Jade 1.0 Technical Note

Accuracy of Readings

Jade Capture records the noise levels by converting the voltage received from the meter into a corresponding decibel level. The accuracy of the data read from a sound level meter during data capture is therefore heavily reliant on the accuracy of the hardware and the care taken by the user. The following list contains those things that will affect the accuracy of data capture:

Laboratory calibration - Regular (typically annual) calibration of equipment in a laboratory in accordance with national standards and manufacturer's recommendations will help keep the equipment as accurate as possible.
Regular software calibration of the sound level meter by the user - Users should perform a software calibration before every data capture session to ensure that Jade Capture reads the correct decibel values.
Type of calibration - Dual level calibration makes for much more accurate data readings than single level calibration (see below). Users should perform dual level calibrations for at least some, if not all, of the calibrations.
Validation after capture - Although validating the meter after data capture does not affect the accuracy of the readings, it does show possible slippage during data capture and can be used to assess the accuracy of the readings.

Single versus dual level calibration

Jade Capture uses the results of the software calibration to determine the mapping of the voltage levels output by the meter to decibel levels. Figure 1 shows the possible difference in accuracy of single and dual level software calibration.

The purpose of a software calibration is to calculate the voltage level for the bottom of range or 0dB (BOR in Figure 1). To find this value, Jade Capture calculates the slope of the line that plots the mapping of voltages to decibel levels. From the diagram it can be seen that if two calibration levels (C1 and C2) are known, calculating the slope of the line is easy. This is shown by the solid sloping line. Once the slope is known, Jade Capture can calculate the "milliVolts per dB" value and enter the correct value in the meter library for use during data capture.

If there is no second calibration level, Jade Capture extrapolates the line backwards from the first calibration level (C1 in Figure 1) using the "milliVolts per dB" value entered by the user in the meter library. The effect of doing this is shown by the dotted line in the diagram.

As you can see from the diagram, the final slope from both types of calibration may be considerably different. For example, in Figure 1, if a voltage level (V) is read from the meter, the single level calibration results in the decibel level shown by D1 and the dual level calibration results in the decibel level shown by D2. In this example, the dB value is too low if the results of the single level calibration are used.

If you have only a single level calibrator

If you have only a single level calibrator, you should do one of the following:

Borrow a dual level calibrator to perform an initial dual level calibration. This will produce an accurate milliVolts per dB value. Subsequent calibrations can then be performed using a single level calibrator because the extrapolation to the base voltage will be reasonably accurate as the milliVolts per dB value is known. Note, however, that repeat dual level calibrations will be needed at least every three to six months to maintain accuracy.
Perform a pseudo-dual calibration in which a single level calibrator is used as a dual level calibrator. Note that although that this may produce more accurate results than a single level calibration, it will not be as accurate as a proper dual level calibration. Also, this works only with certain sound level meters.

Note that neither of these alternatives will be as accurate as a proper dual level calibration, so you should obtain a dual level calibrator at the first opportunity.