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Colour Expert 30th December 2012 08:50 AM

Back loaded horn, with folds, for Alpair 10
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I’m thinking of building a cabinet for an Alpair10, based on the back-loaded horn in Martin King’s Mathcad worksheet. It would use the same path length, horn expansion rate, port size and so on. However, it would be folded so that it curls around the driver and the coupling chamber, about one and a half times, as shown in the accompanying diagram.

I plan to include reflectors at the corners, to prevent standing waves. If I’ve understood how these things work, without the reflectors, the short lengths of tube would act as Helmholtz resonators, and there’d be a whole lot of standing waves at frequencies right in the middle of the music.

Before I embark on this project, I have a couple of questions.

Can I expect the resulting speaker to have the same characteristics as an unfolded horn built to Martin King’s Mathcad dimensions, or would the folds be detrimental?

Would reflectors at the corners be beneficial?

And a third question. Does anyone have a better design?

xrk971 30th December 2012 10:30 AM

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What you have is very similar to the Cornu spiral horn but with one horn instead of four wrapped around. It's a neat idea and I am glad you have an MJK sim. Putting corner reflectors will increase efficiency but will let higher frequency stuff through. In case you may have missed it, there are the two threads dealing with the Cornu spiral that you may want to check out. Post 188 in this thread may be of interest to you.
Are you using stuffing to smooth out the peaks because your sim looks great and may be the answer for redesigning the Cornu from scratch in order to smooth out the bass peaks. It looks like the Alpair 10 may work very well for this application. Note that if made flat against wall, you don't need the 45 deg reflector just let it exit along wall plane for bass enhancement and then no need for BSC. In post 191 of same thread I was thinking of doing four mass loaded transmission lines with slow expansion through a spiral without use of any curves specifically for purpose of filtering out HF. See my sketch - looks similar to what you propose. The foam core thread with lots of info on the Cornu horn is here:

dekiller 30th December 2012 11:12 AM

Does the Alpair 10 suitable for BLH application?

Scottmoose 30th December 2012 11:16 AM

The response plot posted above is an in-room one for Martin's original enclosure design for the first-generation Alpair 10 drive unit. It assumes a specific box size and shape (just to clear up any confusion, it's already folded), and a specific placement in the room -if you're interested, a room with an 8ft ceiling height, a carpeted floor, the box rotated 45 degrees in from the corner, with the edges of the front baffle 2ft from the side & front walls, respectively.

Changing the box shape, the location of the driver & terminus, and its placement within the room will alter the response considerably. The original horn in question is lightly stuffed for about 3/4 of its length.

Scottmoose 30th December 2012 11:16 AM


Originally Posted by dekiller (Post 3304153)
Does the Alpair 10 suitable for BLH application?


xrk971 30th December 2012 11:19 AM

Based on the sim and the lowish Qts of the Alpair 10, it seems to be a nice driver for BLH. That is impressive to have bass extension down to 40 Hz. I am very interested to see how this turns out when built.

Scottmoose 30th December 2012 11:56 AM

Note that the sim. above is for the defunct model, not the current unit, and is an in-room plot for Martin's original enclosure in a specific position & room dimensions.

A low Q is not necessarily advantageous for a back horn. If the mass corner is higher in frequency than it is practical to run a back load (> ~300Hz) then you'll either need some help from the amplifier, Eq, or a short front horn to fill in the drop in SPLs between the practical acoustic low-pass on the bass horn & the nominal direct radiation efficiency of the drive unit.

Colour Expert 30th December 2012 03:16 PM

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Yes; I’d seen the Cornu design, and had considered making it. As I recall, though one of the descriptions suggested that the sound was quite coloured. (Perhaps I won't notice; I'm colour-blind). However, I still might make a pair; they sound like an interesting project. There’s one thing I don’t understand, though. From one of the descriptions, the horns were intended to even out the bass response by being different lengths. However, the diagrams I’ve seen have two symmetric spirals (call them A and B), which each split into two (A’ and A’’, and B’ and B’’). Now, I can see that the length(A’) <> length(A’’), and length(B’) <> length(B’’), but because of the symmetry of the arrangement, length(A’)= length(B’) and length(A’’) = length(B’’).

Why not remove the symmetry, by, for example, starting the A and B spirals on the same side of the central chamber instead of diametrically opposite each other? Then there’d be four spirals of different lengths.

Thanks for your comments. I hadn’t realised the Martin King analysis was done with an earlier version of the speakers. He claims somewhere that he measures his own Thiele and Small parameters, and I assumed he was using his own measurements rather than those supplied with the drivers. Anyway, I’ve now put the Alpair 10.2 parameters into the spreadsheet, and the results have changed a little – for the worse, unfortunately. There’s a dip in the frequency response between about 80 and 200 Hz.

However, as you point out, there are differences between the King cabinet and my design (which was intended to cope with a lack of space). My more recent analysis is as close to the actual situation as I can manage, but the spreadsheet does not allow me to input the location of the driver on the baffle, so I can’t find out how critical that information is.

As I recall from my reading of Martin’s description of his approach, the spreadsheet doesn’t model of the shape of the cavity attached to the back of the driver exactly; rather it models the taper of an idealised (straight) tube, the position of the driver and the terminus along the length of that tube, and the position of the driver on the baffle. If I’ve understood, the folds and corners in the speakers that are constructed on the basis of the model are largely (completely) ignored. My question were intended to evoke some explanation of the extent to which that interpretation is correct – and if there are negative effects from putting bends in the path whether putting reflectors at the bends would alleviate those negative effects. However, aside from Scottmoose’s quite correct observation that I haven’t really modelled my cabinets, none of the responses seem to have addressed that.

I’m struggling with the complexity of it all (I’m a chemist, not an engineer), so any relevant response would be appreciated.

Scottmoose 30th December 2012 04:02 PM

You can't accurately place the driver on the baffle in Martin's worksheets? You must be running a different set to the ones I have.

The MathCAD based worksheets are derived from Martin's work on QW pipes, and can model most geometries. There are limitations when you get to relatively extreme aspect ratios, and the sheets do sometimes underestimate the low-pass effect of multiple bends. Folds in a back horn can be problematic, or they can be actively used as part of the low-pass filter. Depends how you design it. I believe Martin has worked largely on the principle that the LF wavelengths are too long to be significantly affected by bends. Shorter, higher frequency wavelengths are more likely to encounter difficulties passing through a pipe with multiple bends, but since you don't want those anyway, it's not all that much of a big deal. Reflectors or even a completely smoothed horn path will not have much impact on the LF, but will rather promote efficiency at the top end of the horn's functional BW: right where you generally least want it. Nothing of course is cut & dried, but my own designs have multiple bends & internal sharp edges; I design the folding scheme as part of the acoustic low-pass.

Colour Expert 30th December 2012 10:40 PM

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Hey there. I'm running a version of the worksheets that I downloaded a few weeks ago. It's dated 2009. I've attached an image showing the fields that I can't edit; except for "w-mth" and "locate," the values they contain seem to be hard-coded.

As far as building a design with multiple folds is concerned, you seem to be saying that they are, if anything, beneficial; that low frequencies will diffract around the most tortuous of paths, and high frequencies will be filtered out.

My concern, and my reason for including the reflectors at the corners, was that the short straight paths in my design could act as short organ pipes and reinforce the frequencies they're tuned to - and because there are several short sections, each of a different length, the speaker would boost a range of musically significant frequencies. What you're telling me seems to suggest that may not be a valid concern. Any comments?

I suppose I should build a straight tapered tube (except that it would be 10cm taller than the room) a folded spiral with reflectors, and a folded spiral without reflectors, and compare them. But I think we can safely assume that's unlikely. Finding time to actually build something is turning out to be a bit of a challenge...

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