The HOMster! (or How I Learned How to Fix a Horn)

[gedlee]

Originally Posted by Patrick Bateman

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My hypothesis is that bad horns generate more HOMs, and therefore the foam is more effective on bad horns.

I would tend to agree with that. When the house is very dirty just picking up a few things makes it look a lot better. But a clean house is much harder to improve upon.

[kenpeter]

Since I can't find a link to the PDF anymore, and its far too big to up the
whole thing again here. Here are paragraphs relevant to your horn shape.

You might still want to track down the original article in its entirety.

[badman]

I'm going to try some reticulated foam (guy just wanted rid of it at the pet shop, apparently the size was wrong. Score, $1.25 for each piece (about a 5.25" dia cylendar, 1.25" tall) and edge termination on some JBL 2370s.

Earl, would it be correct in assuming that much of the coloration of a diffraction slot itself could be attenuated by the foam? I'd think that reflections back towards the diaphragm would be a significant part of the issue. I'm planning to cut it as well as I can to match the throat profile and extend past the diffraction slit, am I likewise correct in assuming that it can be rather "rough" cut? Should it be fastened to the sidewalls?

I'm also assuming that we'd see a far less significant effect from roundovers on the sidewalls, where the horn flare pretty smoothly transitions into a flat front baffle.

I'm planning to use Aitwood Anderson International - StoreFront for the roundover transitions.

[gedlee]

I've seen those corner pieces. They look very attractive.

I am inclined to believe that the foam would help any horn and the worse that it is the more that it would help. You can probably rough cut it and just hold it in place with compression.

[Sheldon]

Originally Posted by badman

I'm also assuming that we'd see a far less significant effect from roundovers on the sidewalls, where the horn flare pretty smoothly transitions into a flat front baffle.

I's been my subjective experience with my Unity's, that the throat area is most important. I wonder if certain frequencies are more noticeable in this regard.

I originally filled the horn with foam - from the phase plug of the driver to the mouth. I found that I could take out the front third, or so (as measured by depth), without reducing the apparent effect. That front third makes up the bulk of the foam (like the bottom of a pyramid). What I care about are specific reflections that create echos (HOM?). Sounds like, well, like a horn. I don't like the speakers to sound like someone is singing into a metal trash can (even though the effect was slight with the untreated waveguide). Don't like back loaded horns for that same reason.

I never spent much time thinking about a generalized sense of harshness, so I don't know if that's something distinct from the perceived echos, or whether it's just different frequencies that affect our perception differently. I would guess that there is some higher frequency cut-off where an echo no longer sounds like an echo. I would think that it's related to frequency as well as the amount of delay, and probably the number of reflection cycles and rate of decay - and some relationship between all those (and probably more) factors. Pretty complicated, I'd guess.

Sheldon

[kenpeter]

Tried something stOOpid that didn't work, but thought I'd share all the same.

NXT exciter near the throat of my StyroSmith. Does not appear to have any
use or abuse as a unity driver in this application. Maybe its just septums of a
Smith horn damp out the flex modes? Certainly got along plenty well enough
with large flat panels of the same exact pink extruded polystyrene foam.

Proabably comes as no surprise to those who know, that this shouldn't work.
But I was dumb enough to try it anyway. Figured I'd save other dummies the
five bucks price of admission to repeat the experiment.

[Robh3606]

I've seen those corner pieces. They look very attractive.

I just purchased 2 of the 2" radius 8" lengths. Shipping cost to NY was the price of a single piece. It also shipped today. Not bad at all!

Rob

[badman]

Originally Posted by Robh3606

I just purchased 2 of the 2" radius 8" lengths. Shipping cost to NY was the price of a single piece. It also shipped today. Not bad at all!

Rob

I'm lucky enough to live close by, so I don't pay any shipping

[makingmoney]

Hi,

Glad to read some else thought of roundovers, I thought I was alone!

I've been using 4" PVC white pipe cut in half length ways for my roundovers.

Cut to fit the edge lengths of my mid range box (ATC soft dome) and it smooths out a roughness in the sound from diffraction.

Also use it on my tweeter clear plastic baffle for similar good effect.

I use clear 2" wide heavy duty packing tape to tape on the pipe.

Looks like crap but sounds good, not great.

In your horn you might also try using wool from an old sweater or jacket to line some part of your horn. Different foams, wool, felt, etc work at different frequencies. Wool felt is supposedly good for higher frequencies.

Have fun!

[badman]

Originally Posted by Patrick Bateman

The roundover on The HOMster really made an improvement. But what if we filled the horn with foam? And while we're at it, why don't we add a vertical roundover?

So I chopped up about fifty cents worth of PVC pipe, and added about a dollar worth of 30 ppi reticulated foam inside the horn.

Geddes uses reticulated foam to damp the sound which doesn't travel down the axis of the horn. The way that the foam works is that it absorbs the axial wave once, but the reflections inside the horn are absorbed many times. (Since the reflected energy is reflected back down the throat, off the walls, off the mouth, etc...)

Also, just to be a complete lunatic, I put a *full* roundover on the horn. In other words, the roundover extends *into* the mouth. Because the top of the horn is baffled, my hypothesis was that this wouldn't affect the low end dramatically. Besides, if half a pipe is better, how about a full pipe?

Here's what the horn looks like for these measurements:

Note the roundover on the sides *and* the mouth.

The horn has been filled with 30ppi reticulated foam. The 2" PVC pipe extends all the way into the mouth now. In the previous measurements, it was a half pipe, duct taped to the bottom of the horn.

In the side view you can see this is a REALLY small horn. That's a Radian compression driver.

Of course, this is outside the car. The polar measurements were performed inside the car.

And speaking of polar measurements, here they are. The untreated response is at the top. After the second stage of treatments, I measure the horn at the same voltage level. In the graph the second set of measurements were lowered by 10db.

The HOMster with a roundover

The HOMster with a horizontal roundover, a vertical roundover, and stuffed with reticulated foam

What do you think? Here are my thoughts:

• The 0 degree (purple) and the 15 degree (red) curve are virtually indistinguishable now. They track each other within 2db from 1200hz to 10khz. Remarkable for such a funky horn! And a dramatic improvement over the untreated horn.
• The untreated horn has a nasty dip and a peak at 1800hz and 2800hz, respectively. When you have a peak and a dip so close together, it's a challenge to treat with an EQ. But that didn't stop people from trying. These horns were used in dozens of "sound off" cars, where they were frequently paired with expensive Rane equalizers, modified for use in the car. The equalizers alone cost hundreds of dollars, and modding them for the car increases the cost. Now look at the treated horn. Where's the peak and the dip?
• The foam and the roundover has tamed the peak and the dip. Even better, what remains of the dip is fairly consistent on axis and off, and it's shallower and wider. This means that the dip can be treated with EQ (finally!)
• There's a dramatic reduction in the output level. It's down 3db at 1khz, but almost EIGHT db at eight khz. I think this is enlightening, because it illustrates that a significant percentage of the horn's output is being damped by the foam. I have stuffed an OS waveguide with foam, and measured it, and the reduction in SPL wasn't as dramatic as this. I believe this proves that the HOMster has more HOMs than an OS waveguide.
• The response curves of the treated horn fit in a tighter window. For example, the curves of the treated horn fit inside a window that's approximately five db wide. The response of the untreated horn fits inside a window that's eight db wide.
• Without a doubt, the most notable difference is that the treated response is consistent. The response of the untreated horn is consisten for about an octave, but then the off-axis response is subject to wild swings above 1500hz. Psychoacoustic research tells us that this is exactly where we DON'T want wild swings, since we're so sensitive to frequency response discrepancies above 1khz.

In summary, the objective measurements demonstrate that a small investement in PVC pipe and foam can reduce diffraction a great deal. And reducing diffraction improves the consistency of the on and off axis response. I am not able to measure HOMs, but I know what they sound like. In the next post, I'll describe the subjective improvement of these tweaks.

Actually, I have been following a similar path to yours here, playing with what improvements may be available. It looks to me like you have actually made your horn worse, by obstructing the mouth with the second piece of PVC. This would dramatically increase HOMs, not reduce. The arrangement you used on the side of the horn is vastly preferable in smoothing the transition from loaded to unloaded.