Acoustics Glossary

A-Z list of terminology commonly used within acoustics.



L (level): sound pressure level, SPL; in general, it implies the use of decibels related to the ratio of powers, or power-related quantities such as sound intensity or sound pressure.

LA: see under A-weighted sound pressure level.

LAE: see under sound exposure level, SEL.

LAeq,16hour: the LAeq over the period 07.00-23.00, local time (for strategic noise mapping this is an annual average).

LAeq,T: see under continuous equivalent sound level.

LAmax: the maximum RMS A-weighted sound pressure level occurring within a specified time period; the time weighting, Fast or Slow, is usually specified.

LAN,T: percentile level, i.e. the sound pressure level in dBA which is exceeded for N% of the time interval T, e.g. in LA10 and LA90.

Lday: the LAeq over the period 07.00-19.00, local time (for strategic noise mapping this is an annual average)/

Lden: (day-evening-night-level): the LAeq over the period 00.00-240.00, but with the evening values (19.00-23.00) weighted by the addition of 5 dBA, and the night values (23.00-07.00) weighted by the addition of 10 dBA.

LEP,d: see under personal daily noise exposure level.

Levening: the LAeq over the period 19.00-23.00, local time (for strategic noise mapping this is an annual average).

Lnight: the LAeq over the period 23.00-07.00, local time (for strategic noise mapping this is an annual average).

L’nT: see under standardised impact sound pressure level.

L’nT,w: see under weighted standardised impact sound pressure level.

Lpeak: see under peak sound pressure level

LW: see under sound power level.

Late sound reflections (or late sound): reflections of sound from surfaces in a room which arrive at the receiver within 50 ms of the arrival of the direct sound signal (for speech) or within 80 ms for music. Late sound is treated by the listener’s ear/brain system as being unpleasant and discordant and therefore acoustic designers will seek to discourage the production of late sound reflections while encouraging early reflections which are considered to be helpful in reinforcing the direct sound signal response in the listener.

Level difference, D: BS EN ISO 140-4 uses the difference in level between the two rooms as the basic measure of airborne sound insulation.

Level recorder: an instrument for registering and measuring the variation of signals, such as sound pressures, with time.

Linear: a measurement device is linear if its output is directly proportional to its input; in the case of a microphone, for example, this means that the sensitivity is constant and does not change with sound pressure level; linear SPL means unweighted.

Linearity: the degree to which a device is linear.

Logarithmic decrement (δ): a measure of the amount of damping in a vibrating system, based on the rate of background noise increases.

Lombard effect: an effect whereby a speaker will often raise the level of his/her voice when the level of background noise increases.

Longitudinal wave: a wave in which the vibratory movement of the particles in the medium is parallel to the direction in which the wave is travelling; compressional waves in a fluid medium are longitudinal.

Long-term average rating level: average over the long-term time interval of the rating levels for a series of reference time intervals, carried out as described I BS 7445-2 (ISO 1996-2).

Long-term average sound level: average over the long-term time interval of the equivalent continuous A-weighted sound pressure levels for a series of reference time intervals comprised within the long-term time interval, carried out as described in BS 7445-2 (ISO 1996-2).

Long-term sound level, long-term average rating level, long-term time interval: terms used in connection with the description and measurement of environmental noise, and defined in BS 7445.

Long-term time interval: specified time interval for which the results of the noise measurement are representative. Note that the long-term time interval consists of a series of reference time intervals and is determined for the purpose of describing the environmental noise and is generally designated by competent authorities.

Loss factor: a term used to describe the amount of damping in a system or material; it is twice the damping ratio.

Loudness: the measure of the subjective impression of the magnitude or strength of a sound.

Loudness level: the loudness level of a sound is the sound pressure level of a standard pure tone, of specified frequency, which is equally loud, according to the assessment of a panel of normal observers.

Lower exposure action value: a noise exposure level defined in the 2005 Control of Noise at Work Regulations requiring action from employers and employees (a personal daily (or weekly) noise exposure level of 80 dBA, or peak sound pressure level of 135 dBC).

Low frequency noise: a term generally used to refer to sound below a frequency of about 100 to 150 Hz. It is much less well attenuated during transmission both outdoors and indoors than sounds of higher frequencies, and is therefore often heard at considerable distances from its source particularly late at night when other background noise from other sounds has decreased. In such cases it can often five rise to annoyance because it is often tonal in nature.

Low pass filter: a filter, which transmits signals at frequencies below a certain cut-off frequency and attenuates all higher frequencies.

Lumped parameter model: a model of a vibrating system in which mass, stiffness and damping are represented as discrete elements.