B&K 2245 - Terminology

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TERMINOLOGY
A-weighting
filter
Frequency weighting corresponding approximately to the 40dB equal loudness curve, that is to say,
the human ear’s response at low to medium sound levels. It is by far the most commonly applied fre-
quency weighting and is used for all levels of sound.
B-weighting
filter
Frequency weighting corresponding approximately to the 70dB equal loudness curve, that is to say,
the human ear’s response at medium sound levels.
C-weighting
filter
Frequency weighting corresponding to the 100dB equal loudness curve, that is to say, the human
ear’s response at fairly high sound levels. Mainly used when assessing peak values of high sound pres-
sure levels.
Decibel (dB)
The measurement unit for expressing the relative intensity of sound. A direct application of linear
scales (in Pa) to the measurement of sound pressure leads to large and unwieldy numbers. As the
ear responds logarithmically rather than linearly to stimuli, it is more practical to express acoustic
parameters as a logarithmic ratio of the measured value to a reference value. This logarithmic ratio
is called a decibel or dB. The advantage of using dB can be clearly seen in the below illustration.
Here, the linear scale with its large numbers is converted into a manageable scale from 0 dB at the
threshold of hearing (20μPa) to 130dB at the threshold of pain (~100Pa).
Our hearing covers a surprisingly wide range of sound pressures - a ratio of over a million to one.
The dB scale makes the numbers manageable.
Exchange
Rate
Exchange Rate is the increase in noise level that corresponds to a doubling of the noise level. The
Exchange Rate is used for calculation of LavUQ, TWA, TWAv, DoseUQ and projDoseUQ, where U = F
or S and Q = Exchange Rate: 4, 5 or 6dB.
✐ Note: LAeq is always based on an Exchange Rate = 3.
Fast Max
Maximum time-weighted acceleration, velocity, displacement or voltage level measured with Linear
frequency weighting and Fast time weighting. It is the highest level occurring during the meas-
urement time.
Fast Min
Minimum time-weighted acceleration, velocity, displacement or voltage level measured with Linear
frequency weighting and Fast time weighting. It is the lowest level occurring during the meas-
urement time.
'F', 'S' or 'I'
time weight-
ing
A time weighting (sometimes called a 'time constant') defines how the exponential averaging in
root-mean-square (RMS) measurement is done. It defines how heavily fluctuating sound pressure
variations are smoothed or averaged to allow useful readings. The standards define three time
weightings: F (Fast), S (Slow) and I (Impulse). Most measurements are carried out using the F time
weighting, which uses a 125ms time constant.
Frequency
The number of pressure variations per second. Frequency is measured in hertz (Hz). The normal
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