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Elbert 8th January 2009 09:40 PM

RT-2 Horn sub, the result
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Well, time to give something back to the forum now that I finally have something to contribute with.

A while ago, I gave in to the allure of horn construction and decided to have a go at building a horn loaded sub.

In some old swedish "Radio & Television" magasines my father has, there was a series of articles describing various DIY horn projects.

The articles were very interesting and detailed, and I decided to go for the largest design called "RT-2", published in 1978.

Considering the advances in driver technology, it was with a certain anticipation I looked forward to see what this design could deliver with a modern high quality woofer.

With regards to driver selection, I started off another thread:

Now, many weeks later the horn is completed and tested.

So, how was the result?

Well, in general; disappointing...

The problem was that the horn rolled off to early and to steep, the lowest 1 1/2 octave simply weren't there.

On the positive side, what the horn actually reproduced was tremendously loud and clear. As I increased the volume it just got louder and louder with no sign of distortion or rumble. Except from the sheer sound pressure level, the only sign that it was playing loud was the fact that various objects in my living room suddenly started to resonate on different tones.

In that respect, the horn performed as hoped for and a bit more, it was just such a shame that it didn't really play the deep notes.

If the horn had reproduced the deep notes in the same way, I dare say it would have been awesome.

My conclusion is therefore that the horn as such worked very well, but was simply to small with to high a cut-off, and the steep roll-off meant that any room effects were not enough to give the low frequency reproduction initially hoped for.

This has in no way put me off horns as a concept, on the contrary, but what this has tought me is that a horn going all the way down will probably be just to large to be contemplated in a "normal" domestic environment.

So what the heck am I going to do with the thing?

Well, I've just spent too much time and effort building the darn thing to just simply go and throw it in the nearest landfill.

The plan is to close the horn opening on the underside with a MDF panel and convert the whole thing to a "conventional" BR enclosure. This will be done by cutting two holes in said MDF panel and bolting in a pair of Peerless XXSL 12" alu-cone sub-drivers. The volume is right and the internal horn folding structures will double as excellent structural bracing. I will no longer have the bragging rights afforded by an exotic horn-design, but linear response down to the low 20 Hz region at respectable sound pressure levels is not a bad thing either.

I hope this can be informative to anyone else contemplating horn construction.

jbell 8th January 2009 10:13 PM

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The problem with many undersized 1/4 wave front loaded horns, is their lack of flat response. It's not uncommon to see a 13db difference between 40 and 100hz for example. Even worse, about 80-100hz, it's now a 1/2 wave horn and really efficient, so your crossover isn't doing what you want it to do..

Most home plate or subwoofer amps, just don't have the eq tools needed to flatten out the response, and make the horn sound like anything you really want to listen to.

Even dropping your crossover down to a ridiculous 40-50hz doesn't yield a satisfying result.

Pay particular attention to the graph of 80hz cross only... see how response is going UP from 100-120? I'm sure your horn is doing the same thing.

Here's a tip for you. (I've done this, and it works) Go buy a 14-18mh inductor, and put it in series with your horn, and set your crossover at 80 hz. Since you aren't running extreme amounts of power, you don't have to go buy a $200 inductor, one of Parts Express $20-$30 versions will do.

I was using a dual MCM 55-2421 front loaded horn with terrible response, and the inductor flattened it right out... In fact it was within 2db of flat from 40-110hz. That horn would do 126db in my living room, but without the inductor, I wouldn't even turn it on..

Elbert 8th January 2009 10:30 PM

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Hi jbell,

Thanks for your interesting reply!

As I'm in no way an expert on electro-acoustics, I struggle a bit to grasp how such an inductor could save my horn project, but considering the effort and money spent so far, I'm more than willing to give it a try!

May I ask what sort of horn design you actually tried this on and how you arrived at your solution???

Here's another image of the build viewed from above before the lid went on and without the access cover for the internal rear-chamber.

jbell 8th January 2009 10:33 PM

I was editing my post, check out my response graph attached in the above post -- it says it all.... It was a BFM T18. 40hz 1/4 wave horn, that I built 24" wide with Dual MCM 55-2421's.

The general idea (besides the interaction of the inductor with the driver/horn) is that a passive 6db/oct cross at 40-50hz, plus an electronic cross of 12db/oct at 80hz can actually tame the rising response of a 1/4 wave horn. It's amazing, but the combined 18db/oct cross can just barely keep 120hz the same as 80hz in my chart. The good news, is above 120hz it drops like a rock, so that makes for a great cross to your mains.

Elbert 8th January 2009 10:49 PM

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Hummm... That response curve certainly shows a dramatic change!

And like you say, I tried to drop the X-over point down to 50 Hz without any luck.

What you describe certainly correlates well with my experience, but even if I manage to flatten the response like you did, I'm not to certain that I'll get the desired result... One thing is loping of some excessive efficiency in the mid-bass, but I'm not too convinced that this will save the day as I suspect that below 40-45 Hz there just isn't any efficiency left in the horn..

I have a 10 band parametric EQ which I hooked on and despite pulling the 31 Hz slider well up (perhaps to arround +6 dB) those sweet low's didn't really appear. But looking at your graphs, perhaps that just wasn't enough??

do you have any reference to plans or images of the horn you made?

Would be interesting to see what it looks like compared to mine! :)

Attached is another image showing a bottom view of the horn just before the access hatch tho the rear chamber is screwed in place. The black trianglular foot is part of the final horn section partially made up by the the floor and the wall which the horn needs to be placed against. The internal volume of the foot is part of the driver rear chamber

jbell 8th January 2009 11:03 PM

If you are listening to music, you don't need below 40hz, period. Even for listening to HT... below 40hz is only effects, and the harmonics carry the day.

My horn wouldn't do ANYTHING below 28hz... however I never missed it. What really sounds 'low' to most people is really in the 40hz range.

Do yourself a favor, download sine wave test tones, and see if your horn will play what you want played. If it'll actually do a solid 40hz, I think you'll be happy. Put it in a corner, and you'll be happier... the Corner will artificially "lengthen" your horn some, and the .5 pi loading will help dramatically.

Elbert 8th January 2009 11:18 PM

Yes, I do in deed listen to music! :)

I share your view that there shouldn't be too much interesting going on below 40 Hz, which is why I assumed that I'd be happy with this horn despite the fact that it is obviously a compromise in many respects, both in terms of applied horn theory and theoretical cut-off.

I did hook up a sine generator I have and the horn would give output down to around 30 Hz, albeit at an entirely different level than at 70 Hz. 40 Hz wasn't really that great either, had to sweep up past 50, closer to 60 before it really started to give some.

But I guess this is just the driver playing "straight through" the horn without any actual horn loading taking place....

I did actually try the horn moved in to one corner, but the result was only more of what was already there, no perceptible improvement in low end extension. Nearly broke my back hauling the thing across the floor as well! :)

Do you still have your horn and can you compare your impression with joys you've had with other systems?

jbell 9th January 2009 02:31 AM

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I do have it. With the inductor, it's a great HT / music sub. Keep in mind it has 7ms of delay, so it sounds best in a corner behind you. Considering a 55-2421 costs $25 or less, and the horn can be built with just under 1 sheet of 5ply, it's the loudest, cheapest sub that can be built in my opinion. ($100 total) It's the inductor that's key however... other than that, it's a pa sub, and needs pa style eq to get to a good sound.
18x19x24 sub that can play 126 at listening position, is flat out incredible... Things like halogen light bulb filaments, metal hangers for recessed fixtures, and other things you'd never think of vibrate at that level.

In my opinion, BF has no idea what he has with a dual driver T18... Cubic inch per cubic inch, it's the best design he has to offer. I've burned single driver T18's, T39's, given away THT's, and sold off my 36" wide T36's.... I still have the dual T18. It's designed as a 40hz corner loaded horn, in 2pi space it only loads to 50hz. (which explains my charts)

I still overall prefer the sound of my custom HSU cabinet, and even prefer for jazz my NHT sub over the dual T18. But the fact that I still have it, speaks volumes about it's ability. Any cabinet that doesn't perform, gets burned...

tinitus 9th January 2009 03:46 AM

Re: RT-2 Horn sub, the result

Originally posted by Elbert

The articles were very interesting and detailed, and I decided to go for the largest design called "RT-2", published in 1978.

So, how was the result?

Well, in general; disappointing...

So what the heck am I going to do with the thing?

Well, I've just spent too much time and effort building the darn thing to just simply go and throw it in the nearest landfill.

First I am surprised that noone has warned against using an "old" horn with a "random" selected driver

Second you have chosen Eminence Delta Pro 12", right
Most 12" PRO woofers wont do much bass, and yours neither with Fs of 51hz

I was thinking whether you could "reverse" the horn, and turn it into a closed TL
But with your driver it probably wont do much good
Or maybe mount the woofer in the horn mouth making a tapped horn, or kind of

Still, to get down to 30hz or below you need woofers with low Fs

Unless you can beat the hell out of it with huge amounts of Eq, and hope that theres enough Xmax

Your horn would MAYBE do a bit better if you used a woofer with lower Fs, maybe not
Or, maybe your woofers would do better in another design
I find it a bit hard to see through any of it

Actually your horn is remarkable similar to those very popular BVR, ok maybee not quite so, but anyway they are designet with the use of modern calculators
Also Decware have a similar thing, but uses hifi sub drivers with low Fs

Just thoughts, I may be wrong in most of it

Elbert 9th January 2009 08:50 AM

Hmmm.. yes, at first, I was convinved that I'd need a driver witth low Fs, but i was then led to understand that in a horn, it is the actual horrn loading of the driver that determines the result as oposed to driver and cabinet resonance frequency for BR and closed systems.

At the time, it seemed that High BL and a relatively ligh cone-weight was a good priority.

But there's only one way to clarify this, and I'll do that today when I get home.

Se if I can stuff a SEAS 33-WB driver in to the horn, this has a FS of 27Hz, so any distinctive change here should reveal a driver issue.


As for jbell's suggestion, I'll hook up my EQ again (after the X-over) and pull down 6 dB at 120 Hz, that should emulate the inductor-trick he described earlier on.

I'll get back once the results are in! :)

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