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Old 12th February 2009, 06:19 AM   #11
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Default i had an idea for a effecient piezo indoor tweeter

a gas charge chamber the is double the atmosphere with a soft elastic membrane that is connect to one that is a little less that atmosphere pressure that forms a dome. The extra pressure on the vibrating diaphram would hopefully cut the dbs down to where it would not be so intense the lack of total atmospheric pressure would hopefully eliminate the sensitivity to the inherent noise of the electronic be sensed and projected. Plus hopefully the membrane vibrating in all direction would have a good dispersion characteristics.
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Old 12th February 2009, 09:20 AM   #12
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Default Re: bi morph material flex to the electrical charge

Quote:
Originally posted by mcmahon48
the materical arches with the change of voltage so fast and with such effiecency that it achieves too good frequency response and spl. That is why most people do not like them for they will bring out the crosstalk distortion and the noise in the signal that can be introduced by crossover and other noises so clearly and trying to adjust it to personal taste in a room can be a pain, they are best suited for outside where the extra dbs needed to be easily pushed with less load since the bass will need 2 to the n for each 3db increases

What is Xmax on a typical piezo....
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Old 12th February 2009, 09:45 AM   #13
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Default Re: Re: bi morph material flex to the electrical charge

Quote:
Originally posted by electroaudio
What is Xmax on a typical piezo....
Basically there is no Xmax on a piezo since it doesn't make sense to talk about as the cone material doesn't move back and forth as in a regular dynamic driver but flexes instead. You could talk about a maximum curvature before the crystal breaks down though.

On how the flex. Well, when applied with a voltage they warp to form a dome shape in one or the other direction depending on the polarity of the voltage. You can then either center mount it and use this warping much like a ring radiator or you can suspend it in a rubber suspension and use it much like a regular dome shaped driver.
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Old 12th February 2009, 09:53 AM   #14
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Quote:
Originally posted by planet10
http://www.planet10-hifi.com/piezo-XO.html
Hi Dave, you might want to update that piezo x-over file with my idea I posted above. It's a much more simple and better sounding x-over with piezos than trying to shoehorn a piezo to work as a regular driver.

Also remember that a series capacitor will attentunate the piezo just as a resistor attunates a relugar driver, so using a standard filter on a piezo, even after making it electriclly look like a regular driver, will have very detrimental effects on the sound.

Just try it. You'll be pleasantly surprised.
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Old 12th February 2009, 02:43 PM   #15
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Quote:
Originally posted by Datinker
You said the foam keeps the bimorph from rocking back and forth. OK, I can believe that. Seems like it should be easy to add a similar foam ring to one of these units. Could be something to try.
I quoted your text by putting a check in the "quote" box, under your post before hitting reply.

I think it would be interesting to hear a comparison of different materials here. Especially if the 2khz behavior mentioned by Saturnus could be addressed. From Cal's initial pic it looks like there are two different materials used. This seems like a non-trivial amount of manufacturing effort, I expect there would have been a reason to implement it.
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Old 12th February 2009, 03:31 PM   #16
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Quote:
Originally posted by Saturnus
you might want to update that piezo x-over file with my idea I posted above. It's a much more simple and better sounding x-over with piezos than trying to shoehorn a piezo to work as a regular driver.

Just try it. You'll be pleasantly surprised.
Write me up a little article and i'll add it... knowing what typical capacitances one is working with would be good.

I gave all my good piezos to Cal...

dave
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Old 12th February 2009, 03:32 PM   #17
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Quote:
Originally posted by panomaniac
Good idea, Saturnus!

Thanks for posting that.
Thanks.

Forgot to mention that you must use ultra-low inductive film or carbon resistors as even "non-inductive" wirewound typically has to high inductance. It only takes a few hundred nH's to start it working as lowpass filter in the audible range.
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Old 12th February 2009, 04:28 PM   #18
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Quote:
Originally posted by Saturnus
Forgot to mention that you must use ultra-low inductive film or carbon resistors as even "non-inductive" wirewound typically has to high inductance. It only takes a few hundred nH's to start it working as lowpass filter in the audible range.
Firstly ... typo ... uH, not nH

I should explain and illustrate this perhaps. Piezos have capacitances in the 0.1uF to 1uF range depending on size of the diaphram and manufacturer more than on cost per say. You can't say, low is good and high is bad, for instance, it doesn't work like that. Just as you can't say a 16 Ohm driver is better than a 4 Ohm one.

Now if we take my preferred one I used in the Boominator (see sig), that's a Zomax HP100. It's capacitance is 0.30uF

Put into the standard equation, just replacing Z with C gives a 151Ohm resistor if the desired x-over frequency is 3.5KHz.

Since we want almost no phase shift and attunation in the audible range we say that the lowpass x-over frequency automatically induced by the series resistor must be at least 80KHz (2 octave above the audible range). Now take this into the standard equation:

L = R / (6.28*f)

And we see that the series inductance of the resistor used should be less than 300uH.
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Old 12th February 2009, 10:50 PM   #19
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Default Re: Re: Re: bi morph material flex to the electrical charge

Quote:
Originally posted by Saturnus


Basically there is no Xmax on a piezo since it doesn't make sense to talk about as the cone material doesn't move back and forth as in a regular dynamic driver but flexes instead. You could talk about a maximum curvature before the crystal breaks down though.

On how the flex. Well, when applied with a voltage they warp to form a dome shape in one or the other direction depending on the polarity of the voltage. You can then either center mount it and use this warping much like a ring radiator or you can suspend it in a rubber suspension and use it much like a regular dome shaped driver.

I am familiar with bimetallswitches, piezos for servos and such stuff regardless of shape.
what i meant, are they linear att all or how small is the linear movement...
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Old 13th February 2009, 02:05 AM   #20
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Default Re: bi morph material flex to the electrical charge

I will try to use the quote feature here. Thanks to Adam for pointing out how this works.

Quote:
Originally posted by mcmahon48
the materical arches with the change of voltage so fast and with such effiecency that it achieves too good frequency response and spl. That is why most people do not like them for they will bring out the crosstalk distortion and the noise in the signal that can be introduced by crossover and other noises so clearly and trying to adjust it to personal taste in a room can be a pain, they are best suited for outside where the extra dbs needed to be easily pushed with less load since the bass will need 2 to the n for each 3db increases
Mcmahon48,
I searched around using Google and Wikipedia for general information on piezo bimorphs last night and found the same information. The disk goes convex/concave in response to voltage and polarity of the applied electric field.

So, now we know what the motion of the biomorph transducer is. All I/we need to complete the picture is knowing how that motion is transferred to the diaphragm. I took some pictures of the unit as I was taking it apart and made these 6 figures to refer to.

Fig1 is the whole GW-1025 piezo unit.

Fig2 show the back side of the back of the bimorph disk which you see when you remove the top cover or lid. Note the break in the inner rim inside the lid which makes the rear chamber a little larger. You can see this in the next figure as well.

Fig3 shows the center ring containing the bimorph, diaphragm, and terminals, can be removed as a section. There are two alignment pins on it so that the lid can only go on one way. Presumably prevents one from confusing which terminal is plus and which is minus, since those markings are on the lid.

Fig4 shows the horn in profile so you can see the cone shaped section that the diaphragm fits over. It fits very close together with the diaphragm. This is why I raised the question about how the diaphragm can be vibrated by the bimorph if it is touch the tip of the cone right in the center where the diaphragm is glued to the bimorph disk.

Fig5 shows that if I push on the bimorph from behind the assembly of bimorph and diaphragm can be pushed up out of the ring section as far as the wires will allow. The disk is hanging free in space other than for the one attachment in the center to the point of the diaphragm.

Fig6 shows that if you hold this assembly upside down it dangles freely on the wires.

I had a more critical look at this and I think that there is maybe a very small gap between the tip of the cone on the back of the horn and the center of the diaphragm when the center ring section is attached to the back of the horn and the rim of the diaphragm is gripped between the ring and the horn. It can't be a whole millimeter though. X-max has to be small, at least for the diaphragm. Remember that the disk is not pushing against the diaphragm from a solid point so it could move a lot more than the thickness of this gap.

I have to wonder why, if this description of how this unit is put together is correct, the center of the diaphragm would not buzz against the tip of the cone sometimes.
And I have to conclude that the diaphragm is only vibrated against the counterbalancing mass of the freely suspended disk. Given these two conclusions I have to wonder how good an idea it is to put much stuff behind the disk at the risk of forcing contact between the diaphragm and cone tip.
Makes me wonder is shaving a bit of the cone tip and putting a tiny dollop of silicone on there might not be a good idea. Should prevent buzzing at high diaphragm excursions.

The foam ring in the diagram Cal supplied does look like a good idea, depending on whether the diaphragm material needed extra damping. And in that model piezo horn I could see the rubber damper acting as a rear surface for the disk to push off from. A surface that is itself compliant and whose relative stiffness and dampness would have a large effect on the sound quality. That seems like something a tinkerer could try to add to this unit. Since that would be going from the current state of affairs where the disk is pushing off against nothing to one where the disk is pushing off against something, this rubber would have to be pretty light material to avoid overdoing it and forcing the diaphragm to hit the cone.
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