Re: [Phys-L] Resonant One End Closed Tubes

[John Denker <jsd@av8n.com>]

On 4/1/25 15:53, Zeke Kossover wrote in part:

> So, I want to make a one end closed, one end open resonator with an
> adjustable plunger. My design is a wooden plug cut to fit the tube with a
> small wrapping of felt to let it slide. It works okay, but not nearly as
> well as the two ends open version or the one end open version and water.
>
> I don't understand why. Anyone have an idea? 

I'm surprised. I don't have much of an idea, but here are some
partially-baked thoughts:

Given that the water works and the wood does not, the problem
cannot possibly be the wood per se. It has to be the gap between
the plug and the tube, and/or the felt in the gap.

-- Felt is widely used in musical instruments to *absorb* sound.

-- Even a small gap, tiny compared to the wavelength, can absorb
 a lot of sound. Remember the old-style acoustical tile, with an
 array of small round holes?

-- When the Q is high, it doesn't take much dissipation to lower
 it a lot.

===========

Here's an easy experiment: In the tube with the solid plug, pour
some water in on top of the plug. Stand the whole thing in a cup
of water if necessary, so it is in hydrostatic equilibrium, so
you don't need to worry about water leaking through the felt.

This allows you to interpolate between a liquid closure and a
solid closure. In the limit of an infinitesimal layer of water
on top of the solid plug, I predict it will work just fine,
with the water serving no purpose except to fill the gap.

If this trick works as expected, you know what to do: Machine
the plug so it fits super-tightly, with no felt and no gap.

If you don't have access to a proper lathe, you can fake it
with a drill press, or even a plain old drill. Glue a rod
into the axis of the plug, sticking out one end. Chuck up
the rod, and go to work with a file and sandpaper.

The compliance of the thin-wall tube should allow you to
get a seal with essentially no gap, down to something like
the mean free path.


> And do you think that this is
> related to the fact marimbas almost always use two end open tubes for
> resonators?

I don't think so. Many musical instruments achieve a high Q using
a cylindrical bore closed at one end. Flutes, clarinets, et cetera.