7-20 824 Reference Manual 9/29/09Crnt Threshold (SSA, ISM,LOG)In the ISM instrument the CrntThreshold setting is a submenu to theSLM menu selection.The Crnt Threshold sets the threshold for computing thecurrent time weighted average (TWA).This setting affects the System 824’ssecondary or current buffer.In compliance with the requirements of various regulatoryagencies, only those sound levels which are above a certainthreshold level are used when computing the TWA.Accordingly, this setting can take on any integer valuewithin the following range:0 - 255 dBFor hearing conservation measurements, set the threshold to80dB. For most community noise measurements, set thethreshold to 0dB.Crnt Criterion (LOG) The Crnt Criterion sets the TWA level which constitutes100% dose for the current measurement. The 100% doseoccurs if a person is exposed to that level for the criteriontime or full dose hours. Accordingly, this setting can take onany integer value within the following range:0 to 255 dBExposures at higher levels than the current criterion levelwill result in a 100% dose in less time; or, if exposurecontinues, the current dose will exceed 100%.LOG and SSA Ln SettingsLn Settings are available in theLogging SLM (LOG) and SLM&RTA(SSA) instruments.Ln is the SPL which was exceeded n% of the time. The Ln’sare computed from the overall level distribution histogramor each Interval’s level histogram. The measured anddisplayed Ln percentage settings may be changed during themeasurement. The six Ln’s are automatically listed inincreasing order.
