Re: What is the main element in ultrasonic transducer made of ?

This is a little out of my realm of expertise, but at least some of the 
ultrasonic manufacturers use piezzo-electric crystals.  Ultrasonics use 
high frequency, 20 khz vibrations of relatively low amplitude.  I am not 
sure if other technologies would work, but piezzo crystals deform 
slightly when excited by voltages, and make excellent drivers for such high 
frequency, low amplitude waves.  There are more than one type of crystals 
that exhibit this phenomena, so it is impossible to say what each 
application might use. One manufacturer mentions lead zirconate titanate 
(PZT) and lead metaniobate ceramic elements.  He includes jewelry cleaners, 
ultrasonic welders, etc in his applications list.

See the website below for more info.

http://www.piezo-kinetics.com/