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Old 17th September 2004, 07:59 AM   #1
polsol is offline polsol  South Africa
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Default Piezo vs electromechanical tweeters

At first glance, a piezo tweeter would appear to be ideal for the HF section of a speaker system because it does not require a XO.
However by far the majority of drive manufacturers concentrate on the "standard" EM types which would seem to empasize that Piezo's are less than ideal.
What are the shortcomings of Piezo Tweeters?
My first DIY speaker is intended to be a good quality but cheap set of small speakers for use with a PC, initially powered by the PC (in other words direct from the sound card) but later on via a small external amp (say 11W). I hope to make the speakers have a good sound but do not see the reason for making them "Hi-Fi" per se as onboard PC sound cards are not that good to start with. Hence the addition of a crossover would seem "over-kill" and an unecessary compexity.
However, not-with-standing this project, how is the quality of Piezo tweeters for projects that would be considered "Hi-Fi"?

All comments appreciated!

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Old 17th September 2004, 08:43 AM   #2
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Motorola did the work on the recent crop of piezos that have been around for the last 20-30 years. I think CTS now have the rights and market them. The driver that is probably the most popular is the small horn tweeter which begins to be useful above about 4k Hz. It is quite efficient. They used to make a similar one that was marketed as a "Hi Fi" model. It had a smooth throat to the horn where as the other one has quite prominent ribs in the mouth of the horn. As I understand it the "Hi-Fi" version is smoother in response but less efficient. People have reported that they sound better with a cross over although, as you say, they don't technically need one. Alan Smith pubilshed some work in the JAES about 30 odd years ago that examined the tranient behaviour of horns and he made the point that they were better with a decent slope x-over. They have good power handling and high output.
As to their limitations. Some people just won't use horns! That is not so much a limitation as a faith statement. Secondly, as is pointed out in this forum when ever they are mentioned you can't use normal x-overs. A Piezo is a capacitor. The Motorola paper (Bost, I think in the JAES) will advice you to attenuate response with other capacitors in series. Small values such as 0.1uF from memory. Resistors tend to taper the high frequency responce. 20 or 30 ohms being typical. That will tame the top end a bit and ensure a more stable load for your amp.
If you use the small version you need a bass-mid that will go to 4 K Hz. Since the original one was produced they ahve lowered the useful range and there are both round and rectangular versions that will go down to 2 K now. Generally Hi-Fi shops don't stock them so you will get more joy from public address/sound reinforcement people.
I am glad you are thinking using them as I think they have some potential. Let us know how they go.
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Old 17th September 2004, 01:46 PM   #3
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Quote:
Originally posted by Jonathan Bright
...As to their limitations. Some people just won't use horns! That is not so much a limitation as a faith statement....
Most horns have a measurable limitation that is: severe linear distortion. Even simple computer models such as Hornresp usually show sharp notches in the frequency response and phase response. This indicates severe ringing. Similar ringing occurs naturally with various musical instruments, so some people won't be able to hear it except after prolonged listening as the ears finally learn which resonances are characteristic of the speaker, not the song. That's when listening-fatigue sets in.

It's definitely not a "faith" but a human reaction to avoid certain types of speakers that are consistently bad. I'm not saying that "all" horn speakers sound bad, but certainly all of the ones I've heard belong in that category. People don't seem to understand that designing a perfect horn-shape is the easy part. The hard part is designing a stable amplifier with reduced output impedance. I'd in fact be willing to design a horn speaker, if it wasn't so difficult to design a tailored amplifier for it (think: stable negative output impedance with highly reactive feedback, ouch!).

Anyway, who said that piezo tweeters had to be hornloaded? I think Audax makes an elliptical direct-radiating piezo tweeter.

CM
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Old 17th September 2004, 02:05 PM   #4
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There was also once a tweeter available from Motorola that didn't have a horn. Maybe it is still made by CTS as well.
As already suggested, level matching and a simple crossover can greatly improve the quality of a piezo tweeter.

The Audax tweeter mentioned above used a piezo polymer and was used together with an "air-cored" step up transformer. I have never heard it but it showed good measurements. Unfortunately it is not officially available anymore.

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Charles
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Old 17th September 2004, 05:33 PM   #5
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Default Re: Piezo vs electromechanical tweeters

Quote:
Originally posted by polsol
What are the shortcomings of Piezo Tweeters?
My thoughts, non-technical, purely subjective.

Piezo tweeters are great for in your face rock and roll where realism is not paramount. They're inexpensive, easy to produce and have balls. They are suited for a small PA system where a large compression horn is not practical. I would not use them in a house system.

I find their sound harsh and lacking in depth, ie: imaging

That being said, I have used them in many systems over the years, but always with the understanding that you are getting bang for your buck, not something to set the world on fire. You can replace a piezo ten times before you reach the cost of a good compression unit.

There once was a long throat piezo that sounded OK. It may be the one Jonathan is speaking of, ie: the HiFi model.

This may sound a bit strange but the best I've ever heard a piezo sound was when a choke coil, yes, a choke coil was wired in series. About a 5mH if memory serves me correctly. It boosts the high end, much the opposite effect one would expect, and the tweeter actually starts sounding OK.

If your intent is to use them for computers speakers then it's hard to advise you. They may or may not meet your needs. It depends on how much you will be listening to them. If you listen a lot then I'd consider stepping up a bit.

This is the tweeter I use in my computer speakers. Cheap and not bad sound.

Cal

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Old 18th September 2004, 07:10 AM   #6
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Cal is right. I've got a circuit in the files somewhere. The coil had a high DC value, 12 ohms I think and it was preceded by a cap' of a few uF. At first sight it lookled fairly conventional but the high DC coil was needed. Again, Charles is correct there was a non-horn model but I've only seen photos and that was a while ago. I think these units are interesting because of the high impedence and high efficiency. It lend themselves to low power class A amps' with an active Xover. You could have an amp that was designed for a couple of watts into 30-40 ohms. V.clean and no great demands in the way of current etc. Yes CM there are problems but I should have explained that the ringing you mention is precisely what Smith's work identified and addressed in his paper. If your interested I'll dig it up. I think it is from the 1960's.
Ben Duncan is a great believer in horns but he advocates active systems where each horn is used over a v.narrow range say, 2 octaves.
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Old 18th September 2004, 02:36 PM   #7
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There is a fairly expensive 2-way coaxial loudspeaker from France I believe that uses a piezo, non horn loaded, at it's center that I have seen in some nice open baffle speakers on the net. I forget the name.
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Old 18th September 2004, 04:28 PM   #8
dnsey is offline dnsey  United Kingdom
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I have a pair of cheap & nasty Philips speakers which use an ordinary-looking peizo sounder (the flat brass plate kind) as the tweeter. They sound quite reasonable - certainly not as harsh as the typical horn loaded peizos, and have no crossover.
Might be interesting to experiment with these sounders, with and without home-made horns.
I'm old enought to remember when some of the better record players used peizo tweeters, which were usually about 2-3 ins. square X 3/4 in thick, and sounded pretty good to me at the time.
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Old 18th September 2004, 06:10 PM   #9
zed is offline zed  Sweden
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I have built a Worldaudiodesign design KLS3 Mk II gold utilizing above mentioned Audax tweeter HD3P tweeter.
It produces a very sweet and distinct treble quality. Regreatibly it seemes not to be in production any more. Non horn design, kind of dome where the gold layer (on the polymer) is a motor.
Meadowlarks speaker company (among others) had the driver in their top of the line model once.

So: the piezo pricncple has a potential to develop!
Regards
Dimitri
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Old 18th September 2004, 09:12 PM   #10
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Default near field listening?

I don't think anyone's considered whether horn tweeters are appropriate for near field listening? They would be efficient, which is good if you're driving them from the sound card, but if you're close to a horn tweeter they may not blend with the mid/bass driver that well?
Anyone's experiences with these would be appreciated, I've got a pair of Piezos & a pair of EV horn Tweeters which I haven't thought of a good use for yet.
Pete McK
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