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Old 11th February 2009, 09:02 PM   #1
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Default How do these piezo tweeters work?

I am working with some of the cheap KSN1025 knock-offs from parts Express, the GT-1025 3x7 Piezo Horn Mid-Tweeter. I thought this would interesting to try in a 2-way speaker with a cheap 4" woofer, also a Goldwood, that I got some time ago from All Electronics. This piezo can go as low as 2K so there is a lot of range in the combination for what cross-over point to use. The idea of this little project is to see how good a speaker I can make with these cheap drivers. One of the things I like about using a piezo tweeter here, besides it's being probably the cheapest tweeter out there, is the fast transient response and ability to go well above the human hearing range. My son is learning to play the flute and I have noticed that when he plays the flute our dog howls even if she is in another room. But, we can listen to flute music on the stereo and the dog could care less even if she is curled up 3 feet from a speaker. Obviously the stereo is not reproducing the full sound of this instrument. In fact, up close and personal the flute has a lot of sibilance and even harshness, the same sorts of things which are often points of complaint against piezo tweeters. Maybe this aspect of piezo's isn't an entirely bad thing.

Of course, when you are reproducing a pure sine wave tone, then you do not want to hear sibilance or harshness, and I do hear those characteristics if I just hook the GT1025 up to an amp directly with no cross-over components and play a sine wave tone generated on the computer through it. So, I wanted to experiment with this unit to see if I could make it sound more civilized, and that brings me to the question in the subject line -

After searching the forums here for information I found the recommendations to coat the paper cone with damar and to put some wadding in the space behind the cone and piezo transducer. I did not have damar so I used thinned Modge Podge (MP) on the paper cone. BTW, the Modge Podge treatment did wonders for the midrange of those close-out 4" GW woofers. I can raise the cross-over point now if I want to. Anyway, the MP coating did indeed improve the purity of the sound of the sine wave tone from the tweeter, and placing a pinch of polyester stuffing behind the transducer seems to have smoothed out the early peak in the 2-3k range some.

What I don't understand is how this piezo driver works, now that I have seen it's indisdes. The piezo sandwich is glued to the apex of the paper cone, and the rim of the cone is held pinched between two layers of the plastic housing when the screws are tightened down. But that leaves the piezo sandwich dangling in space back there behind the cone. What gives with that? Wouldn't placing something behind it, like the fiber pad, probably press it against the point of the horn which the round rear case sections are screwed onto? You can press against the piezo disk sandwich from behind when you have taken the rear cover off and it does not move forward, so apparently that is how it is supposed to be. But, if the apex of the paper cone is fixed against that point behind the horn throat, how is the paper cone being vibrated? I can't quite picture how this works.

Datinker
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Old 11th February 2009, 09:04 PM   #2
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Are there no dampers as shown here?
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File Type: png picture 1.png (68.1 KB, 744 views)
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Old 11th February 2009, 09:20 PM   #3
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I don't know how they work, but I'll be following the thread to see what your impression of using this piezo is.
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Old 11th February 2009, 11:05 PM   #4
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Default How do these piezo tweeters work?

Carl asked:

Are there no dampers as shown here?

Attachment: picture 1.png
(cutaway drawing of a similar piezo horn with a foam dampener in fron of the bimorph and a rubber dampener behind it)

This GT1025 unit has neither the foam in front or the rubber behind the bimorph. That is why I don't get it. The diagram you supplied must be of one of the more high-class units from Motorola. Even so, in your diagram I still don't see where either dampener or anything other than the rim of the cone is anchored to anything solid. Maybe these things shake the cone simply against the suspended mass of the bimorph?

The unit I took apart and put the MP coating on is a spare just sitting naked on the desk. The ones which are mounted in the 2-way speaker boxes have a 22ohm parallel and 40 ohm series resistor, and this combo is crossed over to the woofer with an AR-SXO cross-over. The electrical components alone already have it sounding much nicer than the bare unit on the desk, but I think I can do better still after these experiments.

I didn't think that this cross-over would work after all the warnings I have read about using an inductor with a piezo, but this sounds pretty good. Much better than just a capacitor for the piezo and an inductor for woofer. The mounted piezos have some modelling clay on the backs of the plastic horns to damp the hollow plastic resonance, but no other mods yet. The plastic sounds nice and dead when I tap it now.

I am interested in trying an ENAbl pattern on the cone but I am unsure about where to put the pattern if I don't know how the moving parts are actually moving.

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Old 11th February 2009, 11:21 PM   #5
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Default Re: How do these piezo tweeters work?

Quote:
Originally posted by Datinker
Carl asked:
Cal asked

Quote:
Attachment: picture 1.png
(cutaway drawing of a similar piezo horn with a foam dampener in fron of the bimorph and a rubber dampener behind it)[/B]
No seeum pic

Quote:
This GT1025 unit has neither the foam in front or the rubber behind the bimorph. That is why I don't get it. The diagram you supplied must be of one of the more high-class units from Motorola. Even so, in your diagram I still don't see where either dampener or anything other than the rim of the cone is anchored to anything solid. Maybe these things shake the cone simply against the suspended mass of the bimorph? [/B]
The foam damper, in part, prevents the element from "rocking" back and forth and to tame the ringing. It doesn't need much more as the element is very light, glued to the diaphragm in the center and held in place at the flare by the cap. It doesn't move in the same way as a dynamic driver does, the element kind of slides back and forth across itself when excited and transfers that movement to the diaphragm. The one on the back of the element I am guessing is a form of stabilizer and acoustic damper.

Quote:
I didn't think that this cross-over would work after all the warnings I have read about using an inductor with a piezo, but this sounds pretty good. [/B]
I am definitely not the one to talk to about piezos. I've been known to do some rather off the wall things with piezos and inductors and I can still hear people shaking their heads wondering what Cal's up to next.

Quote:
I am interested in trying an ENAbl pattern on the cone but I am unsure about where to put the pattern if I don't know how the moving parts are actually moving.[/B]
Talk to BudP and planet10 among others about those things.
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Old 11th February 2009, 11:31 PM   #6
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Most piezo horns have a sharp peak in the 2KHz range and then a deep dip in the 3-4KHz range before becoming more linear above about 5KHz. Generally it's that initial range peak that account for most of the bad qualities in a piezo horn, so generally they should just be used over 5KHz.

Luckily that's an easy fix. Just add a series resistor. That will work a highpass filter in itself and at the same time make the piezo a reasonable load for the amp since a piezo is a capacative load the equation is exactly the same as for a normal 1st order highpass but instead of calculating the capacitor, you calculate the resistor value you need.

R = 0.159 /(C*f)

R = resistor value in Ohms
C = piezo's capacitance in Farads
f = desired x-over frequency in Hertz (a good starting point is 3.5KHz)
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Old 12th February 2009, 01:02 AM   #7
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Good idea, Saturnus!

Thanks for posting that.
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Old 12th February 2009, 04:15 AM   #8
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Bear with me here, as I have not figured out how to get text of a previous post quoted in my reply.

Cal, sorry about calling you Carl in my first reply!

The picture I referred to was the one you had attached to your first post in this thread.

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.

Further, you said that the bimorph element slides back and forth across itself in the side to side direction. I would have thought it expanded and contracted in thickness between the two electric poles since the contacts appear to be the front and back faces of the disk. But if the motion was occurring on that axis then letting the center of the diaphragm rest against a hard point wouldn't make sense. To me, at least. So you may be right about a more side-to-side motion.
Another possibility is that the disk warps in response to the applied electric field. Again, we would need to know in which direction it warps to make sense of this device.

If planet10 doesn't show up here, in which forum should I post a question about this?
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Old 12th February 2009, 04:42 AM   #9
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http://www.planet10-hifi.com/piezo-XO.html

I'm not sure how the crystal actually moves in response to an electrical signal, but i expect that warp is the most likely.

Mod Podge was a much better call than damar IMO. EnABLing these would be interesting.

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Old 12th February 2009, 07:11 AM   #10
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Default bi morph material flex to the electrical charge

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
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