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PFM OGRE 2 thoughts

Thing is it's the making sawdust I like the most . Got some new vacuum forming stuff I'm dying to make something with too :)
 
Been poorly so my brain hasn't been up to giving more details on the design, however I have taken delivery of some shinny connectors to make a breach airline for the vacuum bag, not really HI-FI but here you go.

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Well the air line works, the connector didn't fit my robinair vacuum pump as it turned out to be an Ameriacan hydraulic thread. A quick trip to my local tractor dealer this morning with the pump and the connector and hey presto. I love that sort of place, the owner told me he'd got everything except money :)
 
A really interesting project this which I will follow with interest. A few observations if I may: The Scanspeak 8531 example in a 33 litre cab with a 40 Hz tuning point seems tuned a tad too high for that volume. Its not optimum in terms of efficiency or driver excursion. I have used these drivers to great effect in a 30 litre enclosure tuned to 32 Hz. Group delay is still well within 15 millisecs which whatever way you cut it, isn't slow. The sealed box may be better in this respect at closer to 4 or 5 millisecs (they always are in this respect) but anything under 15 to 20 mSecs is quite reasonable and having built many of these in ported designs, I can assure you that slow sounding they aren't! You can get astonishing bass out of a single 8531 without the need for power robbing BSC filters which can also screw up phase (unless actively done).

I wish you the best of luck with this project and hope it delivers for you.
 
The 18W8531 is a mid-woofer. For a three-way design, I'd be inclined to get a proper 10" or bigger woofer that can shift some serious air and load it in a sealed encolsure.
 
It's also used as a woofer in a great many designs James, thanks partly to quite a low Fs of 28Hz. It's certainly capable of superb bass results which is why designs such as the SF Cremona and Gamut RS3 designs use it. Your inclination my well be right for a genuine full range speaker but it's splitting hairs a bit when this excellent little driver can deliver sub Fs rated frequency response in an appropriate cabinet design. As mentioned, I have used this driver before and it has been the equal of some 10 inch drivers. "Proper" needs a little more flesh on the description ;-). Granted, the 8531 may not be as efficient as a larger driver nor quite as effortless sounding but a cracking driver never the less.
 
Pac1 said:
A really interesting project this which I will follow with interest. A few observations if I may: The Scanspeak 8531 example in a 33 litre cab with a 40 Hz tuning point seems tuned a tad too high for that volume. Its not optimum in terms of efficiency or driver excursion. I have used these drivers to great effect in a 30 litre enclosure tuned to 32 Hz. Group delay is still well within 15 millisecs which whatever way you cut it, isn't slow. The sealed box may be better in this respect at closer to 4 or 5 millisecs (they always are in this respect) but anything under 15 to 20 mSecs is quite reasonable and having built many of these in ported designs, I can assure you that slow sounding they aren't! You can get astonishing bass out of a single 8531 without the need for power robbing BSC filters which can also screw up phase (unless actively done).

I wish you the best of luck with this project and hope it delivers for you.
Click to expand...

Please feel free to comment by all means, yes the 8531 can certainly be tuned lower, and I have used it in a slightly larger cab with lower tuning in the past. The cabinet it is in at the moment is my test cabinet and the port is just tuning it to the frequency the port happened to be uncut in the box, it's not perfect however it has helped me understand quite a few things about the relationships between electrical and acoustic specs while enjoying rather good sound. This box was specifically built to allow me to change baffles out reasonably easily and add shelves to change volume etc so that I could transfer that maths knowledge into an understanding of how things actually sounded easily. It's certainly done that.

You are right, you can get astonishing bass out of these drivers, two is even better, I'm all active here, and for the most part linear phase.

Stefan
 
orangeart said:
You are right, you can get astonishing bass out of these drivers, two is even better, I'm all active here, and for the most part linear phase.
Click to expand...
I'm not sure what you mean by linear phase. The moment you introduce a filter, passive or active, you're shifting phase. It is unavoidable.

Scan-speak makes astonishing good mid-woofers, I will happily agree. They make even better woofers for woofing duties. If I were to build the E-XI and was constrained to 50L or thereabouts, I'd jump at the chance to try out the 32W4878T00. Shame they are not exactly cheap at around €450 plus tax each. Compared to the 18W8531, they shift 3.5x more air, go lower (fs 18Hz) and cost less per displacement.
 
James said:
The 18W8531 is a mid-woofer. For a three-way design, I'd be inclined to get a proper 10" or bigger woofer that can shift some serious air and load it in a sealed enclosure.
Click to expand...

Well I thought long and hard about this, firstly I already had 4 8531 drivers which I have worked quite a lot with and now know reasonably well. Pac is right they do make an astounding woofer and you can make them go quite low with authority,not as low as the 32w you commented on of course but low enough for me on-balance.

When i first started looking at the woofer choice i did in fact model with this driver and for a while i was going to sell my 8531 drivers and buy the 32w's instead. A few things stopped me. firstly i didn't want to cross any lower than I have suggested above and the 32w ideally would ideally be crossed lower ~ 200hz to keep it well behaved. I wanted to keep the baffle width sensible and also wanted the baffle step to be kept to the crossover region. secondly I couldn't really push the little midrange driver much lower without using much more excursion and this would probably have caused distortion higher up which is why I'm getting away from 2 way in the first place. I could of course have used a larger mid-woofer instead of the little mid (probably even the 8531) which would need less excursion and hence cause less distortion higher up but then I would have run into breakup and directivity issues lower for the higher frequencies and would have to put my low pass filter lower.

Using a smaller low distortion mid was the biggest design goal and smaller woofers just are a better match. It's all a juggling game isn't it? always compromises to be made, still I do like that 32w, i'll definitely do something with one at some point i think, a 4 way maybe.....


James said:
I'm not sure what you mean by linear phase. The moment you introduce a filter, passive or active, you're shifting phase. It is unavoidable.
Click to expand...

Not quite James, agreed, using analogue filters, either passive or active introduces the associated phase shifts, however, using DSP when going active, and creating FIR filters, means that phase and frequency response can be manipulated completely independently of each other. I've got some thread's here abouts on the subject if you are interested?
 
orangeart said:
Well I thought long and hard about this, firstly I already had 4 8531 drivers which I have worked quite a lot with and now know reasonably well. Pac is right they do make an astounding woofer and you can make them go quite low with authority,not as low as the 32w you commented on of course but low enough for me on-balance.

When i first started looking at the woofer choice i did in fact model with this driver and for a while i was going to sell my 8531 drivers and buy the 32w's instead. A few things stopped me. firstly i didn't want to cross any lower than I have suggested above and the 32w ideally would ideally be crossed lower ~ 200hz to keep it well behaved. I wanted to keep the baffle width sensible and also wanted the baffle step to be kept to the crossover region. secondly I couldn't really push the little midrange driver much lower without using much more excursion and this would probably have caused distortion higher up which is why I'm getting away from 2 way in the first place. I could of course have used a larger mid-woofer instead of the little mid (probably even the 8531) which would need less excursion and hence cause less distortion higher up but then I would have run into breakup and directivity issues lower for the higher frequencies and would have to put my low pass filter lower.

Using a smaller low distortion mid was the biggest design goal and smaller woofers just are a better match. It's all a juggling game isn't it? always compromises to be made, still I do like that 32w, i'll definitely do something with one at some point i think, a 4 way maybe.....
Click to expand...
Absolutely, it's all about choices and there is no perfect solution. Your four 8531s can be used in the future with a 32W in a M-T-M-W D'Appolito configuration. If you use a suitable waveguide on the tweeter, you'd also lower its practical reach and cross below 2kHz and benefit from a much better dispersion as well.

If money was no object and I had the time, I'd love to build a wide-baffle version of the venerable AR9 with floor-coupled, horizontally opposed 32W woofers (wired in series to make it a sensible load). But rather than a 4-way, have an M-T-M with Scan-speak's finest, crossing at 200Hz and 2kHz.

orangeart said:
Not quite James, agreed, using analogue filters, either passive or active introduces the associated phase shifts, however, using DSP when going active, and creating FIR filters, means that phase and frequency response can be manipulated completely independently of each other. I've got some thread's here abouts on the subject if you are interested?
Click to expand...
I'm unfamiliar with DSP, but will watch your adventure with interest. I'm somewhat cynical of digital manipulations, but I accept the proof will be in the listening. Good luck with your project. I'll be cheering from the sidelines.
 
Hi James, I agreed with the digital manipulation thing until quite recently myself. However since it became possible to do it either in software or a hardware module in the digital domain without resort to DAC - ADC - DSP - DAC which was the case just a couple of years back. It's much more transparent than ever an analogue circuit was and much much more flexible. As you know analogue filter design is always a compromise between getting the transfer you want ideally and complexity/cost. Those barriers are removed somewhat with DSP. I still design using sound acoustic principles of course and use drivers that would work together with a passive filter, and use DSP to finese.

Try it, you like a tight speaker, sorting out those phase issues that as you say are unavoidable with filters and building enclosures REALLY tightens things up. :)
 
OK, a little update. I started building them!!

Here are a few pictures of the progress so far, not much but at least a start.

I needed to build a curved form to vacuum clamp the side profiles to, This needs to be quite sturdy to resist crushing under the full weight of the atmosphere!!

I started with a soil pipe

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Made some supports of the correct diameter with the circle cutter and trimmer them on the small bandsaw

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secured the pipe to the support

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clamped some shuttering to the side, drilled some hole in the base and filled it with expanding foam, hopefully this will help spread the load of the top skin and stop it deforming under the weight of vacuum

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filled the pipe as well as i need it to stay round if possible and hopefully the foam will help resist deflection a bit

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after the foam has set

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screwed a strip of bending ply round the finished form to give the final form shape that I'll use for the vacuum forming.

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here is an old test baffle stood next to the curved form to give and impression

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Here are some of the tenon slots being cut into one of the internal braces. The nail is part of a sled I made up with a simple registration hole to ensure that the slots in all of the braces line up. I used the same holes as pivot points for my circle jig for the router to round the edges to the same radius as the vacuum form. Hopefully it'll all line up properly!!

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Got some lovely sheets of walnut veneer to finish with as well.

All for now.
 
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