Large midrange for OB??? Scott G ?

[ScottG]

Originally posted by MBK
Making axially symmetric horns the easy way would be great, but how do you get the cellulose/foam etc, into a proper symmetrical 3D shape, with little deviation and roughness? If there was a way to make a mould in an easy way it would be nice to pour the horn in gypsum / alabaster (actually one of the LeCleac'h enthusiasts uses synthetic plaster, apparently the best he found during extensive testing). The thing one would have to get right is the passage from a profile cutout to a 3D form, say, rotating the profile in clay or such.

Well basically you are making a "user" negative.

For a horn it would require 2 parts for the negative: 1. the exterior of the horn; 2. the interior of the horn.

Functionally the user pours the material, in an appropriate volume amount, into the exterior form. The interior form then gets placed "into" the exterior horn form cavity where the poured material rests. It compresses the poured material to fill in the surrounding air space and the additional amount purges from a "weep hole" from one of the negative portions. (Note: I've done this before.. not for a horn, but for a baffle. With cement the form requires "tapping" the sides of the form to evacuate any trapped air.)

That process isn't likely to be a lot different than for your "master" forms, but inverted.

In effect you need to create a positive (your horn..not really though, see below) from a very rigid/stable material that also will release from the material you are using for the user's form (..perhaps a cellulose mixture as I mentioned before). Lets say a polished and anodized aluminum. You will also need to create the negative surround to the positive that will encapsulate cellulose mixture.

Of course you are actually making 2 separate forms here one for each of the user's forms. Moreover considering support extensions and weep holes and factoring in uniform compression, you are likely looking at 3 form pieces for each of the 2 separate "user" forms. I.E 6 master form pieces of polished and anodized aluminum:

User form "exterior" composed of 3 master form pieces:

1. Positive exterior of horn with support
2. 1/2 of Negative surround of #1 with screw/vice compression clamping system
3. 1/2 of Negative surround of #1 with screw/vice compression clamping system

User form "interior" composed of 3 master form pieces:

(basically as above).

An alternative to aluminum for the master forms could be with UHMWPE, which is also easy to machine and doesn't need polishing or anodizing.

Cellulose in this instance is nothing more than paper and starch, which after compression and dry time will be extremely stable (..as long as its not around moisture).

You will of course also add in things like some support bracing allowing for "in-fill" of high expansion foam,(and its weep holes), for each section of the user form.

Finally, you will also need to factor in the small amount of silicone needed to coat the interior of both user forms.

And yes, it will require a LOT of thought to get it right. But probably not as much effort as it will to get the right horn profile (..unless you have a software package that will do it.. IF you know how to use Akabak, or will learn, (now free for download) - you could almost certainly do it with that.)

The beauty of it is though that once the master forms are made, (and you only need one set), labor *could* be cheap (..in that it requires no special knowledge to make the user forms), and the master forms would last as long as you cared to continue making the product.

The product would basically look like a box (from its exterior when the two user forms are together), and weigh maybe 5? times the weight of the cardboard box its shipped in.

It would be serious effort though, so don't entertain it unless you have a viable business model.

Oh.. forgot to mention - you have 2 potential markets here if you want it.. 1. for DIY'ers like us for the user forms and a restrictive agreement not to use commercially or available for resale/trade, 2. for making your own forms to use to make a horn for the commercial "pro sound" market (..likely from a pour-in polymer though).

[MBK]

Well, umm, I sort of imagined it would be complicated... this only makes sense for series production. Anything else it's probably best to go to a CNC shop and have the horn simply carved from wood.

Regarding mouldings - my question was actually simpler: how to produce a horn positive (the actual horn used to make the form) *without* access to a lathe or a CNC machine. Because if you do have access to one of those, it seems mightily simpler to just have a custom job done on a pair of "one-of-a-kinds". If there was a simple way to make the template, the outside box for the mould could then be poured from this original positive. I imagined one could laminate it from stacked plywood disks, and then filling in the "steps" with concrete or such, using a precise horn profile cut from sheet aluminium or wood, turned around the horn axis. But all of this still looks like a lot of trial and error...

Commercial production, I wouldn't dream of that. People will expect to buy this at say $50-100 (max), 80% of which will go to dealers and marketing. That is, *if* wou manage to find someone to sell that thing, the harder to find the more idiosyncratic it is. So anything less than a 1000 pcs. run wouldn't make sense.

[ScottG]

I don't think you would try this without a CNC machine. You could CAD the thing with very thin (almost veneer like) sheets of plywood and spend an absurd amount of time tracing and cutting each sheet. Then, after gluing it together, spending even more time sanding down the edges and praying that you don't screw up the profile. The "laminated" approach isn't one I'd recommend though!

For a conical design its a LOT simpler, a lathe based system.. and it doesn't even have to be a real lathe. I've seen horns formed with plaster, a bucket, and the horn profile cut from the edges of a single board of plywood (that the user turned in the bucket to "route out" the horn.. true, but ).

[Ed LaFontaine]

The worksheet has been forwarded to 3 more (happy, I hope) customers! It was developed in the interest of getting people on the same page, speaking of the same parameters, leading to a "finished" profile.

I recognize the difficulties involved in making one of these things...just consider the trouble getting to a consensus for what it looks like!

That being said, my toolbox is open. I've been a woodturner for years and intend to continue until I can't support the tools any longer. How big is this thing going to be?

[Ed LaFontaine]

I look at it this way:

For my own tastes I would build it from hardwood...I'm just nutty about natural materials....ok?

For multiples I see a 2 part negative form.
1) The horn that includes a stub tenon @ the throat and
2) A flange that recieves the stub tenon from the throat and
includes the mounting appointments for the driver.
The user/builder would work with their fiber/binder composite of choice. After lay-up the flange would be removed, allowing the form to be removed from inside the horn.

[Paul W]

Several people have written requesting test results for the 10NDA520, so...

For a fairly well known "reference" I included the Seas W26...besides I have another use for it coming up. Both drivers are centered on very similar 19" wide baffles, closed across the top, but open to the sides and back...so the baffles influence the low end and likely introduce a little diffraction on the top end. No smoothing, no baffle extensions, basically "un-fooled-around-with" driver measurements at 500mm.



First, the W26:



Violent breakup at 4k which is why I like to stay below 800Hz with these guys. However, good correlation with the Seas factory curves which have a reputation for brutal honesty.

Now the 10NDA520:



Again, good correlation with the 18Sound factory curves. Bottom line is that the 18Sound factory curves seem trustworthy.

[ScottG]

Very Nice Paul! Thank you!

[Lynn Olson]

Nice data! Thanks for rolling out the cart and getting the measurements!

Usually I'd grumble about using a 85 by 115 cm IEC baffle with the driver off-center, but there appear to be almost no points of correlation between the two drivers, despite similar baffles. The Seas driver is certainly dominated by high-Q resonances - man, those won't be easy to filter out, a 24 dB/octave filter will just be getting started when it hits the big one at 4 kHz. (Back when I was wrestling with the KEF B139, I found it took 40 ~ 50 dB of filtering to audibly get rid of the severe resonance at 1.5 kHz.)

[MBK]

Jeff,

interesting design, surprising that a 1.4" would go that low. No excursion problems / THD / IMD? I sort of assumed around 800 Hz as the lower practical limit for a 1.4".

Ed,

thanks for the worksheet, will compare the data for similar design specs to see if there are any significant differences with my composite attempt. Not sure if I can patch the LeCleac'h formulas into it, they way he build his spreadsheet is quite complex (plenty of correction factors) and besides expansion ratio and wavefront shape you only really can choose the cutoff. So I had to iterate before I found values where diameter, angle and depth would line up nicely with the OS WG throat.

Paul,

impressive data, again! The 10NDA520 seems to set a very tight standard. BTW the Seas W26, is that the metal dome? The breakup certainly looks like it. If yes the result is even more impressive for the 18Sound, much less peaking *and* better CSD, and that from paper vs metal.

[jeff mai]

Originally posted by MBK
interesting design, surprising that a 1.4" would go that low. No excursion problems / THD / IMD? I sort of assumed around 800 Hz as the lower practical limit for a 1.4".

Altec used this size driver at 500Hz in it's A5 systems using 2nd order filters. They also made 300Hz horns for some of their 1.4" drivers. I haven't taken measurements of these things, but I'll eventually get around to it.

The Yamaha drivers I use have a very unusual suspension - see the patent I linked above - resulting in fantastic compliance and a reduction in side to side motion that allows a narrower gap and a surprisingly small magnet. Another feature of the diaphragm is that the suspension is formed by a photo-resistive etching process out of berylium copper and doesn't suffer from work hardening problems as the pressed diaphragms do.

Meyersound used these drivers in a couple of their loudspeakers. When Meyersound started making their own compression drivers, Yamaha stopped making these. I'm not sure Yamaha ever used them in their own loudspeaker products.