This thread is about my latest build, the Natur Aura.
I completed these just in time to demo them at Burning Amp ‘23. I am proud to say that these got many compliments including a couple “best in show” comments.
Details provided below for the ambitious DIYer.
Summary
A monument to sound.
Aura is an open baffle loudspeaker crafted from a solid slab of Purpleheart wood.
Each side features seven metal-cone wideband drivers from Esoteric Audio Devices in Sweden. We selected these drivers for their impressive detail and realistic presentation of vocals and acoustic instruments.
The narrow and frameless open baffle design yields excellent clarity along with a large, full-bodied sound. The dipole radiation pattern minimizes room reflections. This design acts as a dipole throughout the full frequency range. Dipole bass is especially clean with a natural-sounding rolloff.
The crossover is the result of much experimentation. Our goal was to retain as much detail and clarity as possible, while minimizing the harshness that can be a problem with metal cones.
Aura builds on our expertise designing passive open baffle loudspeakers. The final design is the result of hundreds of hours of prototyping, tweaking and critical listening.
Specs
Motivation
At BAF ‘22 I demoed the Spirit Maple speakers. These have been my main speakers for a few years now. For my next project - Aura - I wanted to make something similar while also trying out some new ideas:
Measurements
Scott Hinton took measurements at BAF and I will post them as soon as I get my hands on them. Off-axis performance was great. Bass was very flat. Mids and high frequencies were also pretty flat but about 10db lower than the bass frequencies. This in part reflects my “basshead” taste. This is consistent with some listener feedback that they need more high-frequency energy.
I ended up tweaking the crossover to boost the highs by about 3db, and it does sound better. However, I still prefer the mids unchanged.
Crossover & wiring
(If the USD loses its value, my plan is to sell some of this copper!)
(The original unmodified crossover is described here)
The crossover is very unique and “breaks some rules”:
Driving inefficient open baffles.
This design trades away efficiency for wide bandwidth on a natural-sounding narrow baffle. So is it a problem to drive?
My $200 60watt pioneer stereo receiver drives these speakers to ear-piercing levels in my living room… I just need to turn the volume knob way up.
At BAF '23 I tried a Sony VFET amp and some tube amps. With a good preamp, 20W is enough to drive these to normal listening levels in a large room. The Zenductor amp I built is underpowered. I would recommend 50-100 Watts for these.
I completed these just in time to demo them at Burning Amp ‘23. I am proud to say that these got many compliments including a couple “best in show” comments.
Details provided below for the ambitious DIYer.
Summary
A monument to sound.
Aura is an open baffle loudspeaker crafted from a solid slab of Purpleheart wood.
Each side features seven metal-cone wideband drivers from Esoteric Audio Devices in Sweden. We selected these drivers for their impressive detail and realistic presentation of vocals and acoustic instruments.
The narrow and frameless open baffle design yields excellent clarity along with a large, full-bodied sound. The dipole radiation pattern minimizes room reflections. This design acts as a dipole throughout the full frequency range. Dipole bass is especially clean with a natural-sounding rolloff.
The crossover is the result of much experimentation. Our goal was to retain as much detail and clarity as possible, while minimizing the harshness that can be a problem with metal cones.
Aura builds on our expertise designing passive open baffle loudspeakers. The final design is the result of hundreds of hours of prototyping, tweaking and critical listening.
Specs
- Seven driver passive open baffle design.
- Symmetric WWMTMWW driver arrangement.
- Pair made from a single slab of solid purpleheart wood.
- Point-to-point wired external crossover containing just inductors and resistors - no capacitors.
- Four EAD E170 MKII aluminum cone woofers with selectable 70/57 Hz (nominal) first-order low-pass filter.*
- * Additional bass wiring options available for tailoring bass response to room and listener preference.
- Two EAD E100HD MKII aluminum cone wideband midrange with 375hz first-order high-pass and 5000 hz (nominal) first-order low-pass filters.
- One EAD E60HD MKII aluminum cone wideband midtweet with 5000hz (nominal) first-order high-pass filter.
- Low-efficiency wide bandwidth dipole design. Impressive FR down to 30hz on narrow baffle achieved with massive 19db attenuation of mid and tweeter.
- Finished with Osmo UV Protection Oil to accentuate the wood’s natural beauty while protecting from fading.
- Dimensions: 20.5” W x 17” D x 61” H - including legs excluding crossover. Baffle width tapers from 20.5” at base to 10” at top.
- 4ohm (nominal) load
- Weight: 70 lbs (each).
Motivation
At BAF ‘22 I demoed the Spirit Maple speakers. These have been my main speakers for a few years now. For my next project - Aura - I wanted to make something similar while also trying out some new ideas:
- I wanted a cleaner look. With the Aura, I chose straight-edge sides and a trapezoidal baffle shape. The Aura baffle is taller and narrower than Spirit’s.
- I wanted a less hodge-podgey driver selection. With Aura I went with all-metal cone drivers from the same manufacturer.
- I wanted to try an array of smaller 8-9” drivers for bass. I experimented with some line array designs but ended up with a WWMTMWW arrangement.
Measurements
Scott Hinton took measurements at BAF and I will post them as soon as I get my hands on them. Off-axis performance was great. Bass was very flat. Mids and high frequencies were also pretty flat but about 10db lower than the bass frequencies. This in part reflects my “basshead” taste. This is consistent with some listener feedback that they need more high-frequency energy.
I ended up tweaking the crossover to boost the highs by about 3db, and it does sound better. However, I still prefer the mids unchanged.
Crossover & wiring
(If the USD loses its value, my plan is to sell some of this copper!)
(The original unmodified crossover is described here)
The crossover is very unique and “breaks some rules”:
- No capacitors. I use “LR” low-pass and “RL” high-pass filters. Since I need to attenuate the mid and tweeter anyway, the series resistor serves double-duty. I find that “RL” high-pass filters are slightly more transparent-sounding than “CR” high-pass filters.
- Large inductance on bass drivers. You can select between 9.1mh or 11.1mh of inductance on the bass drivers (or a mix, as shown below). This corresponds to a (nominal) low-pass filter with a cutoff of 70hz or 57hz. However, the mid has a high-pass filter at 375hz. So what's going on? The -6db slope of the low-pass filter cancels the +6db slope due to dipole rolloff, resulting in flat (albeit very quiet) bass performance.
- Low-efficiency. I use massive 19db attenuation of the mid & tweeter to match the quiet dipole bass output. This provides a full-range sound down to 30hz despite the narrow baffle. While this insane amount of attenuation may sound like a challenge, one just needs to throw watts at the problem.
- Dual mids in series. I run the two mids in series with 375hz-5khz band pass. This in theory has drawbacks such as interference (due to dual-point-sources at high frequencies) but I find it sounds good and don’t notice the drawbacks when listening.
- DVC - The E170 is a dual-voice-coil driver with two 16ohm voice coils. When wiring the E170, I leave one voice coil open. I wire the other voice coils in parallel for a total 4ohm load. This gives a QTS of 0.78 which is perfect for open baffle.
Driving inefficient open baffles.
This design trades away efficiency for wide bandwidth on a natural-sounding narrow baffle. So is it a problem to drive?
My $200 60watt pioneer stereo receiver drives these speakers to ear-piercing levels in my living room… I just need to turn the volume knob way up.
At BAF '23 I tried a Sony VFET amp and some tube amps. With a good preamp, 20W is enough to drive these to normal listening levels in a large room. The Zenductor amp I built is underpowered. I would recommend 50-100 Watts for these.
Last edited:
The build
I picked up a massive solid slab of Purpleheart wood at Global Wood Source in Campbell, CA.
I had them crosscut it near the middle so that it would fit in my car.
I then used my Festool track saw to cut the trapezoid shape, trying to preserve as much sapwood along the edge as possible. I was able to get a 10"-20.5" x 61" trapezoid out of each side. I aimed for a 85 degree inside angle, but was using a small cheap protractor and ended up making the trapezoids slightly asymmetric. I wrote this off as a "feature" because it may minimize edge effects.
I made the cutouts for the drivers. Boy was this wood dense and required a lot of passes.
I then used a rabbit bit to make the recesses for flush-mounting. I then sanded all sides with 60/80/120/180/220 grit. Finally, I applied two coats of OSMO UV Protection Oil (24 hours between coats). After the second coat I installed the drivers and legs.
(Test fitting the drivers)
Unfortunately, I must have applied too much finish because as it dried it started to blister in some places. I plan to scrape or sand the finish off and refinish these at some point.
I picked up a massive solid slab of Purpleheart wood at Global Wood Source in Campbell, CA.
I had them crosscut it near the middle so that it would fit in my car.
I then used my Festool track saw to cut the trapezoid shape, trying to preserve as much sapwood along the edge as possible. I was able to get a 10"-20.5" x 61" trapezoid out of each side. I aimed for a 85 degree inside angle, but was using a small cheap protractor and ended up making the trapezoids slightly asymmetric. I wrote this off as a "feature" because it may minimize edge effects.
I made the cutouts for the drivers. Boy was this wood dense and required a lot of passes.
I then used a rabbit bit to make the recesses for flush-mounting. I then sanded all sides with 60/80/120/180/220 grit. Finally, I applied two coats of OSMO UV Protection Oil (24 hours between coats). After the second coat I installed the drivers and legs.
(Test fitting the drivers)
Unfortunately, I must have applied too much finish because as it dried it started to blister in some places. I plan to scrape or sand the finish off and refinish these at some point.
Feet: https://www.architecturaldepot.com/BKTMMO.html
10” in back and 6” in front.
10” in back and 6” in front.
Yes that’s the idea. Sound from the back of the cone will be slightly different but pretty close.