Suitable midrange cone, for bandpass mid in Unity horn.

[Patrick Bateman]

Originally Posted by Patrick Bateman

After studying this problem for nearly a decade, I believe the formula above fails to take into account a few variables which affect the midrange response of a Unity horn.

Basically, there are things we can do to extend the response of the midranges, and there are things we do which limit the response of the midranges.

Worst of all, I do not believe these factors are accounted for in the hornresp and Akabak simulations.

First, let's look at the predicted and measured response of the midranges in a Unity horn. I believe Paul Spencer has done the best investigation of this:



In Paul's measurement, we see the following:

• At 900hz, the measured SPL is over 5dB lower than the simulated SPL
• At 1500hz, the measured SPL is over 10dB lower than the simulated SPL
• At 3000hz, the measured SPL is over 10dB higher than the simulated SPL

These are not small differences. The scale of the graph masks the fact that the midranges have a broad depression that begins at 650hz.


So why is there so much more output at 3khz? And so much less at 1500hz?

I believe the additional output at 3khz is due to beaming. An 11cm piston, like the Pyle woofers used by Paul, begins to beam at 3090hz, due to the diameter. So we have two factors going on here; the mass rolloff of the horn, at 465hz, and beaming at 3090hz. The beaming doesn't increase the efficiency of the horn; it simply narrows that energy into a beam, where it no longer interferes with what's radiating at the edge of the cone.

The problem that's really vexed me is how to get more output at 1500hz. And I think that I've figured out some things which hornresp and Akabak don't account for. It's the pathlength differences from the edge of the woofer cone, and a series of nulls which occur due to the distances between the four midranges.

In the picture above, I have illustrated the problem.

We have a series of nulls which aren't accounted for in the model. The nulls occur at the following locations:

First, we have a null which occurs due to the pathlength difference between the edge of the cone that's furthest from the midrange holes. In the picture, that's labeled "A" and "C" For instance, with an 11cm woofer, there's a pathlength difference which can be as long as approximately 9cm. (It's a bit less than 11cm because we're measuring from the center of the midrange hole.)

The cancellation from the edge of the cone occurs at one-half wavelength. In other words, if you draw a point from the furthest edge of the midrange cone to the midrange hole, there will be a null which occurs at one half wavelength because the sound radiating from that point is out-of-phase by one half wavelength. In the example above, with a distance of 9cm, we will get a null at 1888hz. (34000cm per second / 9cm / 2)


There's a more nefarious problem which Akabak and Hornresp do not account for. That problem is that the gap between the midranges creates a null, due to the distance between them. The reason that we do not see this in the sims is that the simulators assume that all four drivers are "on top of" each other.

Even worse, the null changes with angle!

The null is illustrated by the blue line "B" in the illustration above.

For instance, with a wall angle of sixty degrees, the midranges in the picture above are seperated by 15cm. A null will appear at a frequency of one half wavelength. In the example above, that null is at 1133hz.

Even worse, we get a series of nulls at various angles. For instance, at the center of the horn there will be a null at 2266hz, because the sound from the top midrange is 180 degrees out of phase with the sound from the center midrange. This is offset by the fact that it's IN PHASE with the opposite midrange.


If reading all of that just made your head explode, I apologize



There is a really simple and elegant solution to this though. First, use a narrower wall angle. Even reducing the coverage angle by ten degrees will move the nulls up in frequency by hundreds of hertz.

Second, I'd really think twice before using a woofer that's larger than 12cm. I've seen some people trying to get away with 15cm woofers, and that's really hard to do successfully due to the distance from the edge of the cone to the midrange holes.

Third, I think that focusing on the mass rolloff causes us to discount some drivers which don't "look good on paper." For instance, I've had good results with 5cm and 8cm drivers which appeared to be lousy candidates. I attribute this to the smaller piston size, which drives the geometry nulls much higher in frequency.

Fourth, the use of a frustum instead of a hole for the midranges serves to equalize the pathlengths. This affect is not insignificant; [b]it can reduce the distance from the edge of the cone by as much as a centimeter or two. (Because the shortest distance between two points is a straight line, and when you use a hole instead of a frustum, the sound from the edge of the cone has to make a 90 degree bend as it exits the coupling chamber.)




In summary: If you want to fill in the range from 900hz to 3000hz in a Unity horn, use a frustum for your midrange holes, reduce the wall angle, and consider using a woofer with a smaller cone. These changes are not accounted for by Akabak or Hornresp, but make a measurable difference.

A few weeks ago I posted the message above, where I argue that the upper limits of the midranges in a Unity horn aren't just dictated by mass rolloff. There are some factors we're ignoring, particularly nulls which occur due to the distance from side-to-side and top-to-bottom. Hornresp can't model this, because it 'thinks' they're all one unit.

Based on this argument, I think that the easiest way to extend the high frequency response of the midranges in a Unity or Synergy horn is to use very small drivers.

Bill Waslo has had good results with the $2 5cm drivers from Parts Express.

I have had good results with the $25 5cm drivers from Tang Band, as well as the $20 5cm drivers from Peerless.

If you want to take my idea to the logical extreme, have I got a driver for you

This driver has some interesting advantages over what Bill and I used:

#1 - It's a sealed back driver. You don't have to futz around with sealing off the basket.
#2 - The FS is in the right range. It's 520hz.
#3 - The size is even better than what Bill and I used. It's not a 5cm driver it's a 2cm driver (!)
#4 - The price is right. My drivers were $25 each, this one is $3


I think this might be the driver that would finally let us do a Synergy horn with a dome tweeter at the apex, along with a horn angle that's not so narrow.

DIY Synergy horn test - YouTube

A member of another forum managed to do a Synergy with a dome tweeter, and it has a lot of that 'unity horn magic.' (Listen to it over headphones, then compare it to the videos on Danley's Facebook page.)



I think the reason that this driver is so promising is that you need to get the midrange driver to play very *high* if you want to mate up with a dome tweeter. Plus, you want to use a wall angle that's wider than what you'd use for a compression driver, because a dome tweeter is better behaved on waveguides with a wide wall angle versus a narrow one. (Check out the measurements on Zaph's site; you can see this with his tests. I think it's also the reason that JBL/Mackie/Genelec use wide angle waveguides with dome tweeters.)

And when you make that wall angle wider, you need to raise the xover point, and this tiny little driver makes that possible.

Soooo...

Do I have to build one of these or will someone else give it a try first

[bwaslo]

PB, I thought about that driver,too,even have a couple of them. onedaunting issue is their sensitivity or lack thereof. For a part of a full range Synergy you might end up having to paddown the woofers!

[Nissep]

Any thoughts about the smallest of tangband extended range speakers?

http://www.parts-express.com/pedocs/...ifications.pdf

Tang Band W0-2008S 1" Neodymium Full Range Driver Pair 264-921

No problem geting it close to the cd with a diameter of 18.4mm.

[Patrick Bateman]

I folded a Unity horn into a Paraline. This has some neat benefits, such as a very small footprint, and it's much easier to build than a conventional horn. (Built mine in less than two hours.)

Video and pics here: Square Pegs

[JLH]

Been way too busy for audio, but just wanted to let you guys know I found an interesting 4" sealed back mid-range. If somebody has the time, look into it and see what it will take to get some. Please share what you find out so everyone can benefit from it. Back to the salt mine now...

Visaton - Lautsprecher und Zubehör, Loudspeakers and Accessories

[Antripodean]

Or you can just contact me for the 4" sealed back Celestion drivers. Might as well use the best

[JLH]

Originally Posted by Antripodean

Or you can just contact me for the 4" sealed back Celestion drivers. Might as well use the best

Shipping from down under to the states will be too costly. Not to mention customs will also rape you on duty taxes.

[Antripodean]

Originally Posted by JLH

Shipping from down under to the states will be too costly. Not to mention customs will also rape you on duty taxes.

Not so! They are available ex USA - Boxes of 12 mids (no split boxes)
As I say, just PM or email me

[drlowmu]

Hello Antripodean and JLH :

May I volunteer ?? I am less busy than JLH.

I suggest we all keep an open mind and investigate BOTH the Celestion and Visiton four inch sealed back drivers, and use the best suited ones !! One brand may be far better than the other, depending if its a two way Synergy horn, or, a three way Synergy horn design. Right !!??

It seems like Viston sells world-wide through dealers on different continents, and usually handles "technical questions" direct in Germany.

Since there presently seems to be a world-wide interest in a DIY Synergy Horn project, and it will likely be on-going, having a world -wide sales network and driver availability may be both cost effective and positive. Each end-user will likely have to buy eight drivers for a pair of stereo horns.

I think its good to speak with " one voice for all " to Visiton, and I volunteer. I am sending an email, and will telephone them on Monday.

Allow me to speak to the USA sales reps initially. We will figure out cost structures and the most suitable way, to make available, first to JLH and potentially to the ENTIRE Synergy DIY group (on an on-going basis), these Visiton four inch drivers.

Lets leave it to JLH and Antripodean to give us the DIY design and driver selections that work the best. They are well-qualified to do this engineering.

I am happy to help, and will openly post results in a timely basis, for all DIYer's information and benefit. If there are any additional topics I should determine from Visiton, please post me up here.

Thanks to all.

Jeffrey Medwin, Warrensburg, MO

[MaVo]

Originally Posted by drlowmu

Lets leave it to JLH and Antripodean to give us the DIY design and driver selections that work the best. They are well-qualified to do this engineering.

Its not that difficult. Just plug the driver parameters into hornresp. Do a conical horn with offset driver. Adjust the corresponding parameters until satisfied with result. Driver has to be relatively small with strong motor, basically every 4-6 inch PA midrange should be workable, as you will have to eq it anyway a little bit. With two midrange drivers, you can go up to about 8 inch. Doesnt have to be closed back either, if the horn is two way.