MARRIAGE SOUNDS GOOD

December 23, 2020November 25, 2023Leave a comment

What’s the right wedding gift with 30 days of lead time when your friends are stranded across the Canadian border because of a global pandemic but they’re willing to risk it all for love and get married in a DMZ? I went with an Ikea cutting board. Well—to start.

It turns out that in places where border boundaries are blurred the acoustic offerings are slim. Without loud music (and strong drinks) no party is bompin, and without a bompin party, it’s not a wedding, so there was really only one thing to do: make a matching & linkable set of portable, hi-fidelity bluetooth speakers:

DESIGN SPECS

Cost of parts: $150 (ea.)
Loudness: 96dBSPL, 1m, @ 10% THD, A-weighted
Frequency Response: 50Hz to 20kHz ±5dB (but look at the curves down below)
Connectivity: Bluetooth 5.0
Battery: 3S Lithium-Ion, 37Wh
Runtime: 10 hours at “half volume” input (92dBA output)
Amplifier: 2x50W TPA3116D2 running @ 24V
Difficulty of Build: Dummy high—approx 120hrs from start to finish, requiring 2 CNCs and a 3D printer

In a lot of ways, this was a 2020 capstone project for me: to make something that’s loud, compact, and full of deep bass, with a 30-day conception to finish timeline, I had to pull out at least half of the dirty tricks I’ve learned over the last six years. Here’s how it went down.

ACOUSTIC DESIGN

When it comes to compact loudness with a lot of bass, excursion and efficiency are the belles of the ball, and although I simulated almost every 2-4″ driver I could find, the Dayton ND91-4 drivers (descended from long-gone AuraSound’s Neo-Radial IP) are nearly unbeatable when you factor in magnet strength, Fs, Xmax, price and weight. E.g. Peerless SLS-85S25CP04-04’s (catchy name huh) are potentially 1dB louder for a similar box size but weigh 285% more, while the Fountek FR89EX win for Xmax but need too much back volume and are 2dB less efficient…etc and so on. Just trust me on this one. In a 1.5-2.5 litre box: ND91-4, tuned low.

Tweeters are a fair sight more efficient, so down selection should be mostly driven by crossover frequency, dispersion and ease of integration. The ND91s break up right after 3kHz:

While the ND16/ND20 tweeters are truly amazing, they have to be crossed higher, and they come with a bunch of extra plastic, which clashes with the ultra compact layout I pushed. LaVoce’s TN100.70 did the trick and can be crossed over at 1.5kHz, which was perfect—the lower a tweeter can be crossed (disclaimer: within its volume displacement limit), the better. Finally, the TN100.70 dispersion is on par with the ND20FA tweeter @ 20kHz (-15dB):

As for the port, in order to maintain compactness and b-b-bass, I had to fit 250mm of port into a 2.5L box while keeping a holdable 4-inch width so I folded it around the ND91 and then crushed the port geometry until it fit in between the driver and borders of the speaker. Tweeter in green, port/body in pink, and woofer in yellow below:

Driving the woofer and tweeter is a 3S 3500mAh battery pack (I use LG 18650s that I order B2B from the factory) paired to a 2x50W Class D TPA3116 D2 amplifier through DC-DC step up converter for maximum power delivery. WONDOM makes a wonderful TPA3116 board with the DSP integrated, which merges with their 3S MPPT Battery Management Board, although to my late-stage chagrin neither of the boards have a step up to power the TPA chipset at an adequate 24V.

BUILD PROCESS

With the acoustic design tucked away 15 days from the deadline, it was time to build. The octagonal outer shell is just a set of 22.5° mitres, tape-clamped, with the patent-pending dual-bevel 8th wall precision cut to match:

The front face was a 2 sided CNC operation, which required calibrating features for aligning the Shaper Origin I used.

Merging the two pieces with the speakers and the front mounted the port was rather easy except for some minor mishaps with a few missing microns; the t-nut I planned to use to rear-mount the woofers were exactly 300 microns short of the planned front face thickness, so after sanding I had two t-nut holes showing on the front face. The port itself had to be printed in 3 pieces because of the complexity of the geometry to fit it both on the border and between the woofer and the back panel:

With space at a premium, but also for aesthetics, I used an LED array for status lights and integrated the on switch into the potentiometer. With that in mind, I also fabbed an ultra slim 6mm bracing/sealing ring for the rear panel mounting, as a butt joint would’ve been ugly but the shell was too thin/weak for threaded inserts. Those loose microns got me again and the flange on the port interfered with the built dimensions of the rear panel, so I slotted that out, but after some truly painstaking finagling of circuit boards, 5 days before the wedding ship date, I was ready for sound test.

That’s when I realized neither the BMS nor the Amp was using a boosted rail which was causing very noticeable voltage clipping, so I had to rip everything open and shove not only a DC-DC buck converter but a giant LC ripple filter (1.3mH L and 100µF C) onto the voltage rail. The only DC-DC buck converters I had in house were straight outta Hua Qiang Bei which means the were both cheap and poorly designed. Buck converters are in general awesome, and about as efficient as one could hope (for 12 to 24V boost, I saw ~85% efficiency depending on load), but the switching causes a lot of load-dependent ripple, which adds both noise and intermodulation into the signal chain. But with that bullet bitten, and with 1 day until ship, it was time to tune. And boy does this design sound good. Sparing the details of the tuning, here’s the final frequency response with a -3dBFS sine sweep @ half input “volume.”

The 2nd harmonic distortion looks pretty high @ 50Hz but this is mostly due to the aggressive non-linear processing I added in for extra kick; a more reasonable measure of THD in this scenario are the 3rd order harmonics, which I kept below 8%. The dips in the mid band (400Hz, 800Hz) are regrettable from a data standpoint (probably due to product baffle dimensions) but overall, these speakers deliver supple bass, smooth vocals, crisp treble, and excellent definition from 50Hz all the way to 20kHz. I added a little bit of level-dependent EQ, so at maximum volume these speakers are loud enough to kick off a backyard party, and at reasonable volumes they deliver a little extra extension for a very full, deep, frequency response. In my book, a thermos-sized speaker that can fill a room down to 50Hz ticks the “bigger than it looks” box; even from another room I found my self saying “damn, these sound good.”

BRAUN V BRAWN

November 6, 2019November 6, 2019Leave a comment

It’s wireless by my mom has a lot of cellphones to charge 💁‍♀️

RELEASED INTO THE WILD!

November 26, 2016November 26, 2016Leave a comment

Much like worried parents will fuss over a child before sending them off into the world, I fidgeted over the details of this lil guy, attempting to delay the inevitable departure, filled with pride and worry at the rigors he’ll face out in the real world. Unlike most worried parents, I eventually said “fuck it,” and dropped this fucker off at the local Fedex, to be shipped cross country in a large cardboard box.

The details were particularly sweat-able on this build, as this was essentially the third iteration on the concept, starting with a beast of the beats that went to Keith, and then a semi-pro configuration that went nowhere. Here are the vague details of the build:

Obviously, the first detail to isolate is the Lego theme. Legos, by the way, are a fairly mediocre permanent construction material. Turns out the 10-micron precision makes them fairly expensive from a cost/volume stand point (a small enclosure requires a lot of legos). Had my little siblings help me build lego boxes to compare the looks. Turns out rainbow is a crowd favorite

Fast forward a few days, after much designing, some deliberation, and then cutting, I’ve got a wood box to match the lego box. I believe I designed for 4L internal volume for each NS3 driver, which, in retrospect, I feel was too much. However, once the wood is cut…alea acta est. I went for a seamless miter approach on this build, to avoid the ugly “end-grain” of the birch plywood. I wrapped the grain around the “depth” of the box, but the grain of the “face” does not flow into the edges. So far I haven’t figured out a solution to this that works out in our boring 3 dimensional Euclidean / Newtonian universe.

Here we can see the translucence of the 3d prints, pre assembly. Originally, I had not planned for there to be a VU lightstrip in this build, but then I realized that since I built all the electronics off of the wood box, fitting them into the lego box, which had 5/6 faces constrained already, would be extremely difficult. At this point, I also realized I miscalculated the amount of space wires take up. Medusa rears her ugly cable management head yet again.

Now you see what I mean.

Analog signal processor demonstrating that I am the particular type of person that loves neat things but does not love making things neat, and so I live in a constant torment of my own devising. I chose a scheme that allows for a bass shelf at lower volumes, but flattens the EQ towards higher volumes—it’s essentially a loudness compensation circuit, except that since I have no reference for the actual loudness of the output (due to lack of information about source gain, listener position). I call it the “party” compensation circuit, because while one might enjoy deep sonorous extension at lower listening levels, once your friends roll through, 14 beers deep each, you’re cranking that fucking volume knob. And while the NS3’s have a lot of allowable excursion before crashing, the garbage bass lines that litter the hip hop soundscape are essentially glorified sine waves that will fuck your shit up. The obvious and simple solution to this is a simple dynamic gain-tied or signal-adaptive high-pass for excursion limiting, or a multi-band compressor. I leave these endeavors as an exercise to the reader until I have the time to implement them on the next build.

Pre-wood finishing. After disliking the gumminess and amber tint of marine spar varnish, I chose to use tung oil to bring out the figure. Then I sealed the box+3d printed parts with epoxy (bar top) for strength/durability, and finally, for UV protection and hardness, I finished with a clear gloss polycrylic. Here’s what it looks like finished:

A BRIEF HISTORY AND MUSINGS ON GRADUATION

August 31, 2016September 9, 2016Leave a comment

Boombox The First

IN the way that most things in life are, there was no moment where the stars aligned and a beam of silver brilliance illuminated me with inspiration; it was not a sudden cataclysm of grand events or even a single moment of clarity. It was more the lucky coincidence of a few small dust mites, just little things, which only in detailed retrospection, could one notice an amount of circumstantial alignment, that lead to this—which, of course, is on par with the scale of endeavor.

I was confronting my post-graduation ennui, as the evaporation of two of the more fulfilling experiences in my life (leading the water polo team, and learning tons of shit with tons of new people) had left me both without purpose and without the satisfaction of achievement of purpose. This was compounded by the fact that I had a degree in Mechanical engineering, which is a certification that I have all the necessary tools to be achieving things, but no job, which rendered those tools somewhat useless. I also other aligned mites; I had a little bundle of cash from graduation, a garage full of random bits of electronics, a shit load of time, and a best friend who wanted portable bluetooth speakers.

So I told him, I could build those for cheaper. He said fuck yeah, and I went to Urban Ore, where a quite lucky mite of dust fell into place; I found a use pair of drivers which just so happened to be the AuraSound NS4-255-4D. If this set of esoteric numbers doesn’t cause heart flutters, the NS4 is one of the best small format full-range drivers in the world, and it’s also impossible to get, because AuraSound was shut down by the feds. Th NS4 some how managed to sneak a little performance past the Iron Law; it’s loud and it’s got bass in a small box, and it’s cheap (well, used to be cheap; the last commercially-available pair was bought by me, 1.5 years ago for $50). And somehow I had found a pair of these fuckers for $4.

And so I built him a damn boombox, and it’s design was mediocre, and the electrical engineering was shoddy, and the panels were cut from slightly different bits of wood, and nothing was quite lined up right, and it took all summer to get the right parts, and I ran up against the deadline of his birthday so I had to build for 2 days straight, and then it was the day of, and I hadn’t eaten all day because I had to finish it, and I was supposed to be leaving to go his house for a BBQ, and I was late, and I was and had been focusing for the last 8 hours to an extent that makes me wonder about the exact nature of my mental health, and I had my doubts about the whole thing because in someway the little guy’s success was entangled a bit with my own soul, but then it was time to go to his house, and since it was too late to turn back, I turned it on, for the first time, on the porch looking out to the sun setting over the Golden Gate, fog and clouds glowing with orange warmth, with my parents drinking a beer and my little siblings watching, and when the power light flicked on, and the beast was awakened with a bit of thunder, and the peaceful calm of a idyllic Berkeley summer sunset was shattered, I grinned.

Here’s the guy from that exact moment:

And Then The Rest

The response was positive, so I built a few more. Though, the first few efforts up until recently were a bit sophomoric, here are a few of them:

Here we’ve got some unfinished pictures of an ultra mini build with a 2 way design using a very surprising 3-inch sub that kicked low-end ass. Built into a (small) cake box.

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Here’s some beer for scale: WP_20150117_11_02_35_Pro

Here’s the second NS4 build, with some Brazilian cherry I found on the side of the street. Shipped this one to the east coast. Not the most finished product I’ve made but it worked ok.

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Eventually I realized that the finish (wood pun intended) is as important as the start so I worked on getting a cleaner look and looking into wood varnishes. WP_20150604_21_01_22_Pro

And the most recent piece, the KrumpKanon for Keith “I’ve been lifting so it can be heavy” Savran, complete with Lego detailing, a 200W sub, missile launch switches, and a marine spar varnish. It’s quite heavy.