USM นี้หมายถึงเสียงมันเงียบกว่า ผมเข้าใจถูกไหมอ่ะ 
Ring Ultrasonic Motor
Beginning in 1987 with the 300/2.8, Canon has been slowly equipping its best lenses with the ring ultrasonic motor, in which the rotor and stator are big circles around the optics. These are incredibly fast and silent. Even more important, they allow simultaneous use of autofocus and manual focus. You can leave the lens in AF mode all the time and override the camera's decision at any time (or move AF to a separate button as with the EOS 5 body).
This is the "right stuff."
Micro Ultrasonic Motor
If you were a big company and figured out how to make something for 1/3rd the cost but still sold under the same name, wouldn't you? Well, that's what Canon did. The micro ultrasonic motor is a tiny little motor that they stick in the lens and then connect with gears to the focus mechanism. It is tough to see how this is any better than the Minolta/Nikon AF method of driving the lens focus mechanism from a motor inside the lens mount.
A micro USM does not allow simultaneous AF and MF. Thus, the largest user-interface advantage of the ring USM is gone. Much of the noise of a standard motor is back too. The micro USM is noticeably quieter than a regular motor, but not silent
a ring USM.
Canon puts the micro-USM mostly into its new cheap zoom lenses.
ก๊อป ความหมายมาให้ USM = Ultrasonic Motor
