Acoustic Glossary

L : Sound and Vibration Terms and Definitions ...

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LA Sound Levels

LA : A-weighted, sound level.

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

LA10,18h : (UK Traffic Noise Regulations) : A-weighted, sound level in dB(A) exceeded for 10% of each hour over the period 06:00 - 24:00 hours.

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

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

See also percentile noise levels.

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LAE : A-weighted, sound exposure level.

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LAeq Sound Levels

LAeq : A-weighted, equivalent continuous sound level.

LAeq,T : A-weighted equivalent continuous sound level, T denotes the time period over which the fluctuating sound levels were averaged, for example LAeq,8h is the equivalent continuous noise level over an 8 hour period.

LAeq,6h (UK Government Environmental Noise Definition) : 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) : 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) : 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 Lday • Lden • Levening • Lnight • daily noise exposure level • community noise equivalent level.

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LAF Sound Levels

LAF : A-weighted, sound level, measured with a fast time-constant.

LAFmax : A-weighted, maximum, sound level measured with a fast time-constant - maximum is not peak

LAFmin : A-weighted, minimum, sound level measured with a fast time constant

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LAIeq Sound Levels

LAIeq : A-weighted, impulse, leq sound level

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LAS : A-weighted, slow time constant response, sound level

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LAT : time average sound level, also known as the Leq (equivalent continuous sound level).

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Lavg : average, sound level, the results may vary due to local exchange rates and threshold levels.

Lavg : Leq (equivalent continuous sound level) when the exchange rate is 3 and no threshold is set.

Lavg : TWA (time-weighted average), measured over 8 hours. In Europe and the UK a 3dB exchange rate is always used.

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LC : C-weighted, sound level

LCpeak : C-weighted, peak, sound level

LCS : C-weighted, slow response, sound level

LCSmax : C-weighted, slow response, maximum, sound level

LCSmin : C-weighted, slow response, minimum, sound level

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See also Lden • Levening • Lnight

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Lden noise level

Lden : day-evening-night noise level, the A-weighted, Leq (equivalent noise level) over a whole day, but with a penalty of 10 dB(A) for night-time noise (23:00-07:00) and 5 dB(A) for evening noise (19:00-23:00), also known as the day evening night noise indicator

See also CNEL - community noise equivalent level • Lday • Levening • Lnight

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Ldn noise level

Ldn : day-night noise level, the LAeq (equivalent noise level) over a 24 hour period with a penalty of 10 dB(A) for noise during the hours of 23:00-07:00, also known as the day night indicator.

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LE : sound energy density level

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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.

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Length a scalar quantity and one of the seven SI base quantities. The SI unit of length is the metre, symbol (m).

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LEP,d : daily personal noise exposure level.

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Leq : equivalent continuous sound level

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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.

See also level difference • sound energy level • sound exposure level • sound intensity level • sound level • sound power level • sound pressure level

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Levening : evening equivalent noise level, the A-weighted, Leq (equivalent noise level) over the 4 hour evening period 19:00-23:00 hours, also known as evening noise indicator

See also Lday • Lden • Lnight.

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LEX,8h : daily noise exposure level

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LFNRV : low frequency noise rating

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Lg : logarithm

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Li : impact sound pressure level

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

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

See also other types of averaging

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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 the sound propagation coefficient

See also elementary attenuation of propagation • elementary dephasing of sound propagation • elementary exponent of sound propagation • propagation loss definition

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Linear Momentum (p) = mass x velocity.

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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.

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Linear Weighting

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

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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

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

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

See also constant bandwidth • constant percentage bandwidths • continuous spectrum • fast fourier transform • narrowband noise • narrowband spectra • octave bands. pink noise • white noise • wideband noise.

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Li,R : pressure-residual sound intensity index

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Live Room Definition (IEC 801-31-14) room characterised by a relatively small amount of sound absorption.

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Lk : sound intensity pressure intensity index

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Lmax should not be confused with Peak.

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Ln is the percentile noise level where 'n' is between 0.01 and 99.9%, of the time, calculated by statistical analysis and usually includes a descriptor i.e. A-weighting. The most common Ln values are the LA10, LA90 and LA95 levels, widely used in the assessment of environmental noise levels and regulations.

See also aircraft noise • industrial and port noise • railway noise • road traffic noise

Ln is also used in sound insulation assessments
Ln : normalised impact sound pressure levels - sound insulation, laboratory measurements.
L'n : normalised impact sound pressure levels - sound insulation, field measurements

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Lnight, the A-weighted, Leq (equivalent noise level) over the 8 hour night period of 23:00 to 07:00 hours, also known as the night noise indicator.

See also Lday • Lden • Levening.

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Lnp : noise pollution level

L'nT,w : weighted standardised impact sound pressure level - insulation, field measurements

Ln,w : weighted normalised impact sound pressure level - insulation, laboratory measurements

L'n,w : weighted normalised impact sound pressure level - insulation, field measurements

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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

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Logarithm, Log, log, lg, Ln, (common logarithm ratios), expressed in decibels (dB) are widely used in acoustics to convert the immense range we can hear into manageable numbers.
For example we can hear sound power levels of 0.000000000001 watts up to 100 watts or more.
Similarly we can hear sound pressure levels of 0.00002 pascals up to 200 pascals.
Converting these levels into dBs results in a range of values 0 dB up 140 dB; much easier to compare and discuss.

The Binary logarithm (log₂n) is used in computer science.

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

The Natural Logarithm is the logarithm to the base of the mathematical constant e, where e is approximately equal to 2.718281828459. The natural logarithm of x is generally written as ln x, loge x,

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).

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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 a dB change. Therefore, noise and vibration frequency analyses are usually plotted on a logarithmic amplitude scale.

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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.

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

See also frequency interval

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Logging is the process of recording the noise data at regular intervals, so a 'picture' of the fluctuations may be studied at the end of a measurement. Traditionally 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 sample at 16 times a second to ensure all the sound levels are included.

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Longitudinal Sound Waves

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Loudness depends not only on the sound levels and the frequencies involved, but also on the individual listener's subjective response to the character of the noise under consideration.

See also calculated loudness level • equal loudness contours • fletcher-munson curves • minimum audible field • phons • pitch • sones • timbre • zwicker loudness

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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 .

See also the comments in the A-weighted sound level entry

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

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Lp : sound pressure level.

LpA : A-weighted sound pressure level.

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LPac : the sound power.

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Lpeak and Lpk : peak sound pressure

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Lpn : a tone assessment parameter

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LRPI : pressure residual sound intensity index

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LSEL : single event noise exposure level

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Lta : tone assessment parameter

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Lv : particle velocity level

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Lw : sound power level

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LZ : Z-weighted, sound level