From: DPierce@world.std.com (Richard d Pierce)
Newsgroups: rec.audio.high-end
Subject: re: Piezo tweeters
Date: 5 Nov 92 17:53:46 GMT
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Scott Dorsey writes:

>Personally, I have found piezo tweeters abhorrent.  The Motorola units are
>the best of the bunch, and far and above the quality of any tweeter (piezo
>or not) in the under $20 price range.  They can actually be coaxed to
>sound good; I have heard a Vandersteen speaker using the things which had
>still a slightly harsh high end, but not as brittle as you normally
>associate with piezo tweeters.  They are difficult to get a good sound out
>of, though. I worked for several months with a set of PA speakers
>attempting to clean up the high end and eventually gave up trying to get a
>flat pattern without extreme harshness. 

First, let me say I strongly disagree that the Motorola piezo's are better
than any under $20 tweeter. The Audax TW60ATI is flatter, wider band,
lower distortion, better dispersion, sounds better and is about $7.00 in
single quantities. The only thing the piezo does better is power handling
(they are as close to completely idestructable as one can imagine) and
they're pretty efficient.

At the same time I agree that the piezos are abhorrant. I attended the
ALMA conference in Chicago CES this year where Motorola was pitching an
new "flat" version where they have two piezo elements that are purposely
tuned differently so as to try to spread the resonance out a bit. The
graphs that they showed, sanitized as they were, were simply awful. In
talking with the Motorola guy afterwards, he almost apologized for their
lousy sound quality but countered by saying that they really intedned them
not to be used in any serious high-fidelity speakers.

The samples I have measured are truly awful. A couple of peaks around 5
kHz, some more up around 10 kHz, etc.. If you're daring, you can say they
have a bandwidth of 5-20 kHz, but just duck when you say that that's +- 12
dB (no joke!). 

>For the price, though, they are worth buying to play around with, and if
>you can't do anything with them you don't feel so bad about pitching them. 
>They will definitely be a good exercise in design, as well. 

I suppose, but I, frankly, have a hard time throwing away anything that
works (no matter how badly). So, for laughs, I tried destroying one of the
samples I had. At the time, I had a Crown DC-300A. I hooked one channel
directly to the piezo. I put it in a lined test enclosure (2 layers of
3/4" MDF separated by 1/2" sheetrock, lined with 10" of dense foam, volume
about 3m^3). I brought out my oscillator, hooked it up, set it to 15 kHz
and slowly started to crank up the volume. God did it sound awful! Even
with the chamber tightly sealed, it was f**king LOUD! With the Crown near
clipping (but showing nice clean sine waves on the scope, at about 40 V
RMS), the piezo was putting out some incredible amount of garbage down
around 5-6 kHz!. My B&K mike was clearly driven farther into limiting than
it ever had before. 

The tweeter took a lickin', but it was still kickin'! It was absolutely
impossible to kill it with the amp.

I finally took a big mother 300uF 500V photoflash capacitor I had, charged
the sucker up and dropped it across the speaker leads. In a very
satisfactory BANG! and a nice curl of acrid-smelling smoke, I succeeded
only in burning out the wire leads to the piezo element. Resoldering them
revealed the sucker still worked as advertised!

Yucko!

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