MTG Designs - Waveguide Shootout

The Great Waveguide Shootout

Objective Tests / Measurements of Various Horns and Waveguides

This project was started in 2013, then nearly abandoned for several years and finally restarted last year (2021). The idea is to test and provide various measurements on a wide range of waveguide and horn designs making it easier to understand the relative performance of each.

Initially I will focus on measuring directivity behavior of the various waveguides providing on/off axis frequency response, normalized plots and polar sonograms for the vertical and horizontal axis. In the future the project may be expanded to include a closer look at different compression drivers (response/sensitivity/distortion/compression) on the more popular horns and waveguides.

There will be a lot to measure so the tests and data will likely come in and be uploaded here slowly. Updates when more data has been added will likely be posted to the original forum thread. Link to the original thread on AVS Forum here.

The Original Test Rig

The initial project hit a snag early on with the initial testing rig providing troublesome measurement data, reflections from the rotating mic boom & baffle edge diffraction issues towards 90 degrees. It was also so large and cumbersome to setup that the whole idea was abandoned for several years. Recently I had an idea for a much simpler more compact test rig that is adjustable making it easier to mount and test waveguides of various shapes and sizes and so the project was restarted.

The New Test Rig

The new setup is a simple structure of 2020 aluminum that allows easy adjustment the width of the rig to conform to the waveguide being tested (no longer the need to make baffles for every waveguide). The smaller size and vertical orientation allows it to be used indoors which is a huge plus as measurements can be taken easily without waiting for the weather to be cooperative.

I made large roundovers out of cut down sonotube mounted to the sides and covered in egg-crate foam in order to minimize reflections and edge diffraction. Though it is evolving as the modular nature makes it easy to change out sections as needed. So far the measurements are clean with minimal diffraction ripples out to 90 degrees which is what I was shooting for. This allows the measurements to be posted without the heavy smoothing often seen on polar plots for various waveguides or horns.

During the planning stage before restarting the measurements I was able to design a small controller using an Arduino and stepper motor to remotely control the rotation of the turntable. This ended up being a huge time saver not having to manually move the turntable during testing and makes the rotation more accurate and consistent as well.

Update (4/7/2021):

Testing has paused as I noticed an issue with elevated 2nd harmonic distortion measurements from my UMIK-1. After ordering two new MiniDSP mics (another CSL Calibrated UMIK-1 and a new UMIK-2 from MiniDSP) I've found they all have issues with elevated 2nd harmonic distortion generated from either the capsule or built in preamp at SPL levels well below their rating (seeing a rise at SPL levels as low as 90dB) making them unsuitable for distortion testing.

My older UMM-6 does not exhibit this same behavior and I was able to create a new calibration file for it testing it against one of the CSL calibrated UMIK-1's so the waveguide measurements may resume soon using that mic.

In the mean time I have removed all the distortion measurements as they are inaccurate.

Update (October-2021):

Expecting to resume measurements in the coming months, my new Earthworks M23 will be used for testing which does not suffer from distortion issues at high SPL.

Loads more waveguides/horns and compression drivers have been acquired. I'll focus on directivity/polars of the various waveguides first then testing of the compression drivers on the more popular waveguides.

Update (February-2022):

Sidetracked for longer then expected but I'm finally back at it. I will progress with the threaded waveguides then move onto the 1" bolt on when those are complete. I'll be slowly adding the measurements as I take them and create/upload the graphs.

I was going to try and use the Celestion CDX1-1731 for the remaining threaded waveguides. However I've found the diameter of threads on the Celestion drivers are slightly larger then average which means it doesn't always thread into every waveguide, in the case it doesn't fit the Blast King driver will be used instead.

I've also switched to using VituixCAD to generate the off axis graphs/polars as it's much easier to import the data and generate the graphs and to do so at a consistent scale. I'll try to update the few older graphs when I get a chance so everything matches.

Horn and Waveguide Tests:

Notes - I tried to be consistent and use a single compression driver for all the waveguides however the compression driver I choose for that, the Celestion CDX1-1731 did not thread into all the 1-3/8-18 TPI horns, in that case a Blast King compression driver was used instead.

The CDX1-1731 is also used for the bolt-on horns mounted with a flangeless adapter. The 1-3/8"-18 TPI CDX1-1731 also has the same exit geometry as its bolt on sibling the CDX1-1730 and the two should theoretically be interchangeable on those horns.

The directivity performance in the very high frequencies will be dependent on the compression driver used and its exit geometry. This occurs above ~12-13 kHz on 1" horns, above 8-9 Khz for 1.4" horns and even lower for 2" horns.

The Great Waveguide Shootout

Horn and Waveguide Tests:

1 Inch - 1-3/8"-18 TPI (Threaded):

1 Inch - Bolt On:

1.4 Inch:

2 Inch: