Brushless DC (BLDC) gearmotors are typically quieter than PMDC gear motors or inverter-duty AC three-phase gearmotor and motor designs. PMDC gearmotors have carbon brushes that continuously "rub" on the commutator of the motor armature. This interaction, along with the DC power from an unfiltered SCR or PWM speed control can result in audible noise. Similarly, inverter-duty, variable speed AC motors and gearmotors emit some noise created by the PWM voltage supplied to the motor from the inverter (PWM speed control). Gear housing style and design, as well as the number of gear stages can also affect overall noise emitted from the gearmotor. Worm gearheads (right-angle gearmotors) are generally quieter than inline or parallel shaft gearmotors due to the differences in gear geometry and how the gear sets interact.