Many times hydrophone recording contains unwanted noise like 'wave noise'. How to deal with this noise at the source? Is there any way to deal with this in the analysis stage? as abrupt noise impacts the detection of targeted sound.
There are a couple of principles to enhance the signal to noise ratio
- bandpass filtering removes out-of-band noise
- median filters attenuate impulsive noise, when one is interested in tonals.
- matched filter is signal has characteristic waveforms
- notch filter if there is a particular frequency interfering.
Edit: As Rasmus indicates, better avoid generation of noise in the first place and design the system such that it does not generate so much noise. OK, it is not always easy. E.G. a chain is a clever way to add weight and to minimize vertical impulsive stress from recorder. But, system design was not mentioned in OP.
You originally asked for 3 different noise sources, and you ask how to deal with it at the source (and later in analyses):
- Bumping on sea floor: If your hydrophone is bumping directly on the seafloor (unlikely given your fine illustration) then that's the real problem, and hard to solve for you without more information on the setup. If the bumping is the noise from your anchor/acoustic release bumping on the seafloor, then either have your top float sit lower in the water column (less wave heave effect) or make your anchor softer (wrap it in something soft and bio-friendly).
- Chain noise: Use heavily bio-fouled chains (leave them out in the water for a long time before deployment), or wrap in other bio-friendly soft material. Alternatively, don't use a chain...
- Wave noise (the currently mentioned issue): This is presumably from the float? is so, try to deploy it deeper. If this is surface wave noise, there's not much you can do at the source.
In analyses: As @WMXZ writes there are many options for DSP solutions, but without knowing more about what sounds your are interested in keeping, it's hard to suggest a good approach. You cannot just eliminate all wave noise in the analysis stage, but you can likely make filters that will improve the detection/quality of the signals you're looking for. Please add some information as to what kind of sound you're interested in detecting.
Not sure if this is something you could consider, but my lab just had these floats custom made to hold a SoundTrap and F-POD with the goal of stabilizing the hydrophones, reducing strumming, etc. They'll be deployed at the beginning of August, so I’ll let you know how it goes!
If you can deploy multiple hydrophones (either on same cable or on separate cables) and synchronize their recordings, it is possible to filter signals based on their arrival angle.
Two hydrophones vertically separated on same cable can distinguish arrival angle in the vertical direction, allowing you to focus on signals coming from the target depth horizontally.
Several audio mixing programs (DAW = Digital Audio Workstation) has noise reductions functions. It is possible that if you use one of them you could get a good reduction. Some of them work on taking a no-signal noise signature first. The algorithms seems to be proprietary. A free example with source code is available in the program Audacity. To say one single product (expensive though) used in recording the Izotope Rx comes to mind. There seems to be specialized tools used in forensic evidence situations, I have no names there though.