> >As you're dealing with wood, I'm not sure electrical
>>means are a good choice, and ultrasonics at this scale
>>and resolution would be a nightmare (a quick
>>back-of-envelope calculation gives 40kHz in air a
>>wavelength of about 8mm, which doesn't bode well for
>>measuring with 0.1mm accuracy)...

40kHz x 8/0.1 = ~ 3 MHz

Modern specialist ultrasound systems operate in the Mhz range.
Once I read about recently (when they were possibly going to use one on me 
but didn't need to) worked at 7 MHz.


        RM

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