Q: The sensitivity of my hydrophone is specified as "-254 dB". How should I interpret it in terms of output voltage per pressure in Mega-Pascal [MPa]?

A: The dB in the calibration data is actually "dB re V/uPa" and should be read as "dB relative to 1 Volt per micro-Pascal", with the following relation:
-254 = 20 *Log ( ML[V/uPa] )
with ML the sensitivity in units of Volt per micro-Pascal. To get the sensitivity ML in units of Volts per Mega-Pascal  [V/MPa] do:
-254 = -240 + 20 * Log( ML[V/MPa] )
-14 = 20 * Log( ML[V/MPa] )
ML[V/MPa] = 10^(-14/20) = 0.2 [V/MPa]
Therefore, you can expect to get 200 mV for pressure of 1.0 MPa at the specified frequency.

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