Casella CEL CEL-281 - Manual (Page 31)

In the field of noise measurement, it is important to know the amount of
time spent at a given noise level (known as "on time"), or the amount of
time spent at or above a particular noise level.
A statistical distribution of the noise levels can be produced by dividing
the total dynamic range of noise parameters measured or calculated dur-
ing a given logging period into steps representing different levels. These
are then plotted as a histogram to show the percentage frequency of oc-
currence of any particular level or calculated parameter.
2.2.7 Probability %
Acoustic specialists are also interested in using data collected from a
short period to predict how things will go over a longer period (i.e. the
Probability that they will occur).
Providing constant conditions are maintained, the statistical presumption
can be used that past events will be the same as coming events. There-
fore, a percentage frequency histogram may be regarded as a Percent-
age Probability histogram.
2.2.8 Cumulative %
It may also be required to know how much of the logging period was
taken up when any particular noise level was reached or exceeded. This
information could be obtained from a percentage probability histogram,
but is more easily read from a separately produced Cumulative Per-
centage histogram.
In the cumulative% histogram shown in Figure 3, it can be seen that
100 dB is reached or exceeded for 65% of the sampled time. Similarly,
the commonly required values L
10 , L 50 , and L 90 can be read directly from
the cumulative% histogram (116, 104, and 91 dB respectively).
To cover the full 70 dB dynamic range of the CEL-281 (70 - 140 dB), each
histogram is divided into 1 dB steps (cells), plus one over range and one
under range cell, giving a total of 73 cells. The following five parameters
are stored as histograms.
CEL-281 Operator's Handbook - Page 23
_Introduction