Water analogies [Re: one for the theorist]

[Martin Czech]

:::Gravity doesn't work well in this analogy. Here's the one I've always used:
:::imagine a cylindrical container with pipes attached at the ends. In the
:::middle there is a flexible membrane. If the pressure is greater on one
:::side, this will distend the membrane, displacing water from the other side.
:::Too much pressure and it will burst. :o
:::

Wait... Almost.
But I think this model does not explain the frequency dependency
of the water stream.

Would it be better if the membrane is replaced with a air cussion?
The faster the pressure rises the more the cussion displacement will
pump the water ?

I think maybe a valve/piston/spring arrangement would serve.
(Now I really need a dictionary). The valve has to outlets, one left one right.
We have two pistons in the valve, they are connected with a spring.
The volume between the pistons is empty, no water in here.

If we now add a pump on the LHS and a narrow pipe at the RHS (resistor),
and a wider pipe back from the resistor to the pump, well that's a highpass circuit.

Allmost ... , no , doesn't work because the RHS presure depends on displacement now.
Oh come on, I had two semester mechanics, there was mass for the acelleration terms,
a spring for the displacement terms, and a DAMPER (oil or something) for the speed
term.

I think that is it: the two pistons have to be connected via fluid damper,
so a slow LHS displacement will not show up at the RHS, the faster
the LHS piston goes, the better the RHS will follow.

But this arrangement will fail in the lp case ...

Any better ideas?

m.c.