Measurement History and Event Detection in Noise Monitoring

While time histories are typically logged at 1 sample per second, it is convenient to view longer term averages of measurement data to more easily ascertain trends. For example, you may want to compute 10 min or 1 hour averages of various noise parameters. 831-ELA Firmware enables Measurement History (MH) which logs parameters similar to the Time History (TH) yet looks at the average, minimum and maximum over the selected interval time. MH and TH can be run in parallel or independently.

When the Model 831 is set to MANUAL RUN mode, MH history can be used to construct a noise survey. Data for each measurement or location is saved individually and may include the Leq, maximum and minimum SPL and statistical distribution of the SPL (Ln).

MH records are available for easy review on the Model 831. The current measurement is visible on the “CURRENT” display tab, while completed measurements can be browsed in the “MEASUREMENTS” tab. When combined with 831-LOG, the time history detail of each measurement can be viewed quickly using keypad shortcuts. Finally, an automated sound recording at the beginning of each measurement can be achieved with 831-SR firmware.

Automatic Event Detection and Event History

In the Model 831 events are defined as either:

• Exceedance of a fixed threshold level for a minimum duration
• Exceedance of a dynamic threshold level for a minimum duration
• External trigger set by the digital input signal
  

The standard Model 831 includes basic exceedance logging functionality (see “Threshold Triggers”). However, only the number of exceedances and cumulative time above threshold values is provided. With 831-ELA software, you are able to define the attributes of an event including threshold level, duration and hysteresis. An event is considered “VALID” when it meets these criteria and ends when the SPL drops below the threshold level for a specific period of time (CONTINUATION PERIOD). The user is provided triggering status updates via triggering icon graphics, helping to identify event progression and qualification (see graph above).

The “EVENT” tab contains detailed information on each event including:

• Time and duration of Event
• Leq, MIN, MAX and PEAK SPL
• Frequency analysis of event (with 831-OB3)
• .wav format sound recording for source characterization (with 831-SR)
• Event time history (with 831-LOG). Time history period can be different then that used in overall TH measurement.
  

A DYNAMIC trigger method can also be selected in Model 831. The dynamic trigger is set to trigger when L85, L90 or L95 is exceeded by a predetermined number of decibels. A rise time can also be specified to determine how quickly the meter responds to changes in the background noise level. By utilizing the dynamic trigger, the number of false triggers is reduced and events (significant noise above background level) are better determined.