What is the difference between built-in HSM lens and DC-motor lens?

With conventional auto-focusing, auto-focus is driven by electromagnetic force of a DC-motor, gear and shaft mechanism. However, Ultrasonic Oscillation Energy operates the HSM lens and creates high-torque drive. Since this motor has no reduction gears, it has much quicker response to start and stop compared to DC- motor lenses. HSM system has virtually silent auto focus function, very quick responsive auto focus function and “full-time” manual focus, wherein AF can be overridden without disengaging the AF mode altogether.