by Bob Richards E. E. T.
The Mid/Tweeter baffle of my other "2019 Hi-Fi speakers" should be great, but is a bit experimental. Putting them against a wall, which I want to do, might have a slightly less than ideal psycho-acoustic effect, because the rear radiation reflection won't get the 6mS delay (3 feet out from the wall behind the speaker) that is required for best effect. The bounce board in that design may improve that enough to still be great, but time will tell (haven't actually fired them up yet as of 8-17-19).
I already had the drivers for this "Plan B" project, so I built these.
I love using white oak for cabinets, because it's beautiful, light colored, very heavy and very hard. It's therefore very hard for the acoustic energy inside the cabinet to re-emerge through the cabinet walls. I recessed the front 3/8 inch so I could glue 3/8 inch thick felt to the front surface, to absorb some of the emission reinforcement effects of the panel. There will still be "baffle step" effects, but you can't really get around those unless the drivers are mounted on a very big wall. the purple felt on the Seas Millenium tweeter drivers is thin regular felt. Treble is so directional, I doubt if that felt makes an audible difference, but I like how it looks.
The wiring just comes out to Banana Jacks on the rear, so they can be bi-amp'd with an active crossover/EQ, or a passive crossover could be attached externally to the rear of the cabinet, and thereby be easy to mess with during calibration.
Here's the specs on the drivers:
With Frequency Response (FR) graphs this smooth and flat, I could use a one pole passive crossover at 2-4kHZ no problem, and then just have to worry about baffle step, and low end rolloff below 100HZ (unless there a separate woofer that does the bottom end - which there will be with X=100HZ). The Seas Millenium tweeter was arguably "state of the art" back in 2000 when it came out. I doubt if there's a one inch dome tweeter made since that is audibly better. It's said to have about twice the Xmax of most other one inch domes. It's one of very few one inch dome tweeters that can perform real well down to 1.4kHZ (with a 4 pole active filter). The five inch Focal drivers are not state of the art, but have enough Xmax to be used down to about 80HZ and perform real well. With active EQ I would expect them to do well down to 40HZ in a smaller room at moderate levels.
Here's a bunch of pictures I took during the build process:
The internal acoustic padding is where a lot of speakers fall short. You want to completely dissipate that rear energy, which otherwise re-emerges through the diaphrams FREQUECY SELECTIVELY. In other words it "colors" the sound in a less desireable way. A "gaussian" dissipation mechanism would be close to ideal. To achieve that, I glued the relatively stiff 3/8 inch thick wool felt tight against all internal surfaces. Then put a layer of 1/2 inch thick foam rummer on top of that. Then some dense wads of "Acousta-stuff" (like fiberglas insulation, which would work fine too) (the white stuff). then more foam rubber (the pink stuff) toward the rear. It's all glued in place as in the photos. The two shelves are there for their abilty to break up the potential end to end resonant issues, and to make the center part of the front baffle board (3/4 thick MDF) more rigid; less likely to ever vibrate significantly. Their openings don't line up, so any resonance is minimized.
I don't use the second voice coil winding in the Focal drivers for servo feedback of any kind. The two windings are wired in series to create a 16 ohm driver (spec sheet is wrong - I measured them), and the two 16 ohm drivers wired in parallel gives me an 8 ohm load for the power amp. This is one of the simplest speaker projects I've done in a long time, but should be quite excellent (haven't hooked them up yet as of 8-17-19).
I modified the circuitry for my earlier, since discarded, Open Baffle speakers (The Aurium Waveguides). Now it's a 3 way 4 pole Linkwitz/Riley, with active EQ for both the woofer (to be acoustically relatively flat down to 25HZ), and Baffle Step for the midrange drivers (see graph), and there's still the 2.7kHZ dip of about 2-3dB that Linkwitz recommended on his system due to the way the ear works.
Above on the left is the SPICE model graph for the Baffle Step EQ. The result is in the graph below.
The crossover/EQ circuit for the midrange and tweeter is above on the right. You can see how I modified the open baffle circuit for this system.
The levels in the above graph are with the output level trimpots centered. Not how they ended up after acoustic calibration with pink noise and calibrated mic.
Since I don't have an anechoic chamber for testing, I won't include the final acoustic Frequency Response graph.
Like with the vast majority of the listening rooms, the FR gets a bit ugly by the time the acoustic energy gets to the couch. Every room reflection causes a comb filter effect.
The Peerless brand 12 inch XXLS woofer below was top of the line about 20 years ago, and is still very competitive with any of todays woofers.
Below you can see the very well vented spider. On the right is a better look at the cabinet.
The very low bass is quite juicy. Especially noticable on percussion.