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L-weightingLinear weighting. All values are weighted equally in the specified range.
Lateral LocationThe physical location of a rotor relative to the fixed, or non- rotating parts of the machine.
Law of the First WavefrontThe first wavefront falling on the ear determines the perceived direction of the sound.
LdnA 24-hour Leq, except 10 dB is added to all levels measured between 2200 and 0700 hrs.
Leading EdgeThe part of the signal near the start of the time window.
LeakageIn an FFT analyzer, 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; the signal energy essentially “leaks” from a single FFT line to adjacent lines. Leakage 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, which forces the signal level to zero at the ends of the data block.
LEDLight emitting diode.
LEPNEffective Perceived Noise Level. A complex rating used to certify aircraft types for Flyover Noise. Includes corrections for pure tones and for duration of the noise.
LevelA sound pressure level in dB means that it is calculated relative to the standard reference level of 20 µPa for airborne sound. The word “level” associates that figure with the appropriate standard reference level. In common usage the level of a signal is its amplitude, but strictly speaking the term should be reserved for the amplitude expressed on a decibel scale relative to a reference value.
LifteringA technique to separate harmonic vibration families. The main application for Cepstrum analysis is analysis of gearboxes. Liftering is used to separate the different harmonic vibration families from the various rotating shafts.
Line AmplifierAn amplifier designed to operate at intermediate levels. Its output is usually on the order of one volt.
Line DriveAn input socket that can also provide power to drive a transducer.
Line SourceA 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.
Line SpacingIn an FFT spectrum, the frequency difference between two adjacent bin centres or lines.
Line SpectrumA line spectrum is a spectrum where the energy is concentrated at specific frequencies (lines or bins), as opposed to a continuous spectrum where the energy is smeared out over a band of frequencies. A periodic deterministic signal will have a line spectrum, and a random signal will have a continuous spectrum. Spectra generated by machine vibration signatures are always a combination of these two types.
LinearA device or circuit with a linear characteristic means that a signal passing through it is not distorted.
Linear TaperAttenuation by a numerical factor that increases or decreases linearly with time.
Live End-Dead EndAn acoustical treatment plan for rooms in which one end is highly absorbent and the other end reflective and diffusive.
LNThe dB(A) level exceeded N % of the time, for example, L90, the dB(A) level exceeded 90% of the time, is commonly used to estimate ambient noise level.
LNPNoise Pollution Level. A variation of Leq which accounts for short-term variability in noise level.
LogarithmThe exponent required (power to which a number must be raised) to produce a given number. For example, used in acoustics, 10 to the exponent 2 = 100; the log of 100 = 2.
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.
LoudnessAn observer’s auditory impression of the strength of a Loudness: The subjective judgement of intensity of a sound by humans. Loudness depends upon the sound pressure and frequency of the stimulus. Over much of the frequency range it takes about a threefold increase in sound pressure (a tenfold increase in acoustical energy, or, 10 dB) to produce a doubling of loudness. The unit is the Sone. The Zwicker method of calculation of stationary loudness is described in ISO 532 B and DIN 45631.
Loudness LevelMeasured in phons it is numerically equal to the median sound pressure level (dB) of a free progressive 1000 Hz wave presented to listeners facing the source, which in a number of trials is judged by the listeners to be equally loud. Loudness level can be calculated according to ISO 532B.
LoudspeakerAn electro-acoustical transducer that changes electrical energy to acoustical energy.
Low-pass FilterA filter that passes signals with less than 3 dB attenuation up to its cut-off frequency, and attenuates the signal above that frequency. The attenuation slope is called the Roll Off. An Anti-aliasing Filter is an example of a low-pass filter.
LSELSingle Event Noise Exposure Level. The dB(A) level which, if it lasted for one second, would produce the same A-weighted sound energy as the actual event.Brüel & Kjær Sound & Vibration Dictionary End User Agreement