question; does the piezo type 'transducer' (passive) have a hi Z?
Yes, very. It basically looks like a fairly hot (0.5-1Vpk) voltage source in series with a small cap.
I've just measured three disks I have here;
Disk dia, piezo dia, cap.
15mm 9mm 3.23nF (3230pF)
20mm 14mm 7.9nF (7900pF)
20mm 14mm 16.4nF (16,400pF)
A series C with a following shunt R forms a First Order high-pass circuit. The cut off frequency or hinge point, already -3dB down on mid-band, is where the reactance Xc of this inherent capacitive source equals the input resistance.
To take an example using a smaller wafer, and assume an input Z of 1Megohm and a Cpu = 5nF, the hinge and -3dB point will be;
Xc = 1 / 2 Pi f C
transpose to make f the subject
f = 1 / 2 Pi Xc C
1 / (2 * Pi * (1*10^6) * (5*10^-9)) = 31.8Hz
Just a few more quick sums suggests that you might be looking at 10nF for a bridge-slot piezo.