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
Dec 11, 2009 1:05:43 GMT -7 @chrisbee said:
This goes completely against the grain of everyday experience. So I'll persevere in the hope we may learn something useful.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.
Dec 11, 2009 1:05:43 GMT -7 @chrisbee said:
Let's take a very simple example: A sheet of plywood we intend to use for our absorber membrane.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.
Dec 11, 2009 1:05:43 GMT -7 @chrisbee said:
How small do you want to go? 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.