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

I'm using Mabat's software to make the waveguide, and then I add the midrange taps in Autodesk 123D. But you could do it in Fusion as well.

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
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. [/url]

An inside out frustum. Acoustically - allows smaller chamber volume - traded off against effect on HF?

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.

Oh, one more thing:

See that dip at 1800Hz? That's a crossover problem, not a waveguide problem. Basically that dip indicates that the midrange and the tweeter are about 45 degrees out of phase at the xover point. This can be fixed. I just didn't have time to get too obsessive about it. I spent all of twenty minutes making this crossover.
I made a new revision to Metlako. It features the following improvements:

1) The main reason I did it, was because the first Metlako has a seam that runs right through the throat. I wasn't thrilled about that because the throat must be PERFECT. Metlako V2 has *two* seams not one, but both are over an inch from the throat.

2) By going with three pieces, I was able to make it wider vertically.

3) I basically made the waveguide as wide as I could get away with, while still horn loading the tweeter. It's JUST narrow enough to load the tweeter, and no more.

4) I did some fairly outlandish things to make the midbasses fit. It's crazy looking, two of the midbasses are actually *behind* the tweeter. I literally spent hours shaving off a millimeter here and a millimeter there.







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I got 1.28kHz -3dB out of FaitalPRO 3FE25 (Qes=0.8) with a very small rear chamber and front volume reducers. Without reducing the front volume the -3dB was ~100Hz lower.

My understanding is that the upper frequency limit is set by a number of factors:
Mass corner frequency
Distance of taps from horn throat (for which a compression driver adds extra length so the dome tweeter has an advantage)
Front chamber/port volume low pass filter
What is the efficiency of this design?

Hornresp says 92.5dB

How low does it maintain pattern control


With a waveguide that's 13" in diameter, I would expect it to lose pattern control at 1038.4Hz. But based on this measurement of the Metlako v1 polars, it maintains control to 800Hz.

How large / small is it ?

The waveguide is approximately 13" x 9" x 3.5". It's literally the widest waveguide I could make that would load the tweeter down to the crossover frequency. See : The Preference for Direct Radiators

The enclosure is approximately 20" tall x 15" wide x 9" deep. That's 1.56 cubic feet. I'm using a QB5 vented enclosure maximize the power handling.

It should be good for 110dB maximum.

Apologies if I overlooked the answers elsewhere in this thread


No worries!
Hi Patrick, I'm curious about the orientation of the midbass ports, comparing the black and orange waveguides on post 1. It seems to me that you can get the slots physically closer to the tweeter and more symmetrical in the X and Y direction if they are oriented radially like the black waveguide as opposed to concentrically like the orange waveguide.

Also I wonder of the sharp edges of the slots on the orange waveguide will cause some diffraction since they are perpendicular to the direction of the tweeter sound waves.

Maybe this is all a result of the new printing configuration, or the larger midbass drivers being used?
Thanks, Sixto