piezo sonar : impedance matching ? anyone ?

Hi all,

I'm building a short-range sonar, not unlike the fishfinders. my first
go at it was actually interfacing a fishfinder to the arduino. nice,
but limited success.

Now I have obtained a few pieces of piezo composite material.
In order to get a maximum electrical response on the occurrence of an echo, I understand I have to impedance-match the sensor and the
leads. this would ensure maximum power transmission.
I have seen professional piezo composite transducers producing a single wave echo, mine is still ringing happily at 100 khz and 500 khz.
needsless to say I cannot afford the transducers, I just have the
material. I did make a reasonable transducer however.
it's backing is chunks of eraser gum and it's matching layer is
PVC (Z=3-dot-something). The first return on a 5volt 3-wave ping
is about 0,4 volts top-top, and about 12 waves long @500khz.

I'm not able to calculate this electrical impedance stuff (yet)
so I am trying a bit of trial and error. I'm halfway, i.e. I've mastered making errors :smiley:

my question : is there a way to see/judge the impedance match from an oscilloscope screen ? I have a single channel one. what should I look for ?

second q : the sensor is behaving much like a small capacitor.
does that mean that there should be an equal capacitor on the other side of the cable for maximum impedance match ?

third q : I have read up about filters, but is there maybe a trick/circuit
to actively combat a specific frequency ? I would like to suppress the 100k khz "ring", and I noticed that "bleeding off" energy using an output pin (kept at 0 volts) of the arduino seems to be effective.
it greatly improves the decay of the ringing of the original ping. however, as soon as the sensor is stimulated again (by an echo), the "ring" re-occurs.

any impedance guru help (or references to that knowledge) greatly appreciated.