# How to measure acoustic pressure level under water with a hydrophone?

this is my first post here and I am also new in signal processing.

Let's say I have a hydrophone with amplifier which is connected to a sound card plugged in to a computer. I gather the sounds to a WAV file every 3 seconds and analyze each file in a Python program. The system is supposed to detect loud events which are dangerous for sea animals.

I would like to know how to reliably measure acoustic pressure (loudness?) of events occuring under water based on WAV data. I am not sure if I should use exactly the same formulas as measuring SPL in air. I do not have the hydrophone yet so I can not really tell anything about it at this moment, but I am pretty sure the catalog notes of the hydrophone will be available for me when I finally get it.

It would be good enough if the measurement had an error under 5 dB SPL. It does not have to be perfect, I just want it to be relative and adapted to under water characteristics.

Could the simple measurement be calibrated by shifting measures by single constant in dB?

I found a similar solution in Determine Sound Intensity in SPL. But the problem is I do not know if I will be able to have all the data needed for this, mainly about amplification levels. Would the calibration-like solution be good in this case? How can I do it?

• I doubt you can without knowing any information about the response of the hydrophone, the impedance of the cable, and the specs of the DAC in the soundcard. If you do have information from all of those things, then you can probably get a reasonable estimate. Commented Aug 16, 2018 at 2:01

A hydrophone should come with at least a typical response curve.

The thing to aware of is that that response represents an idealized far field response in an infinite space referenced to an wave of an ideal point source taken at one meter (or yard).

A good reference that covers the conversion of acoustic to electrical energy is

Horton, Joseph Warren. Fundamentals of sonar. United States Naval Institute, 1959.

but his book is scarce as hen's teeth and requires luck or access to a good University library that collected physics and engineering books from the 1950's.

There are other books on transducers. Essentially an accurate SPL measurement is a bit more than a hydrophone hooked into a sound card. The field expedient calibration method is to use a calibrated source at a known direct path distance at least 100 wavelengths in distance.

In most cases the cable you need is going to be too long for a unamplified transduction signal. It's best to have the converter at the sensor and send over a digital signal but have weak analog signals close to digital circuitry.

Long cables usually imply higher voltages to minimize resistance losses. Sharks are attracted to cables that have high voltages so don't be surprised when you see cables offering Kevlar with steel reinforcement.

Ideally access to pressure chambers to test your the water-tight integrity of your hyrophone cable is a very good thing to have if you expect to dangle more than 20 feet and the deeper you go the more important it is to pressure check your set up.

• Aside from that I really wonder the following: acoustsical sound waves on air are carried as compression waves; air molecules are compressed and decompressed so that the pressure increases and decreases as wave passes a point. Now I wonder what's the nature of underwater acoutical waves considereing that water is virtually an incompressible fluid ? How does the pressure wave travel accross water? Commented Aug 16, 2018 at 20:16
• It’s been a long time since I looked at the first principles derivation. It’s probably in Morse but there aren’t any infinitely incompressible materials. There is particle displacement
– user28715
Commented Aug 16, 2018 at 20:33
• I checked the wiki page and it states it as a pressure wave... So that slight compressibility is accounting for the pressure wave in it... Yet the shear forces required to compress it just a little bit makes me wonder how powerful should the transducer be? Commented Aug 16, 2018 at 21:28
• Well, I’ve seen pictures of one system where each transducer was a little smaller than an old Volkswagen Beetle and the array consisted of a few tens of these dangling in a vertical line. High frequency side scan sonars that look like a torpedo can be easily lifted by 2 men. There are pingers that around the size of a beer can. There are all sorts of sizes. It depends on the application
– user28715
Commented Aug 16, 2018 at 22:56
• never seen one :-) Commented Aug 16, 2018 at 23:43