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Underwater sound

A member asked on IBSNET for advice on hydrophones.

Chris Woolf responded:-

Hydrophones vary quite a bit in quality from high grade phantom powered  devices to some pretty poor plug-in power versions.

The are invariably omnis and since the propagation of sound in water is very different to air you can expect a lot of LF from a long distance away and somewhat less HF.

The sound won’t be “natural” since you don’t normally hear that way underwater – so it doesn’t really matter what it sounds like. Results can be interesting and good fun.

An ordinary pressure omni (waterproofed) can work but you can’t dunk it more than about 15cm and expect anything useful.

Roger Long offered:-

I recommend the DPA hydrophone. I did some under ice recordings in Greenland with the presenter on an UW radio link and mixed in the DPA for amazing reverb from the ice field.

Hired from Hazardous in Plymouth.

Terry Meadowcroft amazed everyone with:-

I was lucky enough to have some time to think about this problem, and built an apparatus which, surprisingly, gave stereo images underwater.

Each individual transducer consisted of a hard rubber ball (you know, the ones that bounce amazingly well), drilling a hole just a little wider than a Tram mic., to roughly the centre of the ball. A Tram mic was pushed into the centre of the ball, and sealed into the ball along with a small air space, using plumbers’ silicone sealant (clear).

My thinking was that water is incompressible, the plastic of the ball quite incompressible too, so pressure changes at the surface of the ball would pass fairly unreflected, into the ball (whose specific gravity was a little greater than 1, more or less the same as the water, and the air space around the mic. would be the only compressible thing around.

Imprecise, I know, but boy, does it work!

The underwater sounds in Lake Taganyika were all there, HF and LF alike, no LF overloading, brill!

I mounted four such balls on the ends of a cross made from about 2.5ft. long aluminium aerial rods, flattened and swivelled in the cetre, held tight by a wing nut and bolt.

The four mics were connected to the four channels of the Deva II, and lovely recordings resulted. I know that sound travels much faster in water than in air, but the stereo (4.0 as well) was undeniable, and I’m sure that if I had had more time to experiment more (I built the thing on location in my spare (!) time), stunningly good results were to be had.

As it was, the bubbles of a diver did move around the picture, and the voice of the diver sounded, well, like an underwater voice.

Give it a try; cheap, good quality, and I think you’ll like it!

Chris Woolf commented:-

All pressure omnis are essentially waterproof in the sense of the capsule being a sealed box. The way the connections, electronics etc are handled and certain other refinements are what makes the difference between "waterproofed" and standard mics.

The limitations of most pressure omnis is that they are also effectively barometers. They can cope with slow changes in the pressure of surrounding air but suddenly increase the pressure (slam a car door, or dunk a couple of foot underwater) and the diaphragm whams into the backplate and stays there for a long time.

Terry’s technique is akin to a bathysphere and lets the mics stay at something like surface pressure in absolute terms while allowing the relative pressure of sound waves in the water to give you something to listen to.

Last word from Terry:

There was one problem which I found, and did not find time to overcome.

The apparatus was connected solidly to a boom pole, and of course sounds from anything in contact with these little ‘bathyspheres’ (thanks, Chris) pass every little creak and rattle to the mics, and the pole is effectively ‘in contact’ with the balls via the incompressible water, even if it is elastically mounted under the water. So I think that the pole itself ideally should not enter the water (even though the excellent Ambient pole showed it can take a good bit of dunking – unlike my old VDB ones).

I suppose you could think of the pole as being as near to the Tram as the walls of the hole in which it sits, all lonely, because everything in between is relatively inelastic, and so not very lossy to sound vibrations.

When you consider that the lovely squeaking and nibbling sounds made by the local fish could easily be heard, as could very distant boat engines, any vibrations from the pole – which is very solidly connected to the mic by the inelastic water – are easily heard.

So my next step would have been to evolve an elastic mounting going down into the water from the pole, yet allowing manipulation of the array.

Much more fun ahead, I’m sure!

 

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