# Sound made by Euler's disk toys

There are lots of videos on youtube etc showing Euler's disks. Why do we hear that periodic clattering sound? If it's a uniform rolling motion we would expect a smoother sort of whirring sound. (Like a monocycle running along a tight circle, maybe). What determines the frequency of the clatter?

Why do we hear that periodic clattering sound?
Sounds like the clatter of the mirror wobbling on an uneven table surface.

anorlunda
Staff Emeritus
Could it be that the velocity cycle is elliptical, not circular?

I guess one would have to first eliminate 2m4r35n357 's possibility of the base (usually a mirror) not resting firmly on the table.

Assuming that part is ok, then a non-circular motion might explain it. But anorlunda, by elliptical, do you mean like a wheel rolling along an ellipse on the ground instead of rolling along a circle? What would cause the wheel to turn more sharply at the ends of the ellipse, and less sharply in the 'middle' of the ellipse? Could it be the shape of the mirror, mimicking a gravitational force?

Somehow, the sound gives the impression of larger forces at work than what the very slight curvature of the mirror would suggest.

I wonder if it is just position (of the listener) dependent? IOW, the sound is constant (like the rolling wheel example above), but since the disk is changing its "face" relative to the listener, the listener is hearing the sound change as the "face" changes.

Ever see the rotating horn on a Leslie™ Speaker (mostly used on Hammond organs, to provide a complex vibrato/tremolo/comb-filter effect)? You get a sort of whoop-whoop-whoop-whoop sound out, with a static tone applied. A little like the sound of a Euler disk, especially if you input a filtered noise sound that the disk or rolling wheel is probably making.

If you plugged one ear, and placed your other ear (or better, a microphone) directly above the axis of the spinning disk, would the sound be more constant? I wouldn't expect the variation to go away completely, you will get reflections from the room. Try it in an an-echoic chamber! :)

If I remember correctly, the chop-chop of a helicopter is also something to do with where you're listening from.

OK, I just watched a couple videos to refresh my memory. I think my earlier post explains some of it, but I do hear more 'clattering', usually near the end. My guess is that at that point, the disk is not just rotating/wobbling constantly. I think it might actually be 'jumping' up, to release air underneath, and the 'falling' down and banging against the mirror, and this repeats.

Ahhh, just saw your note on helicopters - yes, I think that is a similar effect. We should expect just a constant whirring sound as they rotate at a constant speed, but we hear the chopping sound.