Detuning/widening bandwidth of piezo transducer?

Hi all,

I recently posted about a bat detector (read ultrasonic pitch
downshifter) I'm building from the following schematic:

http://home.kabelfoon.nl/~bertrik/bat/simphet.htm

My goal is to build it without the indicated coils. Bob Eldred
showed me the light on getting rid of one of the coils, reminding
me that the 47mH coil and its cap were simply a passive LPF, which
I then replaced with an active Sallen Key LPF (TL072 and some
resistors/caps).

Now my detector works quite well. I can easily hear several
harmonics from my TV, my pocket chain clinking sounds REALLY
weird, etc. But I want to see if I can improve the bandwidth
of the piezo UST.

Bertik (author of the schematic) has a good page on how to
do this with caps, resistors, and a 5-8mH coil, but before
I get into trying to make the coil, I was wondering if there
are any active/non-coil ways to do it.


Any answers/tips/links appreciated,

Torben

P.S. I've googled/teoma'd/mootered for this a million times,
but either the answer's not out there or I didn't
recognize it for what it was.

 

I think you can mechanically broaden the response of the piezo and
also increase its sensitivity by putting an exponential horn on it.

 

Agree. Fixing it mechanically is likely better than trying to patch
up a high Q response after the fact.
First thing I'd try is to run the piezo into the virtual ground of an
op-amp. But the intrinsic series impedance of the transducer may make
this unfruitful. May have to resort to mechanical damping. Glue
the piezo to a piece of foam rubber and load it with a horn as
described above.


I have a commercial ultrasonic leak detector. Can't see in the hole too
well, but it looks more like a plastic dome on a voice coil than a piezo
transducer.
The transmitter and receiver transducers "look" to be the same, but
that's not necessarily a valid conclusion. I never found any data on
the setup, so don't know what the claimed bandwidth is.

Might be interesting to plug on a capacitor microphone element and see
what that does. Most of the microphone specs I've seen are still going
strong at the 20 KHz. end of the graph.
mike

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OK, that's two votes for the horn, so it's probably not a bad idea at
all. However, I have both a space limitation and a horn-manufacturing
limitation, not to mention a limitation in my understanding of acoustics
which I figure would help me work out the size needed for best
performance from ~30kHz to ~100kHz. Quicky paper horns (pretty much
linear) seem to help with the sensitivity, though.

I'll give that a shot on my breadboard. I still have to check the actual
response of the transducer; I've been assuming so far that it has a
fairly tight spike around 40kHz, but I've also read that some work
just fine with no detuning at all. Given that I'm already fairly
happy with the sound, what I'm trying for now is to solidify my
understanding of the detuning. I'm currently under the (probably
wrong) impression that it's mostly phase games. Am I close?

Hm...that sounds neat. I'll see if I can find one.


Thanks for the tips,

Torben

 

Lars Torben Wilson wrote:
snip

Good place for microphones is an old cellphone, cordless phone, cassette
tape recorder...
mike

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