You’ve probably seen “USM”, “SWM”, “SSM”, “HSM”, “SWD” written on DSLR lens barrels and feature list, usually to imply that a given lens focuses fast and silently.
Each sounds as if it came from some marketing guy with an overactive imagination.
R&D Guy: “Hey, we’ve got this new lens motor technology. It’s fast and it’s quiet!”
Marketing Guy: “Fast? Let’s for supersonic, like the-faster-than-sound-Concorde, but faster! Let’s call it ULTRAsonic! VRWOOOOSH!!”
However, as much as “Ultrasonic” sounds like a gimmick, there’s more to it than a marketing blurb.
Conventional autofocus camera lens used to be driven by motors with gears. These are characterized by a somewhat loud whirring/gears grinding sound while it focuses.
In the 1980s, Canon developed an electric autofocus lens motor that used the ultrasonic vibration to move things. In simple terms, it was using vibration to move stuff inside the motor, instead of using ordinary mechanisms.
How does an Ultrasonic Motor work?
Here is an animation of how the concept works, taken from Canon’s Camera Museum.
- Click on the Japanese sentence with “USM” in it to get started.
- It will show you two rings. The top ring is called a “rotor” which is the part you want to rotate (a lens assembly maybe). The bottom ring is called the “stator” which is what vibrates at ultrasonic speed.
- Click on the Left Arrow to for counterclockwise movement. As you can see, the stator ring vibrates in a counterclockwise fashion, and this in turn makes the rotor twist counter clockwise. Click on the Right Arrow and the opposite will happen. This is how a lens is made to focus. The stator vibrates too quickly for a human to really feel it, the demonstration just shows a very-slowed down vibration.
The result was a lens that autofocused faster and more silently than a conventional lens. Ultrasonic motors are usually found in the better-made lenses.
Other lens makers call this technology by different names and some of them use the word “Wave” instead. This is because they’re referring to the “traveling wave” of this ultrasonic vibration.
Here’s a list of the common ones, which I linked to their respective manufacturer’s page if available (I can see an overzealous marketing guy naming some of them 
):
- Canon’s Ultrasonic Motor (USM)
- Nikon’s Silent Wave Motor (SWM)
- Olympus’s Supersonic Wave Drive (SWD)
- Sony/Minolta’s SuperSonic Motor (SSM)
- Pentax’s Supersonic Drive Motor (SDM)
- Sigma’s Hyper Sonic Motor (HSM)
- Panasonic’s Extra Silent Motor (XSM)
- Tamron’s Ultrasonic Silent Drive (USD)
Regardless of how they call it though, they all mean one thing to lensmen all over: fast and silent autofocus lenses.
More technical reading can be found at Wikipedia’s entry for Ultrasonic Motor.
