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.

LAT : Time Averaged Sound Pressure Level : the equivalent steady level over a given period of time that contains the same amount of noise energy as the actual fluctuating sound level

Lday : Day equivalent level :
A-weighted,
Leq.
Sound Level, measured over the 12-hour period 07.00 - 19.00 hours.

Also known as Day Noise Indicator and other terms.

Lden : Day-evening-night equivalent level :
A-weighted,
Leq.
noise 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.

Ldn : Day-night equivalent level :
A-weighted,
Leq.
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.

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

Level : in acoustics, it is the Logarithm ratio of a sound or vibration 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.

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.

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.

Lmin : Minimum Sound Level : during a measurement period 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 Min dB(A).

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

Logarithm : Log, log, lg, Ln : Common logarithms are widely used in science and engineering, 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.

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 Frequency Interval : logarithm of the ratio of two frequencies.

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.

Loudness : that attribute of auditory sensation in terms of which sounds may be ordered on a scale extending from soft to loud. Loudness depends primarily upon the sound pressure of the stimulus, but also upon its frequency, waveform and duration.

Loudness Level : 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.

Calculated Loudness Level : loudness level calculated by a specified procedure. Procedures are given in ISO Recommendation R 532-1975.

Equal Loudness Contours : have been arrived at by group consensus. 0 dB is the threshold of hearing at 1 kHz. However we are less sensitive at low and high frequencies, so the SPL at 10 Hz, for example would have to be increased to 70 dB for the sound to be just detectable, or of equal Loudness - the unit is the
Phon.

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.