Back To Horns... Building DRC Frugal-Horn Mk3s ~ Bandsaw Chord Cuts & MA Pluvia 7.2HD Drivers 28Feb25

The Frugal-Horn Mk3 (FH3)... is a tapped (driver placed somewhere along the line of horn taper; on FH3, that's very early) hypex (hyperbolic exponential ~ meaning the flare of the horn increases monotonically and yields a smooth bass transition) corner-horn (where you put it in a room) with an internal choke serving for low pass filtering. It was born from 2005-2007 DIY enthusiasts seeking to design an easy to build (more later) corner horn... let's say the primary designer was Lindgren and I used base drawings from DLugos, but many from the DIY community (diyAudio.com) were involved, at least in stuffing and testing. That team has done all the acoustical physics math and tons of testing ~ I'm adopting the gen 3 variant directly and will just be the craftsman, listener, and tweaker. The designers don't release hornresp or other math models and I can't get the final segment of the horn (with the curved mouth) to render correctly in hornresp, so I'm flying a bit with faith. The design is very successful and lauded for accommodating a variety of drivers with just some damping changes and room placement adjustments. There are even $2K commercial versions as well as $750 "flat packs." I haven't built a horn since the original DRC BLHs in 2020. What's not to like? I have 2 more Mark Audio Pluvia 7.2HD drivers (~$50 each) for the initial buildout, expect 1 sheet of 15mm Baltic birch for $90, $40 binding posts, and I'll use leftover damping material (or a Target pillow).

Cutting Wood... The first task was cutting a tight layout from a 5'x5' piece of 15mmm Baltic birch. Given the tricky nature of the long side chord cuts and some other angles I first asked Matt @ CO Lumber to do all the cuts, but the price was way too high @ ~$450. I understand the many setups but golly. A friend with access to CNC offered cuts too but that didn't pan out I guess. So, within the viable pricing window, it was possible to purchase a home WEN bandsaw and do the cuts myself! I also bought 2 additional inexpensive tool benches for the WEN benchtop stuff in Silverthorne. With a new saw (mounted on a bench, as I did with the WEN drill press) and a new cut sheet, I first made some gross cuts in COS in order to get the pieces in the car with the doors closed... I did that with a rough wall powered jigsaw to the "big dotted green" cuts on the sheet... the baffles, bottoms, and tops had already been sliced off to even just get the plywood home from CO Lumber - with bungee chords on one door. Make no mistake, this cut sheet is challenging as precious little is square at the end of the day ~ keep your wits.

I got all the new stands, tools, and gross-cut parts to Summit ~ Crusher Too was packed to the gills. I made additional "gross cuts" between the 3 abutting side pieces with a corded 7.25" rotary saw, and at other times when the table saw rip fence was too close to accommodate a larger width.

The bandsaw was essential. I made front, top, and bottom cuts with the rotary saw, or the table saw when 90°, but the long chord cuts were made on the new bandsaw with two pieces (sides) clamped together. I used a bendy board to score a simple chord line... this curve is actually important in restoring low freq sound waves to spherical as they exit the back. Sure I needed to sand the chord too after cutting it but I had "drill bits" with floppy sandpaper on 'em for the occasion.

The parts are all cut now, but for the driver holes and some router work. Whew... this was not an "easy" day (weird angles and many cuts). When done after a day on the wood tools my back was talking to me... I closed with a fine Roam DDH Nelson from ORB, my Feb can club selection, and a spa ride in the snow.

I dry fit each speaker ~ all good. All the end angles have been cut and the driver holes have been cut and eased on the inside using a 45° bit in my router. All the parts are ready. My table saw angle adjustment is gross so I bought this keen tool to offer greater accuracy. A new inclinometer was used to assure end-angles were righteous... it's magnetized and stuck right to the blade after zeroing on the bed. The supra-baffles got eased driver holes and angle cut edges, for smoother transition from 2pi space. All the parts are ready for a glue fest tomorrow. I will first assemble a non-supra-baffle variant and measure it. I "know" there may be baffle diffraction step (BDS) issues, but I want a baseline before acoustically adjusting performance ~ I can always combat BDS loss with some miniDSP tweaks anyway.

Let The Adhesion And Stuffing Begin... I began gluing the FrugalHorns together on one side first ~ they need to be left open in order to add damping materials. Assembly was pretty easy (as compared to cutting) and after glue dried overnight I began lining with 1/2" damping felt, installing the binding posts, and stuffing in key areas. I didn't have enough polyfluff from prior builds so I bought a $4 pillow at Target ~ DIY baby. The FH3 cabinets are assembled! It's time to load the Pluvias and do some initial measurement and listening.

Initial Listen & Measure Of DRC FH3s... I did the bulk of needed raw sanding already and will do finishing later in warmer weather. As before with MA Pluvia metal cone drivers, I ran them in for 32hrs straight before measuring... they'll keep running as I think I really need 3 days. I don't really have permanent or even good corner locations for these in Summit but chose to set up in the Nest Study with its cathedral ceiling and somewhat available corners, to first listen and measure them. Whoa... even just driven by a simple T-Amp, this corner horn bass sound rocks with the MAP7.2HDs... very good. There may be better drivers with more rigid baskets or expensive cone designs or surrounds; heck, I've used 'em. But, make no mistakes, Mark Audio's range is killer in sound and value.

I quickly went to guns to measure the FH3s using my normal miniDSP mic 1 and REW. My listening position in COS will be at ~15' so I mimicked that here (I know, the room will change everything but this space is mid-volume as in COS); I closed the blinds. I confirmed what I heard, that the bass response of these new corner horns is very fine for a single tiny driver (BTW ~ I used 30g of fluff in both the trap and the throat... seems right for the MAP7.2HD. The dip 3KHz - 10KHz might yet be the unfolding of the driver... more later but I hear it and don't like it. Still, remarkably solid from 70Hz - 23KHz, naked with a bad amp: I love horns... more later.

Sidesaddle... It's noticeable to observers (both of them), I constrain my builds to efficient, full-range, single dynamic driver variants of speakers... the FH3s are the latest speaker of this ilk I've built. Why do I do this? ~ That's a topic for another post, but one reason is that constraints help me dive deeper in understanding and allow me to more easily achieve perfection/selection in a narrower category. I may miss a few things sure, but within class I can crush it. There's more 'cause single full range driver systems, in killer accompanying acoustical cabinets have efficiency, coherency, and dynamism galore.

Black Ice Amplification... As a final Nest test for the FH3s, I reallocated the sweet Black Ice FX10H tube amp from the Nest Desk to drive the new speakers. No measuring, just a past catalog rendered anew from the new horns. Mostly Americana from Nashville. Needs more top end but yeah, these drivers should ultimately render that. Completely awesome with a full sound and large soundstage ~ I am very ready to tote the towers to the Front Range and stage with the TU8900R.

Baffle Design... I've had baffle diffraction step "loss" issues with narrow full range designs before (like this one) so I added an extra, larger, optional baffle around the driver. I cut my octagonal baffle from scrap Baltic birch and eased the edges. From prior measurements of the 4" MA Pluvias I saw I wanted to get down to 100Hz reflected into 2pi space. I'd leave the rest of the bass to the corner horn. So, 13560in/sec /8 /100Hz ~ 17". However my biggest scrap piece right now was only 12.8" wide so I'll start with that and make others if I feel it's warranted. The baffle hangs out over the sides and top about 3" (13560/8/12.8 = 132Hz). Measuring will tell the tale of this idea. The supra-baffle is mostly ready... I did change the design to one which hangs around the speaker top, so I won't have to change drivers to the supra-baffle.

Meh... they not only look kindof goofy but my measurements didn't show the improvements in the frequencies I'd hoped (see supra-baffle vs none below) in mid-bass... the back horns already carry the day. Any adjustments needed can be done in EQ, and I will keep the cool FrugalHorn naked look. I am not delusional, there is not much below 70Hz... the kit relocated to COS will get full support from the NanoSub... but I will tune it down from the much higher cutoff required in support of the NanoOnkens.

System Integration @ Blodgett... Mated to the nearly infinitely variable Elekit amp I should be able to come up with something very nice sounding. My next experimental space for the FH3s is the cathedral living room @ Blodgett ~ it's not an ideal side wall setup but will have to work. I first tested the right speaker positioned against the side wall and 6 ft off the wall. There was insufficient difference to warrant the "wide" placement. The bass response of the FH3s in either position is excellent. Music is tighter with the narrower position.

I did subwoofer integration adjustments with the miniDSP. The NanoSub is only a modest improvement augmenting the already solid low end of the FH3s when in sufficient corner spots.

I added PEQ filters on each channel of FH3s (which wasn't quite warranted from measurements but I wanted to see how close to optimal I could get). I removed the NanoSub, at least temporarily, and tried 2 ideas, EQ and EQ', as shown below against the NoSub NoEQ sweep. Again, not a lot of "improvement" versus the FH3s running unadulterated.

I reviewed again the bass responses of the FH3s, the NanoSub, and both together. The NanoSub does not add sufficient freq coverage over the FH3s alone. Yet, distortion is higher when "forcing" the FH3s to produce the low notes via the back horn ~ hmmm, who cares, isn't the point of the FH3 to get "more out of less" - I removed the NanoSub completely.

At the end of the day I also "retired" the miniDSP from the Blodgett Living Room stereo, it wasn't needed as it was with the NanoOnkens and without a need to drive a sub at line level, see ya. I also removed the AirPlay wireless widget... it stuttered and was lower quality than a righteous BT AptX HD wireless stream so I wasn't using it. The current "final" stereo in the Blodgett Living Room (BLR) is much less complex, wholly unadulterated, and a very cool full range sounding kit. I may yet take another pass at EQ/DSP but for now this thing sings, and it's so cool too ~ the DRC FH3s loaded with MA Pluvia 7.2HDs, driven by the Elekit TU8200R with Tung Sol 6550 power and Valvo ECC82 pre-amp, are GR8.