Disclaimer: These designs are simple prototypes and in no way reflect accurate and functional models. They are supplied as sketches only. The hydrophone DIY sourced from DOSITS as an example. The ROV images are from user dezbot on thingiverse of an earlier version of the OpenROV kit. Sketchup Make was used for CAD rendering.
Hydrophone design is varied. The above image depicts 3 mount and enclosure designs based off of the DOSITS DIY hydrophone, found here. This design, with the simple mounting configurations, will be the used as prototypes for hydrophone use, system design, and data communications/software interfacing experiments with the OpenROV kit. The hydrophones are oil-filled with a potted interface module attached to the underside of the kit. Cabling, not shown, connects the hydrophone(s) to the interface enclosure, while a single communications line interfaces with the ROV kit. The yellow blocks are buoyancy compensating foam that will be needed to offset the added weight of the hydrophone(s). A stereo and mono hydrophone setup is given. The stereo system allows for directional information to be gleaned from passive reception of sonic vibration. A frequency filter, in the potted interface enclosure, will deal with motor noise and other ROV induced EM sources, ensuring near uninhibited/unmolested signal reception. The shape of the hydrophones may be deceiving. The black longer component under the green cones are the actual hydrophone housings, whereas the cones house wiring and coupling components. I'm not much of a mechanical designer, and my CAD skills are basic due to limited time, but you get the idea.
Actual build and implementation is currently in progress. In the above image, we have three prototypes that will be tested. The single hydrophone mount, and two styles of the stereo rig. The lower ROV uses a single bracket, hosting two 'phones while the above right ROV has two independently attached 'phones. The independently attached version will most likely go away, as the idea of a tool sled adapter to the ROV is under consideration as well, and would conform better to the plug-n-play concept of the kit. I'm not a huge fan of these designs, they seem bulky and cumbersome. You can adapt flatter and wider 'phones in an enclosure, similar to the interface box, which would act as a very nice single hydrophone module. The key here is that you do not want an air interface between the 'phone and surrounding water. This is two fold: 1. pressure concerns with depth. 2. Sound dynamics change when interfacing between air and water.
I will not go into the interfacing characteristics in this article, however, in will be addressed. Simple hydrophones are easy to make, yet consideration should be made to the frequency range of studies you intend. Design of Experiment is always a good thing.
Dolphin Pod Observation
Salt Water Environment
Hydrophone Frequency Range
Hydrophone Response and Ambient Dynamic Characteristics
Data Management and Logging
General Animal Interaction ...
The above list represents a subset of considerations needed in such an expedition. Take for instance, hydrophone frequency range. Dolphins communicate between 75 Hz up into the ultrasonic 150kHz. Typical dolphin social interaction lies in the lower range, where as echolocation is in the upper ultrasonic. A simple search on say, Digikey, and you find that typical microphones range from 20Hz to about 20kHz, with one or two 1MHz to 4MHz variations available. Another search for ultrasonic microphones, yields some rather pricey results typically around $1,000.00 USD. (However, there is an interesting DIY ultrasonic that I recently stumbled on that I think will be quite adaptable.)
But for a low cost, rapid prototype, we shall select a microphone within the 20 - 20kHz, and build from there.
Build notes and prelim. testing on it's way. Software interface and piping up through the data stream using Node.js in the next few months. In the mean time,
Make and make often. Make the world you want, not the one you were given.
-- Jim N.