TUNING
A simple transducer shape, such
as a circular piston transducer, has a wonderfully complex
electrical impedance. This can be a problem when it comes
to driving this transducer with a power amplifier. Most
PA's are designed to operate into some fixed, usually
resistive, load impedance. Driving some other load
impedance, especially if it is reactive, results in less
power being delivered to the load and, hence, less sound
output into the water.
I have tried a couple of approaches, including winding an
output transformer to match the transducer impedance.
Eventually, I gave up on this approach in favor of matching
a 50 ohm output transformer to the transducers with an L-C
network. This approach works particularly well with
narrow-band systems where a single frequency is
transmitted; in this case, an L-network is all that is
needed to match the transducer with, perhaps, a series
reactance to cancel the reactive portion of the transducer
impedance. When using a switching power amplifier, you can
use the low-pass style of L-network to additionally filter
out the odd harmonic structure of the applied square-wave
signal.
It is also possible to match a transducer over a wide range
of frequencies. Possible, but not necessarily easy. I hope
to pull together some of the methods here -- maybe after
Speed Week.