Bull roarers

This is an edited transcript of an exchange which took place on the Internet list "Science Matters", hosted by the ABC, with the quotations from Glen Moore (Wollongong Science centre) and Sally Edwards (Curiosity Co) reproduced with their permission.

I wrote:

I have just added in the bullroarer, but I lack a good all-round explanation of the aerodynamics of the bullroarer's action. Why does it spin, and why does that spinning make a sound? Can anybody help?

In case you have never used one, drill a hole near the end of a 30 cm ruler, or similar piece of thin wood, insert ~1.5 metres string, tie off, an swing it around.

In stepped Sally Edwards:

stumped I'm afraid

I've checked in all my experiments/demonstrations type books - (I held out great hope that Jearl Walker's The Flying Circus of Physics would help) but to no avail and my innate talent at physics concepts is seriously dodgy.

I kept thinking that the ruler must be moving back and forth about its long axis and that this rhythmic movement might be what's giving the noise its distinctly "roaring" quality. The spinning part seems to fit instinctively - water wheels certainly create a roaring noise but I am not sure how. A roar seems to me to be a "shaky" kind of noise - one where changes in sound as regular almost as if the vibrations themselves were vibrating. This fits to some degree at least with the different effects you get with different sized pieces of wood (bigger SA = lower frequency and higher amplitude) but I haven't tested to isolate changes in length, width or overall SA.

(Note from pm: here's a good idea for a project!!)

I have found several books which refer to this demonstration but none bother to explain it in any sense whatsoever except with something like "the ruler makes the air vibrate and creates a roaring noise". (Didn't anyone tell these people the devil's in the detail...). Books which have better explanations don't seem to include this experiment at all! Is there something we should be learning from this?

Peter - if you do manage to find something on this I'd love to hear about it. BTW - my 4 year old insists on calling this activity the "Rule-boarer" which seemed as good a name as the original somehow!!
Sally Edwards [Curiosity Co]

I'm stumped too -- on all counts. Stumped, read the books, thought it would be easy, and as a physicist, I make a very good garden gnome (except I've known some of them to be quite good around QCD).

The bull roarer rotates, and as it rotates, it roars. What aerodynamic effect makes it rotate, and is the direction determinate? Why does the turning make a roar, and is the frequency determinate? This is silly -- somebody must know.

This is getting serious -- somebody HAS to know. I want to know, Sally wants to know . . .


Glen Moore to the rescue:

'Bull roarers' require some reasonably advanced physics to understand them but here are some thgings to try. If we consider the motion of a falling body such as a ruler (an approximation to a bull roarer shape) there are some interesting effects. Try cutting a small piece of paper approx 50mm by 15mm and throw it into the air. When it falls it undergoes a surprising motion. It will rotate about its long axis like a paddle wheel and also follow a spiraling path as it falls. Depending on the way in which it is thrown the motion will not be exactly the same each time. The reason for this is that a body is stable for rotations about two of what are called it major axes, through its centre of mass). The unstable rotation is about the axis with the intermediate moment of intertia. For a ruler the stable motions are about the axis perpendicular to its plane (like a helicopter blade) and the one about its longest axis (like a paddlewheel).

The bull roarer has the added complication that it is a 'driven' motion (not free fall) and there is substantial air friction but the ideas are the same.

When you swing the bull roarer around it acts much like the falling paper, taking on the paddlewheel motion and producing lots of noise as it moves through the air, but soon you will have a very twisted string.

Glen Moore

Hmmmm. Works for me -- sort of.
>through the air, but soon you will have a very twisted string.
Not me -- I always use a swivel from the fishing junk box downstairs in the garage.
peter And there's another hint for smart readers!

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