How do bed shakers actually work?
Short answer: a small motor spins an off-centre weight, the imbalance shakes the motor's housing, and that housing is pressed against your mattress or pillow so the shaking transfers into whatever you're lying on — and from there, into you. It's a simple mechanism, but where you put it and what it's shaking through change the result more than most buyers expect.
The motor: an off-balance spin, not a piston
Open up almost any bed shaker and you'll find the same part that's inside a phone or a game controller: an eccentric rotating mass motor, usually shortened to ERM. It's a tiny electric motor with a lopsided weight bolted to the shaft instead of a balanced one. Spin a balanced weight and nothing happens — the forces cancel out. Spin an unbalanced one and the motor's whole body gets thrown a little sideways with every rotation, dozens of times a second. That wobble, repeated fast enough, is what you feel as a buzz rather than a series of individual knocks.
A bed shaker's puck is essentially that motor in a flat plastic case, wired to a controller in the clock. Turn the alarm on and the motor spins up; the case shakes; whatever the case is touching shakes with it, roughly in proportion to how firmly it's pressed against it and how much of that vibration the material in between lets through.
How vibration gets from a puck into you
A shaker doesn't touch your body directly — it touches a mattress, or a pillow, or the frame underneath, and the vibration has to travel through that material before it reaches skin and bone. This is the part that determines whether a shaker "works" in someone's bedroom, more than the motor spec ever will. Every bed shaker alarm compared looks at how different models handle the puck-and-cable design; the physics underneath is the same for all of them.
Vibration moves as mechanical energy — a wave of compression and release passing through whatever's in its path. Rigid materials pass that wave along efficiently, losing relatively little of it as heat. Soft, springy, or absorbent materials do the opposite: they flex, and flexing converts some of the vibration's energy into heat instead of passing it on. That's the entire story behind why placement matters so much, and it's a large part of why bed shakers and wrist-worn bands end up compared on such different terms — a wrist band skips the mattress question by touching skin directly.
Under the pillow vs under the mattress
The two placements most people try produce genuinely different experiences, and it's not just about the manufacturer's picture on the box.
Under the pillow, the puck sits close to your head, with a pillow between it and your skull rather than a full mattress. Pillow fill is generally thinner over that short distance than a mattress's internal layers, so more of the vibration tends to arrive with less lost along the way — you feel it mostly as a buzz near your head.
Under the mattress, the puck has to shake the entire mattress structure — foam layers, springs, or both — before any of that motion reaches your body through the surface you're lying on. Depending on what the mattress is made of, a meaningful share of the vibration can be absorbed before it gets anywhere near you, which is a large part of why the same shaker feels strong on one bed and barely noticeable on another.
Neither placement is objectively correct. Under the pillow tends to be the more forgiving choice on softer or thicker mattresses, precisely because it routes around most of the material that would otherwise eat the signal.
Where the energy goes: foam and springs as absorbers
Materials science has a word for what a mattress does to vibration: damping. A damping material converts mechanical energy into a small, harmless amount of heat instead of passing it along as motion. Viscoelastic memory foam is a particularly effective damper — it's slow to spring back exactly because it's absorbing energy as it compresses, which is the same property that makes it good at absorbing vibration. A steel innerspring behaves differently: springs are mechanical linkages built to transmit force efficiently, so a shake introduced at one point can travel across the structure with comparatively little of it converted to heat along the way. Hybrids and toppers sit somewhere in between, depending on how much foam sits over how much spring.
None of this means one mattress type is "better" in some absolute sense — it means the same device will feel different depending on what it's shaking through, which is worth knowing before assuming a weak result means a weak product.