front port Fonken floorstander

[robtwickenham]

Following brief correspondence with David D concerning a front ported Fonkenesque floorstander, he suggested reversing the port direction on the mk.2.

Having now completed a job requiring 15mm ply sheets, I had enough offcuts to lay out and cut a trial set of parts, incorporating self-jigging, easy assembly rebates all over (the wonders of auto-nesting software and a CNC machine!)

As I am now fabricating the boxes together, my mind is turning to the issue of damping. David's notes suggest lining the walls with 1/2" wool felt and lightly stuffing the enclosure.

Has anyone experimented with other damping materials eg. Dedsheet style self-adhesive bitumastic sheet, or would this effect the internal volume?

Regardless of the type of damping chosen, should this extend down onto the internal baffle faces and port area, or just in the main cavity?

All opinions welcome. I am desperate to get the backs on and report back on the sound of this variation.

Cheers,

Rob

[chrisb]

Originally Posted by robtwickenham

Following brief correspondence with David D concerning a front ported Fonkenesque floorstander, he suggested reversing the port direction on the mk.2.

Having now completed a job requiring 15mm ply sheets, I had enough offcuts to lay out and cut a trial set of parts, incorporating self-jigging, easy assembly rebates all over (the wonders of auto-nesting software and a CNC machine!)

As I am now fabricating the boxes together, my mind is turning to the issue of damping. David's notes suggest lining the walls with 1/2" wool felt and lightly stuffing the enclosure.

Has anyone experimented with other damping materials eg. Dedsheet style self-adhesive bitumastic sheet, or would this effect the internal volume?

Regardless of the type of damping chosen, should this extend down onto the internal baffle faces and port area, or just in the main cavity?

All opinions welcome. I am desperate to get the backs on and report back on the sound of this variation.

Cheers,

Rob

The P-10 philosophy to enclosure construction includes;

1) use the thinnest high density plywood (Baltic Birch etc) you can, depending on the enclosure /part size (i.e. a 72" tall double mouth horn such as Woden Avebury or Vulcan would need 18mm for side panels, but 15mm would probably suffice for the multiple internal panels)

2) incorporate asymmetrical bracing to increase enclosure rigidity and distribute amplitude and Q of multiple smaller panel resonances

The results, particularly on an enclosure with the panel sizes involved here shouldn't require "deadening" by mass loading that your method would approach.

Rather, we line all the panel walls with compressed wool or cotton felt, or thick carpet underlay, and in some designs add conventional BAF wadding / Acoustistuff etc to reduce internal reflections, and in the case of tall /skinny enclosures such as these, to control 1/4 wave induced ripple.

Frankly, I can't remember the amount and location of additional stuffing on the pairs that I built - I almost never get around to shooting pictures or taking those kind of notes. Dave might well have a suggested starting point, but you mind end up "tuning by ear". That was certainly the case with a recent build of Alpair Pensil7 - thank goodness they were built with removable back on cleats.

[planet10]

Originally Posted by chrisb

1) use the thinnest high density plywood

That should read "stiffest" If faced with 2 materials of the same stiffness, the ligher one is preferred as its resonances will be at a higher frequency.

Things like dead-sheet actually make things worse (unless you can use it to tune your cabinet panels so that resonances happen in the "voids" of the well-tempered scale). They add mass without adding stiffness so lower the frequency & Q of any panel resonance, making it more likely to get excited & be audible.

Ignoring the high aspect ratio, one would line the cabinet walls with 1/2" wool or cottom felt (or thicker BAF/poly batting or fiberglass insulation, not more than 1"). In the floorstander, the aspect ratio is sufficient to invoke some 1/4 wave action. To help counter this, the bottom of the enclosure (below or away from the vent exit) should be filled with loose damping material.

dave

[robtwickenham]

Thanks for the guidance, Chris and Dave.

That's a pity about the sound deadening sheet. I have several metres of it left over from furniture project! I will stick with felt - most likely a wool carpet underlay, in the first instance.

Whilst I experiment with lining and stuffing, I think that I will run an O ring groove round the rebate on the back panels, so that I can seal the boxes up without committing to glue.

I am intrigued by Dave's comment about panel stiffness. I coincidentally have a quantity of aerolam (aluminium skinned, aluminium honeycomb cored panel). Super stiff, for minimal weight, used commercially in the aircraft building industry amongst others. I seem to recall that it was experimented with by Spendor or Celestion about 20 years ago.

Any thoughts on this subject would be gratefully appreciated. maybe a Mk2 comparison is in the offing.

Cheers,

Robert

[planet10]

The aerolam should make a very good panel material. Expensive & harder to work with keeps the number of actual builds minimal.

KEF's latest Blade concept speaker using a balsa core with carbon fibre us a somewhat analogous structure

dave

[nrg2009]

Rob, what are the dimensions of your build?