Two numbers describe one motion
A sonic brush head moves side to side, and that motion has two independent measures. Frequency counts how many sweeps happen per minute - the strokes figure that headlines the box. Amplitude measures how far the bristle tips travel on each sweep, and it is quoted as a displacement, roughly plus or minus 1.5 to 3 millimetres of bristle movement across the category. The pair works like a pendulum: frequency is how often it swings, amplitude is how wide. A handle can sweep fast with a short throw or slower with a long arc, and the two combinations feel entirely different even at the same strokes figure. Buyers who spec only frequency are half-blind; amplitude is the number that decides whether a handle feels assertive or gentle.
What sets the amplitude band
Amplitude is not a software setting floating free - it comes from hardware. The motor type comes first: eccentric-weight micro motors and linear voice-coil drives both run the 20,000 to 40,000 cycles-per-minute band, but they convert rotation into sideways motion differently, and the drive choice caps how far the shaft can throw the head. The shaft and coupling come next: a stiffer coupling transmits more of the throw to the bristles, a compliant one absorbs it. The head closes the chain: collar weight, bristle length and tuft density all damp the tip movement, which is why the same handle measures a different amplitude with different heads fitted. The sonic motor line lists the drive families and the sonic toothbrush line shows how amplitude appears on a nameplate.
| Factor | What the plant controls | Typical choice in the category |
|---|---|---|
| Drive type | Motor family and its throw characteristic | Eccentric micro motor or linear voice coil, 20,000-40,000 cycles/min |
| Quoted sweep | Bristle tip displacement target | About 1.5-3 mm, per model datasheet |
| Head platform | Collar mass, bristle length, tuft density | Slim elongated heads for longer throw; compact heads for control |
| Mode profiles | Firmware steps within the motor band | 2-5 modes from gentle to full intensity |
| Handle damping | Shell ribs, overmould grip, weight balance | Overmould and ribbing to keep shake out of the grip |
Amplitude is also the half of the pair that marketing under-communicates, and that gap confuses buyers. Frequency is printed in large digits because a big number sells; amplitude rarely appears outside the datasheet because it needs a unit and a condition to mean anything. The result is a market where handles are compared on the easy number and chosen on the feel the hard number creates. Distributors who respec products for private programmes do the reverse: they measure or sample the amplitude, write it into their own datasheet, and use it to explain to shelf buyers why two same-speed handles sit at different price points. Making amplitude a stated line in a brief costs nothing and removes most of the guesswork from sample rounds.
Why the shake in the hand differs
What a user calls vibration is partly the head doing its work and partly the handle transmitting it. A well-engineered handle delivers the throw to the bristles and keeps the grip quiet: the shell adds ribs to stiffen the body, an overmould layer damps what remains, and the internal mass is balanced so the reaction force does not buzz the fingers. Cheap-feeling handles skip those steps, so the same motor produces a buzzy grip. Head fit plays its part too - a loose collar clicks and rattles, and retention force is measured with a pull gauge to keep it tight through a full session. None of this shows in a spec table, which is why the desk brushes with sample units before signing anything, and why testing and inspection includes feel-and-noise checks alongside bench measurement.
Rotary handles frame the contrast. A round head turning at 7,000-8,800 rotations per minute puts its motion inside the small head instead of swinging the whole neck, so perceived handle shake is comparatively low even before damping - the motion is local. Some rotary models layer 20,000-40,000 sonic strokes on top, and those hybrids need the same damping discipline as sonic handles because the sweeping component returns the shake to the grip. The lesson generalises: perceived vibration is a system property of drive, head and shell, not a motor rating, and the sonic versus rotating guide shows how the two families distribute the same energy differently.
Amplitude across the range and the modes
The ±1.5 to 3 mm band spans the category's personality range. Flagship sonic handles aim for the upper half, pairing a long throw with the 31,000-40,000 strokes band for an assertive sweep. Entry models often combine the lower amplitude half with the 20,000-31,000 strokes band, which keeps the drive affordable and the feel milder. Kids models sit gentler still, pairing 7,000-20,000 strokes per minute with small heads and reduced intensity, and U-shaped silicone trays take a different geometry entirely at 8,000-20,000 vibrations per minute with whole-arch contact. Within one handle, the mode ladder is mostly an amplitude ladder: a soft mode pulls the intensity down, the clean mode runs the standard profile, and polish programmes trade throw for dwell. The modes note maps the usual ladder and the strokes note covers the frequency half of the pair.
Measuring and writing it down
Amplitude is a bench measurement: the head is fitted, the mode selected, and the bristle tip displacement recorded under stated conditions. Because head fit changes the result, the figure only means something with the sales head named alongside it, which is how the desk asks for it - per model datasheet, per head, per mode. For a program the practical sequence is to shortlist by ranges, then tune by hand: brush the samples, pick the intensity profile that matches the target user, and freeze the firmware map that produces it. A sensitive-gums brief, for instance, usually lands on the lower amplitude half and a soft-mode-first ladder rather than the top of the strokes band; the sensitive gums note walks that selection.
Where amplitude fits in a brief
A complete motion brief reads: drive family, strokes band, amplitude target, head platform, mode ladder with the intensity of each step, then the acoustic target - quiet handles sit at 55-60 dB or below, and pushing amplitude up tends to cost noise budget. Those five lines let plants quote precisely and let the desk compare answers like for like. Bring them to the quote form and the program frame - quantities, samples, schedule - sits on MOQ and lead time. Confirmed in writing per program, the amplitude target becomes an inspection line item rather than a marketing adjective.