Helium Filled Loudspeakers

[rajama]

An idea I had some time ago...

 

The idea is reduce energy storaged internally by a loudspeaker - we do this by making it acoustically 'smaller'. The speed of sound in helium is nearly three times that in air at sea level & around 20c [hmmm - need to check that] so this makes the volume of the cabinet act like an acoustic spring through most of the range over which the mid-low driver operates, as standing waves can now only occur at a higher frequency [for a compact enclosure these would cut-in at around 2kHz instead of 700Hz].

 

This also speeds up the dissemination of energy by the cabinet, reduces reflected sound passing out thru the driver cone, and a host of other useful things... when you actually hit a standing wave mode for the cabinet, the energy stored is something like -4dB lower than with air, and at higher frequencies, as wavelength shortens and the interior takes on a 3 dimensional acoustic charactor, the efficiency of the driver falls away, by something like -16dB.

 

This is all based on some shaky calculations I did 20 years ago, so... and I haven't even gotten into the thermal implications and reduced audio compression!

 

Dull, isn't it.

 

I thought it might be a fairly obvious thing to try, yet I've seen no commercial examples, not even DIY. Since the idea exploits the properties of a gas, I imagine the patentable part would be in sealing the cabinet and driver, but as helium is usually used to check for leaks... no I haven't built one.

 

Anyone seen anything like this?

[atinymonkey]

I have never seen anything like that, and I think it sounds very interesting!

 

Most of the modern devices revolved around an adapted acoustic chamber, but they seem to be about shape and not content. I imagine if you have knocked up a design, and have the helium chamber as a unique selling point (companys hinge investment on that point) then you could sell your concept for a large chuck o' cash.

[nomadd22]

Actually, the speed of sound in helium at one atmosphere is less than it is in air. The speed of sound goes down with density.

[jdurg]

nomadd22 is correct. As the density of the gas decreases, it's ability to conduct sound decreases. This is because the gas molecules are further away from each other so sound can't travel as well. As you increase the density of the gas, the ability for the gas to amplify and transfer sound increases. This is why popping a balloon filled with carbon dioxide sounds a LOT louder than a balloon filled with normal air or helium.

 

For an optimal gas, try Sulfur Hexafluoride. It's an inert gas and is incredibly dense, so it will transfer sound very effectively.

[mmalluck]

Now, I'll admit I don't know a great deal about speaker box design, but why would fill the box with helium as opposed to just building a smaller box?

 

For a box with the least amount of stored energy, you'd have no box at all. You'd transfer the sound directly into a solid. Think transducer.

[rajama]

Actually, the speed of sound in helium at one atmosphere is less than it is in air. The speed of sound goes down with density.

 

Take a look at this reference

 

I didn't really explain fully, hence - perhaps - the confusion: we do have a transducer - the usual kind of cone - fixed to a cabinet or box in the usual way, nothing different from the usual hi-fi design.

 

The purpose of the box is simply to prevent the rear travelling wave interferring with the sound reaching the listener from the front of the cone.

 

With hi-fi (so I'm told by various texts) the problem is that the box makes noises. It stores sound energy for a relatively long after the speaker cone has stopped moving. This causes all kinds of problems, especially when you've spent a lot of time and effort making the speaker cone behave itself (choice of materials, motor selection, etc.).

 

So, we want to cut down the output of the rear of the cone. We can't dump the box, and we can't make it smaller, as it provides most of the suspension for the cone and it needs to have a specific volume, but what we can do is reduce the energy stored by the volume and increase the speed with which it is dispersed by the cabinet.

 

Sorry jdurg, I only mentioned sound speed before, to keep things simple [mistake], but yes of course Heliums density is as importent here: the product of the density and the sound speed determine the specific acoustic impedence of the gas, and this is the governing physical term in the equations relating to the efficiency of the rear of the cone as a transducer...

 

There's a lot more, but that covers the points made above... or does it?

 

Have I missed something?

[Murilo]

rajama, have you predicted the effects of Helium over the loudspeaker parameters ?

 

While helium's density is has 0.137 times air's density and speed of sound in Helium is about 3 times FASTER than in air, Sulfur Hexafluoride (SF6) is in the opposite extreme.

 

SF6 has a density 5.1 times higher than air's density and speed of sound 3 times SLOWER than in air. I have predicted some advantages from it's use.

 

I created a long html page that describes my conclusions, my thoughts and the known facts. Please have a look:

http://murilo.dyndns.org/audio/gas.htm

 

Please send me your critics (by forum or email). I have more material.

 

Do you think the main energy dissipation comes from:

(a) the heat generated by the compression of air or

(b) by the vibration (deflection) of walls?

What about energy dissipation on enclosure with ideal gas?

The highlight below explains that sound propagation in ideal gases is an adiabatic process (no heat will be generated or lost). Do you think this will also be valid for the lower frequencies inside a sealed enclosure filled with ideal gas ?

In case it is valid for all frequencies, then the energy inside it will stand untill an object absobs it (a damping material or the enclosure walls).

 

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe3.html

"When a sound travels through an ideal gas, the rapid compressions and expansions associated with the longitudinal wave can reasonably be expected to be adiabatic."

 

Regardless of this fact, an SF6 enclosure aided with damping material may reach your goal. The slower sound speed and higher density of SF6 will make the absorbtion of sound energy by a damping material more efficient.

Reasons:

- The waves will remain for longer at a given "damped area" before leaving it

- The damping material will "shake harder" due to the higher density

OBS: I suppose that the lenght of the movement of molecules in SF6 would be the same as in air.