Something stirs in the Undergrowth

Dedicated to those large boxes at one end of the room
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IslandPink
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#1711 Re: Something stirs in the Undergrowth

Post by IslandPink »

Two more significant insights while deciding on the final details of this horn expansion .
Reading Romy's thread on the roof-mounted 40Hz bass horns, I realised :

(i) Horn loading lowers the resonant frequency of the driver. This is unlike a sealed box which raises it, as the Q is raised from eg. 0.4 to 0.7 ; also a ported box raises the main resonance and adds another lower resonant peak from the port. I checked this situation with the Fostex on this folded horn design, and the resonance without a back-volume behind the driver goes from 42Hz (free-air) down to about 25Hz. Then you add a back-volume and raise it again until it starts to counteract the failing loading ( =reactance ) as the horn length and/or mouth size cease to load properly ; but it works best when the driver Fs is about the same as the horn expansion to begin with. In my case I am a little high for the horn as the horn expansion is ~55Hz and the mouth size is only sufficient for about 90Hz. Hence the back volume has to be a bit smaller to do anything useful, say around 60Hz or so . Hopefully this will all sound OK but there aren't many drivers out there with 60Hz Fs and a Q of around 0.2 to 0.25 to use .

Reading Martin King's comments from years ago on BLH design I also found :
(ii) The back-horn on these typical ( eg. Fostex ) designs only works as a horn down to a certain point, then turns into a transmission line,as the mouth is too small for the expansion rate. I had wondered about this , but now it makes sense and also explains why the FE208Ez horn ( I simulated )drops about 6dB below 100Hz as the loading turns into TL which is less efficient. So, in effect you only get about two octaves of proper horn loading ( 100Hz to 400Hz ) between the TL part and the fuzzed-out cancelled bit above 400Hz where the throat volume and 180 deg bends kill the response, to allow the direct forward cone output to take over. Still, the horn-loaded bit still adds something nice to the mix !
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#1712 Re: Something stirs in the Undergrowth

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Paul - so he had the big sub there as well, did he ?
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#1713 Re: Something stirs in the Undergrowth

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IslandPink wrote:Hopefully this will all sound OK but there aren't many drivers out there with 60Hz Fs and a Q of around 0.2 to 0.25 to use .
Wrong :roll: - Eminence to the rescue - can fall back on this :
http://www.eminence.com/pdf/Delta_Pro_8A.pdf
...if I find the Fostex unconvincing at the lower-end of the horn range .
Cone mass is ~17.5g and the Rms is ~1.2, which is lower than the B&C and Fane 8" drivers I've used previously.
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#1714 Re: Something stirs in the Undergrowth

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Slow going with hols and visitors, but the kit of parts is taking shape :
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#1715 Re: Something stirs in the Undergrowth

Post by Paul Barker »

IslandPink wrote:Paul - so he had the big sub there as well, did he ?
Yes. It was best bass of vsac. Ruined the sound in adgacent rooms.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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#1716 Re: Something stirs in the Undergrowth

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He he !

Today I sawed another piece of wood and then trimmed with the router. Then I went round the bend ( the second bend ) :D .
ps. Last night I ordered another essential part of the project - a folding luggage trolley with 60kg capacity and wheels big enough to go over the back-door step, either way.
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#1717 Re: Something stirs in the Undergrowth

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Each one of these projects is more difficult than the last. I have no idea how to make some of the pieces of wood required around the two smaller bends. I expect I'll find a way when it comes to the crunch !
Various pieces of wood in different stages of manufacture at the moment.
The trolley has arrived and is good.
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#1718 Re: Something stirs in the Undergrowth

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IslandPink wrote:Paul - so he had the big sub there as well, did he ?
yes and the reason for the best bass ive heard outside Westminster Cathedral! Sorry if not mensioning that gave you the wrong direction at any time. I forget its so long since vsac material about it will be less accessible and assumed knowledge isn't reliable.

I haven't heard the Tightans without the best sub in the world. The brilliance of Dr Edgar was demonstrated in the whole package on that day in my life.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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#1719 Re: Something stirs in the Undergrowth

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No that's OK it's good to get the full story. Yes, the big sub would add a huge amount to the sound, especially in bass extension but also in adding to the quality in the 70-200Hz region. His standard upper bass horns (pair) go to about 80Hz , and have some shortcomings as they are rather short and don't load properly at the low end. The straight ones also have quite a bit of output from the dustcap of the big driver ; I would hazard a guess that the folded ones ( probably one fold ) you saw may be a bit better ( and Bruce confirmed this to one of his customers somewhere ) despite the fold.

Since I did the wooden 120Hz tractrix horns I've gone back and read more of the earlier horn literature ( Keele, Dinsdale ) and realised that that horn was not going to properly load the drivers I used , much below 150Hz or even 200Hz . In that respect Hornresp is a bit confusing, in that it can show a good response sometimes when all the conventional 'horn-loading' criteria are not met. The one in particular is the length , where it's recommended that the horn length should ideally by 1/2 the wavlength of the target bass extension frequency, or 1/4 of the wavelength for partial ( not ideal ) loading. The current horn is twice as long as the 120Hz tractrixes and is 1/2-wavelength at 100Hz.
The advantage of this horn is that it can get usefully below 100Hz , which allows much easier use of the tapped horn that a built a while back. I'm hoping that fairly soon after I get this horn working, i can get the tapped horn back from Nick's place and see how it works as a combination. The tapped horn is going to be right in the middle of its working range, where the folded uuper bass horn will be rolling-off, around 80Hz. How close this can get to Bruce's sound remains to be seen :!:

ps. Here's a basic folded upper-bass horn built by Peter Empson ( of Vitavox fame ) :
http://website.lineone.net/~empson/Owners1.html
This is probably similar to the Vitavox horns like the Thunderbolt and Oracle, and early Living Voice stuff. It's relatively crude in that it doesn't have any width expansion ( ie. parallel sides ) which gives rise to more resonant dips in the response. I noticed Kevin Scott mentioning that the Vox Olympian had 'non-parallel sides' in one article. The horn also doesn't use a phase plug near the driver to improve upper-end response. It's also a bit flimsy using 3/4" ply & mdf & not much bracing.
So I'm aiming to go better in 3 respects - width expansions, phase plug and more solid construction.
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#1720 Re: Something stirs in the Undergrowth

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Blumming router brushes are finished again :roll:
Plenty to do, but have to change plans of what parts I wanted to finish this weekend.
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#1721 Re: Something stirs in the Undergrowth

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Bloody hell, it fits :!:
Just finally got round to laying out the last ( smallest ) bit of the horn line , with the fostex driver attached to its rather bulky 'transition' piece with the phase plug. I knew it was tight and was rather worried - but it can be done with mm to spare . The edge of the driver just clears the line ( with 18mm wall assumed ) as it comes round the first clockwise fold and on towards the lower RH corner. The remaining air space around the driver ( if sealed ) is small, but with the 38cm internal width, it should comfortably exceed the required ~6 litre back-volume. The red wire traces ( more or less ) the centreline of the horn path. If you're confused, on this pic the driver fits into the lower right corner area, facing upwards and left.

Back on track today with the router fixed. Fitted the upper panel onto the main board ( see pic ) at the top, using two biscuits and three honking great chipbaord screws about 4" long. Seems solid. More work on Sunday after I come back from a visit to my mum's.
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#1722 Re: Something stirs in the Undergrowth

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Every time I'm working on the parts for this project, all I can hear in my head is the bass of Jaco Pastorious, from 'A chair in the sky' and 'Goodbye pork pie hat' from 'Mingus ' ( Joni Mitchell ) Let's hope it lives up to the sound in my head !
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#1723 Re: Something stirs in the Undergrowth

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"It's nice to stick something up them"
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#1724 Rod for own back

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Just been brushing up on my brass-rubbing skills, making the template for the side-pieces( left ) to go around the second bend. The original for this was the pencil markings ( 2B ? or not 2B ... ) on the baseplate, transferred to the paper, then to two pieces of 18mm ply which took an hour or two to cut & finish late this afternoon. Mr. Snouty came in useful.
Plenty more prgress this last two days. Will put up a photo of the 'kit' of parts either tomorrow or Monday.
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#1725 Re: Something stirs in the Undergrowth

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Plenty of progress this weekend ; time + weather ideal.
Here some pics of the throat ( small ) end of the horn.
i : view from where driver sits
ii : the internal volume and initial expansion , where the waves feed into
iii : 'assembled'

What the pics don't show is that I varnished all the internal surfaces in this area with 4 layers of yacht varnish. This was following Kevin Scott's observation that birch ply sounds dry & lacking tone, in both the upper mid horn and the bigger upper bass/ mid horn , if untreated. Beech is much better, but multiple layers of varnish ( OK in his case 3 to 4mm of build-up ! ) improve the sound of birch significantly. I figured that it was worth doing something like this in the throat area where the pressures are highest. Certainly if you rap your knuckles on the wood in untreated/treated areas it does sound different.

Last pic 'bits' shows some other parts for the 2nd fold and last ( big ) fold. Each of these bits took at least a couple of hours work to make. I haven't counted up how many other pieces i still need to make !
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