>modern power supplies and motors rely on the ever more powerful and cheap power transistors that we have these days -- they are able to switch power on and off at very high frequencies and thus reduce the size of the rest of the circuitry needed to raise or lower voltage. but this often results in extremely high pitched sound coming from parts that, for whatever reason, physically move or expand in response to current or voltage changes. in cheap electronics, this effect is not accounted for and controlled with vibration dampening materials applied to the vibrating parts, and the result is a maddening high pitched squeal.
Battery-powered motors almost always operate in the range of 80 to several hundred kHz, which is basically unavoidable. Those make up a minority of motors in your home. Wall-plugged motors use AC, in which case they have no high-frequency harmonics, or they are DC in which case the ultrasonic noise comes from the brushes scraping and arcing on the commutator. This is the source of noise in vacuums and the one you're most likely to hear but it's basically static at a very high frequency. Supposedly this is one of the reasons pets hate vacuums, because it's a loud noise of a rarely-heard pitch. Vacuums will be much louder than almost any other motor in your house.
The difference between cheap an expensive power supplies is often down to the use of fully integrated PSUs. The expensive ones operate in the MHz, which lets them use very small capacitors that are built into the chip. Cheaper supplies operate at closer to audible frequency, but they aren't actually louder despite using larger components. The efficiency is relatively close to a more expensive supply so the amount of energy loss is also similar.
>ive been meaning to buy something that will let me detect ultrasonic sound so that i can smash those devices with a hammer.
It's actually very easy to make one! Cheap (<50 cents) electret microphones can hear up to 100 kHz and are pretty common in DIY bat detecting microphones[1]. Note that you can't use normal ultrasonic sensors like those used in rangefinders (about 1 cm wide, black plastic cylinders). Those are heavily tuned to resonate at 40 kHz and can't pick up sound outside that range at all.
>ive always wondered what it would be like if you used a dual motor system in a car, where one motor is wound and sized for very high torque and the other motor is wound and sized for very high speed.
That's exactly why 4wd Tesla models have a longer EPA rated range, or at least speculated to be why, anyway. The EPA range test is done at a set speed and Tesla optimized on that by having one motor for lower speed and one for higher speed. It's also useful for vectoring torque to the front/back for braking/acceleration. You can't use it like a variable transmission though, since there's very limited benefit to splitting the power up.
The benefit in practice is pretty unclear since you won't be driving the exact speed it's optimized for and the difference in efficiency is only a few percent anyway.
Battery-powered motors almost always operate in the range of 80 to several hundred kHz, which is basically unavoidable. Those make up a minority of motors in your home. Wall-plugged motors use AC, in which case they have no high-frequency harmonics, or they are DC in which case the ultrasonic noise comes from the brushes scraping and arcing on the commutator. This is the source of noise in vacuums and the one you're most likely to hear but it's basically static at a very high frequency. Supposedly this is one of the reasons pets hate vacuums, because it's a loud noise of a rarely-heard pitch. Vacuums will be much louder than almost any other motor in your house.
The difference between cheap an expensive power supplies is often down to the use of fully integrated PSUs. The expensive ones operate in the MHz, which lets them use very small capacitors that are built into the chip. Cheaper supplies operate at closer to audible frequency, but they aren't actually louder despite using larger components. The efficiency is relatively close to a more expensive supply so the amount of energy loss is also similar.
>ive been meaning to buy something that will let me detect ultrasonic sound so that i can smash those devices with a hammer.
It's actually very easy to make one! Cheap (<50 cents) electret microphones can hear up to 100 kHz and are pretty common in DIY bat detecting microphones[1]. Note that you can't use normal ultrasonic sensors like those used in rangefinders (about 1 cm wide, black plastic cylinders). Those are heavily tuned to resonate at 40 kHz and can't pick up sound outside that range at all.
>ive always wondered what it would be like if you used a dual motor system in a car, where one motor is wound and sized for very high torque and the other motor is wound and sized for very high speed.
That's exactly why 4wd Tesla models have a longer EPA rated range, or at least speculated to be why, anyway. The EPA range test is done at a set speed and Tesla optimized on that by having one motor for lower speed and one for higher speed. It's also useful for vectoring torque to the front/back for braking/acceleration. You can't use it like a variable transmission though, since there's very limited benefit to splitting the power up.
The benefit in practice is pretty unclear since you won't be driving the exact speed it's optimized for and the difference in efficiency is only a few percent anyway.
[1]: http://www.wildlife-sound.org/equipment/technote/micdesigns/...