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Multi-Way Conventional loudspeakers with crossovers

Acoustic Horn Design - The Practical Way
Acoustic Horn Design - The Practical Way
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Old 21st May 2020, 12:57 PM   #11
Ro808 is offline Ro808  Netherlands
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The cnc-mold could be used to make a small batch of horns.
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Old 21st May 2020, 03:20 PM   #12
Bill Brown is offline Bill Brown  United States
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I have been refreshing this thread frequently since started as I am quite excited about seeing possible construction techniques!

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Old 22nd May 2020, 03:20 AM   #13
bmc0 is offline bmc0  United States
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I recently completed a speaker that I suppose would fit here even though I didn't actually use Ath4 (I wrote my own tooling). The waveguide profile is OS with an angle- and curvature-matched Euler spiral (a.k.a. clothoid) termination starting at 40% of the total depth. Throat angle is 10, which matches the compression driver I'm using (Celestion CDX1-1747). Here's the completed speaker:

Acoustic Horn Design - The Practical Way-finished_speaker-jpg

The cabinet is 73cm x 41cm x 30cm (approx. 29in x 16in x 12in).

It took me a while to figure out how to make an accurate waveguide of this size since I don't have a big lathe or CNC router. I have a 3d printer, but I wasn't too keen on the idea of printing the whole waveguide due to the time required and the fact that the finished product would be a bit flimsy unless a lot of material was used. The basic process I arrived at is gypsum cement (basically plaster of paris, but much harder and stronger) over an MDF substrate coated with a suitable bonding agent. After some experimentation I found that type II water resistant yellow (carpenter's) PVA glue works remarkably well as the bonding agent (more on this later).

I shaped the gypsum cement using 3d-printed running molds that pivot on a 1/2in stainless steel rod held in place by this jig (which is also 3d-printed):

Acoustic Horn Design - The Practical Way-jig-jpg

Here are the templates:

Acoustic Horn Design - The Practical Way-templates-jpg

Two of the templates have a wide angled edge which helps to spread the plaster, kind of like a plasterer's trowel:

Acoustic Horn Design - The Practical Way-template_detail-jpg

I applied the plaster in two coats, similar to two coat veneer plaster. The basecoat is about 3mm thick, cut 50/50 by weight with fine silica sand and the finish coat is 1mm neat. Here's the bare plaster waveguide. You can see the stainless rod in the middle:

Acoustic Horn Design - The Practical Way-plaster-jpg

This particular one was actually a failed attempt as the plaster didn't bond properly to the substrate. Due to the fact that gypsum plaster expands slightly as it sets, you need a bonding agent that provides some bond strength even when damp to prevent the plaster layer from lifting off the substrate near the waveguide mouth. For the attempt above, I sealed the MDF with a waterproof primer and then coated that with Elmer's Glue-All (a white PVA glue without water resistance). The problem is that the Glue-all turns back into a PVA emulsion on contact with the wet plaster and thus has no bond strength until dry. What I found works well is two coats type II water resistant PVA (I used Gorilla wood glue (NOT the polyurethane stuff), but Titebond II or any other similar product should work just as well), sanded lightly between coats, and one coat of Elmer's Glue-All. The Glue-All is probably not required, but I figured it would improve the bond once dry. Type II PVA adhesives get a bit soft when exposed to water, but don't completely re-emulsify. This property appears to be what makes it work so well for this application.

The gypsum cement I used is a G-P product called Densite (K-5, I think), which you can buy in small quantities at dickblick.com. USG Hydrocal White should also work well. Do not use USG Ultracal 30 or any other gypsum cement with portland cement (check the SDS). Based on my experience with Ultracal 30, it will shrink slightly over time and crack.

So how does the final product measure? Very well, in fact. The data seems to agree pretty well with the ABEC3 simulations I did. The measurements were done inside with a short gate, so I'd ignore below 1kHz for the most part.

Acoustic Horn Design - The Practical Way-directivity-png

Acoustic Horn Design - The Practical Way-directivity_h_pos-png

Acoustic Horn Design - The Practical Way-directivity_contour_h-png

You can see that the sound power is very flat from the crossover frequency (1050Hz) to about 16kHz. Also, the 0 response is about 1dB from the listening axis (18), which I'd say is pretty good for an axisymmetric device. I think that this is pretty good evidence that mabat is right about how important the termination is. The axial response could be improved further with a more gradual termination, but the DI would be more tilted. I don't know which would be better in terms of perceived sound quality.
Attached Images
File Type: jpg finished_speaker.jpg (211.2 KB, 272 views)
File Type: jpg jig.jpg (404.5 KB, 219 views)
File Type: jpg templates.jpg (447.4 KB, 215 views)
File Type: jpg plaster.jpg (187.8 KB, 274 views)
File Type: jpg template_detail.jpg (321.8 KB, 214 views)
File Type: png directivity.png (125.5 KB, 274 views)
File Type: png directivity_h_pos.png (346.4 KB, 215 views)
File Type: png directivity_contour_h.png (369.6 KB, 213 views)
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Old 22nd May 2020, 06:11 AM   #14
mabat is offline mabat  Czech Republic
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Wow. Thank you for sharing. This is just brilliant.
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Old 22nd May 2020, 08:04 AM   #15
mabat is offline mabat  Czech Republic
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You could offer the printed parts with the rod as a kit
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Old 22nd May 2020, 11:23 AM   #16
wesayso is offline wesayso  Netherlands
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Excellent work! great use of practical materials!
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Old 22nd May 2020, 03:03 PM   #17
Ro808 is offline Ro808  Netherlands
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Big thumbs up for a stunning result!
It looks like a factory built loudspeaker and those plots are excellent.


Thank you also for your detailed description of the construction, a source of inspiration for other forum members.

The woofer is an Eminence, judged by the cone and surround?

What's is your (initial) subjective impression of the speaker(s) in the room?

Last edited by Ro808; 22nd May 2020 at 03:25 PM.
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Old 22nd May 2020, 03:05 PM   #18
Bill Brown is offline Bill Brown  United States
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Yes, excellent/impressive work. Thank you for sharing in such detail.

Bill
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Old 22nd May 2020, 04:11 PM   #19
diyiggy is offline diyiggy
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WOW !

I am jealous about your skills ! It's just beautifull. Have class, because it's not glossy. We share similar looking tastes Bmc0...

The mid seems a 10". Why did you choose (and many diyers) a compression driver for the horned uppermid/tweeter instead a classic dome driver which can benefits of the horn reenforcment and perhaps more linear that a compressiopn driver ? Am I wrong to prefer classic driver if I would like to horn as well ? (home listening level, no pro studio related).


@ Mabat : thank you for the impressive work and sharing. Gave me the envy to draw my first horn (and also the huge pedagogic efforts of Dr E. Geddes, Ro808, Docali and others fellows I forget the name right now)

Last edited by diyiggy; 22nd May 2020 at 04:19 PM.
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Old 22nd May 2020, 04:55 PM   #20
mabat is offline mabat  Czech Republic
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In fact I would like to try a classic dome tweeter in such a waveguide some day. As I showed several times, in a similiar waveguide with a simple conical throat the performace should be pretty good actually, even with a biggish tweeter. Probably nowhere near the dynamics of a compression driver but I can't say how big that would be a problem at home listening levels. Maybe not so big.

I'm thinking about SEAS Excel T35C002. I would use BlieSMa T34B-4 right away but that's already somewhat costly.

So in the end, compression drivers are actually fairly cheap for what they can do.
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