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 thiswhich, 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.


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.


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.



So after all that talk of efficiency v extension, I figured I’d actually build something. It worked out that I ended up building on both sides of the coin, as the “Extension” build ended up being pretty gigantic. But the review of both approaches is in and it turns out that in some regards efficiency is more pleasant; I used the Faital Pro 5FE100 in a ~8L enclosure tuned to about 45-50 hz with a 3 inch x 15 inch port which I 3D-printed.  It WOMPS. Sounds great, crisp, powerful. Really pushes the “effortless” bass feeling, until you get below resonance and the woofer starts whacking around. It becomes excursion limited quite fast on songs with a lot of low frequency content, and it sounds pretty alarming. This forces my hand in ASP to put in a high pass around 32Hz-20Hz, adding complexity and heartache. But all in all, not a bad design.

As for the extension, which also sounds good, a different set of issues comes into play. I went with the Tang Band W5-1138SMF ~14L tuned to around 40 hz with a PR, and for starters, the thing is fucking gigantic. Despite the apparent size, 40 hz in 14L is pretty damn good, and it definitely has a presence visually and sonically. It sounds large, powerful, and it’s exceedingly hard to push the woofer itself into distortion. However  pushing the low efficiency of that driver is it’s ridiculous excursion capability, which brings an unexpected issue into play: high excursion means a lot of acceleration. A lot of acceleration mechanically means all kinds of shit is moving around, including things coupled to the boombox through the surface it’s on; put the boombox on the counter and all of a sudden the beer bottles on the counter start wiggling around too. Additionally, in the electrical domain, a lot of acceleration means a lot of voltage, which means more batteries.

These factors push the design to seem a lot less effortless than the 5FE100, potentially because the 1138 design seems to promote higher THD. It’s not necessarily a fair comparison, because the ASP, amp, and battery management on the 5FE100 design happens to be a lot better (I built it second, learned from previous mistakes). Normalizing for those factors, though, the smaller size is pretty great as it turns out that there is usually very little content from 40 Hz and below, and because larger boxes tend to be less efficient in terms of materials.


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After modeling 20+ woofers, including the attractive ScanSpeak classic series, the illustrious semi-pro Morel and FaitalPRO designs, and the uncategorizable SB Acoustic woofers, I’ve decided that there are only two viable designs. For the purpose of listening to  party music in party situations (Keith’s BuffBox), I want something like 104 dB at 48-42Hz at max, solid directivity, and about 3dB head room to handle over-compressed music and power compression that comes with party scenarios. For Elise’s use (KuadKrumpKannon), I’m probably going to want something slightly smaller and with slightly less performance. We can go with 100dB 50-40Hz with 3 dB of head room.

That leaves me with two options, the classic crux, the bane of boomboxes, the designer’s dilemma,  the iron law. For a given size, you can either have more bass but lose sensitivity, or have less bass bandwidth but more loudness. The third unspoken option is get both and pay $$$.

One the one hand, I could go for a 2.1 type set up with the boombox-friendly micro-subwoofer designs like the Tang Band 1138-SMF with it’s extreme 45 Hz free-air resonance, it’s lovely 9.25mm linear x-max, 5-something liter Vas and 82 dB sensitivity. Yeah, it sacrifices sensitivity for low Fs+high xMax at a decent cost point, but with 2^6.3 watts power handling we can potentially get to 99 dB at 1m down to 32 Hz in a 15L cabinet. In my experience, having deep reach with slightly less output can be more enjoyable despite not being as overwhelmingly loud. Essentially, I’d give up loudness for an extra  bass half-octave.

Alternatively, I can go for sensitivity over bandwidth. The FaitalPRO 5FE100 is a great example of this, as it maintains decent excursion and could potentially hit low 50’s in a small box while still being 6 dB more sensitive than the 1138-SMF. In fact, it could  hit 103 dB  in a 8L box, for a 51 Hz-3dB points, or and 40 Hz -3dB in a 15L box. It actually could handle producing about 108 dB (that’s at 100W, it can handle 160W) but above and below the port resonance we’d be we’ll out of the linear excursion range of the woofer. It starts to get directive at 1.5 kHz (beam forming begins at f=c/λ which is 2.7 kHz for a 5 inch woofer), so I could easily cross it over with some equally sensitive 1-2.5 inch speakers around 300 hz and call it a day.  Lots of out put, lots of head room, good package. Only two factors to consider: 1) whether it will work with available  passive radiators and 2) What Would Heathor Trainor Think? About the missing half octave?

On the other hand, I could go for a more sensitive design dual-fullrange or dual woofer tweeter with slightly less reach, but I get 3dB off the bat because I’ll have two woofers. The NS4 would be perfect for this but alas, they are gone from the world. The second best thing is the AuraSound NS3-193-8A , and it’s copy cat brothers: Dayton ND90, ND91, ND105, and DS115 are all great candidates for this. But after modeling I found:

  • AuraSound NS3: Great response, pretty design, 50hz in a 4L box. Play nice with passive radiators. All around solid drivers at a good price. Definitely on the table. The only issue is they’re not that sensitive and they don’t hit super duper hard.
  • ND90: good for this application, but still, lots of ripple. -3dB at 40-38 in 9-7.7 liters. Good news is xMech for days, so bottoming out shouldn’t be an issue for these ND series. Doesn’t work
  • Dayton ND91: great for 1-4 liter enclosures. Looots of ripple otherwise, still a lot of ripple.
  • ND105: lots of ripple, needs at least 14L but can get down to 33 hZ  at -3dB, or in 9L, 38 Hz with a bit of extra efficiency in the pass band.
  • DS115: Kind of a middle ground. In 6 liters, could go to 50 hZ -3dB. They look pretty, and with two of them, I could get 104 dB while staying in the linear region.

The downsides to this approach are: raised complexity (I’ll have to fit 2 4th order systems in the same form factor), limited design flexibility (long ports are almost out of the question and passive radiators are harder to fit), potentially greater cost, and of course, half the space which usually ends up meaning less bass.

Here is a graphical explanation of all that jazz ^.

TF ELise session

Transfer Function of Speaker. -3dB point marked in red.

But what do the colors mean? See next photo.

20w legend

Legend of which colors match up with with speaker/box designs.

Transfer functions are great but we want to know the absolute level.

20W comparison

Absolute level. Compare the sensitivity vs the bandwidth of the various drivers. The ND105 manages to be a good combination of both.

But how loud do they really get with linear excursion constraints?

maxSPL Elise session

Maximum (linear excursion) loudness. Note the FaitalPROs (magenta, green, teal) manages massive power handling until the excursion limit kicks in. The previously impressive ND105 (orange) has little-to-no advantage over the 1138SMF (yellow) due to its lower power handling and limited excursion.

So…what to do?