Stuffing a Pensil

[roscoman]

Hi guys
Following advice from folks on here last year I am about to embark on a Pensil build - Pensil 7 with Alpair 7.3 drivers to be precise. (It's a year late because I've had setbacks on the penny amassing front).

The only thing I'm not clear on is stuffing, because I've never done it before. Some basic research tells me that I want long-haired sheep's wool but I'm very hazy on what to actually do with it. My main issue is how to keep it in place. I just can't imagine that it won't gradually fall down under its own weight and gather at the bottom of the cabinet. I am unlikely to use the brace because of build complexity and I gather that it is less necessary for the narrower pensils.

Are my fears well founded? Will lining the walls with some rough material help to hold the wool in place? Or am I missing something about stuffing techniques?

Many thanks in advance
Ross

 

[roscoman]

Hmmm, not getting much response to this very noob question. I can't believe no-one on here has stuffed a speaker cabinet before Am I asking in the wrong way?

I don't want to duplicate if that would annoy people - should I move this (or request it to be moved) to the Markaudio forum since I'm asking about the Pensil design?

 

[fastbike1]

Stuffing is often held in place by resting on, or otherwise adhered to a brace.

While several of the popular single driver designs have an elaborate brace, that is as much a design "trademark" as absolute requirement. A simple cross brace or series of cross braces will suffice. The idea is to kill of move panel resonances out of the passband by dividing the panel into smaller, unequal panels.

Stuffing can also be held in place by netting, mesh, etc.

Polyfill (pillow stuffing), recycled denim (Ultratouch in the US) or fiberglass all work.

I would think that bracing is necessary for the pensils because they have some long panels that will be happily resonant.

 

[roscoman]

Thanks fastbike, that helps. Maybe I will go with a brace. I did read somewhere on here that the original designer of the Pensil series says not to use horizontal braces because they will interfere with the transmission of sound down the line.

A vertical brace would still leave me thinking that the stuffing will be liable to sink down over time so perhaps I would need to add some nets to hold it in place but I've never heard anyone talking of doing that. Is it common? Or am I worrying too much about stuffing sinking?

 

[fastbike1]

Measured responses from many TL builds demonstrated that the line has to be significantly constricted by bracing in order to impact the sound. An 18mm wide brace going side to side or front to back is hardly going to disturb the quarterwave effect. You are generally going to use a fair bit of stuffing such that it will be self supporting. Also not a lot of mass for gravity to work on.

Lining three sides (not the baffle) with dense felt (at least 12mm thick), rockwool, fiberglass, or recycled denim will also work.

The trick is in the quantity of stuffing. Might be worth a removable panel so the amount can easily be adjusted.

Thanks fastbike, that helps. Maybe I will go with a brace. I did read somewhere on here that the original designer of the Pensil series says not to use horizontal braces because they will interfere with the transmission of sound down the line.

A vertical brace would still leave me thinking that the stuffing will be liable to sink down over time so perhaps I would need to add some nets to hold it in place but I've never heard anyone talking of doing that. Is it common? Or am I worrying too much about stuffing sinking?

 

[roscoman]

That's great info, thanks fastbike! Does the felt lining need to be taken into account for the size of the cross-section? (i.e. do I need to make the wooden part larger so that the cross-section between the lined walls is the as the Pensil design?) Or do I ignore the lining for the purposes of the cross-section?

 

[Scottmoose]

Thanks fastbike, that helps. Maybe I will go with a brace. I did read somewhere on here that the original designer of the Pensil series says not to use horizontal braces because they will interfere with the transmission of sound down the line.

Did I? No, I don't recommend them because they don't work very well, simple as that. Longitudinal bracing is much more effective than transverse.

Since the box is designed to be stuffed at a uniform density throughout (try not to have it right up against the driver though to prevent mass-loading the cone), it should stay up reasonably all by itself. You can stick a couple of dowels across the box above the vent to hold things up a little if you feel so inclined, or similar.

As for material, forget long hair wool. It works fine, but it costs a packet, and there's no mystical properties to it, any more than anything else. All damping materials have their own idiosyncracies, and require somewhat different packing densities for a given volume. The pensils assume straightforward dacron (hollow-fibre pillow stuffing, available from any good craft shop). I say 'straightforward' because even then, all is not created equal. The plans all indicate a default density to get you started, and a removable back to the box is recommended. Adjust the density to preference; it's deliberately intended to be flexible in this regard so you can tailor things as you wish.

As for lining the box, I designed them to be stuffed, not lined. Lining will not provide the required damping levels.

 

[roscoman]

Did I? No, I don't recommend them because they don't work very well, simple as that. Longitudinal bracing is much more effective than transverse.

Whoops, sorry to misquote you! My memory obviously isn't as good as I imagine it is

Thanks for chipping in, scottmoose. This is all very valuable info. And thanks so much for designing them in the first place. Very generous of you. I haven't touched a single piece of wood yet and already I'm having such a lot of fun. Now that I can stop fretting about the stuffing I can get on with the build and finally have a pair of good quality speakers.

Huge thanks to you and fastbike