Metlako: A Small, Affordable Two-Way Unity Waveguide
Theoretically, it's not possible to make a small, affordable two-way Unity waveguide.
The problem is the midranges:
There aren't many midranges that work well on a Unity waveguide. The ones that are usable tend to have a high FS and a low QES. Due to this, it's very challenging to get a suitable midrange to play lower than 300Hz on a Unity horn.(1)
Over the years, I've tried a gazillion different shapes for the midrange taps on a Unity waveguide. I settled on a shape that's largely inspired by the shape of the L'Acoustics DOSC. Basically the entrance to the tap is circle, and the exit is a ribbon. (2)
This specific shape allows for something seemingly impossible in a Unity waveguide, which is that you can 'push' the midranges to play much higher than they normally would.
With this innovation in mind, I wanted to find out if it's possible to make a small, affordable two-way Unity waveguide. The theory says it can't be done, but sometimes you can bend the rules.
Mabat's software is truly a game changer. It's literally cut down the time I need to make a waveguide by around 75%, and it's also allowed me to simulate them before I design and print them. It's unreal. (3)
I came up with a new way to 3D print a waveguide that allows you to make one that's about twice as large as normally possible. Basically I slice the waveguide in half, and then I put one half inside of the other, like a Russian doll. The black waveguide was the biggest I could formerly print on my printer; the orange waveguide, hot off the printer, is about twice the volume. Same printer.
I have no idea if this is going to work. If it DOES work, it will simplify the process of making a Unity waveguide. Basically you will be able to have the midranges cover the entire spectrum. It will be a two way instead of a three way.
There HAVE been variations on this idea before:
1) Sound Physics Labs sold the "SPL Runt" which is a two way Unity horn. I could have gone that route by using a more expensive compression driver and midbass. Basically by using a midbass with a high FS and a low QES along with a compression driver that can handle some abuse. But I wanted to keep this project cheap; the total driver cost for all five drivers is under $75. Copying the SPL Runt would have quadrupled the driver cost, plus I don't like copying things :)
2) XRK did a two-way by putting a full range SB Acoustics at the apex of the horn. By basically pushing the tweeter down lower he was able to get it to play full range. Very cool! It blew my socks off when I saw it.
3) Art Welter has a two-way MEH horn that follows a similar recipe as the SPL Runt. (High FS and low QES on the midbasses along with a beefy compression driver.)
But I'm trying to take a different route, I'm trying to see if the geometry of the midrange taps can be modified to a point where they'll play full-range.
I don't know if this will work, but I'm eager to find out!
So if I understand it correctly, you want to combine 4 fullrangers in a horn with no tweeter?
Interesting idea. So one would have a full range point source with controlled dispersion. I will definitely follow this project.
It's a plain ol' Unity Horn with the midbasses covering four octaves.
In a conventional Unity Horn, the mids cover about two octaves (350hz - 1400hz) and the midbasses cover two octaves (87.5hz - 350hz.)
I'm trying to see if I can get the midbasses to play up to 1400Hz.
Hey patrick, when using the simulator, are you simulating it with the midrange taps in the horn? because I kind of thought Mabat's SW was for basically auto-generating a waveguide from a preset
Ok, thanks. I was confused by what looks like a closed throat on the 3D render. Still an interesting project:)
Since Mabat's software produces a STL file, you can manipulate in any program you want. It's the greatest thing ever.
I wrote a guide on how to do it here:
3D Modeling Tips and Tricks
Theoretically, my midbasses should NOT be able to play high enough to reach the tweeter. See (1) for an explanation why. But if I'm going to try, I need to make the volume of air in the chamber as small as possible. Basically the more air in the chamber, the more the highs are rolled off. Like a bandpass box. There's five *millimeters* of clearance here.
Lo and behold : IT WORKED.
This is the polar response of Metlako with four midbasses mounted to the waveguide, and the waveguide mounted to a 1M x 1M baffle. High frequencies go up to 1300Hz. I could probably go even higher if I squeezed the midbasses closer together, which is possible because my tweeter is the size of a quarter.
Metlako is my attempt to take my "Unitized Image Control" waveguide (2) and simplify it, by making it a two-way instead of a three way. For comparison's sake, here's a picture of said waveguide and a measurement of it's tweeter and midrange response. Oddly enough, the Metlako midbasses actually play higher than the UIC midranges. I'm guess this is because their QES is lower. On the downside, the Metlako midbasses roll off steeply. That could be a "defect" or a "feature" really.
(1) Suitable midrange cone, for bandpass mid in Unity horn.
(2) "Unitized" Image Control Waveguide
An inside out frustum. Acoustically - allows smaller chamber volume - traded off against effect on HF?
Here's a Vifa NE19 and an adapter I made for it, so it will work on a 1" horn. In this case, an 18Sound XT1086.
Here's the polar response of this "Metlako" project, using the same tweeter and four midbasses mounted on the waveguide.
Here's what I see:
The 18Sound IS a bit smoother. Particulary at 45 degrees off axis, we can see some diffraction happening off the edges of the midrange taps.
Having said that, the rest of the polar response is pretty good IMHO. Particularly when you consider how cheap this is: the tweeter is $20, the midranges cost $48 (total) and the MiniDSP is $80. Filament costs about $20.
So that's well under $300 for a full range Unity horn.
The "Metlako" project was largely to eliminate some complexity. But an unexpected side effect is that midrange output is higher, and distortion is lower. The last project certainly didn't have much distortion, but this one is even lower.
Here's the phase response.
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