Jade 1.0 Technical Note

Pseudo-Dual Calibration
Using a Single Level Calibrator

Introduction

If you only have a single level calibrator, it is sometimes possible to perform a "pseudo" dual level calibration. This is only possible if the sound level meter produces a DC voltage that depends on the displayed dB value rather than on the actual dB value.

Note: Although a pseudo-dual level calibration will probably produce better results than a single level calibration, it is not as accurate as a true dual level calibration. You are strongly recommended to use true dual level calibrations.

Performing a Pseudo-Dual Calibration

To use pseudo-dual level calibrations, do the following:

1. Attach the calibrator to the meter. Set the meter to an appropriate range and turn on both the calibrator and the meter. You do not need to connect the meter to the computer.
2. Using the calibration adjustment on the meter, alter the adjustment control until the meter displays a dB value at least 1 dB below the nominal calibration value. If the calibration adjustment does not have sufficient range to achieve this, you will not be able to do a pseudo-dual level calibration. Ideally, you should find a value at least 2 or 3 dB below the nominal calibration value. Make a note of this lower value. This will be the first calibration level for your pseudo-dual level calibration.
3. As a result of step 2, the meter is now uncalibrated. You should therefore use the calibration adjustment to readjust the meter so that it displays the nominal value of the calibrator.
4. Create a dual level calibrator in the calibrator library. The first level is the level obtained in step 2. The second level is the nominal value of the calibrator.

Tip: It is a good idea to use the Note text box to record the fact that this calibrator is a single level calibrator used as pseudo-dual level calibrator.

You can now use the pseudo-dual level calibrator to calibrate the meter as normal:

• For the first calibration level, you use the calibrator on its only level (ignore the message telling you to set it to the pseudo first level) but you do adjust the meter to display this pseudo first level. This means that from an acoustical point of view the meter is now incorrectly calibrated. However, provided the meter outputs a voltage dependent solely on the displayed value, Jade Capture registers a valid first calibration level voltage from the DC output.
• For the second calibration level, you again use the calibrator on its only level (this time matching the message). You must also adjust the meter to display this true level, ensuring a correct acoustical calibration for any subsequent data capture.

Summary

To summarize, you can only use pseudo-dual level calibration if the following conditions are met:

• The voltage on the meter's DC output reflects the displayed value rather than the actual sound level.
• The meter can be adjusted using the calibration adjustment to read a value at least 1 dB and preferably at least 2 or 3 dB below the nominal calibration value.
• When you edit a meter in the meter library, editing any details (except the meter's name and note) invalidates the last calibration for the meter. You must therefore calibrate the meter before using it again for data capture.

Copyright © Ptolemy Services 1997
Most recent revision 8 September 1997