Hello, I am using stepper motors for some neuroscience experiments on rats. The problem is that our experiments are being corrupted by high-frequency noise. The acoustic frequency of the noise is high enough that humans can't hear it, but rats can (humans can hear up to 20 kHz, whereas rats can hear up to 80 kHz). Using a microphone, I measured the acoustic frequencies of the noise coming from the motor, and I observed peaks at 22.5 kHz, 43.25 kHz, 88 kHz, 110 kHz (note that the larger frequencies are harmonics of 22.5 kHz, with the exception of 65 kHz which is missing for some reason). The weird thing is that the motor does not need to be moving for this sound to happen; simply supplying idle current to the motor generates this sound. I checked to make sure that this sound was not present when no current was supplied to the motor, and I also ensured that the sound is indeed coming from the motor and not another piece of equipment. The stepper motor we are using is the 2-phase Vexta PK223PA (http://www.orientalmotor.com/products/pdfs/2009-2010/C/usa_st_2pk_pv.pdf), and the driver is Applied Motion Products Si3540 (although I don't think the driver matters since no movement is necessary for this sound to occur). My questions are: 1. Why would simply supplying idle current cause such high-frequency noise, regardless of whether the motor is moving? 2. Would a 5-phase motor also produce this sound? 3. Are there any quieter options? How do servo-motors, linear motors, and solenoids compare to stepper motors, in terms of acoustic noise levels?