforms to the specification of a European Directive.
  • Dose or Dose %: This is the level of noise exposure expressed as a percentage of a fixed level for 8 hours.
    For example, if the noise limit is 85 dB and a person is exposed to a constant or equivalent sound pressure level of 85 dB for eight hours, then the result is 100% noise dose.
    In the UK, a 3dB exchange rate, or Q, is used. This means that a noise level of 88dB has twice as much energy as a level of 85dB. Therefore, a constant level of 88dB over eight hours is a 200% dose.
    The 8-hour average level is known as LEP,d (Daily Personal Noise Exposure) or LEX,8h.
  • dB(A): This is the decibel level, A-weighted.
    This is the most commonly used standard frequency weighting, designed to reflect the response of the human ear to noise.
  • dB(C): This is the decibel level, C-weighted.
    A standard frequency weighting commonly used for the measurement of the Peak Sound Pressure level.
  • dB(Z): This is the decibel level, Z-weighted.
    Z-weighting is a flat frequency response between 10 Hz and 20 kHz ±1.5 dB, excluding microphone response. It replaces Flat and Linear.
  • Decibel (dB): The Decibel is a unit used to measure the intensity of a sound or the power level of an electrical signal by comparing it with a given level on a logarithmic scale.
    In the case of noise measurement, the measured sound pressure, p (in Pascals) is compared to a reference value p0 of 2×10-5 Pa using the equation: Screenshot 2026 01 19 122259
  • Data Logging: The storage of measurement information into a sound level meter or noise dosimeter that can be downloaded into software on a PC such as NoiseTools.
  • Dynamic Range: All noise measurement instruments are limited in the range of levels that they can accurately measure by inherent noise at low levels and by overload at high levels.
    The usable region between these two is the dynamic range of the instrument, expressed in dB.
  • DIN 45641: German standard that defines the additional measurements LAFTeq & LIeqT.
  • Equivalent Continuous Sound Level (Leq): Leq is the equivalent continuous sound level, and represents the total sound exposure for the period of interest, or an energy average noise level for the period of interest.
    Leq is often described as the “average” noise level during a measurement, which although not technically correct, is often the easiest way to think of it.
    If the noise is varying quickly, the average energy over a period of time is a useful measurement parameter. It is for this reason that Leq is often called the equivalent continuous level.
    Leq values should be written with a frequency weighting, such as dB(A,) along with the measurement duration.
    For example: LAeq, 5min = 90dB
  • Estimated Dose or Est. % Dose: This is the % dose projected forwards over an 8-hour period.
  • Estimated Exposure: This is the noise exposure projected forwards over an 8-hour period. Also displayed as Est. Exposure.
  • Exchange Rate (Q): This is the increase in noise level that corresponds to a doubling of the noise level.
    LAeq is always based on an Exchange Rate, or Q, of 3.
    In the US, the exchange rate as defined in the OSHA standard is 5 dB. Using the 5 dB exchange rate, the 8-hour average level is known as TWA or the Time Weighted Average. For other exchange rates, the average level for the measurement duration is known as Lavg.
  • Exposure: This it the measured noise exposure expressed in Pa2h (Pascal Squared Hours).
  • Exposure Time: This is the actual time that a person is exposed to noise during a workday – used for the calculation of LEP,d
  • Fast, Slow and Impulse Time Weightings: The time weightings of Fast, Slow and Impulse are defined by the standards to which an acoustic measurement instrument is designed, such as IEC 61672. They determine the rate or speed at which instruments respond to changing noise levels.
    For example, an instrument set to Fast will respond quickly to changes in the noise level, whereas another instrument set to Slow will, by definition, respond more slowly.
    If the noise level is constant, both instruments will display the same level.
    An instrument set to Impulse will respond very quickly to an increase in the noise level, but will take much longer to fall when the noise level decreases.
    Time weightings are only applied to Sound Level, Maximum Sound Level and Minimum Sound Level readings. Also, the Ln percentile levels are calculated from Sound Level, and are therefore also affected by the selected time weighting.
    Measurement parameters that use these time weightings will reflect it in the way the data is presented. For example, LAFmax. This shows that the values presented are the maximum A-weighted Fast time-weighted sound levels.
  • Free Field Microphone: At frequencies above 1 kHz, the wavelength of sound is small enough for a standard half-inch microphone to ‘disturb’ or affect the sound field being measured.
    Free field microphones are designed to compensate for this effect.
  • Hearing Protection: Devices such as earplugs or earmuffs used to reduce noise exposure to the ears.
  • Hearing Protection Zone: An area where hearing protection must be worn due to high noise levels.
  • ISO: International Standards Organisation. An international standards body that issues measurement standards such as ISO 1996 for environmental noise and ISO 20906 for aircraft noise.
  • ISO (EU): This is a noise standard as set out by the EU Physical Agents (Noise Directive).
    This affects the threshold level, exchange rate and time weighting, used in the calculation of these values.
  • Integrating Averaging Sound Level Meter: A sound level meter that accumulates the total sound energy over a measurement period and calculates an equivalent average value, usually displayed as Leq.
  • IEC: International Electrotechnical Commission. This is the international standards body responsible for issuing technical standards for instrumentation, such as the IEC 61672 standard for sound level meters.
  • IEC 60651: A standard for Sound Level Meters, now superseded by IEC 61672. In the UK this was known as BS EN 60651.
  • IEC 60804: A standard for integrating and integrating-averaging sound level meters, now superseded by IEC 61672. In the UK this was known as BS EN 60804 and previously BS 6698.
  • IEC 61652: The international standard for personal sound exposure meters or noise dosimeters. In the UK this is known as BS EN 61252. The standard for noise dosimeters has no Class or Type levels.
  • IEC 61620: The International standard for 1:1 octave & 1:3 octave band filters.
  • IEC 61672: The International standard for sound level meter and integrating averaging sound level meters, which has superseded both IEC 60651 and IEC 608.
  • IEC 651: An international standard for sound level meters, replaced by IEC 60651 and now superseded by IEC 61672.
  • IEC 804: An international standard for integrating averaging sound level meters, replaced by IEC 60651 and now superseded by IEC 61672
  • IEC 60942: The international standard for sound calibrators (acoustic calibrators).
  • IEC 942: An international standard for acoustic calibrators. Replaced by IEC 60942.
  • Key Noise Indicator: A metric used to describe or assess noise impact, such as LAeq or LAFmax.
  • L10: This is the noise level exceeded for 10% of the measurement, calculated by statistical analysis.
  • L90: This is the noise level exceeded for 90% of the measurement, calculated by statistical analysis
  • LA10: This is the noise level exceeded for 10% of the measurement period with ‘A’ frequency weighting, calculated by statistical analysis
  • LA90: This is the noise level exceeded for 90% of the measurement period with ‘A’ frequency weighting, calculated by statistical analysis
  • LAE: This is the Sound Exposure Level (SEL) with ‘A’ frequency weighting. See LE
  • LAeq: See Leq.
  • LAeq,1s: This is an A-weighted 1-second Leq value.
  • LAeq,t: See Leq.
  • LAF: This is the Sound Level with ‘A’ frequency weighting and Fast time weighting.
  • LAF10: This is the noise level exceeded for 10% of the measurement period with ‘A’ frequency weighting, calculated by statistical analysis from samples of the Fast time weighted sound level.
  • LAF90: This is the noise level exceeded for 90% of the measurement period with ‘A’ frequency weighting, calculated by statistical analysis from samples of the Fast time weighted sound level.
  • LAFmax: This is the maximum Sound Level with ‘A’ frequency weighting and Fast time weighting during the measurement period.
  • LAFmin: This is the minimum Sound Level with ‘A’ frequency weighting and Fast time weighting during the measurement period.
  • LAFTeq: Takt maximal sound level as defined by DIN 45641.
  • LAI: This is the Sound Level with ‘A’ frequency weighting and Impulse time weighting.
  • LAImax: This is the maximum Sound Level with ‘A’ frequency weighting and Impulse time weighting.
  • LAImin: This is the minimum Sound Level with ‘A’ frequency weighting and Impulse time weighting.
  • LAS: This is the Sound Level with ‘A’ frequency weighting and Slow time weighting
  • LASmax: This is the maximum Sound Level with ‘A’ frequency weighting and Slow time weighting during the measurement period.
  • LASmin: This is the minimum Sound Level with ‘A’ frequency weighting and Slow time weighting during the measurement period.
  • LAT: See Leq.
  • LAVG: This is the Time Averaged Sound Level with an exchange rate other than 3dB.
  • LCE: This is the Sound Exposure Level (SEL) with ‘C’ frequency weighting.
  • LCeq,1s: This is the C-weighted 1-second Leq value.
  • LCeq,t: This is an Leq value measured with ‘C’ frequency weighting.
  • LCF: This is the Sound level with ‘C’ frequency weighting and Fast rime weighting.
  • LCFmax: This is the maximum Sound level with ‘C’ frequency weighting and Fast time weighting during the measurement period.
  • LCFmin: This is the minimum Sound level with ‘C’ frequency weighting and Fast time weighting during the measurement period.
  • LCI: This is the Sound Level with ‘C’ frequency weighting and Impulse time weighting.
  • LCImax: This is the maximum Sound Level with ‘C’ frequency weighting and Impulse time weighting.
  • LCImin: This is the minimum Sound Level with ‘A’ frequency weighting and Impulse time weighting.
  • LCPeak: This is the Peak Sound pressure level with ‘C’ frequency weighting.
  • LCS: This is the Sound level with ‘C’ frequency weighting and Slow time weighting.
  • LCSmax: This is the maximum Sound Level with ‘C’ frequency weighting and Slow time weighting during the measurement period.
  • LCSmin: This is the minimum Sound Level with ‘C’ frequency weighting and Slow time weighting during the measurement period.
  • LE (SEL): This is an Leq normalised to 1 second.
    It can be used to compare the energy of noise events which have different time durations.
    For example, if a noise level of 90 dB lasts for 1 second, then the LE = 90 dB.
    If the same noise event lasted 10 seconds, the LE would be 100 dBA.
    If it lasted 20 seconds, the LE would be 103 dBA and so on.
    The LE is the Sound Exposure expressed as a logarithm; Leq is the LE divided by time.
    This will usually be displayed as LAE, LCE or LZE
  • LEP,d: This is the daily amount of personal noise exposure. Also see LEX,8h.
    The LEP,d is the average A-weighted noise exposure level for a nominal 8 hour working day. This is also known as the LEX,8h.
    LEP,d is calculated from the measured sound exposure, the measurement time and the reference 8 hour day.
  • LEP,w: This is a measure of the total noise exposure received by an employee during a working week.
    It is similar to the daily noise exposure but is calculated for a 40-hour week (five 8-hour days), instead of an 8-hour day.
  • Leq: Equivalent Continuous Sound Level
    This is the most commonly used value used to describe sound levels that vary over time.
    An Leq is the level that would produce the same sound energy over a stated period of time when using a 3 dB exchange rate.
    It is defined as the sound pressure level of a noise fluctuating over a period of time T, expressed as the amount of average energy.
    Commonly written as Leq, LAeq, LAeq,t or LAT.
  • Leq,t: See Leq.
  • LEX,8h: See LEP,d.
  • LleqT: This is the Impulse-weighted Leq,t as defined by DIN 45641.
  • Lmax: Maximum sound level.
  • Lmin: Minimum sound level.
  • Ln: This is the Statistical analysis of noise levels. The n denotes the percentage exceedance, for example the value of L90 shows the noise level that was exceeded for 90% of the measurement duration.
    The percentile level where ‘n’ is between 0.01 and 99.9% is calculated by Statistical Analysis. Ln values usually include a descriptor that shows the frequency weighting i.e. A-weighting and the time weighting i.e. Fast.
    The most common Ln values are LAF10 and LAF90.
  • Lp: Sound Pressure Level.
  • Lw: Sound Power Level.
  • LZE: This is the Sound Exposure Level (SEL) with ‘Z’ frequency weighting.
  • LZeq,1s: This is a 1-second Leq value with ‘Z’ frequency weighting.
  • LZeq,t: This is an Leq measured with ‘Z’ frequency weighting.
  • LZF: This is the Sound level with ‘Z’ frequency weighting and Fast time weighting.
  • LZFmax: This is the maximum Sound level with ‘Z’ frequency weighting and Fast time weighting during the measurement period.
  • LZFmin: This is the minimum Sound level with ‘Z’ frequency weighting and Fast time weighting during the measurement period.
  • LZI: This is the Sound Level with ‘Z’ frequency weighting and Impulse time weighting.
  • LZImax: This is the maximum Sound Level with ‘Z’ frequency weighting and Impulse time weighting during the measurement period.
  • LZImin: This is the minimum Sound Level with ‘Z’ frequency weighting and Impulse time weighting during the measurement period.
  • LZPeak: This is the Peak Sound Pressure Level with ‘Z’ frequency weighting.
  • LZS: This is the Sound Level with ‘Z’ frequency weighting and Slow time weighting.
  • LZSmax: This is the maximum Sound Level with ‘Z’ frequency weighting and Slow time weighting during the measurement period.
  • LZSmin: This is the minimum Sound Level with ‘Z’ frequency weighting and Slow time weighting during the measurement period.
  • Maximum Sound Level (Lmax): This is the Maximum Sound Level, which is the maximum noise level during a measurement period or a noise event.
  • Minimum Sound Level (Lmin): This is the Minimum Sound Level, which is the minimum noise level during a measurement period or a noise event.
  • MSHA: The Mine Safety and Health Administration. This is a United States organisation that has specific guidelines for the measurement of noise.
  • MSHA EC: The Mine Safety and Health Administration Permissible Exposure Limits.
    Instruments such as the Optimus/Optimus+ sound level meters have settings, which allow the instrument to calculate the Lavg, TWA, Dose and Est. Dose values in accordance with these regulations. This affects the threshold level, exchange rate and time weighting used in the calculation of these values.
  • MSHA HC: The Mine Safety and Health Administration Hearing Conservation values.
    Instruments such as the Optimus/Optimus+ sound level meters have settings, which allow the instrument to calculate the Lavg, TWA, Dose and Est. Dose values in accordance with these regulations. This affects the threshold level, exchange rate and time weighting used in the calculation of these values.
  • Microphone Capsule: The microphone capsule is the part of the noise measurement instrument that converts the acoustic pressure, or noise, into an electrical signal that can be measured and displayed by the instrument.
    This is often the most sensitive and fragile part of a noise measurement instrument as it has to deal with both very small and very large changes in pressure, with great accuracy and precision.
  • NC: Noise Criteria.
    This is a single number for rating the sound quality of a room, used extensively by the air conditioning industry, for example, to test the background levels in offices etc.
    The measured octave bands are compared with the NC curves, which are based on equal loudness curves. The NC rating is the value of the highest NC curve touched by the measured octave bands. The NC Decisive Band is the frequency band touching the NC Curve.
  • NR: Noise Rating.
    This is a method for rating the acceptability of indoor environments for the purposes of hearing preservation, speech communication and the reduction of nuisance noise.
    Sound Pressure Levels measured in octave bands are compared with curves from which a noise rating (NR) is obtained.
    The NR rating is the highest NR Curve touched by the measured octave band spectrum.
    The NR Decisive Band is the frequency band touching the NR Curve.
  • Noise Floor: The lower limit of measurement capability of an instrument, calculated from the addition of all noise sources and unwanted signals within a measurement system.
    Signals beneath the noise floor cannot be measured.
  • Octave Band Leq,1s: This is when 1:1 octave band filters are shown numerically. No frequency weighting is applied.
  • Octave Band Leq,t(Graph): This is when 1:1 octave band filters are shown graphically while the instrument is measuring. The cumulative Leq in each band is shown. No frequency weighting is applied.
  • Octave Band Leq,t (Numbers): This is when 1:1 octave band filters are shown numerically with the cumulative Leq in each band shown. No frequency weighting is applied.
  • OSHA: The Occupational Safety and Health Association. This is a United States organisation that has specific guidelines for the measurement of noise.
  • OSHA HC: The Occupational Safety and Health Association Hearing Conservation values.
    Instruments such as the Optimus/Optimus+ sound level meters have settings, which allow the instrument to calculate the Lavg, TWA, Dose and Est. Dose values in accordance with these regulations. This affects the threshold level, exchange rate and time weighting used in the calculation of these values.
  • OHSA PEL: The Occupational Safety and Health Association Permission Exposure Limits.
    Instruments such as the Optimus/Optimus+ sound level meters have settings, which allow the instrument to calculate the Lavg, TWA, Dose and Est. Dose values in accordance with these regulations. This affects the threshold level, exchange rate and time weighting used in the calculation of these values.
  • Octave Band Filters 1:1 &1:3 : When detailed information about a complex sound is needed, the frequency range can be split into sections, or frequency bands.
    A sound level meter may provide 1:1 (single) octave band filters or 1:3 (third) octave band filters.
    An octave is a frequency band where the highest frequency is twice the lowest one.
    For example, an octave filter with a centre frequency of 1kHz has a lower frequency of 707Hz and an upper frequency of 1.414kHz.
    A third octave band is 1/3 the width of a first octave band.
    An instrument with 1:1 octave band filters, such as the Optimus+ Red (CR:162C), would typically provide 10 bands, from 31.5Hz to 16kHz.
    An instrument with 1:3 octave band filters, such as the Optimus+ Green (CR:171B), would typically provide 33 bands, from 12.5Hz to 20kHz, although some additional bands such as 6.3Hz, 8Hz and 10Hz may also be available.
    Analysing octave band filter data can help in choosing the most appropriate and effective hearing protection. Hearing protection isn’t just about safeguarding people against loud noise, but the frequency of the sound must also be taken into account.
    Octave band data help avoid either under-protection, or over-protection.
  • Overload: The input to the sound level meter is too high for the current measurement range.
  • Peak Sound Pressure: This function is often confused with the Maximum Sound Level, or LMax. Whereas the Maximum Sound Level is the highest level recorded, the Peak level is the physical peak level of the sound wave.
    The reason for this is that the Maximum Sound Level is the RMS (root mean squared) level with a time weighting (F,S or I) applied. The Peak is the highest point of the pressure wave before any time weighting is applied.
    The measurement of Peak sound pressure levels is required by the UK Control of Noise at Work Regulations 2005, where it is C-weighted. In this case, the value would be written as LCPeak = 134dB.
  • Pa2h (Pa2h): This is Noise Exposure in Pascal Squared Hours
  • Peak: This is the maximum value reached by the sound pressure at any instant during a
    measurement period (in dB, usually with C frequency weighting).
  • Percentile Levels (Ln): This is the Percentage exceeded levels, where ‘n’ is between 0.1 and 99.9% calculated by statistical analysis.
    The most commonly used Ln values are L10 and L90 levels.
  • Projected Exposure: This is the measured LAeq projected over a range of durations to give the equivalent daily exposure values.
  • Pa: Pascal. This is the SI derived unit of pressure.
  • Preamplifier: The preamplifier is an electronic circuit, which takes the electrical signal from the microphone capsule and converts it into a signal that can be used in the sound level meter.
  • Q: See Exchange Rate (Q).
  • Quiet Period: A time during which noise measurements are taken without significant interfering sounds.
  • Real-Time Monitoring: Continuous noise measurement with immediate data access and alerts.
  • Reflection: The bouncing of sound waves off surfaces, affecting measured sound levels.
  • Sound Exposure Level (SEL): See LE.
  • Sound Level Meter: An instrument for measuring various noise parameters.
  • Sound Power Level: This is a logarithmic measurement of the sound power as a relation to the threshold of hearing, and makes the values more manageable i.e. 0 to 160 dB and the symbol is Lw.
  • Sound Pressure Level: SPL, or sound level Lp, is a logarithmic measurement of the RMS sound pressure of a sound, relative to a reference value. It is measured in decibels (dB).
  • Statistical Analysis: A calculation performed by a sound level meter on the noise levels measured during a measurement period, to describe the statistical spread of the noise.
    The resulting statistical levels, of Ln values, are displayed in dB.
  • Third Octave Band,1s: This is when 1:3 octave band filters are shown numerically. No frequency weighting is applied.
  • Third Octave Band Leq,t (Graph): This is when 1:3 octave band filters are shown graphically when the instrument is measuring. The cumulative Leq in each band is shown. No frequency weighting is applied.
  • Third Octave Band Leq,t (Numbers): This is when 1:3 octave band filters are shown numerically with the cumulative Leq in each band shown. No frequency weighting is applied.
  • Threshold Level: A number of occupational noise regulations specify that for the measurement of noise at work, sound levels below a certain limit (the threshold) should be disregarded.
    These include the OSHA and MSHA regulations commonly used in the USA.
  • TWA (Time-Weighted Average): Using a 5 dB exchange rate, this is the total amount of workplace noise exposure expressed as an equivalent standard 8-hour working day. Used by the OSHA specification.
  • Third Octave Band: A frequency band whose cut-off frequencies have a ratio of approximately 1.26.
    The cut-off frequencies of 891 Hz and 1112 Hz define the 1000 Hz third-octave band in common use.
    In modern sound level meters, 1:3 or 1/3 octave band filters are usually available from around 12.5Hz to 20kHz although some instruments may also provide lower bands.
  • Time Constant: A standardised time constant used in exponential time weighting for acoustical analysis. The standard time constants for sound level meters are Slow (1000ms), Fast (125ms) and Impulse (35 ms while the signal level is increasing or 1,500 ms while the signal level is decreasing).
  • Time History Rate: The speed or rate at which noise levels are sampled and stored in the instrument.
    These samples, or time history, can be downloaded to software, such as NoiseTools, and displayed on a graph.
  • Tonal Noise Detection: A feature available on certain sound level meters, including the Cirrus Optimus/Optimus+, that allows tonal noise according to ISO 1996-2:2007 Simplified method
    (Annex D) or the Cirrus Improved Method.
  • Type 1: Laboratory and field-grade for sound level meters defined in standards such as IEC 60651 and IEC 60804. These standards have been superseded by IEC 61672, which uses Class 1 rather than Type 1.
  • Type 2: General field-grade for sound level meters defined in standards such as IEC 60651 and IEC 60804. These standards have been superseded by IEC 61672, which uses Class 2 rather than Type 2.
  • Under Range: The input to the sound level meter is too low for the current measurement range.
  • Z Weighting: This has replaced Linear or Flat, and is defined as being a flat frequency response of 8Hz to 20kHz ±1.5dB. Measurements made with this frequency weighting will be displayed as dB(Z) or dBZ.
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