to an air raid siren, or a very large leaf blower. It appears to be the most common on units built by EMD (General Electric's locomotive division) equipped with the 710 series prime mover, although any turbocharged locomotive may develop this problem. Electric traction motors often produce electromagnetically induced noise. This high-pitch noise depends on the speed and torque level of the machine, as well as the motor type. Variable-frequency drives use pulse-width modulation, which introduces additional current harmonics and results in higher acoustic noise. The switching frequency of the PWM can be asynchronous (independent of speed) or synchronous (proportional to speed), but always results in acoustic noise varying with speed. See also Noise health effects Active noise control Roadway noise Environmental noise References Bibliography Mechanical vibrations Noise pollution Trains