How Do 'Singing Magnets' Work?

dpbsmith asks: "Singing magnets are available at all of the usual geek-toy emporia, and, for all I know, ordinary toy emporia as well. They consist of a pair of magnets made of a polished substance with the general appearance of hematite. What is surprising, pleasing, and unexpected is that when the magnets strike each other under their own power, they produce a sharp, loud buzz that rises in pitch. The sound lasts a good fraction of a second and climbs somewhere into what sounds like the 200-500 Hz range. The exact sound and its duration are somewhat unpredictable and depend on how the magnets happen to strike each other. It is a little like the sound that you get when you mash a pingpong ball against a pingpong table with a paddle. What physics are involved in the production of these sounds?" "Google searches turn up some forum postings that indicate that it is a synthetic magnetic substance similar to hematite that's available cheaply in China as an industrial byproduct. The singing magnets are a little larger than size of olives; the shape is similar to a (U. S.) football but slightly more elongated. Their major axis is about 5 cm long, their minor about 1 cm. They are fairly powerful and will jump together when placed on a desk about three inches apart. They can distort the colors on a CRT display from a distance of over 20 cm.

Contrary to expectation, the poles of the magnets are oriented along one of the minor axes of the ellipsoid, not the major axis.

Neodymium magnets in 'ordinary' shapes produce boring 'plinks' when they snap together. Something about the shape of these magnets makes the sound much longer-lasting and entertaining. It is not simply the bounding rebound of two objects made of stiff-but-elastic material. Transfers of linear to angular momentum are clearly involved.

If course, I'd love to know whether these things were 'invented' or 'discovered', and by whom, trying to do what.

Comments

Re:Bounce.

[Wandering Wombat]

It's also true. I had these when I was a kid, and by pushing them towards eachother, I could make the sound louder, and by muffling them with my hands, I could damp them out. That has to be vibratory acoustics. Plus the sound TOTALLY trips out my son, makes him giggle like you wouldn't believe.

Re:Yup.

[Bastian]

I have a pair, and this is the only explanation that makes sense to me. I imagine that the effect would also happen on a whole range of rounded magnets of this type, but with the length of the "buzz" varying - the stretched football shape is probably just one of the more effective ones.

If you try to isolate the system by throwing them up in the air so they pull together and strike each other while airborne, they will generally buzz for quite a long time - generally, it stops because the motion was dampened by your hand (or whatever else they land on) rather than coming to a stop on its own.

The whole effect is made even more fun because if you throw them in the air, they will spin around each other like a cat in zero gravity.

I have an idea...

[syukton]

Go here: http://liquidmetal.com/news/dsp.multimedia.asp

View their "Ball Bouncer Demonstration"

I have a hunch that these "singing magnets" are coated with a Liquidmetal-like material which preserves some 99% of the impact force and returns it quite energetically, causing the magnets to come together, be pushed apart, come together, be pushed apart, fast enough to create a "tone" of sorts.

When the ball gets to the end of the ball bouncer demonstration, it does largely what the original poster was asking about with regard to the sound it produces.