Noise and Vibration
Glossary of Terms, Units and Definitions
The world of acoustics is overflowing with parameters. Take a simple example - Lmax is the maximum sound level but was the measurement A-weighted, C-weighted, Z-weighted and was a Fast, Slow of Impulsive Time Constant used. Already we have introduced up to 9 variables for one simple measurement, without mentioning the measurement duration.
To describe A-weighted, Fast Time Constant, Maximum measurements, correctly, we should use the form LAFmax and not Lmax or Lmax.
For simplicity some manufacturers use XY to indicate a range of possible descriptors where X = dB(A)), dB(C) or dB(Z) Frequency Weighting and Y = Fast(F), Slow(S) or Impulse(I) Time Weighting. Using this convention LXYmax describes the range of possibilities for a maximum measurement.
The table below is for customers who come across a term they do not. If the term is not strictly correct we do not go into great detail explaining where it differs from the strict convention, we just say what the term means. So this table is not for the professional acousticians who will already know and use the 'correct' terminology.
Leq and Ln are commonly used but there is some confusion about these parameters so we have expanded our descriptions accordingly.
We welcome feedback, suggestions and corrections,
| Parameter | Description |
| % Dose | noise exposure expressed as a percentage (%) of a fixed level for 8 hours |
| 1/1 Octave Band Filters | a division of the frequency range into bands, the upper frequency limit of each band being twice the lower frequency limit. |
| 1/3 Octave Band Filters | octave bands sub-divided into three parts. |
| ANSI | American National Standards Institution |
| 'A' weighting | the most commonly used frequency weighting, designed to reflect the response of the human ear to noise - also written as A-weighting or dB(A) or dBA. |
| Acoustic Calibrator | an instrument providing a reference noise source used to calibrate and check the performance of sound level meters. |
| Broadband Noise | measurements using parameters which include all the audible noise, such as dB(A) and dB(C) |
| CCLD | Constant Current Line Drive for IPC type accelerometers |
| CE Marking | a label to show that the Meter conforms to the specification of a European Directive |
| CNEL | Community Noise Equivalent Level - the 24 hour average noise level of all hourly Leq measurements with a 10 dB penalty added to the levels between 2200 and 0700 hours and a 5 dB penalty added to the levels between 1900 and 2200 hours to reflect people's extra sensitivity to noise during the night and the evening |
| Community Noise Equivalent Level | the 24 hour average noise level of all hourly Leq measurements with a 10 dB penalty added to the levels between 2200 and 0700 hours and a 5 dB penalty added to the levels between 1900 and 2200 hours to reflect people's extra sensitivity to noise during the night and the evening |
| Criterion Level | the Leq sound level measured over exactly 8 hours that corresponds to the 100% noise dose value |
| 'C' Weighting | a standard weighting of the audible frequencies. Commonly used for the measurement of Peak Sound Pressure level - also written as C-weighting or dB(C) or dBC. |
| Daily Personal Noise Exposure | LEP,d or Lep,d is an A-weighted measurement and the default time is 8-hours. If the Leq is measured over 8 hours then Leq and Lep,d would be the same. Lep,d is also known as LEX,8h or LEX,d in some countries. |
| Data Logging | measurements can be stored in the sound level meter for downloading to a PC |
| dB | the decibel - units of sound level and noise exposure measurement, derived from the log of the ratio between a value and the reference value of 20 μPa - 20 micro-pascals. Click here for more information on decibels |
| dB(A) | most commonly used standard weighting of the audible frequencies designed to reflect the response of the human ear to noise - also written as A-weighting or dB(A) or dBA. |
| dB(B) | B weighted decibels, a weighting curve between A and C. Not used very much these days, was popular in the motor industry. |
| dB(C) | a standard weighting of the audible frequencies - also written as C-weighting or dB(C) or dBC. Commonly used for the measurement of Peak Sound Pressure level |
| dB(Z) | Z weighting is a flat frequency response between 10 Hz and 20 kHz ±1.5 dB excluding microphone response. Replaces the old Flat and Linear curves. Alternatives Z-weighting, dB(Z), dBZ |
| DIRAC | Dual Input Room Acoustics Calculator - Software to measure impulse responses and calculates various room acoustical parameters, according to the ISO 3382 and IEC 60268-16 standards. Dirac supports a wide variety of measurement configurations. |
| Dose | the noise exposure expressed as a percentage (%) of a fixed level, usually A-weighted and over 8 hours |
| Decibel | a unit derived from the log of the ration between a value and a reference value. For sound pressure the reference quantity is 20 μPa, the threshold of hearing is around 0 dB and 120 dB+ is the threshold of pain. Click here for more information on decibels |
| Dosimeter | a pocket sized instrument designed to be worn during all or part of the day as a personal monitor in order to check an operator's personal exposure to workplace noise |
| EA | total sound exposure in Pa2h : Pascals squared hours, A-weighted |
| Equivalent Continuous Sound Level | Leq - a measure of the average sound pressure level during a period of time T, in dB. A measure of energy level of a time-varying noise - more detail - alternatives LAeq.T, Leq dB(A). |
| Exchange Rate | Q - the number of decibels that noise in the workplace has to increase by to be considered as being twice the harmful risk to hearing, usually has the value of 3 for equal energy measurements for European regulations (or measurements following the ACGIH recommendations) and a value of 5 for noise measurements to the US OSHA or MSHA noise regulations |
| Fast Time Weighting | a standard time weighting applied by the sound level meter - 125 ms averaging time - also known as Fast Response and Fast Time Constant |
| FFT | Fast Fourier Transform is a digital signal processing technique that converts a time record into a narrow band constant bandwidth filtered spectrum. Measurements are defined by specifying the frequency span and a number of lines (or filters). |
| Frequency Weighting | standard frequency weightings applied by the sound level meter. For details see under the relevant type - A, B, C and Z-weighting. |
| Hz | Hertz - the unit of frequency, or pitch, of a sound. 1 Hz is one cycle per second. |
| IEPE | Integrated Electronic Piezo Electric - PE accelerometers with built-in electronics to transmit the output signal over 'long' cables. |
| Impulse Time Weighting | a standard time weighting applied by a sound level meter. In this mode the meter responds very quickly to an increase in noise level but responds slowly when the impulse stops - a hang over from the days you had to watch the meter needle and the human eye was not fast enough to 'catch' the maximum level - also known as Impulse Response and Impulsive Time Constant |
| Impulse Weighted Leq | Lleq,t - used in Germany as defined by DIN 45641 - alternative LAIeq.t |
| Integrating Sound Level Meter | a sound level meter that accumulates the total sound energy over a measurement period and calculates the 'energy' or logarithmic average |
| L | Instantaneous Sound Level |
| L10(1 hour) | noise level exceeded for 10% of the one-hour period, calculated by statistical analysis. LA10 generally used for road noise at peak traffic flow. |
| L90(1 hour) | noise level exceeded for 90% of the one-hour period, calculated by statistical analysis. LA90 often used as a measure of the background noise levels |
| L95 | noise level exceeded for 95% of the measurement period, calculated by statistical analysis. Sometimes the preferred measure of background noise levels. |
| LA10 | noise level exceeded for 10% of the measurement period, A-weighted, calculated by statistical analysis - the descriptor of choice for traffic noise. |
| LA90 | noise level exceeded for 90% of the measurement period, A-weighted, calculated by statistical analysis - used for background noise surveys. |
| LAE | Sound Exposure Level (SEL) - A-weighted |
| LAeq,T | equivalent continuous sound pressure level. A measure of the average sound pressure level during a period of time T, in dB and A-weighted. |
| LAF | sound level with 'A' Frequency weighting and Fast Time weighting |
| LAFmax | the maximum sound level, A-weighted, Fast Time Constant |
| LAFTeq | Takt maximal sound level as defined by DIN 45641 |
| LAI | sound level with A-weighting and Impulse Time weighting |
| LAImax | the maximum sound level with A-weighting and Impulse Time weighting |
| LAn | noise level exceeded for n% of the measurement period with A-weighting, calculated by statistical analysis - where n is between 0.1 and 99.9%. |
| LAS | Sound Level - A-weighted, Slow Time constant. |
| LASmax | the maximum sound level with A-weighting and Slow Time weighting |
| Lavg3 | Lavg - averaged sound level with Exchange Rate of 3 dB - Lavg(4) and Lavg(5) used in other parts of the world. |
| LCE | Sound Exposure Level (SEL) with C-weighting |
| LCeq,T | equivalent continuous sound level, over a period of time T and C-weighted. |
| LCF | sound level with C-weighting and Fast Time constant |
| LCFmax | the maximum sound level with C-weighting and Fast Time weighting |
| LCI | sound level, C-weighted, Impulse time constant |
| LCImax | the maximum sound level with C-weighting and Impulse Time weighting |
| LCpeak | Peak sound pressure level with C-weighting |
| LCS | sound level, C-weighted and Slow Time weighting |
| LCSmax | the maximum sound level - C-weighting and Slow Time weighting |
| Lday | Daytime noise levels - excludes over-night levels |
| Lden | Day, evening and night levels - a single figure to cover 24 hours |
| Ldn | the 24 hour average noise level of all hourly Leq measurements with a 10 dB penalty added to the levels between 2200 and 0700 hours to reflect people's extra sensitivity to noise during the night |
| LEP,d | Daily Personal Noise Exposure, A-weighted and the default time is 8-hours. If the Leq is measured over 8 hours then Leq and Lep,d would be the same. Lep,d is also known as LEX,8h or LEX,d in some countries. |
| Leq | Equivalent continuous sound pressure level. A measure of the average sound pressure level during a period of time T, in dB. A measure of energy level of a time-varying noise - more detail - alternatives LAeq.T, Leq dB(A). |
| Leq(1 hour) | equivalent continuous sound level measured over 1-hour |
| LFNR | Low Frequency Noise Rating for example - LFNR 35 |
| LIeq,t | impulse weighted Leq,T as defined by DIN 45641 - alternative LAIeq,t |
| Lmax | the maximum noise level during a designated time interval or a noise event - should also include other descriptors i.e. A, C, L or Z weightings and F, S or I time constants. Sometimes written as Max dB(A) |
| Lmin | the minimum noise level during a designated time interval or a noise event - should also include other descriptors i.e. A, C, L or Z weightings and F, S or I time constants. Sometimes shown as Min dB(A) |
| Ln | percentile level where 'n' is between 0.1 and 99.9% calculated by statistical analysis. - may also include other descriptors i.e. A, C, L or Z weightings . Most common Ln values are A-weighted L10 and L90 levels |
| Lpeak | peak sound level - may also include other descriptors i.e. A, C, L or Z weightings |
| Lpkmax | maximum peak level during the measurement - may also include other descriptors i.e. A, C, L or Z weightings |
| LZE | Sound Exposure Level (SEL) with 'Z' frequency weighting |
| LZeq,t | equivalent continuous sound pressure level. A measure of the average sound pressure level during a period of time, t, in dB with Z-weighting. |
| LZF | sound level with Z-weighting and Fast Time weighting |
| LZFmax | the maximum Sound level with 'Z' Frequency weighting and Fast Time weighting |
| LZI | sound level - Z-weighted and Impulse Time constant |
| LZImax | the maximum Sound level with 'Z' Frequency weighting and Impulse Time weighting |
| LZS | Sound level with 'Z' Frequency weighting and Slow Time weighting |
| LZSmax | the maximum Sound level, Z-weighting and Slow Time weighting |
| MLS | Maximum Length Sequence. It is basically a pseudo-random sequence of pulses. MLS measurements are quite standard in many different application fields, one of them acoustics. For more details click here. |
| NEC | Noise Exposure Category - used by local planning authorities to determine the suitability of a proposal for residential development. The 4 NECs described in PPG24 take account of both day and night time noise levels from road, rail and air transport. The NEC levels should not be used for assessing the impact of industrial noise on proposed residential development although at a mixed noise site where industrial noise is present but not dominant, its contribution should be included in establishing the appropriate NEC. |
| NEF | Noise Exposure Forecast - a complex composite measure of exposure to aircraft noise taking into account the peak noise level, the duration of flyover, the total characteristics and the number of aircraft movements in both the daytime and night-time period. |
| Overload | The input to the Sound Level Meter is too high for the current measurement range. Change the range |
| Overload % | Percentage of time an overload occurred |
| Overrange | The input to the Sound Level Meter is too high for the current measurement range. Change the range |
| Overrange % | Percentage of time an over range occurred |
| Pa2h | Sound Exposure |
| PE | Piezo-electric - some materials (notably crystals and certain ceramics) generate an electric potential in response to applied mechanical stress. Accelerometers utilize this feature. |
| Peak | the maximum value reached by the sound pressure at any instant during a measurement period (in dB usually with C frequency weighting) |
| PPV | Peak Particle Velocity - the maximum or peak velocity value during a given measurement period. If measurements are made in 3-axis then the resultant PPV or PPS is the vector sum = the square root of the summed squares of the maximum velocities, regardless of when in the time history those occur. |
| Percentile Levels | Ln - percentage exceeded levels where 'n' is between 0.1 and 99.9% calculated by statistical analysis. - may also include other descriptors i.e. A, C, L or Z weightings . Most common Ln values are A-weighted L10 and L90 levels |
| Q | Exchange Rate - the number of decibels that noise in the workplace has to increase by to be considered as being twice the harmful risk to hearing, usually has the value of 3 for equal energy measurements for European regulations (or measurements following the ACGIH recommendations) and a value of 5 for noise measurements to the US OSHA or MSHA noise regulations |
| RASTI | RApid Speech Transmission Index - A transmitter [loudspeaker] broadcasts a modulated noise test signal from the talker's location. A receiver gives a direct read out of the RASTI value at the listener's position. The receiver is moved around to survey the whole listening area to identify any difficult locations. |
| RT | Reverberation Time - time required for a sound in a room to decay by 60 dB (called RT60). |
| RW | the weighted sound reduction index which characterises the airborne sound insulating properties of a material or a building element over a range of frequencies. |
| SEL | Sound Exposure Level - an Leq measurement normalized to 1 second i.e. Leq + 10 Log T where T is in seconds. Used to compare the energy of noise events that have different durations - displayed as LAE, LCE or LZE |
| SIL | Speech Interference Level - a calculated quantity providing a guide to the interference of a noise with the reception of speech. The speech-interference level is the arithmetic average of the octave band levels |
| Slow Time Weighting | a standard time weighting applied by the sound level meter - also known as Slow Response and Slow Time Constant |
| Sound Exposure Level | SEL is an Leq measurement normalized to 1 second i.e. Leq + 10 Log T where T is in seconds. Used to compare the energy of noise events that have different durations - displayed as LAE, LCE or LZE |
| Sound Level | Sound Pressure Level, frequency weighted, such as dB(A) |
| Sound Power | W - the total sound energy radiated by a source per unit time. The unit of measurement is the watt, SWL, Lw. |
| Sound Pressure | the instantaneous difference between the actual pressure produced by a sound wave and the average or barometric pressure at a given point in space. |
| SPL | Sound Pressure Level - the basic measure of noise loudness, expressed in decibels = 20 times the logarithm, to the base 10, of the ratio of the pressure of the sound measured to the reference pressure, which is 20 micro newtons per square meter. In equation form, sound pressure level in units of decibels is expressed as SPL (dB) = 20 log p/po. |
| Spectrum | the description of a sound wave's resolution into its components of frequency and amplitude. |
| Statistical Analysis | a calculation performed by a Sound Level Meter on the noise levels measured during the measurement period to describe the statistical spread of the noise - more details. |
| STC | Sound Transmission Class - the preferred single figure rating system designed to give an estimate of the sound insulation properties of a structure or a rank ordering of a series of structures. |
| STL | Sound Transmission Loss - a measure of sound insulation provided by a structural configuration. Expressed in decibels, it is 10 times the logarithm to the base 10 of the reciprocal of the sound transmission coefficient of the configuration. |
| SWL | Sound Power Level. |
| Threshold of Hearing | the threshold of hearing is 0 dB at 1 kHz, extrapolated from tests on young people. It varies with frequency as we are less sensitive at lower and higher frequencies, hence the use of A-weighting etc., to relate meter measurements to subjective assessments. |
| Threshold of Pain | the threshold of pain varies, in the literature from 120 dB to 140 dB. |
| Time Weighting | standard time weightings applied by the sound level meter - also known as Time Constant. For details see under the relevant type - Fast, Slow, Impulse and Peak. |
| TL | Transmission Loss - the difference between the noise levels across a partition between two or more rooms |
| Type 1 | Laboratory & Field Grade for Sound Level Meters - class 1 |
| Type 2 | General Field Grade for Sound Level Meter - class 2 |
| Underrange | the input to the Sound Level Meter is too low for the current measurement range. Change the range |
| Underrange % | percentage of time an under range occurred |
| VDV | Vibration Dose Value - cumulative measurement of the vibration level received over an 8-hour or 16-hour period |
| Z Weighting | A flat frequency response between 10 Hz and 20k Hz ±1.5 dB excluding microphone response. Replaces the old Flat and Linear curves. alternatives Z-weighting, dB(Z), dBZ |
1 pascal = 1 newton per square metre = 10 dynes per square cm
More parameters to be added - when time allows
| Gracey & Associates 01933 624212 |
| BSI approved for the hire of noise & vibration instruments |
Acoustic Parameters - some more details
Leq - equivalent continuous sound levelNoise levels often fluctuate over a wide range with time. For example in the middle of the night the level might go down as low as 30dB(A) with occasional passing vehicles of 70dB(A) or more. Later comes the dawn chorus followed by the milkman clattering around for several minutes. Then the general noises of the day occur, before relative peace returns in the late evening.
One of the general noise sources may be a factory with different noise emissions throughout the day or week, with deliveries, intermittent compressors, and lots of varying noisy processes on top of the routine production noise levels.
How do you measure these noise levels and come up with an overall value?
This is where the Leq or equivalent continuous sound level meter comes in. This meter faithfully follow all the fluctuations, stores them in it's short-term memory and at the end of the measurement calculates an 'average' value called the Leq value. When we say average, this is not a simple arithmetic average because we are measuring in decibels which logarithmic values. So our meter converts the dB values to 'real numbers', adds them all up then divides by the number of samples and finally converts this true energy average back to dBs. A good Leq sound level meter samples and 'captures' the noise levels 16 times a second which means over an hour it makes 16 x 60 x 60 = 57600 calculations, not difficult for a modern meter but quite an achievement a few years back.