Nest Music Theater Part Thirty One ~ Pure Fostex EΣ Soundfield & Building Metronomes 01Apr24-08May24

I've kept going ~ I found another lightly used pair of Fostex FE108EΣs on eBay for $250 all up. This time I've invested more time on the cabinets (though not horn-level investment) and chose to build a pair of mass loaded quarter wave quadradic tapered transmission line cabinets called Metronomes (ML-QQWT). I'm using them in the surround spots in the Nest Music Theater, though they could easily be mains. I've rotated the miniOnkens previously there to the presence/height spots, and given the Tang Band Tri-Ports to Matt (he was somewhat enamored when hearing them initially), and he is just starting to build out his theater. I decided these Metronome towers took no more space than the miniOnkens on stands, and I had a chance to employ Fostex EΣ drivers throughout the NestMT. Plus, I'd never built a transmission line speaker before and the FE108EΣ (low volume but strong) is somewhat applicable in one.

Here are the drivers and Theil-Small parameters for it. I didn't remeasure these used drivers and it's likely some aspects have changed slightly with age ~ not enough to effect enclosure design though I believe. Performance will be more impacted by the amp to which I connect them and the room and their position in it.

Mass Loaded Quarter Wave Transmission Line... Quarter wave transmission lines are speakers where the cabinet's line length is 1/4 the wavelength of the driver's resonant frequency, Fs. So for my drivers with Fs=77Hz and converting to wavelength with speed of sound = 343m/s then the line should be about (1/4)*(343/7) = 1114mm. Such designs assure bass reproduction out the port, from the rear of the driver, is in phase with the front. Just basics; so far so good. Mass loading implies moving the air in the cabinet in total (in mass). Tapering pace & direction, driver placement on the line (stopped end vs ported end), port dimensions, and damping are a few other key factors. Many modern speaker engineers (Brines, King, Lindmen, Dlugos, Cresswell, and others) have worked a long time on transmission line attributes, modeling, experiments, and calculators. Many DIYers have trialed and perfected various full range and subwoofer TL designs as well. My hornresp modeling of the Metronome v1.1 showed it appropriate for my driver (good thing, it was made for it) and yes, hornresp works under Wine on macOS. Numbers in my drawing are actually a decimal off so actually 9/60/200/20 on the cross sections. The graph actually depicts the undamped bass response but I will line the entire interior. The offered design fit my needs and I adopted it directly. The Metronome was created by Cresswell and is an attractive version of a full range (non-folded) transmission line speaker for a 4" driver and it fits my surround locations perfectly.

Building The Metronomes With FE108EΣs... I bought a 5'x5' sheet of pretty 18mm Baltic birch. When I asked for it at the wood shop they asked for thickness in US inches in ratios... grrr. When I said 23/32nds they double-taked and I revised to 3/4", but it's not really. At Colorado Lumber it was $130. I could not fit the full sheet in the Crusher Too in one piece so I had them cut it in half to fit.

The trickiest part of the build is getting the correct angle cuts on the main cabinet pieces. Sure the fabrication would be pretty easy using just lapped joints, but there are tough angles on the rips. I got it set up fine clamping an aluminum level against the fixed fence to guide the pieces past the blade at just 3.84°.

05Apr24 ~ However, I had to abandon the rips. My home table saw was not up to the 18mm Baltic birch @ angles and popped the breakers twice. I packed it in, drew up cut sheets, and went to see Matt at Colorado Lumber for some precision cuts with the panel cutter and CNC machine (+$180). I've used him very successfully before.

02May24 ~ Once the pieces were cut, I believed assembly would be pretty straightforward. Matt finished cutting the cabinet sides today and I retrieved them. I will personally cut the tops and bottoms and do assembly. That's all in Summit Co, and will be attended by some final ski runs in the slush @ Breck.

06May24 ~ I double checked my design and then cut the tops and bottoms with angled edges with my table saw. I drilled 2 3/8" holes in the bottoms with a hole saw and fabricated the ports in them ~ I used simple 2" ID Sched 40 PVC and epoxy; OD is 2 3/8" for the hole saw.

After a couple of dry fits, including the tricky driver clearance and my top & bottom pieces, I began assembly. I attached front to sides with tops and bottoms dry in place. The back stays off to in the beginning as I need access to the inside to lay damping material. Lots of clamps along the transmission line length during drying.

I used $4 of 6mm carpet acrylic felt underlayment for damping (so much cheaper than the wool felt I've used prior); damping was secured by 3M 77 spray adhesive. Messy but cleanup is breezy with citrus cleaners. Proper shears/scissors are key as is using a straightedge to sharpie the cut lines.

I then added two heavy gold-plated binding posts to each of the lower backside ($35 for all), avoiding the driver location and bottom with port itself, which are too tight.

After wiring for the driver and adding carpet to the backs, I glued and clamped the backs in place. Fortunately my stuff is square and the driver clears the rear, and I'm ready to install the drivers.

08May24 ~ Once the glue dried I installed the drivers as usual with foam gasket tape and #8 1" black wood screws. My small Metronomes look pretty cool, even raw in Baltic birch; I'll wait for warmer weather to finish them in the NestMT satin true black scheme. I'm using grill covers for these since they are right at a height and somewhat in a position to be accidentally bumped.

I measured them naked in free space and again at my 2m listening distance angled against a wall in the surround spot. The sweep is pretty solid with the dip at 1KHz I notice in these drivers in other cabinets too. Still, they sound great and do not display a rising SPL really. One cannot buy this stuff ~ you have to make it. They sound contained, but awesome, to my ear. They'll kill as surrounds.

Fostex FE108EΣ Cabinet Bakeoffs... I even rolled them into a front wall/free spot as main L/R and listened and they were absolutely great on acoustic stuff. They aren't quite as open as the Fostex FE168EΣ BLHs but they image well and have a decent SPL sweep even without EQ. I then staged a listening and measuring bakeoff among the three pairs of FE108EΣ-based speakers I've built: new Metronomes, the MiniOnkens, and the DRC Onkens. I turned off the sub and the BLHs and just ran the 108s flat and unadulterated. All three were brilliant, primarily of course from the driver. All the mid-high behaviors were similar but the DRC Onkens were the most efficient and they also had the smoothest and deepest bass response. Metronomes were probably 2nd best in the lows but the MiniOnkens don't suck.

Back in their respective spots as shown in the very 1st pic above, the 5.1.2 (really 6.1.2 with two center channel speakers... but they get the same signal) soundfield is amazing and can really load your head with music and discrete soundtracks are phenomenal. I don't think I can take this any further... time to do final integration and simply enjoy.

NestMT 5.1.2 Final Integration... My first "final integration" task was to listen and take sweeps with and without the TV augmenting the DRC Onken center channel. I did the sweeps with all channels rolling, all EQ (front BLHs) and delays applied, and the sub "on." At the end of the day, while the Sony speaker array is awesome and it actually gets me some front field expanse, as well as some very high end frequency boost, it is more finicky (Sony doesn't have a specific delay setting feature) and variable than without it. So, I disabled this feature of the NestMT and will just listen with 9 speakers... all full range Fostex EΣs.

My 2nd task was to again check volume balance amongst the channels (done) and assure delays are appropriate. The latter required some impulse measurement assessment ~ I did so under several different DSP settings in the Yamaha (7 channel stereo, Dolby ProLogic/Digital+, and Yamahas MusicVideo). I then tweaked the delay for the ~27msec out and listened & measured again. Perfecto.

I swept the frequency range again with all channels going, all EQ applied, the sub "on," and delays used... so this SPL graph is the integrated sum of all drivers. The NestMT is stellar. 2 channel PCM audio is subtle from the surrounds when Dolby PLIIx is used and more pronounced when Yamaha MusicVid is used.

Finally, I added the DVD Player into to the remote control capability and added some features of the Yamaha AVR to be accessed directly in my integrated remote.