I read a few years ago in a scientific article that a new way to
produce clearer sound was found using arrays of piezo transmitter to
create audible sound by using the interferences of the ultrasonic
waves emmitted. A while later, I had a discussion with a friend on the
subject who told me that this technology was much older and that he
has seen himself High-end consumer grade speakers made this way about
20 years from now. I belive i recall they were manufactured by a
belgian company ( subject to caution).
I would like to know 
-if such loudspeakers are still manufactured and/or sold
-if the sound quality ( theoretically perfect) produced is really
different from what we get with good 'classic' speakers nowadays.

I believe this is the site you are seeking, however there is not
enough data to offer a complete answer to your question.

http://www.sonitron.be


Pizo Speakers Page
http://www.sonitron.be/index.cfm?ThemeID=3537

Hi sgtcory.
Out of all the google researchers, it is pretty funny that _you_
should answer my question since I just discovered google answers today
and actually spent aout two hours reading nearly all your previous
answers. I don't know how I got to one of your answers, I think I was
just browsing by, I happened to read one of your answers anc licked on
your name because I really liked the style and clarity of it. Well, I
will stop here the compliments (for now) but reading your answers made
me discover a few interesting things and put off my time to go to bed
by an hour or two ( i live in CET zone... work tomorrow... aarrggh).


To summarise : Thanks for the link, but I was looking for Hi-Fi
speakers, the kind you could hook up on your stereo. Then again, maybe
my friend (who is normally very cultivated and knowledgable) made a
big mistake and there never was such a thing.

Howdy vax!

Might this be the article you read?

The Recreational Boat Building Industry Home Page aka RBBI web site
has:

http://www.rbbi.com/folders/tech/basic/basic.htm

"The December 2, 1996 issue of Business Week pages 108-109 has an
article
titled, "The Cookie-Size Concert Hall". It describes a new technology
that
may soon replace loud speakers. The units are similar to Piezoelectric
quartz
crystals that keep time in watches."

More about the Amercian Technology Corporation website at:

http://www.atcsd.com/

That said, your friend was probably referring to piezoelectric
speakers,
which is not quite what you are describing, as they use piezoelectrics
to drive a traditional (paper, etc.) cone or in some case, some type
of diaphragm instead of interference patterns.  They've been around
since
the 1980's and the basics of the technology goes back to the 1920's.

The Audio Equipment Circuit web site has information on:

http://www.audiocircuit.com/9109-ribbon-circuit/Commercial/Sawafuji-SAW/9109CMSAW.htm

Commercial Ribbon Loudspeakers

"... model SFI Digital 20. My store sold these in the 1980s ..."

The above page references various patent numbers, including:

4597099

You can go to:

http://patft.uspto.gov/netahtml/srchnum.htm

and use the above patent number to look up the details on:

" ... invention relates to a novel sound producer using a
piezoelectric
crystal diaphragm ..."

and from there you can peruse earlier patents on piezoelectrics.

You can "still" buy piezoelectric speakers, but mostly in the format
of "tweeters" since they are better at producing high frequencies
than low frequencies:

http://www.coolspringsdesigns.com/item/KLH_SE_6000_6_Piece_550_Watt_Home_Theater_System/i85999.htm

has this SE 6000 6 Piece 550-Watt Home Theater System which includes:

"Satellites w/4" Dual Cone Woofer & 5/8" Piezo Electric Tweeter"

More on piezoelectric tweeters can be found at the CTS Corporation
site:

http://www.ctscorp.com/pzt/speakerappnote.htm

I think there were several factors holding back piezoelectrics being
used in "hi-fi" speakers, which included high cost, power consumption
and difficulty in producing mid and low range frequencies.

Search terms:

"piezoelectric speakers"

://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22piezoelectric+speakers%22

"piezo electric" speakers hi-fi

://www.google.com/search?q=%22piezo+electric%22+speakers+hi-fi

Hope this Comment helps!

Looking Forward, denco