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Old 3rd June 2012, 05:11 PM   #1
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Default Need an advice on compression driver use for pneumatics

Hello,
I'm looking for solution to create pneumatic pressure oscillations of about 100Hz inside 0.5 liter rigid tank.
So i came up with idea to drill a 1" hole in the tank wall and seal a compression driver. Then, if i'll drive 100Hz sinusoidal signal throw power amp, its diaphragm will oscillate and this will produce the desired 100 Hz pressure oscillations.

But the 1" compression driver is basically a tweeter....

So i thought (i would appreciate if someone would say if i'm right or wring) that 1" compression driver can't produce low frequency ACOUSTIC waves because its small size, but the coil would operate well in-and-out at low frequency...
So if this tweeter is located on sealed tank. it would force PRESSURE oscillations inside the tank.... am i right?

And if it is so, can anyone advise what type of compression driver has the largest stroke if my power drive is about 30 watts?

Thanx,

Eli.
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Old 4th June 2012, 01:26 PM   #2
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I can see a couple of problems, both having linearity implications. Firstly the static pressure due to the head of fluid could displace the diaphragm off centre in the linear excursion region. Secondly the diaphragm will be loaded very differently on each side. What ammount of non linear distortion are you prepared to tolerate?

Keith
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Old 5th June 2012, 05:24 AM   #3
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I would try using a polypropylene driver or a smaller speaker that has a plastic cone the water shouldn't bother it as long as there is no salt or other corrosive additives.

You can also seal the back of such small drivers with a plastic cup of some sort or some PVC pipe.

I used to have a few of such drivers in the 2" to 4" diameter range.

Many dog toys and simple little noise makers and somtimes old remote phones have such a driver in them of 3/4" to 1.5" diameter range as well.
This may work well for your application but with a limited amount of excursion.

If you do find a poly driver to use and choose to seal the back of it and are afraid if the surround is porous, You can seal it with a very thin coat of flowable silicone sealant,the stuff used to seal windshields that you can find in any hardware store.

This is the same stuff that I repair my torn surrounds with.
It works great and is very flexible as long as you create a very thin layer.


Good luck!

Jer
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Old 5th June 2012, 09:35 AM   #4
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The distortion isn't so big issue to me because this isn't hi-fi audio but pressure oscillations and i can filter out the harmonics from the sensor output.
The question is what maximum stroke can i get with commercial 1" - 2" speakers?

On the other hand, the 2" speakers easily come to 12KHz and up...
So if i'll add additional mass to the membrane, i guess it still would be good for up to 300Hz...
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Old 5th June 2012, 10:41 AM   #5
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Hmm. I doubt a commercial CD diaphragm would stand up to the load of driving an incompressible fluid like water. They are designed for air loading, and aren't physically that strong. You might need to graft the VC and suspension onto a disc of something like 1mm ali.
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Old 5th June 2012, 11:59 AM   #6
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Guys, why are you talking about water? This is a pneumatics problem and the OP doesn't mention water at all...
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Old 5th June 2012, 12:35 PM   #7
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Doh...
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Old 5th June 2012, 12:45 PM   #8
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Yup, water would be hydraulics.

A CD would certainly introduce a 100hz frequency into the tank, however, it may not be very durable. What kind of pressure are you looking for? If you need higher pressure then you could use a diaphram pump or modify a belt drive compressor since it seems like you are after pressure oscillations at a certain frequency. Does it need to be variable?
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Old 5th June 2012, 01:26 PM   #9
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Oops your right, My Bad !!!!

A compression driver driver is what was used in my Heil Talk Box unit.
It would produce some lows but I am not sure how durable it is.
As mine was quite raspy sounding when it came to the lower notes on the Guitar.
Knowing me I probably cracked the diagphram prematurely.
So I will never know if it was supposed to sound clean or not,But it still worked well though.



jer

Last edited by geraldfryjr; 5th June 2012 at 01:38 PM.
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Old 5th June 2012, 02:28 PM   #10
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Default Some clarifications:

The device will operate in air, not in water. At future it would be needed to be durable in high humidity environment, but not this time.

I'm looking to conduct an experiment, therefore i'll need to sweep the frequency from less than 100 Hz to 25 Hz.
Maybe after i'll find the desired frequency i'll be able use another type of oscillations source - something which operates on mechanic resonance for instance...

The pressure in the tank is expected to be +/- 30 millibar (+/- 0.5 psi). It isn't so much, but razing the question if large speaker diaphragm can hold it.
On the other hand, i need to compress very quickly +/- 12cc of air by this diaphragm, so in case of 2" speaker the stroke would need to be around 5mm...
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