Acoustic Glossary


L : Sound and Vibration Definitions, Terms, Units, Measurements ...

La : acceleration level

LA : A-weighted, sound level.

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

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

LAn : 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 background noise

LAE : SEL (sound exposure level) with "A" frequency weighting.

LAeq : A-weighted, equivalent sound level.

LAF : A-weighted, fast, sound level.
LAFmax : A-weighted, fast, maximum, sound level.
LAFmin : A-weighted, fast, minimum, sound level.

LAIeq : A-weighted, impulse, leq, sound level.

LAmax : A-weighted, maximum, sound level

LArT : Sound Rating Level, the a-weighted, leq sound level of an industrial noise during a specified time period, adjusted for tonal character and impulsiveness

LAS : A-weighted, slow, sound level.
LASmax : A-weighted, slow, maximum, sound level.
LASmin : A-weighted, slow, minimum, sound level.

LAT : time-average sound level : Leq

Lavg : averaged sound level with selectable exchange rate, results also vary depending if a threshold level was used.
Lavg : Leq - equivalent sound level measurements - but only when the exchange rate is 3 and the threshold is set to none.
Lavg : TWA - time weighted average when the exchange rates and threshold levels are the same and the measurement period was 8 hours.

LC : C-weighted, sound level.
LCE : C-weighted, sound exposure level
LCeq : C-weighted, Leq - equivalent sound level
LCF : C-weighted, fast, sound level.
LCFmax : C-weighted, fast, maximum, sound level.

LCpeak : C-weighted, peak, sound level.

LCS : C-weighted, slow, sound level.
LCSmax : C-weighted, slow, maximum, sound level.
LCSmin : C-weighted, slow, minimum, sound level.

Lday : Day equivalent level,

A-weighted, Leq equivalent sound level, measured over the 12-hour period 07.00 - 19.00 hours, also known as the Day noise indicator and other terms.

Lden : Day-evening-night equivalent level,

A-weighted, Leq eqivalent sound level, measured over the 24 hour period, with a 10 dB penalty added to the levels between 23.00 and 07.00 hours and a 5 dB penalty added to the levels between 19.00 and 23.00 hours to reflect people's extra sensitivity to noise during the night and the evening.

See also • community noise equivalent levelevening equivalent level

Ldn : Day-night equivalent level,

A-weighted, Leq equivalent sound level, measured over the 24 hour period, with a 10 dB penalty added to the levels between 23.00 and 07.00 hours, also known as the day-night noise indicator and other terms.

LE : Sound 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 worst 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,d : Daily personal noise exposure

LEPN : Effective perceived noise level

LEP,w : Weekly personal noise exposure

Leq : Equivalent 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 kind of level is indicated by use of a compound term such as sound power level or sound pressure level.
Note 2 : 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 equivalent level

, A-weighted, Leq. sound level, measured during the evening period 19.00 - 23.00 hours, also known as evening noise indicator

See also • LdayLdenLdn

LEX,8h : Daily exposure level

LF : Force level

LFNR : Low frequency noise rating

Lg or Log : Logarithm

Li : Impact sound pressure level

Li : Sound intensity level

LIeq : 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

, 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 under 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,R : Pressure-residual Intensity Index

Live Room

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

LK : Sound Intensity Pressure Index

Lmax : maximum sound level

, during a measurement period or a noise event, Often includes other descriptors, for example LAFmax

Lmax should not be confused with the Peak.

Sometimes written as Max dB(A).

Lmin : minimum sound level,

during a measurement period or a noise event. Often includes other descriptors, for example LAFmin

Sometimes written as Min dB(A).

Ln : Logarithm

Ln : percentile level where 'n' is between 0.01 and 99.9% calculated by statistical analysis.

Usually include a descriptor i.e. A-weighting. Most common Ln values are LA10 and LA90 levels.

Ln : Normalized impact sound pressure level

Lnight : night equivalent level, Leq. A-weighted, sound level, measured overnight 23.00 - 0700 hours
Also known as the night noise indicator and other terms.

Lnp : Noise pollution level

LnT : Standardized impact sound pressure level.
L'nT : Standardized impact sound pressure level.
LnT,w : Weighted standardized impact sound pressure level.
L'nT,w : Weighted 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 0 dB reference sound level - see the decibel for a more detailed explanation and practical examples in acoustics.

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).

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


, 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 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 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

Lp : Sound pressure level

LPac. Sound power

Lpeak (Lpk) : Peak sound pressure

LPN : Perceived Noise Level

Lpn : Tone assessment parameter
Lpt : Tone assessment parameter
Lpti : Tone assessment parameter

LRPI : Residual pressure intensity index

LSEL : Single event noise exposure level

Lta : Tone assessment parameter

Lv : Particle velocity level

Lw : Sound power level


LZ : Z-weighted, sound level.
LZE : Z-weighted, sound exposure level
LZeq : Z-weighted, Leq equivalent sound level.
LZF : Z-weighted, Fast, sound level.
LZFmax. Z-weighted, Fast, maximum, sound level.
LZFmin : Z-weighted, Fast, minimum, sound level.
LZS : Z-weighted, Slow, sound level.
LZSmax : Z-weighted, Slow, maximum, sound level.
LZSmin : Z-weighted, Slow, minimum, sound level.

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