Hi.
A speaker half submerged in water emits a signal. Two equal, equidistant, vented microphones, register their readings. The submerged microphone should show larger, equal or smaller amplitude ?

 

I think the answer depends on how well matched the acoustic impedances of the speaker/mics are to air and water.
Mind you, you'll have to do the experiment quickly, before the water gets in and wrecks the speaker/mic .

 

I would say that, since the water is a much denser medium, the speaker would be constrained to very small motion, and most of acoustic energy would go into the water.

 

But water being incompressible transmits the wave hydraulically so what
prevails, better transmission thru media or as Crutschow says lowered
wave amplitude...?

Regards, Dana.

 

I vote with Danadak. The water will transmit sound with less loss due to it's relative incompressablity. However it requires more power to put into the water because it is a much heaver(denser) medium. If you assume for the purpose of this question that the lower half of the speaker (in water) has exactly the same cone deflection as the portion in the air AND that the sensitivity of the two microphones is identical then there should be some amount of increased signal on the water side. Note that this imbalance is not linear with increasing frequency. This will change as a log function I think, with air having much less transmission at the higher frequencies. I don't know about transmission frequency in water but it might be similar.

 

Replace the speaker with a bell and hammer.....and the results will make sense.

Applying the speaker cone at surface interface introduces huge asymmetry in the cone reaction.

Air is elastic and the water isn't.

 

If you replace the single speaker with two identical speakers, one submerged and one above, would the answer to that question address your original question? As is, you'd get distortion of the cone, splashing at the interface, and all sorts of unpredictable stuff.

Is the root question really about power transfer versus distance for audio signals in air versus in water? I mean, it's kind of interesting to ask how far away can you hear a 1W audio signal in the two media. I think/predict that air will win. Loss of signal is related to momentum loss, and this will be greater in the denser medium. Then again, whales communicate over miles and miles.

I swim almost every day and despite having quite a bit of college-level physics, one question that I can't answer is why waves move slowly the way they do while sound travels very quickly under water.

 

Hi.
Yes, two identical speakers driven at the same power would work.
I believe there is zero splashing on the submerged one, like any waterproof transducer. The wet cone will not extend fully or fast enough as the one in air if the frequency reverses amplitude before full extension as takes more time to fight against the incompressible fluid, the sound will reach the wet microphone first -but that is only the initial wavefront- (1.48 Km/s versus 340 m/s)
Sound attenuation in water is a small fraction of that of air.
Suppose 1 Pascal SPL; the dry microphone diaphragm moving forth and back 0.01mm... would the wet one vibrate less, same or more ?

The frequency obviously plays a role to overcome inertia of fluid.

For the second part of your comment - not related to the original post- attenuation is much, much less in water, reaching way farther to a given level. An interesting observation you pose is Watts delivered to speaker(s) and the SPL difference that each can impose in wet and dry cases.

 

I swim almost every day and despite having quite a bit of college-level physics, one question that I can't answer is why waves move slowly the way they do while sound travels very quickly under water.

It depends on the type of wave and the depth. A deep gravity Tsunami wave can move at 500 miles per hour, much faster than any surface gravity wave from wind, weather, tides, and currents at the uppermost layer of the water with air bubbles in water.

It's important to ships that sink and come back up.
https://en.wikipedia.org/wiki/Gravity_wave

 

Hi,

The compliance of the speaker becomes part of the problem.
Replace that with a hydraulic piston and repeat the experiment. It would be like an 'open air' hydraulic system,which means lots of secondary things come into play llke gravity which normally is not part of a hydraulic problem (except of course when it comes to just lifting things).

 

Hi Al.
Now you got me thinking that a solenoid driven vented piston at audio tones can equally send sound underwater instead of a wet speaker...

 

Yeah I was sort of taking for granted that the ideal device for imparting a 1W signal under water is going to look a lot different than a 1W speaker cone. I was thinking more about the attenuation with distance and apparently I guessed wrong. My experience is that humans certainly don’t hear very well underwater but that doesn’t mean the signal doesn’t carry.

 

Hi,

I think if you put your head under water and someone knocks on the side of the tub you can hear the thump. Whales dont seem to have any problem communicating over large distances under water.

Yeah a solenoid piston might work. It would take a lot to move it fast, and the area of the piston would make it harder to move as the area got bigger. It would definitely travel, but how well it is hard to say and what depths would work and how that would vary the signal would be interesting too. It's a job for finite element analysis i guess.

It would definitely set up a wave, but the wave would spread out just like with a speaker and air. That means much less signal at a given point at a given distance from the source.

 

Here's my reasoning: the output of each microphone is proportional to the change in pressure of its diaphragm. The acoustic impedance of water is significantly larger than that of air, so for an equivalent pressure amplitude, the submerged microphone will experience less Δρ than the in-air microphone. Likewise for the speaker: for a given excursion amplitude, the Δρ will be greater above the water than under. These two effects combine, thus the submerged microphone will produce a significantly lower output.

 

Yeah a solenoid piston might work.

Sounds reasonable. Water is roughly 1000X denser than air, so at first blush I would expect the area of a speaker cone under water should be ~1000X smaller than a regular speaker to achieve a similar relationship between power supplied and displacement of the cone.

On the other hand, water is incompressible and that may change everything. A very small displacement would make a large pressure wave since the water can't compress at all.

What does a submarine's sonar transducer look like?

 

They look like hockey pucks of varied diametres. They are piezoelectric, not dynamic, embedded in silicone to adapt impedance to water. The Furuno sonars I have installed are like 3 inch wafers, in inverted periscopes trough hull bottom, orientable in azimuth and elevation as they are gimbal mounted inside the domes.

----> http://www.caravan-ny.com/transducer_tr_232b.html

 

Hi,

They say that water is not compressible, but we have to remember that is with water that has no dissolved oxygen. Water itself is actually compressible, but only very very slightly so without dissolved oxygen. In any case, there will be a wave front set up and we even see this in solids like steel.

Another idea would be to use a large plate, then tap the plate at the center to send a signal. The large surface area should cause a larger signal amplitude. Tapping with a hammer for example should send some information outward, tapping with a solenoid would allow electrical control.

I never tried this but it seems reasonable that it should work to some degree. Lower frequencies would probably travel farther.