Alu honeycomb panel

[Dave Zan]

...where to place the drill bit for each hole would be my concern.

I just assumed that it wouldn't be too critical, don't have to hit every cell even if they aren't interconnected.
A bit of variation in the pattern may even be beneficial to break up wave patterns. I doubt it would make any audible difference but it seems unlikely to hurt.
I would use a few different drill sizes to detune any Helmholtz resonances.
Probably also inaudible for the small hole sizes under discussion here but also unlikely to hurt.

Best wishes
David

 

[SamAnytime]

At 'local' prices, a 25mm thick aluminium honeycomp core would cost $440 + delivery (2,500km), which is a lot of money to save a couple of kilos.

To keep the cells of that honeycomp open to the inner airspace, I'd have to use perforated alu as the inner skin (since I don't want to drill 10,000 holes by hand). A 1.6mm sheet would cost $150 (+ delivery), and weigh 4kg more than the ply equivalent.

This would essentially nullify the weight saving of dropping the plastic, so I'd basically just be quadrupling the total price - and getting a slightly thinner panel for it.

Agreed. You could consider cutting the sheet in half, thickness wise, so you'll have two sheets of half the thickness with skin on one side and open honeycomb cells on the other. You'll lose a lot of strength, though, but aluminium might be strong enough. Won't do for cardboard.

 

[hollowboy]

sheets [...] with skin on one side and open honeycomb cells on the other [...] aluminium might be strong enough. Won't do for cardboard.

Anecdotally: with the box that I'm currently working on (pix in post 24), this doesn't seem to be effective.

In some of the cutouts, where the wall is (now) a thin skin, I've glued a layer of plastic grid to the inside. On a knuckle test, this has no effect / very little effect. It sounds very hollow relative to the bits that are ~45mm thick. I think a 'sandwich' material needs something rigid on both sides of the core material.

I want to see if making the inner layer as an open lattice (a few light braces, rather than a continuous skin) is enough to give that rigidity.

The lattice image attached here is not my work, but just illustrates how I think it might look, if crafted with care. I'm using random offcuts for my bracing, and hence it is much less pretty.

link I got the picture from

 

[hollowboy]

My work in progress.

If you peer in there, you can see the plastic grid being glued in at the bottom, and the haphazard way I make braces out of scraps.