Converting dbFS to dB SPL A weighted

Currently studying the differences in boat noises and have audio files of each that were recorded from the same microphone at the same distance underwater. With this knowledge am I able to use audacity to convert my negative dbFS to dB SPL A-weighted. Lets also presume that I’m comparing the boat noises to the lowest sound a human can hear

If anyone knows id this measurement would change because it’s in the water please let me know thanks!

It’s going to be VERY hard finding electronics that quiet. But you shouldn’t have go do all the way to 0dB SPL, because the only place that quiet (on earth) is some anechoic chambers.

You’ll need a calibrated measurement mic. USB measurement mics have a known-published relationship between SPL & dBFS. Or you can make your own calibration with an SPL meter. But a measurement mic is calibrated for frequency response.

REW (FREE) can do A-weighting.

Do you have a calibrated hydrophone?

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If I used my baseline as the average noise of water without any ships and compared them would that be possible?

My hydrophone is the “Subsea acoustic Aquisition system Jason High Blue” or Jason High Blue for short. Could clarify what you mean by calibrated? I also can acess the specifications if needed

Thanks again,

That should be possible.

It means you have a known relationship between the sound level and the electrical or digital level.

Measurement mics are also calibrated across the frequency range. They give you a digital file of the frequency response so the errors/variations can be taken out of the measurements.

USB measurement mics are calibrated for level and frequency. Analog measurement mics are only calibrated for frequency response and it’s up to you to calibrate the level.

With acoustic mics it’s “easy” to get a mid-frequency level measurement/calibration (ignoring frequency response). Take a reading with an SPL meter and a dBFS reading. Then you just have to subtract. A 3dB reduction in dB SPL will result in a 3dB reduction in dBFS reading, etc.

If you have a digital hydrophone the specs might give the dBFS level in relation to the SPL level. (I don’t know if it’s called “SPL” under water.)

With an analog mic the spec usually gives you a certain mV at a certain dB SPL level. For regular acoustic mics the standard reference is at 94dB SPL. The most famous microphone of all time, the Shure SM57 puts-out 1.6mV at 94dB.

But you’ve usually STILL got two unknowns - You don’t know the gain of the preamp and you don’t know what voltage into the analog-to-digital converter corresponds to 0dBFS.

So usually, you need an SPL meter.

MAYBE you can put an SPL meter in a zip-lock bag and drop it in the water with the hydrophone? You’d also need a speaker underwater to generate a constant test-tone.

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To be more specific the hydrophone is the C75 sorry for confusion

The hydrophone has a preamplifier Gain of 20. I’ve been looking for the analog to digital converter voltage that corresponds to 0 db you speak of but having a hard time. I found this statistic that says SPL Equiv. Self Noise at 1kHz [dB, re 1µPa/√Hz]

Here’s is all the statistics I could find on the hydrophone