Tactile transducers- couch shakers and the Hoverboss

Tactile transducers- couch shakers and the Hoverboss

Like many people in the last couple of years I have been expanding my home theater. Back in 2009 or so I experimented with some Aura bass shakers on my couch and I didn’t like them very much. I never figured out how to get them to integrate smoothly and I forgot about them, but as I started to dig more deeply into subwoofers I saw that a lot of people were supplementing their lowest octaves with more involved methods.

The first thing I tried was a Clark Synthesis T239. Since I already had a Minidsp 2x4HD for the pipe subwoofer project it was simple to control the level and lowpass with some precision and this let me achieve much better integration. When it was turned on it was a subtle effect that I almost couldn’t feel but gave bass the impression of being deeper, louder, bigger- it was exciting!

I’m not sure why the Clark was so much better than my first trial with the Aura. It could be that the quality of the transducer, or it could be the processing. Someone out there has done an extensive comparison:


One thing that I noticed was that as the frequency dropped lower and lower I felt less and less shaking. Although the overall effect was fun and exciting, I could tell that with things like dinosaur stomps and spaceships crashing it could be better. That’s where the hoverboss comes in.

“BOSS” stands for “baffle open subwoofer shaker”. The way a subwoofer normally works is a large cone pushes and pulls on the air and creates sound waves within the room. As a side effect the acceleration of the cone generates a Newtonian reaction force on the box. Somebody figured out that you could build a platform for your couch, support it on basically some tennis balls, and then use cheap subwoofer drivers as a mass that would get accelerated by the bass signal . Here’s one build thread: https://www.avforums.com/threads/boss-baffle-open-sub-shaker-mini-riser-build.2269113/

So what’s the hoverboss? Someone else figured out that if you could trap the air being displaced by the woofer it could actually push directly on the ground to give more low end shake. This is really useful because as the frequency goes down so does the available Newtonian reaction force. This lets your couch shake down to 5hz, or even lower, without bottoming out the woofers. Here’s somebody else’s build thread: https://www.avsforum.com/threads/hoverboss-riser.3174695/

I built mine to match the size and shape of the couch, and I was able to pick up the woofers for $30 each on a Black Friday sale. Here’s what the mostly complete platform looks like:

Hoverboss platform

The best part is that when it’s in place you barely see it. The worst part is that before I put some sticky pads under the couch legs the couch would slide all over the place and fall off.

Couch on the hoverboss platform- can you even see it?

Barn Speaker Redo

Barn Speaker Redo

Back in 2003 I worked with a college friend to build a big set of speakers, and in 2015 I did some updates to make them somewhat less terrible- I wrote about that here: https://totallyshould.com/?p=870

One big problem with these speakers was that they were really not capable of producing much bass, and it took so much EQ to balance things out that they really couldn’t play very loudly. They sounded kind of boomy and hollow, probably due in combination to the walls flexing, the ported box tuning, and the sparse batting that was intended to (but did a poor job of) absorbing internal resonances. I wanted to see if I could really make them deliver on the promise of a loud barn speaker with some hifi sensibility.

There’s a project out there that I’ve heard of but didn’t pay much attention to, the Econowave . I didn’t copy it, but I took it as inspiration and found some drivers and a horn. I used the GRS 15PT-8 as a 15” woofer, the PRV D280-ti and the Dayton H6512 waveguide. At the time of this writing this was about the cheapest combination of parts that I could find that would give significantly higher efficiency and bass output in the existing 4.25 cubic foot box.

Power supply, Minidsp, Sure 4×100 watt amp board
No extra bracing yet, but lots of fiberglass

I decided to skip the port with these. I no longer have access to the Mathcad worksheet for simulating mass loaded transmission lines, and I didn’t think it was worthwhile to do learn how to do it in Hornresp – it really would have taken something like that instead of WinISD to do it predictably because of how tall the box is. Instead, I checked the numbers for excursion limited bass SPL and decided that a Linkwitz Transform was the best way to go. According to WinISD I would be able to extend the bass another half octave lower without running out or power or excursion as long as I also added a steep highpass. I took -3db point from about 60hz down to 35hz, and then added a 24 db/octave highpass at 30hz.

Old speaker with new drivers ready to go in

I used a new (to me) measurement technique to dial these in. I used something called the “MMM moving mic method” to perform something resembling an anechoic on-axis measurement of the speaker. The way this is executed is by playing periodic pink noise while taking an average reading on a real time analyzer while randomly moving the mic around the zone of interest. In this case I swung the mic up and down several inches in front of the tweeter and woofer, taking care to keep it moving and not linger in one area too long to too often. In other speakers I’ve checked it’s pretty repeatable. Shout out to Joentell for pointing me toward this method and working with me to get it going.

On-axis close mic MMM measurement of the final iteration of the barn speaker

Ultimately the on-axis response that I’ve created should be a flat horizontal line. it’s not, and there are a number of problems with it. Rather than tear it apart and talk about what’s separating it from a $10k reference monitor like the JBL M2, I’d like to focus more on what it’s able to get done for this all-in price of around $250 per speaker (not counting discman, labor, or wood).

Tuning the barn speakers a little closer than the final distance

One thing that I noticed with these speakers is that the tweeters and woofer are fairly directional. This is to be expected, and I’m sorry I didn’t have time to measure that, but it means that as you move away from the speaker the highs don’t fall off as quickly as a lot of the home speakers I have experience with. For this reason I kept the absolute level of the tweeter just a bit lower than the measurements might suggest. In the future I’ll do a re-tune when they are in their final home, but for now they’re still just a bit bright since I anticipate they’ll get used from a greater distance than the chair.

Close-mic response (blue) and at about two meters, MMM with psychoacoustic smoothing

Finally I did some distortion tests to make sure that the bass boost wasn’t too aggressive and the tweeter wasn’t stressed from the low crossover. The above graph may not be what people are quite accustomed to seeing.

Relative distortion at ~2m. Call it about 85db average

Here’s another way of presenting the same data. It shows that above 100hz distortion mostly stays below 1% at 85db at two meters. Compared to home audio equipment that’s respectable!

Distortion represented as a percent relative to fundamental

That just about wraps it up. These things have some new life and in their latest form make Pink Floyd and Emerson Lake and Palmer a lot of fun. Thanks for reading!

Wall Speaker Project

Wall Speaker Project

When I built our home theater I used the JBL LSR-308p MKii as the front left and right speakers. Overall they sound very good, but they don’t quite hold up when the action is at its peak. I’ve never had a very successful non-subwoofer speaker design so I set out to design something that would play a bit louder than the JBLs, while sounding at least as good as when they’re at their best.

A while ago I built the DIYSoundgroup HT-8, and in many ways it’s already exactly what I want. My main complaint is that I snatched up a single copy of it and it’s been almost continuously out of stock for the duration of the pandemic so I can’t build any more of them. The waveguides aren’t available for purchase, and what this speaker has in common with the JBL is a great waveguide.

One thing that really helps a speaker sound its best is for the off-axis sound to be very similar to the on-axis sound. This is because much of the sound you hear bounces around the room for a while before it makes its way to your ear. If the sound that goes off in various directions is significantly different from the sound that makes its way directly to you then the result sounds less natural. All woofers (and mids, and tweeters for that matter) exhibit something called “beaming” where they transition from omnidirectional radiation at low frequencies to more and more directional radiation at higher frequencies. When a woofer is larger than some small fraction of the wavelength of the sound it is reproducing the sound starts to tighten up into a narrow ‘beam’, where those frequencies would make their way straight ahead without going out into the room. When the sound transitions from the woofer to the tweeter the smaller driver has a much wider dispersion than the larger one and there’s a sudden discontinuity in the off-axis sound. A special type of horn tweeter that we call a waveguide can make the tweeter’s low frequency sound a much better match to where the woofer starts to beam just a little bit, resulting in a much better behaved off-axis response.

There are other benefits to waveguides; they reduce the stress on the tweeter and let it play louder with less distortion. You can read more about that on Zaph Audio. He modified some off-the-shelf horns to fit a conventional dome tweeter, which seems to give a more refined sound than a lot of pro audio compression drivers. In the years since his project we have had advances in free boundary element simulation software and 3D printing, and somebody has taken advantage of this and has begun designing, simulating, and printing custom waveguides to make this conversion even better. His site is: Waveguides | somasonus, and I have printed some of his waveguides to test. This has the advantage of allowing me to modify them to suit my needs, and they can’t be out of stock because I can print whatever I want.

I decided that the drivers I wanted to use first are the Dayton RS225-4 woofer and the Dayton RST28A tweeter. A test bench report on the tweeter is available on AudioExpress, but I’m having trouble find third party reviews of the woofer to link.

The speaker that I’d love to emulate with this would be the Procella P8, which I understand sell for over $2500 each. Those definitely play a good bit louder than I require, so even if the drivers I’ve chosen are sure to come up short of the maximum output then I’ll be in good shape if I come close.

Here’s my first prototype, I’ll post more as I’m able.

Pipe Sub

Pipe Sub

Since my last post we’ve moved and had a baby, who is now a rambunctious toddler. I’m getting to do some projects again so it’s time to bring the blog back!

This project might be a little more “totally should” than my other recent audio posts, so I thought I’d share some of the design process. This project started with brainstorming how I might build subwoofers for our new TV room.

My first idea was to build some modular cabinets across the front, with sort of a repeating cube shape. It seemed neat, but didn’t leave a ton of flexibility for positioning things or experimenting with different speakers such as different center channels or larger left and right speakers. Here’s what it might have looked like.

Next I had the idea to move the receiver elsewhere, and have just the subwoofers and speakers at the front of the room. An idea I had been thinking about for a while was to use pipes as enclosures since the curved walls are extremely resistant to flexing and re-radiating extra sounds that distort the output.

I was considering using two ten or twelve inch drivers in each pipe, using this Excursion calculator to see what the maximum sound pressure level might. THX reference is a peak of 115db at 30hz, and my goal was to be able to achieve that as low as 20hz. I found that four pieces of the Dayton Ultimax 12” woofers would be able to get close to this if they driven to their limit and had some help from room gain. A 14″ pipe would work for that, and would like something like this:

I started calling around looking for pipe, and it was hard! This stuff gets expensive and they only sell it in 20 foot lengths and won’t cut it. Finally I found a place with a broken piece of pipe in their scrap pile, and I could have it for free! But… it was 24″ schedule 40 and I had to figure out how to pick it up. I thought to myself, I totally should!

I called my friend with a truck and the folks with the pipe were kind enough to help us get it loaded. It was an amazing score, but I had to go back to the drawing board.

It looked like the pipe could hold as large as a 21” woofer fairly easily, so I did some more modeling with WinISD to see how much output I could expect at different power levels assuming different volumes and equalization filters. It became clear that I would have to spend a lot more on amplification to use most of the better 21″ drivers that would work in a small enclosure. I also noticed that the large moving mass might have the whole device dancing across the ground slamming into walls. The pipe weighs 32 pounds per foot and has a cross section of 2.77 square feet, so it would be feasible to make a relatively light weight sub with this stuff.

I played with the idea of putting the pipe right under the TV, but that really took up a lot of visual space and makes it much harder to use a center channel. The nice part of this iteration was that the dual-opposed woofers achieve force cancellation to keep the sub stationary (and reduce distortion), and the large volume allows sufficient low frequency output without requiring enormous power levels to overcome the stiffness of the trapped air inside the enclosure.

I selected the Dayton Ultimax 18” driver which was on sale and scaled the design to a pipe length of about 40″ long and nine cubic feet of internal volume. Based on room mode simulation in Room EQ Wizard as well as measurements with a borrowed subwoofer I decided that the sub could fit behind our couch and act as a second row of seating.

So how big is this pipe?!?

Really big. I had to use a jig saw to cut it to length and it took me about ten minutes per cut. I ended up using a piece of aluminum flashing as a flexible straight-edge to mark a guiding line for the cut. I guess a piece of twine pulled tight would work too, but this is what I did.

After cutting it to length I created some 1.5” thick plywood baffles to follow the axial and radial edges to create a large area for glue to hold the pipe in place. Here’s one flipped upside down so you can see what I’m talking about.

The pipe isn’t actually round though, maybe because it was damaged when it fell off the truck, maybe it was sitting around too long, or maybe that’s just the tolerance. Regardless, I used a sharpie taped to a rake to trace the actual contour. After making a circle jig for the driver cutout I glued the pieces together using the pipe to keep them centered and oriented. This was important since I wanted the square ends to stay aligned enough for the sub to sit flat on the ground.

I had to go through several glue trials to find something that would stick to both the wood and the PVC. I found that liquid nails didn’t stick to the PVC hardly at all, and I ended up using marine silicone after roughing up both surfaces with 60 grit.

Skipping ahead to the end, I used a Neutrik Speakon four pole-connector, some hurricane nuts, and Ultratouch denim inside the enclosure to help absorb higher frequency resonances. It was a success!

Next I needed to apply the Linkwitz Transform processing to get that wonderful low end. The brown trace on the graph is a close-mic measurement of the sub with zero processing. The green one is with the Linkwitz transform applied in a Minidsp 2x4HD. The in-room response is strong to below 10hz now, and the subwoofer is very hard to localize behind the couch. I think that this is due to the nonexistent structure borne resonance and lack of spurious noise and distortion. It could theoretically play at least 10db louder than a push it for movies, and I think that’s very helpful for preventing the kind of misbehavior that draws attention to a sub.

Acoustic treatment

Acoustic treatment

A lot of time has passed since my room equalization post in August 2016, and the stereo is now in a different room. Looking back, I think that one of the biggest problems that I had in my attempt at equalization was that I was aiming for the wrong target response, and wasn’t doing enough to actually treat the acoustic problems before equalizing. With the two of us basically living in one room and a bedroom I wouldn’t have been able to really add a lot of acoustic treatment without it being too ugly for Kaitlyn to put up with, but now we have an office in the house where I can make things as ugly as I want. So, here’s the ugly office…


There are a few major acoustic problems here.

First, it’s a square room. Any reflection or reverberation that goes from the front to the back wall also happens from side to side, so the shape is fundamentally bad for even bass reproductions. This is a 12x12x8 foot room, so since the speed of sound is 1125 feet per second you’d expect the first resonance to happen at about 47 hz. The math behind that is 1125 feet per second/12 feet = 93.75 cycles per second, and the first resonance happens with the pressure fluctuation (zero velocity) on the wall, and velocity fluctuation in the center of the room, so a half wave, so the resonance happens at half of 93.75.

Here’s a raw measurement of one of the speakers playing before I’ve done anything to help:


So, what’s wrong? Obviously there’s a huge peak in response a little lower than that predicted 47hz, but there’s something else going on too. As I said, my first attempt at correction in the old place was aiming at the wrong target response. When I applied the correction filters it sounded weak in the bass and I didn’t understand why. It turns out that there’s something called the Harman Target Curve that describes the frequency response that sounds most natural. You’d think that it would be perfectly flat, but when a sound system is set up that way it sounds weak in the bass and too bright in the highs. The preferred shape is a bass boost starting below ~150hz, flat response up to 1-2khz, then gradually falling response above that.

It turns out that the high frequencies were pretty good, and when you line up the bass according to the desired curve, that big peak at 47 isn’t even the worst problem.

Target response

In the new office, this is something I can hear. The bass between ~60 and ~200 is what gives bass that tactile feeling, where you can feel the drums, and that’s what not coming through like it used to, or like I know it could. The problem is that you can’t just boost frequencies that are too low. They’re too low because things are physically cancelling out or are driving modes that have dead spots at the listening position, and increasing the drive level doesn’t help much and adds distortion, cone excursion, and power usage without much benefit.

In order to address some of this, I looked at SBIR, speaker boundary interference response, and found that I should have a pretty big cancellation around 90hz because my chair is 30″ from the rear wall. It’s really hard to absorb frequencies below 500hz, and most of the acoustic foam panels that you can buy do very little at these frequencies. You need something that goes deeper, a lot deeper. I looked at a page of a bunch of absorption coefficients, and played with a porous absorber calculator to see what it would really take to be able to address these frequencies and kill the reflected signal and get my bass back. I settled on [Roxul Safe’n’Sound](https://www.homedepot.com/p/ROCKWOOL-Safe-n-Sound-3-in-x-15-1-4-in-x-47-in-Soundproofing-Stone-Wool-Insulation-1-Bag-RXSS31525/202531875) which was cheap, relatively non-hazardous, essentially fireproof, and capable of absorbing very low frequencies. In order to hold it in place at a distance from the wall, I used some Ikea Ivar shelves that I already had available. This produced a 6″ thick absorber with 6″ of air space between it and the wall.

Here’s a graph showing the effect of the absorber.


The smoothing is a different scale to make things easier to see as averages, but the main takeaway is that the green line is about 6db higher at 100hz than the purple line. That means that by adding these sound absorbing panels behind my chair the bass notes at 100hz got twice as loud. There were other benefits too. The overall response got a little more even (see 500hz), and the clarity of the sound was improved because I no longer hear the reflected sound behind me delayed by ~4ms. Any early reflections happening before 6 milliseconds will blur the apparent location of the sound source; our brain/ear averages those out. Reverberations happening between 6 and ~50ms are integrated into the perceived sound as loudness, but the first occurrence of the sound is what is used by our brain to determine location in space. This is called the precedence effect, I hope I’ve explained it right. This is why speakers and chairs really need to be at *least* three feet from any walls or major reflecting surfaces for optimal stereo sound, and why most households just don’t get to have that type of sound reproduction.

This is what it looks like on a graph; this is the “Energy time curve“or ETC graph. The graphs shown above show magnitude of sound with respect to frequency, but these show magnitude of sound with respect to time. A perfect source would look like one big sharp spike with nothing after it. A real room shows a series of peaks afterward, the reflections.


Ideally you’d want nothing sticking up within 6ms of that first peak, and nothing sticking up above the -20 mark at all, and everything steadily decaying. I’ve circled that first peak at a little after four milliseconds, that’s my reflection from the back wall. Here’s another ETC graph showing what the absorber was able to do.


That first peak is totally gone, and many of the later peaks are lower in level. I suspect that the two large peaks that are till hanging out at around 8 and 10ms are reflections from the floor and ceiling. I think that the other peaks that went away were from sounds that made a round trip from the back wall to the front and back. I can hear a difference, and the stereo image now sounds clearer, with a more realistic presentation of the performers and instruments occupying specific points in space. Bass is fuller and more even, and things like plucked double bass sound more realistic. That 47hz resonance is still there, but it’s very easy to use an equalizer to knock just that one down a little bit without robbing the rest of the bass.

So, what’s next? There’s still a big dip in response from 100-300hz, and 100 could be louder still. My next step will be to add more absorbing material into the corners of the rooms to make bass traps to absorb some of that resonant energy. This isn’t a high priority since we are making preparations for Baby Carrow to join us soon, but I may do it by the end of the summer since it won’t cost very much or take very long. In the longer term, I may purchase some software to further process the sound. I had a free trial of one package that made a very noticeable and favorable improvement, but it’s really not a priority right now and if I wait then the price might come down or a competitor could come up with something better.

Room equalization

Room equalization

This is a topic that I’m still fairly shaky on, so anybody out there reading this as a possible reference should probably take this all with a grain of salt. After building the LX521 speakers I could tell that they weren’t delivering precisely the sound that I know that they’re capable of when they’re set up correctly. As you’ve probably seen in photos, Siegfried Linkwitz’s home does not have any visible acoustic treatment, just some normal ‘stuff’ around the room. Furniture, a nice rug on the ground, nothing like walls full of acoustic foam or avant garde looking diffusers. It’s just a regular room, and most people who build his designs agree that you don’t to do anything special to get the good sound.

Before we go too much farther, I should say that there are two different things that I’ve noticed about the sound in my system that differ from what I heard when I auditioned the original. First, I hear a bit more of the room in the bass response than expected. Things boom/echo just a bit, and its’ the difference between a natural sounding reproduction of a sound that can truly fool you, and the sound of a speaker in a room. We’re not *quite* there yet, though it’s miles better than what I had before. The second part is that there’s a bit of imprecision in the imaging. In the demo I could very plainly localize sounds in space, like I could hear a xylophone player moving up and down the keys and tell the left/right position of each key. That might sound like a wild claim, but that’s what I heard and what convinced me that this thing was worth building. Having my TV between the speakers may be hurting the sound a bit, but I think there’s more that needs to change than just that.

I’m setting out to learn how to improve the sound of the speakers in my room, and I want to be able to do this well enough that when we eventually move I’ll be able to repeat the process with a reasonable certainty that I’ll be able to make the stereo sound good there too. I’m using a program called Room EQ Wizard (REW) to perform measurements. I use a Behringer ECM8000 room measurement microphone that I’ve had for years, a small powered mixer, and my computer sound card to capture the sound. Based on the reputation of the microphone and a loop-back measurement that I did with the rest of the signal chain, I think I have a fairly accurate measurement system…. if I can figure out how to use it right.


For my first measurement I put the microphone on a tripod positioned at the spot where my head would usually be, and ran a test sweep from 20hz to 400hz. I did a few more, but for the purposes of room equalization I thought that I was interested in things below 200hz.

raw measurement

The first thing that you’ll notice is that this response looks totally ragged and nasty. It’s hard to say exactly where the average is if it was ‘flat’, and there are clearly peaks and dips of 10db. That’s partly because this is a real in-room measurement, the room isn’t treated with much of anything, and it’s like a big echoing box. Concrete floor, flat ceiling, thin rug that doesn’t cover much, and not much stuff on the walls yet. Regardless, there are some good things about it. The response doesn’t drop like a rock below 30hz, and I can tell you that the woofers really made some impressive sounds in the mid 20hz range. The system can do bass, but the room still needs help. Lets see what we can do.

After collecting this data I opened the REW equalizer window, and I selected “miniDSP” as my equalizer so that the program would know what it’s allowed to do in terms of suggesting correction. I assume that some of the other selections have more or less EQ bands available. Next I had to come up with the ‘target settings’. This is what tells the program what you want the ideal system to do. I have some trouble with this one, knowing what to set. I think maybe this is where a sweep to more like 20khz would have given me more information about where the average is supposed to be. Regardless, lets give this a shot.


I picked 72db as a target level, set the target as flat with a 24 db/octave rolloff at 20hz. For a full range equalization I hear that you are supposed to set a gradual rolloff above 1 to 3 khz, but I’m mostly interested in taming room modes here, and those are all well below 1khz.

In Room EQ wizard I clicked on the “filter tasks” tab and then “match response to target”. It twiddles things around for a second, and then gives five parametric EQ settings that shifts the response to something closer to the target.

target response

Here you can see a brighter red line that is closer to the blue target, and a brownish line that’s the original response. The bright red line is the predicted response if I load the EQ settings. I’m able to save those to a text file and then use my MiniDSP software to read those, or I could enter them manually. Clicking “eq filters” let me see what those are; they looked like this:

filter suggested

Overall it’s a few sharp cuts at specific frequencies, and a broad boost at 40hz, giving the flatter response. I loaded these into the second configuration slot on my MiniDSP, and now I can flip back and forth between the unmodified and equalized versions. That has been very interesting.

Subjectively it sounds as though most of the room ‘boom’ is gone. However, I still somewhat prefer the unaltered version. The equalized version seems to take too much of the bass away. That may have to do with my target curve, or it may have to do with the settings when I clicked “match response to target”. The MiniDSP 4x10HD allows four configurations to be stored, so I could even try a few different versions and audition them each without leaving the couch.

I’m going to hopefully meet with some local audiophiles to discuss this more, and when I get a chance I’ll repeat the measurements with a few changes. Other than dialing in the available EQ, I think that my next step might be to hang up something cloth on one or more walls to absorb some of the sound that’s bouncing around. It’s a lot to research, but I’ll keep reporting back as I figure it out.


LX521 Speaker build- the Hypex multi-channel amplifier

LX521 Speaker build- the Hypex multi-channel amplifier

It’s about time I had an update! This is basically skipping to the end from the last post, building cables, but I’ve been pressed for time in getting them built so I didn’t keep blogging as I went. I have a few pictures of putting the cabinets together, but it’s boring. I didn’t design that part, and I’m not a master craftsman with fancy insight into how you’re supposed to do woodwork. Hopefully the amplifier is a little more interesting.

The LX521 is a four-way speaker with an active crossover, which means that a pair of them need a total of eight amplifier channels. In order to provide uniform sound, all eight channels need to be identical. The amplifier that the designer uses is the ATI 6012, a 73 pound monster that costs about $2800. It’s an excellent piece of gear, but I was trying to keep the budget a bit more reasonable. A pair of Outlaw Audio Model 5000 could have also worked for about $1200 total, but then I’d need to put two 50 pound amps someplace. For just a little more than that I was able to put together a 25 pound amp that keeps everything in one box, and theoretically measures a little bit better. Also, I think that the more you build for yourself the more fun it is!

The amplifier I put together is based on a Hypex UCD180HG class-D module. I couldn’t really figure out precisely what their fancy voltage regulator upgrade would do for the sound, so I skipped it. If you have a dedicated listening room with soundproofing and are *really* into this stuff, then maybe you can give them a try (or go hog wild building with the Ncore modules). Now that I’ve built it I realize that this is dumb, but I used two of the 1200 watt switching power supplies, the SMPS1200A180. There is no way in hell I need that much power. Maybe I could throw a rave some day, but for now it just means that I have a bottomless pit of reserve power that I’ll never touch. By my best estimate, as loud as I’ve turned it up I don’t think I’ve used more than a few dozen watts yet. Hey dad, that means you could run these off of a solar panel pretty easily!

Anyway, on to the case and construction! I used the Modushop Mini Dissipante as the basis for the enclosure. I did some calculations of waste heat and fin area and all of that, and I must have messed up a decimal place or something, because I definitely didn’t need all of that cooling area. Oh well, I guess the parts will just last longer since they run cool.

The finished amp looks like this in my living room:


It’s just a plain aluminum panel with an on/off switch. Minimalist, right? The next step will be to make the guts look so nice.

The amp modules have a blue aluminum T-shaped piece with two M3 holes tapped in it. I decided to put all of the holes into an interface plate, and then bolt the plate to the heat sinks. First I drilled and tapped some holes in the aluminum heat sinks, as shown here: note that I used an old T-shirt to protect the kitchen table, and the glass of beer with a lime in it was in fact a mexican style beer by the local brewery, 21st Amendment.


Next I laid out the interface plate using some calipers and an automatic center punch to give even spacing. You can see that tool in the upper left corner of this picture. This is really useful for achieving a medium level of accuracy in an apartment with hand tools. It’s not like when I had a techshop membership and would do this stuff on a mill, but it at least keeps the hole positions accurate enough that I can generally get them to line up.


There are two 1/4-20 pan heads at each end of the interface plate, and if this were going to have a really significant heat load then I would have used more of them (and a different head) to ensure solid contact along the full length of the heat sink base. It was reasonably convenient though, and I’m not producing that much waste heat, so i thought it was a corner worth cutting.

The power supply module comes with six M3 screws already installed, holding a compression piece against an L-shaped plate. You have to take these out in order to have access to tapped holes to get this face against the heat sink interface plate. You could mount another way, and the manufacturer even says you can just let this thing hang out in free air, but I know that if I keep things cool they’ll last longer (like to pass down to my kids some day).


The amp module is surprisingly small for something that can do 180 watts and drive a one ohm load. I slathered on the heat sink grease before attaching them to the interface plate.


Once they’re all mounted it’s ready to mate up with the heat sinks assembly. You might be able to see that I did some really rough and rowdy countersinks to get those M3  screws to sit flush below the surface of the interface plate. I have some slight concern that some day one of them could be pressed against a capacitor and wear through from vibration and short something out, but probably not. I’m just documenting that concern in case anybody cares, and as something to think about for next time.


I know you wanted to see this all greased up. Look at those nasty countersinks! Eww!


IMG_6465 IMG_6466

There it is! Four 180 watt modules with a 1200 watt supply, all ready for assembly.

And…. Oops. I built them the same, and that’s not going to work for wiring. I had to redo one of these.


This is a really useful tool that I wanted to take a minute to talk about. This is a ratchet offset screwdriver, and it lets you reach into tight spaces that a regular screwdriver can’t reach. A well designed product won’t require this, but sometimes the designer just can’t be bothered to think about tool access… so that’s when you break this out. The best one I know of is what they used to work on medical linear accelerators at Siemens, and it’s the Chapman. You’ll notice that compared to mine, the ratchet area is much smaller, and the divisions between ratchets are signficantly smaller. That lets you reach into smaller spaces and tighten screws when you don’t have space for much rotation.

This is how I connected the mains power to the front panel toggle switch. It’s a DPDT switch rated for 20 amps, and I made sure that it was large because I wasn’t sure what kind of inrush current the power supplies would draw. As it turns out, they seem to have a nice polite soft-start, or else their inrush is much smaller than the spec sheet indicated because they’re just getting US voltage instead of the 230V that europeans are working with.


This ended up being a little easier than I expected. The power supply sends a few leads over to the terminal blocks, and each amp takes a positive, negative, and ground input for power. The worst part was all of the crimping. I think I ended up doing about a hundred crimps. There’s a reason they make machines to do that.

In the lower right of this image you can see the twisted pairs of wires going to the speaker output jack, and the black cables soldered directly to the XLR jacks. I need to go back and get everything twisted and trimmed more appropriately before I’ll say I’m done, but for now it works.


The last thing I did was put channel numbers on each of the amps so that I can keep things straight, and I checked a test signal on each to make sure it wasn’t putting out a measurable DC voltage and a full range music signal sounded right with one of my old speakers. During this step it was really promising to hear music coming out of my old speaker that (I think!) sounded better than the eight year old cheapie receiver that had been driving them.

One thing you’ll notice here is that I used blue paper painter’s tape as a temporary measure to keep anything from shorting out. The maximum voltage in here is 48 volts relative to ground, and 96 volts from the positive to negative terminals. Everything is firmly connected to screw terminals inside of insulated blocks, but a little bit of metal peeks out at the edges, and so I didn’t want to take any chances on something shorting out. The end goal is to have the terminal blocks bolted to the bottom panel of the enclosure in a neat row so that the wires can be routed in a clean and proper way.

The 120V mains line goes to the switch on the front panel using a tightly twisted pair of 14 gauge wires in some ring terminals. I used fork terminals for the rest, but for 120VAC I used insulated ring terminals so the screw would have to fall completely out before the crimped part could fall and contact something else. The rear panel has a three-prong jack, and there’s an earth ground that’s similarly connected to the chassis (you can make that out in the photo above).


The back of the amp looks fairly neat thanks to Front Panel Express making the panel for me. The large cutouts in thin aluminum and added threads saved me a *ton* of work, and I know they made it look nicer than I could have. I’ll add more labels when I’m done, but for now I know how they’re laid out well enough that a prompt of “1” and “2” is enough that I won’t plug something in wrong. If I did do that, it would blow up my tweeters.


Ok, so that’s all I’ve got for now. I’ll leave you with a cell phone picture of the speakers and amp in the living room. They sound pretty good. I hope you’ve enjoyed reading!


LX521 build- speaker wires

LX521 build- speaker wires

This is is the very first post in my LX521 build, not counting a couple of background posts saying what I’m building and why I’m building it. Tonight I got started on making my speaker wires. The LX521 is a four-way speaker with an active crossover, meaning that for a stereo pair I need eight amplifier channels and I need to plug in a total of sixteen wires in the correct fashion or I risk blowing up some drivers. If I swap the tweeter channel with any other channel it will probably destroy the tweeters right away. If I switch any of the others it might not break them right away, but it will sound very wrong. If I flip the polarity of anything, it will sound wrong but I’ll probably have a hard time putting my finger on it at first.

One way to tackle this would be to get a bunch of the typical binding posts that you see on any speaker, and just label everything carefully. That’s a little tedious, it takes up some space, and it won’t look very good. Another way to do it is to get a speaker cable with eight conductors in it and use a single plug with eight terminals. I got a set of 8-pole Neutrik Speakon connectors, and once I get those set up correctly I won’t have to worry about it again.


The connector has eight bars around the inside and outside of the barrel that are locked into place when the connector is inserted into its receptacle and rotated about a quarter turn. They’re rated for 30 amps per contact and 250 volts, so one of these plugs could pass over 25kW of musical power if it had to. These are commonly used for the kinds of speakers used at concerts. They’re overkill, but they were basically the same price as using four times as many of the regular connectors.

It was a little tricky to figure out how to assemble them at first, but it’s not so bad. They have eight screw terminals to accept the wires, which is nice because I don’t have to crimp or solder anything.


I wasn’t able to find any eight-conductor cables sold in small quantity, so I’m just going to double up on some four conductor wire from monoprice. I don’t think that it’s terribly important for the wire pairs to be individually twisted, but I do plan on running the tweeter and sub channels through two different lengths of cable just to be safe. I’m putting the two lengths of cable through some Techflex to keep them tidy.


I consider all of this to be a safe level of overkill, even though it’s by no means the pinnacle of what could have been done. It’s a twelve foot run of 14AWG stranded wire, so that should add about 0.06 ohms of resistance in series with each driver. The woofers will be in parallel and will present a two ohm load, so this is about 2.5% of the speaker impedance. Because this system uses an active crossover the increased resistance won’t have an effect on the crossover frequency or slope, but it will attenuate the woofers by about 0.2 dB. The mids and tweeters are 4 and 8 ohms so the impedance will affect them less. When I’m done I may measure the inductance of the cables to verify that they’re not causing any rolloff in the tweeters, but that’s it. A lot of speaker cable vendors go on about all kinds of magnetic/metallurgical voodoo, but I work with a lot of electrical engineers with PhDs who spend all day designing and building sensitive high performance analog circuits, and I have never once heard them talk about the kinds of things that audiophile speaker cable companies base their products on. As far as the EEs are concerned, a wire has resistance, capacitance, and inductance. If anybody out there wants a really honest measurement and has some expensive cables, get in touch and maybe I can bring them to someone who knows how to use the fancy equipment so that we can compare.

I think that’s probably enough posting about speaker cables, you’ll see more of them when it’s all installed. If you want to read a real EE’s opinion, go back to Linkwitz.

Just a few details on the build… It is a little hard to see the markings on the black plastic  that say circuit and polarity, so I marked up an image.


For my build I’ll do:

1 – Tweeter (green circuit, cable 1, red positive, black negative)
2 – Upper mid (yellow circuit, cable 1, green positive, white negative)
3 – Lower mid (blue circuit, cable 2, red positive, black negative)
4 – Woofers (red circuit, cable 2, green positive, white negative)

Lets hope I don’t cross any wires!

Speaker Projects – Part 2, aiming higher

Speaker Projects – Part 2, aiming  higher

What am I building? It’s the LX521. Here’s a picture of me feeling starstruck.


Part 2a- The LX521, compared to what else?

Ok, so they cost about $3k to build on a tight budget… what are they competing against? The LX521 speaker was created by Siegfried Linkwitz, and he’s the original designer of the Audio Artistry Beethoven. These went for about $25k brand new in the late 90s, and the LX521 was designed to surpass them. The Beethovens might play the tiniest bit louder than the LX521, but in every other way the designer considers the new design to be superior.

Another speaker that you could call a direct competitor would be the Jamo R909. These sold for about about $19k new, circa 2009, and as far as I have been able to sleuth out they use a pair of Peerless SLS 15″ subwoofers, and so the math indicates that with their 8mm of excursion and narrow baffle they should have nearly identical low-bass capability as the LX521, reaching nearly 100db at 30hz for a pair playing together.

What these speakers have in common is that they’re open baffle dipoles- they don’t have a box, just a board that separates the front of the woofer from the back. This means that when the speakers move toward you and create a sound wave, they are also producing a sound wave of opposite polarity moving away from you. The positive and negative waves cancel out when they meet each other at right angles to the direction of motion, so the radiation pattern is like a figure 8. This reduces how much of the sound bounces around the room, and it prevents the woofers from setting up resonances in the room. It’s hard to describe what this sounds like, but what I  heard was very neutral and clean. Played side by side with regular monopole speakers, it’s pretty obvious how much you hear the room’s contribution.

An entirely different kind of speaker that operates as a dipole is a range of Martin Logan ESL speakers. Most of their speakers include standard box woofers for the lower frequencies because it’s hard to make a big membrane behave properly when it’s moving very far from its centered position. The CLX ART series does this with a special double membrane, and runs you about $25k before you buy the very-beefy amplifiers that are required to run the them.

Part 2b- sound byproducts

All of these speakers deal with the sound radiating from the rear of the speaker by allowing it out into the room where it can bounce around, and it reaches you after some delay and with some attenuation. I’m not going to give a full explanation of why that’s OK, but Linkwitz gives some useful explanations on his pages describing speaker/room interaction, and has many more pages on room acoustics and dipole bass. What’s important for my blog readers to understand is that this is just one way of dealing with the half of the sound that must (by definition) be produced by a membrane or cone oscillating in the air.

Another reasonably good way of dealing with the rear wave is to completely absorb it. This doesn’t help reverberation or resonances caused by the room (you can get rid of many of those with extensive room treatments like foam wedges) but it does clean up the midrange. A typical bookshelf speaker might have internal dimensions ranging from 8″ to 18″, and has three sets of parallel walls. The parallel walls allow standing waves to resonate inside the box at frequencies corresponding to even fractions of a wavelength. Full wavelength, half wavelength, and quarter wavelength for a typical box would span from about 180 hz up to 1600 hz, and that’s the range of frequencies where the air inside the box will show a resonance. There are a couple of problems with that.

First, that frequency range is notoriously hard to absorb. Take a look at the ratings for some sound studio acoustic foam; it has a coefficient of absorption of 0.96 at 1000 hz for a 2″ thick piece, and it has to be twice as thick to have that performance at 500 hz. That four inch thick piece only has a noise reduction coefficient of 0.5 by 250 hz. As the frequency goes down you need a thicker layer of foam or fiber to affect the sound because the wavelength is longer and it interacts with the fibers less.

Second, it happens to be right in the middle of the frequency range of the human voice. Our ears are very well tuned to this range and we do a good job of noticing sounds here. We also tend to listen to a lot of music with voices (and I watch Netflix with dialog), so this frequency range gets a lot of exercise.You can see that if we line the whole inside of a speaker cabinet with 4″ thick foam it won’t even absorb all of the sound in a key frequency range where the box has resonances, which is also a frequency range where the human voice lives.

Part 2c- Transmission lines

Another class of speaker deals with the rear-wave in a different way altogether. A transmission line speaker (mh-audio, t-linespeakers.org) guides the rear-wave through a long tube that is near the quarter wavelength of the lowest note that the speaker can produce. This, combined with damping material, allows the majority of the sound to be either attenuated or redirected with a delay. If the sound is allowed to escape from the end of the tube, typically only the lowest frequencies would make it through the bending turning tube to get there. It’s not a way to magically get more bass, and it’s not clear to me that the bass you get is automatically better. What it does do is create a cabinet that is mechanically rigid due to the short unsupported wall length, and a column of air that doesn’t reflect resonating frequencies back out through the cone. I encourage you to read the sites that I’ve linked, and quarter-wave.com, since I’m sure that I’ve butchered the explanation of how they work.

Regardless, one of the more famous transmission line speakers that I know of, and perhaps one of the best regarded, is the Bowers and Wilkins Nautilus. A pair of these will run you about $50k. They’re art, I think they’re gorgeous, the materials are top notch, and the design principles are quite sound. They have some good explanations on their site, and some cutaway views. I also found a brochure with a lot of cool information on the Nautilus. Although this top-tier speaker is a transmission line, I don’t think that transmission lines are not that common as commercial products. They’re necessarily larger than bass reflex or sealed  boxes, and they use more materials so they cost more and weigh more for shipping. They’re popular for DIY, but the main place you’ve probably heard them is in the (not so good) Bose wave radio.

Part 2d – The sound in the room

The last part of all this that I want to talk about is what happens once the sound is in the room. If you’ve been following the links I’ve been including in this post, and especially if you are following them one or two clicks deep, you’ll have noticed that what happens after the sound gets out of the speaker has a big influence on your perception of it. You obviously want to have a flat frequency response at the listening position, but the speaker doesn’t just send all of the sound straight to you. It also important that the sound bouncing around the room before it gets to you should have a similar frequency response. If it doesn’t, it sounds less natural because your ears expect a sound around you to radiate the same way into the room in all directions. The way the frequency response changes with direction is called the polar response. It seems that we want both the frequency response and polar response to be well behaved.

Comparing the speakers that I’ve linked above, some to better than other at this. The Beethoven speakers probably had some problems because they ran an 8″ woofer up to 2khz, and the Jamo R909 might have some slight issue with running the 15″ woofer up to 250 hz. The Martin Logans have a different radiation pattern because they have such a large radiating surface. I’ve heard a lower end set of ESLs before, and it was a little bit like wearing headphones. There was a small sweet spot, and not much interaction with the room.

The LX521 and Nautilus both split the sound up into four driver groups that are acoustically small and fairly close together. I haven’t seen a polar response of either of them, but I’m guessing that it’s pretty good. Another way of controlling this is with more speakers and DSP to send different sounds in different directions. The Beolab 90 (about $80k) is one such speaker. As processing and amplification becomes cheaper, I think that we’ll see more like this.

What DSP can’t do a very good job of, and what maintaining a good polar response won’t help with, is resonances within the room. Similar to how a speaker box will have standing wave resonances in the midrange, a listening room will have standing waves in the bass. There are calculators available to show what frequencies will be a problem for a given room size and shape, and there are more sophisticated simulation tools to take that to the next level. What’s cool about dipole bass is that it excites the room modes a lot less. You hear more of the bass and less of the room, it’s less boomy, more clear and even, and you don’t need to bend over backwards to get good bass at your listening position. That’s a large part of what made me zero in on the LX521 as the system to build. I believe it’s going to hang with systems that cost 10x as much that I’ll probably never be able to afford, and couldn’t justify buying if I could.


Speaker Projects – Part 1

Speaker Projects – Part 1

I’ve been interested in speakers and sound for a very long time, and if I had to pick some point in time that started this it might have been in the summer of 2000 when I was working for Radio Shack. I only worked there for that summer before college started, but I had been there long enough to think it was weird when a guy came in and walked straight to the register to ask if we had a certain part in stock. He knew the number without having to check notes, it was the 40-1197. I can remember that number still, he wanted to know if we had any in stock since they were discontinued. I checked for him and had a good look to see if one was lying around, and when I didn’t find any he let me in on why he was looking for it. Apparently this driver could be used in a DIY design that rivaled commercial designs costing many time the cost of materials.

We didn’t really have Google yet in 2000. It had been founded, but I don’t think I knew about it. I was probably still using Infoseek or Ask Jeeves. Regardless, I found some websites and forums dedicated to DIY sound projects, and learned about the Pioneer B20FU20-51FW  and I saw that people were discussing all sorts of designs that used this driver. People were painting thin coats of glue onto parts of the cone to change the frequency response and tune resonances, people were putting modeling clay onto magnet and basket structures, and they were putting this driver into all sorts of boxes and tubes and… *not* boxes, just putting them onto flat panels called ‘open baffles’. More on that soon.

At the time it seemed that most people agreed that the most Audiophile thing to do was to minimize the number of steps between the original sound being recorded and it hitting your ears. Traditional speakers have a tweeter and multiple woofers with electronic components called a crossover to split up the sound, and people on websites like Audio Asylum and the DIYAudio full range driver forum kept saying that the best crossover is no crossover. It made sense at the time, so I ran with it. In 2001 I built some Tapered Quarter Wave Tube enclosures out of 1×10 pine boards and some smaller drivers similar to the Pioneers. They had some interesting sound. They had no bass at all, but there was a kind of clarity in the sound they did make and I was hooked on the idea of it. This is the only existing picture that I have of them.


There they are, CD for scale. It wasn’t quite a success, but I was hooked. In 2003/2004 I tried again, and built some “mass loaded transmission line” speakers using Eminence Beta 8 pro woofers and some Morel dome tweeters. This is what they ended up looking like.


They had some really serious problems. The bass was too weak without major equalization, and the eight inch woofers were relied on to run up to about 5khz- they were doing almost the entire job of reproducing the sound. At the time I designed them I thought that was a good thing- the crossover frequency was well outside of the vocal range, and so it was supposed to be really natural sounding. It turns out that they were beaming like crazy. As you move around the room the character of the sound changes drastically, and even as you sit in place and move your head. The other problem with this is that there is a sharp discontinuity in the polar response of the speakers. This means that the frequency response of the sound reaching your ears traveling directly from the speakers had a drastically different frequency response than the sound that bounced around the room and reached your ears indirectly. This doesn’t sound very natural.

In 2015 I managed to fix these speakers (Most of the way) during a visit to my parents house, and now these speakers keep their barn cheerfully noisy. I used a MiniDSP 2×4 to replace the crossover, and a four channel amplifier to allow an active crossover. This let me move all of the bass equalization into the DSP, and fiddle around with the crossover frequency. By bringing the crossover down to about 2khz where the woofer is better behaved, and by using a sharper crossover slope, the sound was improved a lot. They still have some other problems, but I think those can be addressed with some extra panel bracing and acoustic stuffing.

When I moved from spacious upstate New York to the crowded SF Bay Area, I had to leave those beasts behind and start over. The first speakers that I tried building were some cardboard box speakers, just an experiment with junk parts.


Would you expect those to sound good? I didn’t. The next design I tried out was a little bit more unusual. I tried using heavy duty foamcore as the enclosure walls. I used a little internal bracing and my theory was that if the walls were stiff enough it wouldn’t matter much that they weren’t so strong and weren’t very heavy. Here’s what those looked like.


That’s right, I even used a little PVC pipe as a port. The drivers were the 3″ full range Aura NS3, and the boxes shook all over the place when bass played. I recycled these into surround speakers for the home theater, and although they were ugly as sin they actually kind of worked in that capacity.


They didn’t have any tweeters or anything to disperse the high frequency sound out at right angles so that it didn’t  have to bounce off the ceiling to get to you, but it was a bit of a step up from the foam boxes. They were at least partially inspired by the Linkwitz Pluto, though I may have had some other influences at the time because I was coming out of a long period of thinking that transmission line speakers are the best.

I can’t remember if it was before or after, probably slightly before the pipes, I built a pair of Modula MT speakers using aluminum cone woofers and a sophisticated passive crossover. The original HTguide.com forum post requires a login to view, but I recommend signing up to see it. The speaker was designed by an extremely experienced electrical engineer, and it allows an unusually steep slope in the crossover to allow a lower crossover point to keep the aluminum woofers happy. Here’s my miserable picture of what that looked like.


I purchased the cabinets and almost all of the parts from Parts Express, and I just had to cut out and counterbore the two large holes for the drivers. I also had to cut out the holes for the ports in the back, but you can’t see how ugly those are. These are the speakers that I still listen to today, almost nine years later, and now that I  have them set up on stands with some good distance from the wall I can say that they sound pretty OK. All in all I spend about $300 building them, and I compared them to some commercial products at twice the price that I didn’t think were clearly better. They scratched an itch, but I knew that I could do better. There are plenty of times I’m watching Netflix and I have to strain to hear what the male actors are saying, although some of that has to do with room interaction.

In the summer of 2015 I finally had a chance to hear the Linkwitz LX521 speakers at the home of Siegfried Linkwitz, and I decided that those would probably be the last set of speakers that I’d ever want to build. They’re expensive, expensive enough that I feel reservations about building them when that money could go toward something so much more practical and humanitarian, but if I divide that cost over the next ten or twenty years then it’s not so bad. That’s what I’m telling myself.