Argon

Overview:

Argon is a monitor that offers true full range performance in a moderately sized package. One advantage in a small three-way is that all drivers can operate within their "comfort zones" on and off axis. This allows the designer to target a relatively flat frequency response without having to de-emphasize problem areas that might arise in a two-way design. Ultimately, this helps to preserve all of the details in the recording without introducing harshness. The 8" wide, 20" tall cabinet will allow placement in smaller rooms, but the output capability will support use in medium to large rooms, provided that enough amplifier power is available. Sensitivity is low at approximately 81db/2.87V.

Driver Selection:

I have been interested in trying a 3-way design with the Anarchy woofer more-or-less since it came out. It is a 7" woofer that has a nicely designed motor, aluminum cone and over 12 mm of 1-way x-max. As a result, it has bass distortion levels similar to a good 8" woofer, but can reach to 30 Hz and below in less than 1 cu. ft. The necessary tradeoff is lower efficiency - there is no free lunch. At the DIY event where I first heard the Anarchys, I won a pair of Airborne wood cone midranges as a door prize donated by Solen. There haven't been a lot of projects published using this driver, but fortunately Voice Coil magazine did a full review. Based on their test results, I felt that the driver would perform best between 800 Hz and 4 kHz. This made it a good candidate to use with the Anarchy, which should be near its best up to 1 kHz. For the tweeter, I decided to go with the neo magnet BC25. I wanted a mid/tweeter crossover near 4 kHz and the slight horn loading of the BC25 gives a good directivity match at that point. It also is one of my favorite budget tweeters, offering performance well above expectations for a sub-$20 tweeter.

Cabinet Design:

The Parts Express 0.75 cu. ft. MTM enclosure was used for this project. I used the curved maple version that has been discontinued, but other than the finish, it is the same as part number 302-743. External dimensions are 8" wide by 20" tall by 14" deep. The baffle is 1" thick, while the other walls are 3/4" mdf. The curved side walls, coupled with a vertical brace running the length of the enclosure results in a very inert cabinet. The inside of the cabinet is lined with R8 Bonded Logic Ultratouch denim fiber insulation. The Airborne midrange requires a separate enclosure for optimum performance. I used common pvc parts to form an enclosure, similar to what I did for the Hafnium design. I used a 4" pvc coupling, which has an OD of 5" as the primary enclosure. To that, I attached a pvc street cleanout adapter, which allows adjustment to the stuffing as needed. I stuffed the enclosure with a handful of standard polyfill. The rear of the woofer and midrange cutouts were chamfered to improve airflow around the relatively large magnet structures of those drivers.

Crossover Design:

For this design, the approximate crossover points of 800 Hz and 4 kHz were largely dictated by the midrange performance. The crossover between the woofer and mid has 2nd order acoustic slopes, while the mid-tweeter crossover is 4th order. The steeper slope was needed due to peaks in the midrange response above 8 kHz. The design fell into a very smooth frequency response with only 13 crossover parts, which is noteworthy because the midrange and tweeter required extreme padding due to the low sensitivity of the Anarchy woofer. Five of the 13 parts are resistors.

Measurements:

All measurements were made with the Liberty Instruments Praxis measurement suite. This measurement package works best with Windows XP and has been discontinued, but it is capable and reliable as long as I can keep a Windows XP computer up and running. Frequency response measurements were done in my basement with short 3 ms gating, which means that data below about 500 Hz isn't terribly meaningful. Praxis includes a flex windowing function that attempts to look at longer gate times to give some sense of what the low frequency response is, but accuracy is limited and floor/ceiling reflections play a role in the measured response. Since the lowest crossover point is 800 Hz, the measurements in this state were sufficient to design the crossover.

The on-axis response is respectably flat from the lower accurate measurement limit of 500 Hz to over 15 kHz. Other than a 3 dB narrow band dip centered at 2.4 kHz, the 2.87V response is 81 +/- 1 dB. The reverse null measurement indicates actual crossover points of 750 Hz and 3.8 kHz. The narrow band dip fills out off axis. Although there is some non-linearity in the off-axis response from 2-6 kHz, the overall dispersion from 0 to 60 degrees is pretty uniform as hoped. The tweeter response drops off pretty quickly off axis above 10 kHz, which is somewhat compensated by a slightly elevated on axis response.

Impressions:

This speaker was demoed at DIY Iowa 2015 in Grinnell and InDIYana 2017. Some feedback can be found in the initial design post. In my listening, I find the speaker well balanced. I find the bass response particularly clean and extended compared to other speakers of similar size I have built or heard. I wasn't sure if I would hear any artificial warmth from the wood cone Airborne mid. I don't think I did, but that may be due to the 750 Hz high pass crossover on the mid.