Wayne put me on track with some experimenting with piezo's.

I am using two paralell ksn 1005 piezo's together with a tannoy
cheviot speaker with the HPD 315 dual concentric unit.

The goal is giving some air to the upper octave somewhere between
10 and 20 khz.

First I asked Wayne for the value of the cap in creating a first order
high pass filter starting at prox. 20 khz.

Stimulated by the results I calculated a third order high pass, starting at prox. 11khz.

Setup: 20 ohm resistor paralell to the 2 piezo's, 1.5 ufd in series,
.22 coil to mass and then .49 ufd in series again.

Not altogether unsatisfied with the sonic result (It really gives
"some air"), I connected a sinus generator to the amp and gave it a
sweep.

I put a radio shack Spl meter in front of the Piezo's so that
I also had some visual control over what I was about to hear.

Well, to come to the point, I was a bit disapointed about the output
above 16 khz and, most important, I noticed a sharp audible and
measurable increase in output sharply around 5.7 khz.

In fact, according to the spl meter, the output was somewhere around 5 db higher then on 12 khz.

And that after I thought I am using a third order 11khz high pass!

Is it possible that I have to use something that is called a notch
filter?

Well Wayne and the rest, I hope that you have some time for a sugestion.

Robin.