# Calibration of a hydrophone using SigCalRP

Tech Note: TN0828
Product: SigCalRP
Version: All

## Issue

Hydrophones and underwater calibration equipment use different calibration values than the calibration in microphones. SigCalRP is configured for measuring a signal in SPL (Sound Pressure Level). An SPL of 0 dB is the hearing threshold of a normal human at 1000 Hz. Calibration of a signal in water requires modification to the terms used in SigCalRP in order to match the calibration terminology of a hydrophone.

## Solution

The following example illustrates the conversion process for hydrophone calibration in SigCalRP:

The RESON TC4032 Hydrophone is a typical hydrophone used for underwater sound pressure measurements. Often, sound pressure measurements in water are measured in reference to micro pascal's (uPa. The hydrophone sensitivity is -170 dB re 1 V/uPa. This means that a micro pascal pressure wave will generate a voltage of -170 dBV on the hydrophone. We must first convert the units of reference, in this case micro pascal of pressure to pascal. This is done by scaling the sensitivity by a factor of 1 million (1 Pa = 1e6 uPa). To do this we scale the voltage reference by the equivalent value in dB (1e6 times increase = 120 dB).

-170 dB + 120 dB = -50 dB re 1 V/Pa

SigCalRP calibration specifies mVrms to dB SPL. In order to calibrate the hydrophone, we need a measurement of mVrms for the base sensitivity of the hydrophone. This is accomplished by converting the hydrophone's sensitivity from dB to its corresponding voltage.

To do this we use a voltage ratio conversion: 10 ^( -50 dBV / 20 ) = 3.16 mV

Finally, to calibrate SigCalRP we set the value of our converted sensitivity to 0 dB SPL. This means that 3.16 mV will correspond to 0 dB SPL. For reference, we may equate this 0 dB SPL to 1 Pa of pressure when viewing SigCalRPs output. Therefore, any measurements shown in SigCalRP will calibrate to 3.16 mV = 0 dB SPL or 1 Pa of pressure.

To set the calibration in SigCal:

1. Select Acquisition from the Setup menu to prompt the Setup Acquisition dialog box.

2. In the Microphone Calibration area enter 3.16 mVrms for the signal voltage value and 0 dB SPL rms for the pressure reference.

3. Click OK.

SigCalRP is now successfully calibrated to the RESON TC4032 hydrophone.

Note

It is also important to adjust the gain of the hydrophone to match the A/D converters on the acquisition device.