Part of the show Tuberculosis and Magnetic Bacteria
Mark, Melbourne asked:
I wanted measure how fast my sonic toothbrush actually brushes so I used a sound recorder to measure the sound it makes. I then looked at the waveform that the computer can show up to show how many oscillations per second. I counted 25 per tenth of a second which is only 15,000 cycles per minute but they claim on the packaging that it’s 30,000.
Ben: I’m not really sure I know this is the best way to measure it. If anybody knows get in touch: Chris what do you think?
Chris: I think you may be on to something. When I was driving around in Australia last year a group of school children tried to measure vitamin C in foodstuffs as their school science project. They took an off-the-shelf substance, food or drink cordial which is well known-very powerful brand, full of vitamin C. They thought, ‘we’ll use that as the control because we know that will have lots of vitamin C and if we can detect that we know our experiment’s working.’ They kept getting a negative result. It turned out that the company were lying and saying it was full of vitamin C and there was none in it. They had to pay a very big fine and change all their labelling. It could be that with this sonic toothbrush there’s a little bit of adventurous advertising going on. I think if anyone else has a sonic toothbrush and they would like to record it, analyse the waveform. If in fact you’ve done this, please do experiment and then tell us how many cycles a second is it producing? Is there some misleading advertising going on?
Ben: It would be fascinating to find out and we’re really pleased, Mark, that you’re getting into Kitchen Science in this way.
So what do you think? Is this a good way to find out how fast a sonic toothbrush vibrates?
That was my conclusion as well daveshorts, Mon, 7th Apr 2008
Hi, Gents! Love the show. I'm a fan from Iowa in the US.
Sounds like a plausible argument; thanks everyone. chris, Thu, 17th Apr 2008
If you are recording the waveform digitally then you have to sample at twice the maximum frequency you want to record. Your sound card will certainly not be doing this, if it's an off the shelf audio card. What has probably happened is that your 'sub-sampling' has produced a beat frequency (artifact) at some audio frequency. Buy a system which can record ultrasound and you may get the right answer. One question; did you hear that pitch of sound from your toothbrush that the recording picked up? lyner, Thu, 17th Apr 2008
He may have been able to record the toothbrush okay, if the frequency is, as we suspect, 15kHz - because most soundcards offer up to 48k sample rates; so he was probably ok. Even with 44100, which is standard, it would have eben alright.
Well spotted tm. Fools rush in etc.