Ultrasound Piezoelectric Transducer: Longitudinal or Transverse waves?

[Ipduo]

Hi,

This is probably a silly question but I can't find a simple answer on the web so...

Obviously ultrasound relies upon very high frequency sound waves, but are they transverse (shear) waves or longitudinal, or something more complex. Additionally, do the trandsucer make a difference (the one I'm currently interested in is a Piezoelectric device). Does the vibrating membrane mean it's longitudinal?

Thanks!

 

[Resqueline]

I guess it depends on how the piezo"crystal" itself is made.
The brass speaker/beeper disks supposedly bends into a saucer-shape. I don't know what to call that kind of force but I figure it can't be called transverse or longitudinal.
The piezo-igniter in cigarette-lighters uses longitudinal (compression) forces to generate the spark. Echosounders (fish-finders) also use longitudinal waves.