Here’s a pic of the SoundTraps low frequency calibration setup. SoundTraps factory calibration is performed using two methods. First a tank calibration is done using a sound source and a reference receiver. The pulse method is used to avoid reflections. Unfortunately the test tank size constrains this to a minimum frequency of about 10kHz, below which the required pulse length becomes too short to get an accurate measure.
For low frequency response we use a 250Hz sound calibrator. At this low frequency the response of the hydrophone in air is a very close to that in water, making it a simple measurement.
A Centre technology model 327 250Hz 94 dB sound calibrator is used for the sound source. This is coupled to the hydrophone with a custom coupler, which allows the addition of sound meter. We use a Bruel & Kjaer 2236 Class 1 sound meter to determine the pressure in the coupler, noting that the pressure may not equal the nominal output of the calibrator due to the non-standard coupler volume.
We’re in the process of getting a batch of the couplers made to make available for those that wish to do their own calibrations. These can be be used with most calibrators or piston phones with a 1″ port.
Happy Calibrations
John