A DIY Swarm-based Distributed Bass Array for SOTA Bass

State of the art bass turns out to be a lot more affordable - and achievable- than you think. It just requires a new approach.

The physical realities of low bass in small rooms is discussed by Earl Geddes in "Why Multiple Subs?" http://www.gedlee.com/Papers/multiple%20subs.pdf

Basically, a 20Hz wave is 56.5 feet long. So any bass that low is bound to bounce off a wall long before it even becomes a full wave. Then as it bounces back it runs into itself, either canceling or reinforcing itself, and this all happens even before one full wave can be produced. This is just physics, or acoustics if you prefer, and happens in every room and always regardless of speaker design. As Geddes has shown the shape of the room hardly even matters. At residential sizes they are all pretty much the same. So the conventional approach of one or two subs cannot possibly work. It never could. It was doomed by the physical and acoustical constraints of home-sized rooms.

With four however it becomes almost trivially easy. Not in the sense of being perfect. Its not magic. The quality and performance of the four speakers still matters. But its trivially easy in the sense that the improvement inherent in using four is so great as to swamp these details.

Or at least that is the conclusion that led me to build my own distributed bass array.

The Distributed Bass Array

The system I built is based on the Audiokinesis Swarm subwoofer system http://www.theabsolutesound.com/articles/audiokinesis-swarm-subwoofer-system/ with help from Audiokinesis himself, Duke Lejeune.

It uses four 10" Morel drivers https://www.parts-express.com/morel-ultimate-uw-1058-10-subwoofer--297-130

These drivers at the time included sealed 1.25 cf cabinets. These go together easily with minimal woodworking skill and tools, but that would mean four sealed cabinets. Duke suggested building two ported would give the added option of being able to easily tweak the bass response if needed by simply plugging a port or two.

With ported enclosures there is a relationship between volume, port dimensions, and tuning frequency. Basically, a bigger cabinet volume, longer port, and smaller port diameter yield a lower tuning frequency. A smaller diameter port doesn’t need to be as long. But it needs to be big enough to pass the volume of air quietly, without chuffing. Just a few of the many details designers like Duke know in depth. Me, I used one of the on-line port length calculators. Which just happened to give the same answer as Duke gave me. So I know the calculator was right.

As a result, two subs are 16" sealed cubes, and two are 16x16x24 ported cabs, with 3" diameter by 20" long ports. According to Parts Express this results in an f3 of 52 Hz for the sealed subs, while according to the on-line calculator the ported subs are tuned to 20 Hz. I take all this with a big grain of salt or as an estimated starting point at best, as room reinforcement at low frequencies is so big a factor. The goal is simply to get in the ball park, or close enough for a little port plugging, phase adjusting, maybe even a touch of EQ to do the rest.

The Amps

Duke’s Swarm system uses the Dayton subwoofer amp https://www.parts-express.com/dayton-audio-sa1000-subwoofer-amplifier-rack-mountable--300-811
While one amp would do nicely, because almost all bass is mono, still there is an advantage in terms of flexibility in using two. The Dayton amps are adjustable for crossover, level, and phase. A main reason a DBA works so well is the four source locations results in a smoothing of modes. Another factor is the way we perceive the size of the space we are in is related to the mixing or randomness of the lowest frequencies. The idea is that being able to adjust phase can further smooth or blend modes resulting in a greater sensation of being in a larger space.

At this point however I had a bit of a problem. My Melody 880I amp has no pre-out while the Dayton amps have only line-level inputs.

There are plenty of Line Out Converters but they all look cheesy to me. So instead I opted to convert one of my unused RCA inputs to a Line Out.

Converting speaker output to line level is a simple two resistor circuit. One 10k drops the speaker level, one 1k provides a little impedance to the outboard component. http://www.epanorama.net/circuits/speaker_to_line.html With a foot or two of spare silver wire and $2 worth of resistors soldered inside the Melody retains its stock look and is ready to drive the Dayton sub amps.

The Setup

This is the hardest part with the conventional approach, figuring out where exactly to put the sub to get smooth flat bass. Okay, technically not hard- impossible! With four however...

I spread the four around the room- near the left corner of the front wall, a little further than that from the front corner of the right wall, the remaining two on each side a little further back than the sweet spot. Fairly random. They all face the wall.

One run of ordinary 14 ga wire, at least for now. The two 8 ohm subs on each amp wired in series is 16 ohms, or 4 ohms in parallel. I tried both. The bass is a little more taut and defined at 16 compared to 4. Either way the result is leagues beyond a conventional sub.

The Sound

Finally! Setting up the Talon Roc the DBA replaced I knew would be a chore so for that I used a warbled test CD. Having read so much I was so sure the DBA would not be like that this time that I put on Bela Fleck’s Flight of the Cosmic Hippo. This track features some incredible low bass. Bass that trailed off to nothingness with my Talon Roc. Bass so low I had only heard it done even halfways decent one time- in the gigantic basement of a speaker designer (Linaeum) in Beaverton.

The DBA, speakers plopped down with zero effort, no cones, plain old lamp cord, and only the crudest guesstimate levels set, reproduced the Cosmic low bass of the Hippo so perfectly I could hardly believe it. Everyone talks about sub compatibility, matching, phasing, yada yada- forget about it. If my experience is anything to go by just plop four subs down, sit back, and enjoy. Try not to giggle. Seriously. Try.

The bass from these things is just crazy fast, articulate, powerful and smooth. Uncannily so. The Absolute Sound review above describes this so beautifully I have little to add. What I will add, as amazing as the obviously low powerful bass that you’re aware of hearing is, what is even more amazing is the effect of the very unobviously unpowerful low bass that you’re not.

What I mean is, several times now I have been listening to music that seemed to have no low bass content at all. Why then does it sound so much better? What is the DBA doing??? Sometimes, as with piano, it makes sense. The piano is a physically large instrument. The lowest A on a modern piano is about 27.5 Hz. With string bass its 31Hz to 41Hz, four or five string, depending. All likely with a physical presence that extends a bit lower than their fundamental notes. So with a little thought its easy enough to see why these sound better with the DBA. But what about the rest? Why am I getting the sense of the whole recording being bigger, better, more real and present in the room?

Or should I say, not that the recording is in my room but that I am in the recording room? Duke and others have talked about a sense of envelopment. Something I have experienced for a very long time going back to my first concert hall experience as a youth is the unique sonic signature of large spaces. If you’ve been to a concert you know the feeling. You walk in and not only your ears but your whole body feels the sensation of really, really, REALLY, I’m talking subterranean low bass.

Now honestly, the DBA is not that. Maybe with four 15" subs it would be. If its even on the recording to begin with. Not the point. The point is, the DBA is at least getting to the point where this is where you have to go to try and describe it. Its that good.
Hello millercarbon,

     I like your experimental nature, always exploring and pushing the envelope.  You've become the mad scientist of in-room bass exploration.  I'm fairly sure you now wear a white lab coat when you're experimenting.
     I'm in the process of thinking of a good nickname for you, I'll let you know when the light bulb goes on.
     Very interesting how the Roc as a 5th sub helped so much. It makes sense according to Geddes (the more subs the smoother and better the bass) but the gain of going from 4 to 5 subs is expected to be marginal and not significant like you experienced.
     With the DBA concept working so well being based on using more subs to create more peaks and dips in the room, and our brains being relied upon to process all the resulting bass variances and average them out via psycho-acoustic principles to provide a perception of smoother overall bass response, it's a lot like making order out of chaos.  It makes sense to me in a macro sense but I have almost zero understanding of how this works in detail.
     Maybe the scientific explanation is you just got lucky.  My opinion is that we're very fortunate our brains are capable of making a 'bass smoothie out of a bunch of lumpy bass room modes'.  Don't bother trying to patent that phrase, buddy, I already did.  
     We're also fortunate Geddes, Toole, Welti, etc. took advantage of this brain peculiarity in processing deep bass tones and developed it into the black arts creation of the DBA concept.  It really does remind me of alchemy or an awesome magic trick.
     So, I'd suggest not trying to explain it and just enjoy it.  I'd be concerned I might sprain something important trying to make sense of this one.  I'm just going to refer to it as the Talon Roc Fifth Sub Mystery for now.

Its really only a mystery in the sense of not being able to say for certain exactly why. But I have a pretty good idea.

The marginal 4 to 5 improvement I interpret as "all other things being equal." Which would apply if all I did was add one more sub just like the other 4, run off the same amp, at the same power, and volume. 

Which is not what I did at all. Instead what I did was add a completely different sub, a lot bigger (the Roc is two woofers in an isobaric ported enclosure) and with its own amp, and its own volume level. 

Probably if I was to go to the trouble of measuring and calibrating the Roc level to match the other subs, and then measuring and comparing, probably then it would turn out to have only marginal improvement. But that's the difference between serious guys who earn the lab coat and guys like me who really only care about the bottom line. Or bass line. Whatever.

Also in case I forgot to mention the improvement is mostly in the sense of headroom. The original 4 in other words could be set to give about the same amount of really low bass. But at that setting the amp would sometimes clip pretty bad. When the Daytons clip it sounds really bad. The Roc adds enough headroom to fill in the bottom end without running the Daytons too hard. 

Now if I was worth the lab coat then I would measure and probably find the really low bass that impresses me so much is really only 2 to 4 dB louder. But in human perception terms we experience that as more like 10 dB, which is a lot. 

I guess from reading these posts it seems like all I do is experiment. In reality I hate this stuff. I just want my Seal, Warnes, etc and movies to sound real good. Getting there..... getting there....

     I think perception is reality in a practical sense.  If you consistently perceive the bass as better, then the bass is better.  It'd be nice if a few others perceived it, too, just to validate your sanity but it's not necessary.
     I'm thinking the Roc alone didn't work well previously in your system because there were bass standing waves causing bass modes at your listening seat. Now, with 5 subs, those aren't perceived and the Roc is freed to add its extra and more extended bass to the mix.
     I ordered some new white lab coats for you with 'Duke DBA Disciple' printed on the back.

Alright. But I only wear the Tommy Bahama silk lab coat with subtle palm tree pattern. Goes with the whole Pina colada listening theme. Just so you know. Which reminds me, get that pineapple juice chillin'.

     I was able to change the lab coat order but they were out of the Tommy Bahama silk lab coats.  So I got you 4 sets of their silk pajamas instead.  The pants have footies but I figured that'd be okay just around the lab, right?

     pineapple juice is already chillin'.