Rock wool wrapped in plastic and steel membranes Dec 12, 2009 5:15:47 GMT -7
Post by anonymous on Dec 12, 2009 5:15:47 GMT -7
There are many things that go against the grain, and some of them are still correct - the Sun doesn't go around the Earth In this case - well, what more can I say? Check out the link, it's a very short PDF, with _one_ page of simple formulas that do not go beyond the school's course of elementary physics - Newton's First Law, some minor bits of thermodynamics, that's all.
As a full 8 x 4 sheet it is very floppy and has a very low frequency of resonance. I give it a rap and it responds with a very low, dull thud.
Cut in half, it is only half as floppy. It responds to a tap with a higher note but still rather obviously low.
Cut in half again it is getting stiff enough to support itself safely from its middle. The noise it makes when I rap it with my knuckles is very obviously higher in frequency than before.
This is a good example - but only if that sheet has no air-tight cavity behind it. In this case you are cutting the spring - and the smaller it gets - the stiffer it becomes. As soon as you build a proper membrane trap - the size dependency evaporates: the gas acts as a spring to which the sheet is attached - and the sheet itself acts as a mass attached to that spring.
Obviously, those simple formulas give only a first approximation (the stiffness of the membrane itself adds to the stiffness of the gas), so there could be errors of a few Hz from the computed frequency, especially with thick and rigid materials - like that 3/8 plywood. In general, though, for thinner materials, the physics seems sound and allows one to construct the trap with predictable characteristics.
The steel is sold in 2.5m*1.25m sheets - so I'll just cut one in half to get two traps.
BTW, I'm also thinking about making the trap with a "proper membrane" - basically, attaching the edges of the sheet to the frame with some soft but durable plastic, so the sheet could move as a whole - this would completely eliminate its own stiffness and minimize any errors in frequency.