Originally posted by pop4
I believe HSM uses an ultrasonic ring motor whereas SDM actually uses a ultrasonic micro motor.
The only 'non-ring type' ultrasonic motor drive I'm aware of is Canon's "Micro Ultrasonic Motor" which is essentially a normal winding motor just built smaller. While it is quieter then normal gear driven motor AF, it is still nowhere near the nearly silent ultrasonic ring-type motors.
From my limited experience with the 16-50 and 50-135, they sounded and behaved precisely like an ultrasonic ring-type of motor, including the ability for instant manual focus over-ride.
I admit that I may be wrong - but until I can see an exploded view or breakdown of an SDM lens, I will tend to believe otherwise...