Shaker motors and eccentric weights, explained
Take the back off almost any vibrating alarm — a wrist band, a bed shaker, a game controller, a phone on silent — and you'll find one of two small motors doing the actual work. Neither one is complicated. The reason a wristband buzzes and a mattress shudders comes down to a genuinely simple bit of physics, dressed up by an industry that has strong reasons not to explain it to you.
The two motor types that actually exist
Almost everything in this category runs on one of two designs. The first, and by far the more common in cheap consumer hardware, is the ERM motor — eccentric rotating mass. It's an ordinary small electric motor with a lopsided weight stuck to the spinning shaft instead of a balanced one. The second, found in higher-end wearables and phones, is the LRA — linear resonant actuator — which doesn't spin at all. Instead, a small mass moves back and forth along a fixed axis, driven by a magnet and a coil, tuned to resonate at one specific frequency the way a tuning fork does. LRAs tend to start and stop faster and feel "crisper," while ERMs are cheaper, simpler, and slower to spin up and wind down — which is why a vibration alarm sometimes seems to fade in and out rather than switching on and off cleanly.
Why an off-centre weight makes something shake
The ERM is the one worth walking through, because the mechanism is genuinely intuitive once you see it. A motor shaft spinning a balanced weight — evenly distributed all the way around — doesn't shake anything; the forces cancel out symmetrically as it spins. Stick an unbalanced lump of metal off to one side of that same shaft, and every time it swings around, its momentum tugs the whole motor sideways in the direction it's currently moving. Spin it fast enough — hundreds of times a second — and those individual tugs blur into what you feel as a continuous buzz. That's the entire trick: an off-centre mass, spun fast, on something small enough and light enough for the resulting wobble to move the object it's bolted to instead of just spinning in place.
Everything that follows is really just this same idea, tuned two different ways.
Amplitude and frequency are not the same knob
Amplitude is how far the motor housing physically moves with each cycle — the size of the shake. Frequency is how many times a second it moves — how fast the buzzing repeats. They sound like they should trade off against each other, and to some extent they do, but they're genuinely separate variables, and a motor can be tuned toward either one independently within its physical limits. A heavier eccentric weight, spun the same speed, generally increases amplitude — more force flung further off-axis — while spinning the same weight faster generally raises frequency without necessarily changing how far it swings.
Why both matter for a wake-up device specifically: amplitude is closer to "how hard does this hit," which is what actually has to overcome your skin's sensory threshold and your brain's habituation to reach conscious awareness while you're asleep. Frequency changes the quality of the sensation — a slow, heavy pulse feels different from a fast, buzzy hum even at similar overall energy — and some frequencies are simply more noticeable to human skin than others, in the same way some pitches of sound are easier for the ear to pick out of background noise. A motor optimized purely for one and not the other can end up feeling weaker than its raw power output would suggest, which is part of why two devices with what should be comparable hardware can feel noticeably different against the skin.