Something stirs in the Undergrowth

[IslandPink]

Definitely worth trying Lynn- could be a very cool combo if you can match the curved perspex baffle to the lower box front as per the drawings you showed in e-mail .
Definitely go for the best you can get on cores if you get the special autoformers. I felt that 50% nickel lost a bit of tone & tranparency myself . James D agreed .

Meanwhile , had a successful trip to Bowden on Sunday .
( loan ) Supravox 285GMF's are 'in da house'
I have them by the chair in the sitting room so I can look at them from time to time while I'm watching TV .

[Lynn Olson]

I don't think the esthetics of curve-matching will be too difficult. It's a lot easier to bend perspex/acrylic than plywood, and it machines more accurately too. It was your comments (and comments made by other posters) than made me realize that perspex had acceptable sonic qualities, at least compared to nasty old MDF.

What got me going along this line was the realization that a straight bass horn (no folds or bends) of domestically acceptable size wasn't going much lower than 150~200 Hz. The range from 175 to 700 Hz is two octaves, which isn't all that wide, but there's a lot that goes wrong in those two octaves.

In this frequency range, drivers are well-behaved, but conventional enclosures are not. Efforts to damp large panels quickly became heroic when you're talking about 3~3.5 cubic foot cabinets, far larger (and heavier) than mini-monitors with sophisticated cabinet construction. Lowering the Q and magnitude of interior standing waves (a completely different problem) is also a challenge. The most successful approaches increase complexity, weight, and cost ... and 3~3.5 cubic foot MDF or Baltic Birch cabinets are not that light to start with.

Below 175 Hz these problems are alleviated, since it's below the first panel resonance (with competent construction) and also lower than the first interior mode. If the below-175 Hz driver is close to floor level, the under-floor image effectively doubles efficiency and power-handling, which nicely offsets the drop in LF response of the upper driver. So I think it will work.

[IslandPink]

I'm making good progress at the moment despite no pics - I was waiting for the chokes to arrive from Mundorf to get a shot of everything, but they will be here last week in Jan now, according to Nick .
So, anyway, in the thick of detail work on making the first (Quasar)speaker neatly . Been drilling ( carefully, perpendicularly .. ) the holes through the perspex baffle, countersinking , working on the 18mm ply side-panels - trimming to exact dimensions with router, then rounding .
This evening I drilled the front face of the first side panel with 5 carefully measured holes (+/-0.5mm) and installed the M5 'stegosaurus nuts' ( see pic below ) using epoxy for final fitting , for strength .
The base panel ( 25mm ply ) is done, although I need to firm-up on how to do the 'hump' that James specifies behind the Supravox driver .

If someone ( like eg. Colin ) is out there who has opinions on finishing, here's a Q :
What finish will protect but maintain a fairly pale appearance, on birch ply ?
I'm familiar with how it looks when varnished, or with Danish oil, but if I can keep it lighter, that would be good .

More soon ..

[Toppsy]

If someone ( like eg. Colin ) is out there who has opinions on finishing, here's a Q :
What finish will protect but maintain a fairly pale appearance, on birch ply ?
I'm familiar with how it looks when varnished, or with Danish oil, but if I can keep it lighter, that would be good .

I would recommend General Finishes High Performance varnish. It's a hybrid water based polyurethane varnish with UV inhibiters and comes in several finishes from high gloss to matt. Adds littel darkness to the ply and will not fade with exposure to sunlight.
http://www.generalfinishes.co.uk/index. ... ore&CID=27

[Greg]

[IslandPink]

Thanks Colin ; would you suggest 'flat' , 'satin', 'semi-gloss' or 'gloss' in this context ? I'd like to keep colour fairly pale, but have some wood-grain still visible.

[Toppsy]

Mark,
I had issues trying to post a reply earlier but rebooting my laptop and clearing my temp web browser files sorted things.

would you suggest 'flat' , 'satin', 'semi-gloss' or 'gloss' in this context ? I'd like to keep colour fairly pale, but have some wood-grain still visible.

They all bring out the grain and do not darken the wood as some other varnishes I've used. So basically pick your poison. Me I prefer to use satin but on the Norge prototypes used the gloss which on the darker veneer I think looks better. Brushes on a treat and dries quick.

The problem with using Liberon Finishing oli is it will darken quite some with age as will all oli finishes. You can however, if you prefer the look of an oil finish apply a coat the General Finishes High Performance varnish over the oil when this has fully dried. This will give it UV protection against darkening. I don't know of any other varnish that you can apply over an oiled finish. best to experiment on some spare offcuts of ply to see which you like best.

[IslandPink]

Here some pictures to illustrate what I've been up to . Not far off having speaker #1 complete for the Supravox and Fostex installation . Chokes are now on their way from Mundorf, may reach me by the weekend .

First pic shows one side panel ( 18mm birch ply ) and base ( made from 2x 25mm pieces of birch, with a lot of planing ) to reproduce the 'humped' base panel James has on the perspex ones . Supravox 285GMF ( thanks Steve ) and Fostex FE208EZ behind .

Second pic shows base panel from side, with hopefully just about visible, three slots for biscuit joints , which has worked well to join & hold together for the first side ( PVA glue used ) . The base panel has a biggish cut-out underneath through the first layer of 25mm birch, to reduce the substantial weight a little bit . Yes, there are some filled oblong defects visible on there due to me re-using some 25mm birch from the Onken project. I'll try and choose the wood a little more wisely on the 2nd speaker, but it doesn't bother me too much .

[Dave the bass]

An access ramp?! Radical!

[IslandPink]

More views shown here -

First, perspex front-baffle from rear, showing Fostex cut-out with M5 splined bushings tapped into the through-holes. This seemed worth doing for neater appearance than just using hex nuts on the back . The cutout is also rounded off with a router to aid smooth airflow off the back of the cone - this will need to be polished or blow-torched smooth at some stage .

Next shot, baffle attached onto right side-panel : uses stainless M5 countersunk screws through into M5 socket nuts ( stegosaurus nuts as I call them ) which are embedded flush in the ply edges . Baffle is rounded-off on front edge.
I'm not sure this is the most elegant way to do it, but I think it has a sort of functional engineering-driven style which will look pretty good once I've got the drivers installed, the ply varnish applied, and the protective film peeled-off the perspex .

[IslandPink]

Exit ramp Dave - for the lovely Supravox bass waves

Last pic till the weekend :
Lower edge of baffle and cutout for Supravox . Here there are M4 screws, into 4.5mm through-holes, enlarged to about 5.2mm at the rear into which are knocked similar splined M4 bushings ( with shoulder ) . I need to install a couple more stegosaurus nuts into the front of the base panel to get a solid fit of the front baffle to that section and give a good air-tight fit all round . A job for Friday I think .

[jack]

Silly question, but what are "splined bushings"? Are they the same as "nut inserts" or "blind inserts"? e.g. http://uk.rs-online.com/web/p/inserts/0278607

Quiet at the back there ! (DtB - that means you )

[IslandPink]

It's hard to get the right words for them .... basically a metal insert with eg. an M5 thread on the inside and a splined outside, with an additional flange to stop them going in any further and getting lost in the hole

Perhaps it's a 'Headed Steel Knurled Shank Knock-In Insert' after all :
http://www.ebay.co.uk/itm/M5-x-6-6mm-He ... _477wt_722

[jack]

Ah. Ok. That's a splined insert that goes onto the back of a board.

All these fixings should have defined names. The Victorians were good at that sort of thing. They even had different names for different types of dinosaur poo... They named everything.

[IslandPink]

Pic here from Friday . Since then the house has been full of the smell of varnish . Chokes now delayed till at least the middle of the week