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Sonic electric toothbrush by Oral B and Braun

Electric toothbrushes

by Chris Woodford. Last updated: December 11, 2014.

Do your friends reach for the sunglasses when you smile? If not, maybe it's time you invested in an electric toothbrush. These battery powered teeth scrubbers move their heads back and forth up to 7000 times a minute, shifting twice as much plaque (rotting, sugary food debris) as ordinary, manual toothbrushes. That's called good oral hygiene and it means you won't have to go to the dentist quite so often. The cleaner you keep your teeth (and the gums that support them), the longer they'll last you. Just remember, once your adult teeth are gone, you don't get replacements! Ever wondered how an electric toothbrush works? Let's take a closer look!

Photo: The manufacturers would love us all to buy expensive sonic toothbrushes like this, but do they get your teeth as clean as ordinary electric toothbrushes and manual brushing?

What's inside an electric toothbrush?

Take an old electric toothbrush apart and it's easy to see how it works. I had one that didn't work anymore, so I had a quick look inside before taking it to the recycling center. Here's what I found:

Photo showing component parts of an electronic toothbrush taken to pieces

The first thing you notice is the removable brush. This is the bit that does all the hard work. It has a semi-rotating head at the top so, as the mechanism inside the brush handle rotates back and forth the brush turns from side to side. Directly underneath the brush there's a cam and gear unit, which looks like this (left photo):

How the cam unit in an electric toothbrush works Animation showing how an electric toothbrush head moves back and forth.

The cam and gear unit is the clever part of an electric toothbrush. It converts the high-speed rotary (spinning) motion of the brush's electric motor into reciprocating (back-and-forth) brushing motion that cleans your teeth more effectively. The cam and gear works a bit like this very simplified animation (right). The red motor is always rotating in the same direction at high speed, but the blue and black cam and crank convert that rotation into lower-speed, back-and-forth vibration. So, as the blue cam turns, the black crank pulls first one way and then the other, moving the brush head rapidly from side to side.

Early 1937 electric toothbrust patent by Tomlinson I. Moseley.

The cam and gear unit is connected to a gear built into the top of the motor, so the motor drives it directly. Underneath the motor, there's a rechargeable battery. Attached to the inner plastic case, there's a simple electric circuit board that controls the on/off switch on the outer case. The outer case is made of tough plastic and the on-off switch is set into it in a piece of thin, very flexible rubber. The purpose of the outer case is to keep water and toothpaste away from the circuit, motor, and battery—which would quickly rust if you got them wet. (If you're curious to know how an electric toothbrush can recharge itself standing on a plastic base, take a look at our article on induction chargers.) And that's pretty much all there is to it—a bit of clever, effective engineering technology that keeps your teeth in tip-top shape!

Photo: Early electric toothbrushes didn't use this reciprocating action. Here's the oldest electric toothbrush invention I've found, filed by Tomlinson I. Moseley in December 1937 and granted in April 1940. The electric motor (blue) drives a rack (yellow) and pinion (red) gear that makes the brush head rotate continually in the same direction. Just like in a modern toothbrush, the electrical parts are sealed inside a waterproof casing (brown), which Moseley suggested might be made from Bakelite (a tough plastic popular in those days).

This invention was filed long before cordless, battery-powered appliances and induction chargers became popular, so power is supplied in a more conventional (and more dangerous) way via an electric cable (labelled 15, on the extreme right of both pictures). Artwork from US Patent 2,196,667: Electric toothbrush courtesy of US Patent and Trademark Office.

What's the difference between a sonic toothbrush and an ordinary electric one?

Sonic and ordinary electric toothbrush side by side

Photo: The head from a sonic toothbrush (seen in the lower part of the photo) is just like an ordinary manual toothbrush. Unlike a standard electric toothbrush, it doesn't contain a rotating mechanism. The whole brush head vibrates—and this can make sonic toothbrushes a little more tricky to use at first.

Crudely speaking, sonic toothbrushes work just like ordinary ones: they move back and forth over the surface of your teeth at high speed, scrubbing away the plaque. The most obvious difference is the speed of the movement: sonic brushes typically vibrate at about 260Hz (260 times per second), which translates into 31,000 brush strokes per minute—about 10 times faster than ordinary electric toothbrushes. As we saw up above, in a normal electric toothbrush, the very top part of the brush rotates back and forth thanks to a little cam unit just above the motor. The rest of the brush head is stationary. You can see a conventional electric brush head (actually a special one designed for cleaning the spaces in between your teeth) in the top of this photo. The removable brush head contains a little spring mechanism that lets the brush part turn back and forth, but the rest of the head remains static. With sonic toothbrushes, the entire brush head vibrates very quickly—and the manufacturers claim this produces a different type of cleaning action that makes them more effective.

According to the marketing spiel, because a sonic toothbrush buzzes over your teeth at much higher speed than a normal electric toothbrush, it not only brushes away the plaque but also creates waves of turbulence in the toothpaste and water in your mouth. Tiny bubbles form in the space between the toothbrush and the teeth, and these help to shift more plaque than brushing alone. So, when you're using a sonic toothbrush, it helps if you have quite a bit of fluid near your toothbrush. Then you can actually feel an energetic, tingling sensation around your teeth as the acoustic pressure waves created by the brush do their work. At least, that's what the makers claim...

Why are they called "sonic" toothbrushes?

The bubbling cleaning action sounds similar to what happens with ultrasonic cleaning, where ultrasound (higher frequency sound than humans can hear) is used to generate pressure waves and imploding bubbles (technically called cavitation) that blast dirt off things like industrial machine parts that can be hard to clean any other way. However, as far as I can see, the resemblance between ultrasonic cleaning and what happens with a sonic toothbrush is utterly superficial. Indeed, a 2004 study of five different sonic toothbrushes by British dental researchers concluded that "cavitational activity does not occur around powered toothbrushes. Operating the toothbrushes for periods up to 20 min resulted in no cavitational activity being detected." That's hardly surprising. Ultrasonic cleaning typically uses a much higher frequency (20,000–40,000Hz) where the cavitation effect is much more pronounced. In other words, "sonic" is quite misleading—and little better than a marketing term. Ordinary (rotating head) electric toothbrushes could also be described as "sonic" inasmuch as they make a noise when they're vibrating. In both cases, it's actually the vibrating action of the brush that's doing the cleaning.

Do sonic toothbrushes work and are they worth paying for?

""People are certainly fooling themselves, with the majority of [electric] brushes at least, if they think they are more effective."

Professor Bill Shaw, of the University Dental Hospital in Manchester, quoted by BBC News, 2003.

Don't trust the "customer reviews" expressed on websites selling electric toothbrushes. It looks to me like quite a few of these are submitted by people who work for the toothbrush makers! I've tried normal brushes, electric brushes, and sonic ones. Electric brushes seem to get my teeth cleaner than normal brushing, and I definitely prefer my sonic brush, but I suspect it's a matter of preference. My own opinion, for what it's worth, is that if you use an ordinary electric brush properly, and floss your teeth as well, there's no benefit to be gained from going sonic, especially given the considerable extra cost of a sonic brush. Similarly, if you brush thoroughly with a manual brush and floss regularly, that's probably just as effective as electric brushing for most people. Interestingly, two British scientific studies (in 2003 and 2004), listed in the articles below, found that ordinary (rotating-oscillating) electric toothbrushes are the only electric brushes that outperform manual brushing. But the bottom line is simple: how thoroughly you brush is always more important than what you brush with.

How are sonic toothbrushes different inside?

Take apart a sonic brush and you'll find it's very similar to one with a rotating head, though the circuit board is typically bigger and more complex. In the sonic brush shown below, the circuit controls the brush speed (with two different speeds available at the flick of a button), a timer that beeps every 30 seconds and after two full minutes (to encourage rigorous brushing), and the battery recharging process.

A Braun sonic toothbrush with its outer case removed showing the component parts inside. Side view A Braun sonic toothbrush with its outer case removed showing the component parts inside. Front view.

Photo: Two photos showing the component parts of a typical sonic brush. Note the bigger and more complex circuit in this brush compared to the one up above. This one even has a microchip inside!

How do ultrasonic toothbrushes work?

If you really want ultrasonic cleaning, you need an ultrasonic toothbrush that vibrates about 100–1000 times faster than conventional rotating or sonic toothbrushes to produce a genuine cavitational cleaning effect. Ultrasonic brushes work in a quite different way from rotating and sonic toothbrushes: they have no electric motor inside:

The components inside an ultrasonic electric toothbrush.

Artwork: How a genuine ultrasonic toothbrush works. A battery (1) powers an electronic circuit (2) that produces a high-frequency DC current. Through connecting wires (3), this drives a piezoelectric transducer (4) that converts electricity into high-speed vibration. The transducer expands and contracts at very high speed, producing sound waves in your mouth that create gentle cavitation (5) around your teeth, loosening plaque that can be brushed aside (6).

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Text copyright © Chris Woodford 2007, 2014. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2007) Electric toothbrushes. Retrieved from http://www.explainthatstuff.com/electrictoothbrush.html. [Accessed (Insert date here)]

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