Larson Davis System 824, Reference Manual

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7-18 824 Reference Manual 11/8/00
Crnt Threshold (SSA, ISM,
LOG)
In the ISM instrument the Crnt
Threshold setting is a submenu to the
SLM menu selection. The Crnt Threshold sets the threshold for computing the
current time weighted average (TWA).
This setting affects the System 824’s
secondary or current buffer. In compliance with the requirements of various regulatory
agencies, only those sound levels which are above a certain
threshold level are used when computing the TWA. Accord-
ingly, this setting can take on any integer value within the
following range:
0 - 255 dB
For hearing conservation measurements, set the threshold to
80dB. For most community noise measurements, set the
threshold to 0dB.
Crnt Criterion (LOG)
The Crnt Criterion sets the TWA level which constitutes
100% dose for the current measurement. The 100% dose
occurs if a person is exposed to that level for the criterion
time or full dose hours. Accordingly, this setting can take on
any integer value within the following range:
0 to 255 dB
Exposures at higher levels than the current criterion level
will result in a 100% dose in less time; or, if exposure con-
tinues, the current dose will exceed 100%.
LOG and SSA Ln Settings
Ln Settings are available in the Log-
ging SLM (LOG) and SLM&RTA
(SSA) instruments. Ln is the SPL which was exceeded n% of the time. The Ln’s
are computed from the overall level distribution histogram
or each Interval’s level histogram. The measured and dis-
played Ln percentage settings may be changed during the
measurement. The six Ln’s are automatically listed in
increasing order.