> Radio Shack sells piezo buzzers for a couple of bucks that are nice and
> big. The Schallers can be had at most music stores for around $15, I thin=
k,
> and there are lots of sources on the web, of course.
Keith,
Many piezo buzzers or beepers have a strong resonance around 2 KHz for use=
as sounders. These can be damped when used as contact mics or sealed
hydrophones, but in free air they may not have a flat response.
Piezo mic elements have a rising impedance with frequency (capacitive) as
are piezo gramophone pickups if anyone still has one. Fed into a very high=
impedance, their response rises with frequency, but using a suitable
resistive load, this more or less flattens out. The recommended load should=
be specified or it can be found by testing.
To match this impedance - and this also applies to capacitor mics - a
resistor is connected across the output as with a capacitor mic bias feed
which can be 100 MegOhms or more. A higher impedance gives lower bass
response.
With a piezo, the capacitance is higher and to match this to a medium or lo=
w
impedance, you need to know the actual input impedance of the preamp which=
is rarely the same as the rated impedance for estimating responses. For a
150/200 ohm rated mic input the input impedance is often in the KOhm range.=
Sorry but this gets worse. :-( Mics and inputs don't have to match
impedances, but for the lowest noise, the "noise impedance" has to match.
This is simply the input device impedance which gives the best overall nois=
e
figure. Thus a preamp with a 2000 Ohm actual impedance input can have a 200=
Ohm optimum noise impedance. Hope this makes sense.
Transformer impedance matching is much simpler and I don't know why
transformers have gone our of fashion except for cost. Note input
transformers must be mu-metal shielded as are Sowter's to minimise hum
pickup.
The other proviso is that at one side of a transformer must have a resistiv=
e
load or source. For instance, a capacitive source will form a resonance if=
the preamp output is high.
The transformer impedance ratio is the square of the turns ratio. Guess
that's all you wanted to know. :-)
PS. If the theory fails - experiment.
David Brinicombe
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