How to Field Calibrate a Hydrophone

1. To connect a hydrophone to a SoundAdvisor Sound Level Meter Model 831C with Preamplifier Model PRM831 and microphone installed, remove the microphone from the preamplifier and attach the ADP005 in place of the microphone.

2. Attach the hydrophone cable to the BNC connector end of the ADP005.

3. Remove the standard coupler from the pistonphone. Screw the hydrophone adaptor Model RA0047 onto to the pistonphone.

4. Taking great care, insert the hydrophone straight into the pistonphone as shown below. The hydrophone is extremely delicate, and it should be handled carefully to avoid bending or impacts.

Set Up the Software to Calibrate

In Larson Davis G4 Utility Software, navigate to your meter. The first time you calibrate, you will need to add a new calibrator.

Select the Tools menu in the upper left and then press Calibrate

Click bottom right Settings, then pick “Other” in the Calibrator to Edit field.

Name calibrator to match the pistonphone model

Now, edit the calibrator specs, keeping the following correction factors in mind:

• Adjusted reference pressure, according to the current barometric pressure. In the case of the equipment used here, this value is read from the barometer included with the pistonphone. In the example below, it is 113.9 dB

• The K-factor of the hydrophone adapter (printed on the side of the adapter). In this example, it is 5.2 dB

• Adjustment for the difference of reference pressure in air (20 μPa vs water (1 μPa). Use 26.02 dB for this adjustment.

In this example, enter a calibrator dB level of

 113.9 dB + 5.2 dB + 26.0 dB = 145.1 dB

The frequency is fixed at 250.0 Hz.

Save these changes.

On the same screen, under microphone, select other:

Close the screen.

Go back to the tools menu and turn off microphone corrections by selecting System Properties, navigating to Preferences, and turning Mic Correction Off.

Save these settings.

Perform Calibration

Turn on the pistonphone.

Do Calibration.

Accept the results of the calibration. The results are outside of normal range because of the correction factors applied.

The meter is now calibrated for underwater measurements with the attached hydrophone.

Note: If microphone settings are then edited so that nominal sensitivity matches actual sensitivity, the next time the hydrophone is calibrated, the calibration offset will not be outside normal range:

For subsequent calibrations with the same equipment, the barometric pressure at the time of calibration will still need to measured and accounted for. Then, simply edit the GRAS 42AA Calibrator using the same formula to determine the value to use in the dB field.

 Adjusted reference pressure, according to the current barometric pressure.

 +

 K factor of hydrophone adapter

 +

 Adjustment for the difference of reference pressure in air vs water (26.02 dB)

Then save and choose Tools, Calibrate, Do Calibration as before.

Additional Measurement Tips

You may want to reset the y-axis range of the display graph, by selecting Adjust Graph from the menu.

Use the arrows to adjust Y Min and Y Max as desired.

Physical Set Up of Hydrophone

To protect the hydrophone during measurement, loosen the collar of the hydrophone protection cage, then slide the hydrophone into the collar and tighten.