Underwater speaker...

[Externet]

Hi.

In your opinion, would a plain speaker propagate sound reasonably fidel underwater if lowered to 30m depth inside a hermetic plastic bag filled with light vegetable oil and if needed, a little air left in by the moving coil ?

[Moontanman]

I'd leave out the oil.

[John Cuthber]

At 30 metres the cabinet will be crushed.

[ACG52]

Id guess that the interface between water and the speaker would pretty much prevent the sound transmission.

[Acme]

Why not just buy and/or copy the design of a commercial model?

 

Lubbel Labs

[Externet]

At 30 metres the cabinet will be crushed.

John:

The speaker in the oil filled bag is meant to be like this:

----> http://www.louisvillemusic.org/techcolumn/files/2012/03/speaker2.jpg

 

Not this:

----> http://www.graphicsfuel.com/wp-content/uploads/2010/11/wooden-speaker2.png

[Enthalpy]

Passing sound from a solid to water is waaaaay easier than to air because the hard impedance mismatch vanishes.

 

Then, a standard commercial loudspeaker is optimized for air operation, for instance with a large thin membrane, which fits water less well.

 

You could try to use a tweeter as medium or boomer in water. Just put a stiff panel around it, wide but not closed. Take a stethoscope to check the result.

 

And if your goal is to inject sound in water, a piece of piezo material suffices, no need for a fragile loudspeaker.

[John Cuthber]

John:

The speaker in the oil filled bag is meant to be like this:

----> http://www.louisvillemusic.org/techcolumn/files/2012/03/speaker2.jpg

 

Not this:

----> http://www.graphicsfuel.com/wp-content/uploads/2010/11/wooden-speaker2.png

Do you know why they put speakers like the first picture into cabinets like the second one?

If not you might as well give up on trying to design an underwater speaker system.

[Externet]

Do you know why they put speakers like the first picture into cabinets like the second one?

To isolate as much as possible the rear facing sound waves from interfering when bouncing, with the front waves causing both constructive and destructive fringes producing unwanted acoustical effects depending on the listener location.

 

Which I do not care for the intended tests.

[OptimisticCynic]

At 30 meters depth the high frequencies will be reduced. The pressure of the liquid against the speaker membrane will likely tear the membrane. It would be very easy to overdrive the speaker in liquid. If the speaker survives, at 3 m depth you might get reasonable fidelity, but not at 30 m.

[John Cuthber]

To isolate as much as possible the rear facing sound waves from interfering when bouncing, with the front waves causing both constructive and destructive fringes producing unwanted acoustical effects depending on the listener location.

 

Which I do not care for the intended tests.

No.

it's to stop the air pushed out from the front of the speaker simply moving round to the back.

 

You probably do care about that because it drops the efficiency to near zero for wavelengths smaller than the speaker

With a 20 cm speaker the "sound" can get from the front to the back at about (1.5Km/sec) in something like 100 µseconds.

So, only frequencies higher than about 10KHz will actually be radiated efficiently.

Were you planning to broadcast to underwater dogs and bats?

 

There's another issue. The speaker is designed to work in air.

It's built to be strong and heavy enough that the speaker frame (magnet+ mountings etc) stay still and the (very light) air gets pushed around.

However, under water it will be trying to move essentially the whole pool full of water (the water's practically incompressible so it cant squeeze it- it has to shove it out of the way).

What will happen is that the speaker diaphragm will stay still and the rest of the speaker will move.

[Enthalpy]

At 30 meters depth the high frequencies will be reduced. The pressure of the liquid against the speaker membrane will likely tear the membrane. It would be very easy to overdrive the speaker in liquid. If the speaker survives, at 3 m depth you might get reasonable fidelity, but not at 30 m.

 

Why a reduction of high frequencies at 30m? Seawater attenuates higher frequencies, but it's a matter of distance, not depth.

 

Why 3m? If the membrane has 1atm air behind it, it will be detroyed well before 3m depth. If it has water on both sides at equal pressure, the membrane will resist much more than 30m depth.

[The cabinet's role is] to stop the air pushed out from the front of the speaker simply moving round to the back.

 

You probably do care about that because it drops the efficiency to near zero for wavelengths smaller than the speaker [...]

 

[...] under water it will be trying to move essentially the whole pool full of water (the water's practically incompressible so it cant squeeze it- it has to shove it out of the way).

What will happen is that the speaker diaphragm will stay still and the rest of the speaker will move.

 

I'm a little bit more optimistic here. An obstacle that makes the path from the rear about a quarter wave longer already avoids this loss. If the obstacle is smaller, there are losses, but we still hear sound - any loudspeaker designer has to accept heavy losses, as ausio quality items are about 1% efficient. Only resonating loudspeakers (buzzers) are more efficient.

 

Sound velocity resulting from the compressibility limits the amount of medium that moves in phase to 1/4 of the wavelength, and for a plane wave, the pressure corresponds to the mass of about 1/6 wavelength. Moreover, this pressure is in phase with the displacement speed, rather than 90° in advance as mere inertia would tell.

 

Which doesn't solve the worry of loudspeaker mass, sure. As some wall or cabinet is needed (with something that evens the fore and aft static pressures out), it shall bring inertia too.

 

That's one reason why underwater acoustics engineers like piezo materials. These change not only their shape but also their volume with the electric field, so they can (and often do) operate without a cabinet. Since water's caracteristic impedance isn't so much different from a solid (especially Pvdf), the sound passes rather well from the solid to water without any membrane nor adapter. But adapters are possible, sure - they must just resist the pressure.

[John Cuthber]

One big reason that designers of hydrophones like piezoelectric materials is that they are even stiffer than water; they really can shove water out of the way.

Enjoy your optimism; perhaps you can take video and put it on youtube.

 

You are quite likely to succeed for a simple reason. It's easy to get an amplifier that will dump a hundred Watts of power into a loudspeaker.

And your ear will hear signals of the order of a pico watt per metre squared so (with a roughly 1cm ear hole) you can throw away 99.999,999,999,999,99% of the signal, and still hear it. (I may have lost track of the nines there, but I think you get the point). The signal you transmit is a million million million times bigger than the one you need to receive.

[Acme]

Hi.

In your opinion, would a plain speaker propagate sound reasonably fidel underwater if lowered to 30m depth inside a hermetic plastic bag filled with light vegetable oil and if needed, a little air left in by the moving coil ?

What's the purpose of the speaker? I mean who will be listening and to what?

[Externet]

Hi acme.

Details omitted, the purpose is to send unidirectional voice commands from a boat P.A. to a crew of divers to hook a rope that tows which selected item to another nearby location, and release, repeatedly; the speaker being by the end of the rope, and they do not want to purchase commercial equipment for only an hour of use. Am currently putting together the contraption with the speaker inside an oil filled intravenous bag (it's sturdier), no bubbles, heat sealed, cable trough its hose exit, sealed, all inside a flatty plastic box perforated both sides. Will let know success or failure.

Yes, could be done from the surface too with a floating proper speaker.

[Acme]

Hi acme.

Details omitted, the purpose is to send unidirectional voice commands from a boat P.A. to a crew of divers to hook a rope that tows which selected item to another nearby location, and release, repeatedly; the speaker being by the end of the rope, and they do not want to purchase commercial equipment for only an hour of use. Am currently putting together the contraption with the speaker inside an oil filled intravenous bag (it's sturdier), no bubbles, heat sealed, cable trough its hose exit, sealed, all inside a flatty plastic box perforated both sides. Will let know success or failure.

Yes, could be done from the surface too with a floating proper speaker.

Details wanted. Anyway, you don't need voice or high fidelity for that or even a proper speaker. Morse code or some simple variant. 3 'sounds' (beep, click, clunk, etc.) for item #3 and so on.

[Externet]

Thanks. Correct, but a speaker is what was requested from me as diver and involved on acoustics and electronics. Plus my curiosity on testing such.

 

For simple attention getter, I have used these very successfully for a long time :

 

----> http://www.scienceforums.net/uploads/monthly_01_2013/post-295-0-21064300-1358975176.png

 

I believe the speaker cone immersed in fluid should propagate sound bidirectionally well enough; that is why I asked here. Some valid opinions are contrary and I respect them, but will proceed with the experiment anyway, as it is extremely simple.

[Acme]

Thanks. Correct, but a speaker is what was requested from me as diver and involved on acoustics and electronics. Plus my curiosity on testing such.

 

For simple attention getter, I have used these very successfully for a long time :

 

---->

 

I believe the speaker cone immersed in fluid should propagate sound bidirectionally well enough; that is why I asked here. Some valid opinions are contrary and I respect them, but will proceed with the experiment anyway, as it is extremely simple.

Acknowledged. Mum used to hate those clickers when we brought them home from the fair. I await experiment results with interest.

PS Have you tried using the speaker above water now that it's bagged in oil? I can't stop thinking that the oil will soften the cone and so it won't oscillate except at the very center where the coil is connected. ??