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Underwater speakers

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Doubt me? Submerge yourself in water and have someone standing right next to you scream as loud as they possibly can (with their upper body out of the water) You'll barelly hear a thing because there's an air column still in your ear that meets with the water, and an air cavity at the surface of the water to the person yelling.
No, it's not that simple.

If I'm submerged and you're shouting, of course most of the sound will be reflected but this isn't the case.

If the speaker is placed above the water with no tube, when the cone moves most of the pressure wave will be coupled straight to the air and escape. When the tube is added, when the cone moves, the air is now trapped inside the tube, when the cone moves forward, the air will push the water inside the tube out of it and when the cone moves back, it will suck more water into the tube. This is because the tube has greatly improved the impedance matching between the speaker's cone and the water.

Take a piece of string, hold one end in your hand, get a friend to hold the other end and pull tight. If you shout at the string and your friend puts his ear next to the other end of the string. He'll be able to hear the shouting but only because the sound is travelling through the air.

Now attach a plastic cup to either end of the piece of string and repeat the experiment. If you shout into one cup and he puts his ear to the other cup, it'll be very loud. This is because the cups have improved the impedance matching between your mouth and his ears to the string.
 
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I have to say Sceadwian is right with this:

When I shout in the water, a person even right next to me can't hear me very well. I think this is because my whole mouth is full of air when I'm talking. What they can hear is rather quite, mostly muffled bass.

I think you would have to completely submerge the speaker, with no air involved to actually get some good sound in the pool.
 
That's a totally different situation.

I don't see why the speaker cone needs to be in direct contact with the water.

Here's a drawing which illustrates what I was saying in my previous post.

When the cone moves down, the pressure of trapped inside the tube increases, pushing water out of the tube.

When the cone moves up, the pressure of the air inside the tube decreases, sucking water into the tube.

Without the tube the air will escape so the energy will be lost to the surrounding air.
 

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Hero, why can't I talk to a person on the other side of a wall? You're assuming that ALL of the energy imparted by the speaker in the air column will be transferred to the water simply because it is indirectly coupled to it, this is absolutely positively not the case. The density change between the air and water mediums will act as nearly a perfect reflector as a solid to a gaseous change would, in fact the change might be better if it were TO the solid form of water because the density change is actually lower (Water ice is less dense than water)

In reality most of the energy will be reflected by the density layer (air to water) and simply bounce around the tube dispersing every time it bounces as heat in the materials. As far as your last post about the air pressure differentials, they only equate if the frequency is EXTREMELY low, like bellow 30hz, the bulk of anything above that will bounce around like a mad fiend trying to find the best way to eliminate it's energy.

Don't believe what's been said so far? Do it yourself, you'll see then, don't want to try? Fine no problem, but don't say we're wrong if you haven't. Mind over matter only works in the movies, in reality you have to prove things with data and experimentation. Acoustical lensing like this is very similar to diffraction in solid transparent materials with light.
 
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Hero, why can't I talk to a person on the other side of a wall?
That's a bad example.

Now try attaching glasses to either side of the wall and you'll be able to have a conversation.


In reality most of the energy will be reflected by the density layer (air to water) and simply bounce around the tube dispersing every time it bounces as heat in the materials. As far as your last post about the air pressure differentials, they only equate if the frequency is EXTREMELY low, like bellow 30hz, the bulk of anything above that will bounce around like a mad fiend trying to find the best way to eliminate it's energy.
That makes sense, some of the higher frequencies will bounce back and fourth between the speaker and the water.


I'm now interested, I think I'll experiment with this.
 
Yes it was probably a bad example, but a glass against a wall is not the same as a transducer in an air cavity inside a glass tube against a wall. Hopefully your experiementation will help you understand.
 
I don't see why the speaker cone needs to be in direct contact with the water.

Here's a drawing which illustrates what I was saying in my previous post.

When the cone moves down, the pressure of trapped inside the tube increases, pushing water out of the tube.

When the cone moves up, the pressure of the air inside the tube decreases, sucking water into the tube.

Without the tube the air will escape so the energy will be lost to the surrounding air.

All that will happen is the air within the tube will compress under the influence of the pressure wave from the cone, very little will transfer to the water.
 
Exactly as Ross said, as has been said at least twice now.
 
What about a flexible membrane at the end of the tube?
 
A membrane could help with waterproofing, but it wouldn't change the energy transfer. The mismatch between the densities of air vs. water is about 1:800.
 
The Humpback whale can make sounds heard from miles away, so there must be a way...:)

Wish I took more interest in sonar when I was in the navy.
 
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I have another idea, the schematic should explain all.

You might need more than one rubber suspension if the rod is long.
 

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Hero, I can't see how that schematic is practical at all. You're still going to have the impedance mismatch, you're just adding a secondary impedance mismatch (the rod) to the equation.
 
Have you tried it?

I know that using a couple of cups and a piece of string seems to work well enough and a steel is a better conductor of sound than cotton so it should be better.
 
Hero, I didn't say it wouldn't work, it won't work well, and it's totally impractical for a real world device.
 
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Hero, I didn't say it wouldn't work, it won't work well, and it's totally impractical for a real world device.


Why do you think it's so hard to couple the energy from a speaker's cone to water without it being direct contact with the water?

I don't believe it's that hard to do, it should be easy.

I made a mistake in my diagram above, the cup in the water should be a low smaller than the one in the air because it will loose it's energy much more easily to the water.

Maybe I am going about it the wrong way, a horn might be better?

Unfortunately there doesn't seem to be much information about this on the internet, probably because it's not a very common problem, rather than it being a hard thing to do.

When I have a bath tonight, I'll experiment with a piezo buzzer and a funnel. Hopefully I'll be able to hear the piezo more clearly underwater than without it.

Another thing no one seems to have considered is that, even if you submerge the speaker, it won't necessarily be that good because the cone is designed to be matched to air, not water.
 
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Interesting thread. Someone mentioned whales. Given that they are mammals, it seems they would make sounds similar to us. They don't have to exhale, but they do use air to make their noises.

Whale song - Wikipedia, the free encyclopedia

It seems that they transfer sound to water quite nicely. However, the air is setting up the vibrations in the fleshy parts of the whale, which are in contact with the water. I think the best way is to have the cone in contact with the water. But it would be interesting to see if Hero's idea would work to a better degree than just having the speaker above water without a tube. However, one would think that if a certain tone was played, the frequency of which would be dependent on the length of the tube, a resonance could be set up that would blow out the speaker.
 
Mike, the cheapest unit I could find on that page was 250 dollars, for a unit that was so low powered it wasn't recommended for anything larger than a hot tub sized immersion tank under STILL conditions. The first 'scientific' unit I found had a list price of 1200 dollars, for JUST the speaker.

Hero999 said:
Why do you think it's so hard to couple the energy from a speaker's cone to water without it being direct contact with the water?
Hero, I don't think it's that hard, it IS that hard. You can believe whatever you want. Build it test it and show results that prove otherwise.

Another thing no one seems to have considered is that, even if you submerge the speaker, it won't necessarily be that good because the cone is designed to be matched to air, not water.
That has already been mentioned. It can be partially mitigated by DC coupling the speakers driver rather than AC coupling to help with the massively increased dampening effect of the medium.

Keep in mind the public swiming pool speakers are not 'under water speakers' per say. They just duct enough recognizable audio into the water to understand what is being said/sung, humans under water aren't typically concentrating on what they hear so you'd be shocked at how low the fidelity in general is.
 
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Yes Broz, quiet well, considering the bulk majority of the frequency output is under 1khz, bandwidth for moderately decent human voice signals is around 3khz. Music is 11khz or so minimum 22khz or better for stereo, and the higher the sampling rate the better for multipoint positional audio.
 
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