Glossary of Noise and Vibration
Acoustic Terminology
In order to assist the understanding of acoustic terminology and the relative change in noise, the following background information is provided.
The human ear can detect a very wide range of pressure fluctuations, which are perceived as sound. In order to express these fluctuations in a manageable way, a logarithmic scale called the decibel, or dB scale is used. The decibel scale typically ranges from 0dB (the threshold of hearing) to over 120dB. An indication of the range of sound levels commonly found in the environment is given in the table to the right.
dB (decibel)
The scale on which sound pressure level is expressed. It is defined as 20 times the logarithm of the ratio between the root-mean-square pressure of the sound field and a reference pressure (2x10-5Pa).
dB(A) A-weighted decibel.
This is a measure of the overall level of sound across the audible spectrum with a frequency weighting (i.e. ‘A’weighting) to compensate for the varying sensitivity of the human ear to sound at different frequencies.
LAeq
This is defined as the notional steady sound level which, over a stated period of time, would contain the same amount of acoustical energy as the A-weighted fluctuating sound measured over that period.
L10 & L90
If a non-steady noise is to be described it is necessary to know both its level and the degree of fluctuation. The Ln indices are used for this purpose, and the term refers to the level exceeded for n% of the time. Hence L10 is the level exceeded for 10% of the time and as such can be regarded as the 'average maximum level'. Similarly, L90 is the ‘average minimum level’ and is often used to describe the background noise. It is common practice to use the L10 index to describe traffic noise.
LAmax
is the maximum A-weighted sound pressure level recorded over the period stated. LAmax is sometimes used in assessing environmental noise where occasional loud noises occur, which may have little effect on the overall Leq noise level but will still affect the noise environment.
DnT,w
DnT,wis a weighted single number quantity that describes the airborne sound insulation between two rooms, measured or predicted as an on-site measurement. The value is standardised to a reverberation time of the reception room of 0.5 seconds. The standardisation takes into account that in furnished dwellings the reverberation time is reasonably independent of the volume and frequency of the sound and its value is approximately 0.5 seconds.
Ctr
Ctris a spectrum adaption term which gives an indication of the performance of the element when exposed to particular types of noise, specifically urban road traffic noise and such sources that contain a significant amount of low frequency noise.
Vibration Terminology
Vibration is defined as a repetitive oscillatory motion.
Vibration is often complex, containing many frequencies, occurring in many directions and changing over time. Many factors influence human response to vibration. Physical factors include vibration magnitude, vibration frequency, vibration axis, duration, point of entry into the human body and posture of the human body.
The threshold of perception depends on the frequency of vibration. The human body is most sensitive to vibration in the frequency range 1 to 80Hz and especially sensitive to vibration in the range 4 to 8Hz. As with noise, a frequency weighting mechanism is used to quantify vibration in a way that best corresponds to the frequency response of the human body.
Displacement, Acceleration and Velocity,
Vibration is an oscillatory motion. The magnitude of vibration can be defined in terms of displacement (how far something moves from the equilibrium position), velocity (how fast something moves), or acceleration (the rate of change of velocity). When describing vibration, one must specify whether peak values are used (i.e. the maximum displacement or maximum velocity) or whether r.m.s. values (effectively an average value) are used.
Root Mean Square (r.m.s.)
R.m.s is a statistical measure of the magnitude of a varying quantity. It is so named because it is defined as the square root of the mean of the squares of the values. It is most useful when the set of values includes negatives (especially an oscillating pattern such as a vibration or noise waveform), where the calculation of a simple average (or Mean) would mean the negative values would cancel out the positive values.