Acoustic Glossary


L : Sound and Vibration • definitions • terms • units • measurements ...

Laacceleration level

LAA-weighted, sound level

LA10 is the noise level just exceeded for 10% of the measurement period, A-weighted and calculated by statistical analysis

LA10,18h (UK Government Environmental Noise Definition) is the arithmetic mean noise level in dB(A) exceeded for 10% of each hour over the period 06:00 - 24:00 hours;

LA90 is the noise level exceeded for 90% of the measurement period, A-weighted and calculated by statistical analysis

LAn is the noise level exceeded for n% of the measurement period with A-weighted, calculated by statistical analysis - where n is between 0.01% and 99.99%

The LA10 and LA90 are extensively used for rating traffic noise and background noise respectively.

The measurement period should also be stated. For example, LA10 (18-hour) is considered good practice when reporting road traffic noise measurements.

We also have a full page on statistical noise levels

LAESEL (sound exposure level) with "A" frequency weighting

LAeqA-weighted, equivalent sound level

LAeq,T is the notional A-weighted, equivalent continuous sound level which, if it occurred over the same time period, would give the same noise level as the actual varying sound level. The T denotes the time period over which the average is taken, for example LAeq,8h is the equivalent continuous noise level over an 8 hour period

LAeq,6h (UK Government Environmental Noise Definition) is the equivalent continuous sound level in dB(A) that, over the period 24:00 – 06:00 hours, contains the same sound energy as the actual fluctuating sound that occurred in that period

LAeq,16h (UK Government Environmental Noise Definition) is the equivalent continuous sound level in dB(A) that, over the period 07:00 – 23:00 hours, contains the same sound energy as the actual fluctuating sound that occurred in that period

LAeq,18h (UK Government Environmental Noise Definition) is the equivalent continuous sound level in dB(A) that, over the period 06:00 – 24:00 hours, contains the same sound energy as the actual fluctuating sound that occurred in that period

See also • LdayLdenLeveningLnight

LAFA-weighted, fast response, sound level

LAFmaxA-weighted, fast response, maximum, sound level, note: maximum is not peak

LAFminA-weighted, fast response, minimum, sound level

LAIeqA-weighted, impulse, leq, sound level

LAmaxA-weighted, maximum, sound level, note- maximum is not peak

LArT (sound rating level), is the a-weighted, leq sound level of an industrial noise during a specified time period, adjusted for tonal character and impulsiveness

LASA-weighted, slow response, sound level

LASmaxA-weighted, slow response, maximum, sound level, note: maximum is not peak

LASminA-weighted, slow response, minimum, sound level

LATtime-average sound level also known as the Leq (equivalent sound level)

Lavgaverage sound level, results may vary due to National exchange rates and threshold levels

LavgLeq (equivalent sound level) when the exchange rate is 3 and no threshold is set

LavgTWA (time weighted average) measured over 8 hours. In Europe and the UK a 3dB exchange rate is always used

LCC-weighted, sound level

LCEC-weighted, sound exposure level

LCeqC-weighted, Leq (equivalent sound level)

LCFC-weighted, fast response, sound level

LCFmaxC-weighted, fast response, maximum, sound level, note: maximum is not peak

LCpeakC-weighted, peak, sound level

LCSC-weighted, slow response, sound level

LCSmaxC-weighted, slow response, maximum, sound level

LCSminC-weighted, slow response, minimum, sound level

Lday (day level), is the A-weighted, Leq (equivalent sound level), over the 12-hour day period 0700 - 1900 hours and is also known as the day noise indicator and other terms.

See also • Lden (day-evening-night) • LeveningLnight

Lden (day evening night level), is the logarithmic composite of the Lday, Levening, and Lnight levels but with 5 dB(A) being added to the Levening value and 10 dB(A) added to the Lnight value.

See also • community noise equivalent level

LEsound energy density level

Leakage, in an FFT analyser, the input signal is recorded in time blocks, called time records, and individual spectra are computed from each block of data. Because the input signal period is not synchronised with the duration of the time block, the signal will be truncated at the beginning and end of the block. This truncation causes an error in the calculation, which effectively spreads out, or 'smears', the spectrum in the frequency domain

This phenomenon is called leakage or spectral leakage it reduces the accuracy of the measured levels of peaks in the spectrum, and reduces the effective frequency resolution of the analysis.

Leakage is worse for continuous signals and rectangular window, and it is greatly reduced by use of the hanning window, a form of apodization, which forces the signal level to zero at the ends of the data block.

Length, one of the seven SI base quantities. The SI unit of length (l) is the metre, symbol (m)

LEP,ddaily personal noise exposure

LEPNeffective perceived noise level

LEP,wweekly personal noise exposure

Leqequivalent sound level

Level Definition IEC 801-22-01, logarithm of the ratio of a given quantity to a reference quantity of the same kind. The base of the logarithm, the reference quantity, and the kind of level must be indicated.

Note 1 : the the kind of level is indicated by use of a compound term such as sound power level or sound pressure level.
Note 2 : the the value of the reference quantity remains unchanged, whether the chosen quantity is Peak, RMS, or otherwise.
Note 3 : the base of the logarithm is indicated by use of a unit of level associated with that base.

Level Difference under sound insulation, level difference

Levening (evening level), is the A-weighted, Leq. sound level, over the 4 hour evening period 1900 - 2300 hours, also known as evening noise indicator

See also • LdayLdenLnight

LEX,8hdaily exposure level

LFforce level

LFNRlow frequency noise rating

Lg or Log under logarithm

Liimpact sound pressure level
Lisound intensity level

LIeq, is the impulse weighted average sound level, used in Germany, defined by DIN 45641 : 3 dB exchange rate.

Linear, a device or circuit with a linear characteristic means that a signal passing through it is not distorted.

Linear Averaging, is the process of adding together a sequence of spectra measurements and then dividing the total by the number of samples. The result is a true arithmetic average on a sample by sample basis. Averaging smooths out random noise components in a spectrum.

See also other types of averaging

Linear Exponent of Sound Propagation Definition IEC 801-23-33, with respect to a uniform system, natural logarithm of the complex ratio of particle velocities (or pressures) measured at two successive points separated by unit distance, when this system is assumed to be of infinite length, also known as sound propagation coefficient

Linear Momentum (p) = mass x velocity

Linear System Definition IEC 351-42-11, system the behaviour of which obeys the principle of superposition

Note 1 : the principle of superposition implies that such a system may be described by a set of linear equations.
Note 2 : a system, which does not have this property, is called nonlinear system.

Linear Weighting

Line Drive, an input socket that can also provide power to drive a transducer.

Line Source, a sound source composed of many point sources in a defined line, such as a train, flow of traffic on a motorway, or constant aircraft take-offs and landings. Sound levels measured from line sources decrease at a rate of 3 dB per doubling of distance.

See also • inverse square law

Line Spacing, the frequency difference between two adjacent 'lines' in a line spectrum

Line Spectrum Definition IEC 801-21-16, sound spectrum containing only discrete frequency components.

See also • constant bandwidthconstant percentage bandwidthscontinuous spectrumfast fourier transformnarrowband noisenarrowband spectraoctave bands. pink noisewhite noisewideband noise

LI,Rpressure-residual sound intensity index

Live Room IEC 801-31-14, room characterized by a relatively small amount of sound absorption.

LKsound intensity pressure index

Lmax is the maximum sound level, during a measurement period or a noise event, Often includes other descriptors, for example LAFmax, and sometimes written as Max dB(A).

Lmax should not be confused with Peak.

Lmin is the minimum sound level, during a measurement period or a noise event. Often includes other descriptors, for example LAFmin, and sometimes written as Min dB(A).

Ln is the percentile noise level where 'n' is between 0.01 and 99.9% calculated by statistical analysis and usually include a descriptor i.e. A-weighting. Most common Ln values are LA10 and LA90 levels.

Ln see also • logarithm

Ln see also • normalized impact sound pressure level

Lnight (night level), the A-weighted, Leq., sound level, over the 8 hour night period of 2300 - 0700 hours and is also known as the night noise indicator and other terms.

See also • LdayLden (day -evening-night) • Levening

Lnpnoise pollution level

LnTstandardized impact sound pressure level
L'nTstandardized impact sound pressure level
LnT,wweighted standardized impact sound pressure level
L'nT,wweighted standardized impact sound pressure level

Localisation, the listener's ability to respond to time and level differences between both ears as well as spectrum information, correlation and pattern matching

See also • binaural and our HATS - Head and Torso Sumulator

Logarithm, Log, log, lg, Ln, common logarithms are widely used in acoustics, the logarithm of a physical quantity is used instead of the quantity itself

Presentation of data on a logarithmic scale can be helpful when the data covers a very large range of values - the logarithm reduces this to a more manageable range. For example 120 dB is 'equivalent' to 1,000,000 relative to a reference sound level of 0 dB - see our decibel calculation and examples page.

The common logarithm is the logarithm to the base 10 and is often written as log10(x) or Log(x) but this can be ambiguous or confusing as Log on a calculator often refers to natural logarithms, favoured by mathematicians, with a base of e (~2.718).

The binary logarithms to the base 2 is used in computer science.

To overcome this possible confusion, ISO, the International Standards Organisation, recommend:-
log10(x) should be written lg (x) and
loge(x)   should be written ln (x).

See also the IEC Definition of the Decibel and our sound level calculations and examples page

Logarithmic Amplitude Scale, critical vibration components usually occur at low amplitudes compared to the rotational frequency vibration. These components are not revealed on a linear amplitude scale because low amplitudes are compressed at the bottom of the scale. But a logarithmic scale shows prominent vibration components equally well at any amplitude. Moreover, percent change in amplitude may be read directly as dB change. Therefore, noise and vibration frequency analyses are usually plotted on a logarithmic amplitude scale.

Logarithmic Decrement Definition IEC 801-24-23, natural logarithm of the ratio of any two successive maxima of like sign, in the decay of a single-frequency oscillation

Logarithmic Frequency Interval Definition IEC 801-30-08, logarithm of the ratio of two frequencies.

See also • frequency interval

Logging, the process of recording noise data results at regular intervals of time so that a 'picture' of the variations can be studied at the end of a long measurement. Usually results are logged at 1 second or 1 minute intervals but it can be as much as 1 hour in some cases.

Note : modern precision instruments will be sampling at 16 times a second when calculating the logged results to ensure all the sound levels are included.

Longitudinal Wave

Loudness is subjective, related to the sound pressure level yes, but also subject to the individual listener's hearing, that changes with age and their subjective response to the character of the noise under consideration. The sound level meter is basically a microphone which converts a sound waves into voltages which the meter measures. It is possible to include filter networks in the meter to 'correct' the measurements for an 'agreed' subjective response. However in the early days this complex technology was not available, so the A, B, C and Lin frequency weighting networks were adopted and standardized.

Loudness Definition IEC 801-29-03, that attribute of auditory sensation in terms of which sounds may be ordered on a scale extending from soft to loud.

Note : loudness depends primarily upon the sound pressure of the stimulus, but also upon its frequency, waveform and duration.

Loudness Level Definition IEC 801-29-05, of a sound, in phons, numerically equal to the median sound pressure level in decibels, re 20 μPa of a free progressive wave having a frequency of 1,000 Hz presented to listeners having normal hearing facing the source that in a specified number of trials is judged equally as loud as the unknown sound.

See also • calculated loudness levelequal loudness contoursfletcher munson curvesMethods for calculating loudnessminimum audible field

For modern loudness measurements, see the B&K 2250 sound analyser

Low Frequency Noise Rating (LFNR), we have no experience in this specific rating and just include it for completeness. We are aware of the 'proposed criteria' produced for Defra by the University of Salford in 2005. If you have current information we would be pleased to hear from you.

See also the comments in the frequency weighted sound levels section

Low Pass Filter, signals above the cut-off frequency are attenuated. The attenuation slope is called the roll-off

Lpsound pressure level

LPacsound power

Lpeak (Lpk)peak sound pressure

LPNperceived noise level
Lpntone assessment parameter
pt    ▫ tone assessment parameter
Lptitone assessment parameter

LRPIpressure residual sound intensity index

LSELsingle event noise exposure level

Ltatone assessment parameter

Lvparticle velocity level

Lwsound power level


LZZ-weighted, sound level

LZEZ-weighted, sound exposure level

LZeqZ-weighted, Leq equivalent sound level

LZFZ-weighted, fast response, sound level

LZFmaxZ-weighted, fast response, maximum, sound level

LZFminZ-weighted, fast response, minimum, sound level

LZSZ-weighted, slow response, sound level

LZSmaxZ-weighted, slow response, maximum, sound level

LZSminZ-weighted, slow response, minimum, sound level

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