Unity Horn - budget drivers, active x-over

[jmorken]

I've been dreaming of building a synergy horn (which I am well aware is different from a unity horn) ever since I heard the SH50. The Unity Horn is the closest I've come to finding a fully documented "multi-way horn" in the heritage of Tom Danley, so I thought I'd start there. If this is a success, I'll use what I learned to build a more proper Synergy horn. None of the original drivers are available, so I chose some budget drivers that seemed to fit the bill to minimize my losses. Alright, less talk - more sawdust.

The mids are cheapo GRS 5SBM-8 5" sealed drivers (the grills pried off easily). The compression driver is the trusty Selenium D220Ti. I'm using a MiniDSP 2x4 for crossover. I'll probably crossover to one of the 15" pro drivers I have laying around. I'll worry about that later.

I've got the first horn built up and my measurement microphone calibrated. I built the unity horn as close to the original kit as possible from the information I've been able to gather. I did the port modification as documented here and tuned their length according to my Hornresp sim (0.95cm diameter, 0.5cm long). Measurements of the built horn put the throat-to-port length at about 8cm and port entry area at 56sq.cm.

The results have been encouraging, showing that many parts of the midrange simulation are spot on (light green line). The low cutoff lines right up (300hz) and the notch from the first reflection apearts right where it should (1.9khz). However, the big difference is the roll-off in the mids beginning at 1khz. This where I'm looking for any advice. I've been going back through the sims trying to figure out what could cause such a steep roll-off. The one factor that comes to mind is if I miscalculated the diameter of the ports. Another possibility is a wrong-sized rear chamber.

The compression driver looks promising as well (dark green line). The top end seems to break up earlier than I'd hoped (8khz). The real problem, though, is an irregularity in the off axis response. I setup a quick crossover and measured on axis (purple line) and 20 degrees off axis (gold line). The power response looks real good until you get to 5khz where there is a peak off axis. Any feedback here would be valuable as well. The compression driver was joined to the horn using this adapter. I dremeled off the threads and gorilla glued the remaining plate to the horn. The end of the wood horn is a 1.25" square and the diameter of the adapter plate is 1". I dremeled the adapter to make a conical transition from 1" to 1"25 and I filled in the corners where they meet, but I I didn't go crazy. Perhaps I need to make this transition smoother? The body of the second horn is built, but none of the ports are drilled. I could attach just the compression driver and see what the response is without the midrange ports.

I am having SO MUCH fun!

[bwaslo]

that's pretty impressive.

I can't see why the hf should be so messed up, I'd thipnk that would be the easy part. have you checked with another d220? or measured yours without the horn?

[rvrazvan]

You ARE THE MAN, MAN! ) I am gathering information for the same project. Maybe we can help each other.

Now back to science. The CD entry hole does not look right to me. That part must be very very smooth with no rapid changes in flare. I think that you should try to move the cd further back th throat.

BDW. Can you explain to me what's the big diffrence between Synergy and Unity?

[rvrazvan]

Later edit:

The rolloff at 1000 hz is normal and i dont think there is anything you can do about it. Except for using another driver maybe. The mid is in a bandpass enclosure. anyway..don't expect anything more than 2 octaves.

[dirkwright]

Originally Posted by rvrazvan

Now back to science. The CD entry hole does not look right to me. That part must be very very smooth with no rapid changes in flare. I think that you should try to move the cd further back th throat.

From what little i know about compression drivers and horns, I'd say you're right. The adapter has to make a very smooth transition between the round driver and the square horn throat.

[kessito]

Nice project!

About the mid rolloff;

If you read the akabak manual, it states that the trapped air in the mid chamber-midport forms an extra acoustic filter factor caused by the air molecules moving in a different way in a small volume (Vf in akabak), superimposed on the normal acoustic lowpass filter.

For this reason the acoustic lowpass filter formed by the midchamber-midport is lower in frecuency in reality than in your Hornresp sim, since Hornresp doesn't take this effect into account.

You can rather easily compensate for this by making your midchambers smaller by filling them up.

Succes!

[Tom Danley]

Hi

I would follow the suggestions about a radius at the entry, one should avoid any straight cylindrical portions on the horn (as a hole bored through a plate is) unless it is very short in length.

You might look at a different compression driver that has a lower cutoff as in this design overlap is very desirable if you want to eliminate the crossover phase shift (like an SH-50) and have the result appear to be a single driver. Also, keep in mind that the exit wavefront on a compression driver is not always a plane wave, especially with larger format drivers. I have had very good luck with the BMS drivers which tend to have an expanding wavefront at their exit which makes the transition to the conical horn seamless..

The holes do form a “low pass” acoustic filter which is made of the mass of air in the port and the volume of air trapped under the cone body.
This is very desirable as it reduces the distortion the drivers produce.
The smaller the ports are, the lower the corner will be and also the less difference they make in the horn.
If you want to raise the mid corner F, then making the ports as short as possible (with a tapered or counter bore on the driver side) or increase the diameter is the easy path.
Have fun
Tom Danley
Danley Sound Labs
Danley Sound Labs, Inc. | Facebook

[winslow]

I want Unity horns as my next speaker setup.

[jmorken]

Wow, thank you all for some very constructive feedback. This is great.

Originally Posted by rvrazvan

The rolloff at 1000 hz is normal and i dont think there is anything you can do about it. Except for using another driver maybe. The mid is in a bandpass enclosure. anyway..don't expect anything more than 2 octaves.

Originally Posted by Tom Danley

If you want to raise the mid corner F, then making the ports as short as possible (with a tapered or counter bore on the driver side) or increase the diameter is the easy path.

Looking at my measurements again, my midrange bandwidth is really from 250hz to 1000hz (two octaves). I'll continue to shorten my port length until I get 300hz-1200hz. If anybody knows where the acoustic crossover point of the original unity design occurs, I would be grateful.

Originally Posted by Tom Danley

Hi
I would follow the suggestions about a radius at the entry, one should avoid any straight cylindrical portions on the horn (as a hole bored through a plate is) unless it is very short in length.

This is the part of the construction I took the biggest guess at. From what I've found the original unity horn didn't have anything more than a thin adapter between the driver and horn and some sort of filler in the corners. my adapter is about 1/4" thick and I've beveled it to taper from a 1" diamter to 1.25". I seem to recall you mentioning that you used a 1" pipe to form the radius inside your unity prototype. My original concern was creating too narrow/gradual a transition and causing beaming at the highest frequencies. I'll use some more filler to make a smoother radius.

Originally Posted by Tom Danley

You might look at a different compression driver that has a lower cutoff as in this design overlap is very desirable if you want to eliminate the crossover phase shift (like an SH-50) and have the result appear to be a single driver. A
lso, keep in mind that the exit wavefront on a compression driver is not always a plane wave, especially with larger format drivers. I have had very good luck with the BMS drivers which tend to have an expanding wavefront at their exit which makes the transition to the conical horn seamless..

The BMS 4550 should be on the christmas list if I can get this arrangement to sound half decent. The nice thing is, when I decide to build a larger synergy horn I should be able to use the same mids and compression drivers.

It'll probably be at least a week until I have any updates (this week is going to be brutal at work). Also, It sounds like if I'm going to model these things correctly I'm going to have to bite the bullet and learn akabak

[erjee]

Originally Posted by Tom Danley

I have had very good luck with the BMS drivers which tend to have an expanding wavefront at their exit which makes the transition to the conical horn seamless..

... which minimizes HOM?