Natural Science Forum / Physics / General Physics / August 2008

Every object will have a "natural" or resonant frequency,
as with tuning forks, organ pipes, antennae, bridges,
 http://www.youtube.com/watch?v=eBf0dwXH2Y8
masses on springs:
http://www.kettering.edu/~drussell/Demos/SHO/damp.html

If you bend and flex a piezo-electric transducer at
its natural frequency you'll get a high amplitude output,
with a reduction in amplitude at frequencies both higher
and lower than the natural frequency.
  http://taumoda.com/web/class/Bell.gif

Since in many cases this characteristic is undesirable,
ideally the output amplitude required should be the same
at all frequencies, the partial solution is to dampen the
natural frequency (flatten the bump of the bell curve)
and then amplify the output across the frequency range
externally.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentT
rans/characteristicspt.htm

The backing material supporting the crystal has a great influence on the damping
characteristics of a transducer. Using a backing material with an impedance similar to
that of the active element will produce the most effective damping. Such a transducer will
have a wider bandwidth resulting in higher sensitivity. As the mismatch in impedance
between the active element and the backing material increases, material penetration
increases but transducer sensitivity is reduced.