L : Sound and Vibration Terms, Definitions, Units, Measurements ...
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 • Lday • Lden • Levening • Lnight.
Ldn (day-night noise level)
is the LAeq (equivalent noise level)
over a 24 hour period with a penalty of +10 dB(A) for noise during the hours of 22:00 to 07:00
, 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.
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.
is a scalar quantity
and one of the seven SI base quantities. The SI unit
of length is the metre, symbol (m).
is the 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 the kind of level is indicated by use of a compound term such as
sound power level
sound pressure level
Note 2 : the the value of the reference quantity remains unchanged, whether the chosen quantity is Peak
, or otherwise.
Note 3 : the base of the logarithm is indicated by use of a unit of level associated with that base.
under sound insulation, level difference
LF • force level
LFNRV • low frequency noise rating
Lg or Log under logarithm
Li • impact sound pressure level
Li • sound intensity level
Linear, a device or circuit with a linear characteristic means that a signal passing through it is not distorted.
, 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 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
Line Drive, an input socket that can also provide power to drive a transducer.
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
LI,R • pressure-residual sound intensity index
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.
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).
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
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 long 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.
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 div
Low Pass Filter, signals above the cut-off frequency are attenuated. The attenuation slope is called the roll-off
is the sound pressure level
is the sound power
is the peak sound pressure
is the perceived noise level
is the pressure residual sound intensity index
is the single event noise exposure level
is the tone assessment parameter
is the particle velocity level
is the sound power level
LZ is the
LZE • is the
sound exposure level
LZeq is the
Leq equivalent sound level
LZF is the
LZFmax is the
LZFmin is the
LZS is the
LZSmax is the
LZSmin is the
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