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.


A new joystick

A new joystick

Here’s a quick update of a project I was able to do pretty quickly- one of the BORP players was having trouble reaching her joystick properly to continue playing at a high level, but there weren’t any off-the-shelf products that would allow that to happen, and most of the customizable systems that are available for wheelchairs cost hundreds of dollars, and would probably require a little custom fabrication to even work with the model of chair that she uses.

Based on previous experience, I knew that it might be hard to fine-tune exactly where to mount the thing, and it’s hard to predict if where I put it now is going to be where she’ll need it later. I thought that a locking ball-pivot would be a good way to allow the thing to move around and pivot to get just the right angle, and by using two of these joints I’d be able to have some translation front-to-back and side-to-side.

I recently got a real 3D modeling program on my home PC, and so I went to work finding weldable ball pieces on McMaster Carr and whipped up a concept. Here’s what I had in mind:


new joystick mount

I was able to cheaply harvest a ball on a post out of a quick-release ball joint linkage (it could have been cheaper and better if I was good on a lathe) and welded it up at Techshop. Here’s what that looked like:

arm pivot-002

You can see from the bending 1/4″ thick steel that the trial piece was getting clamped hard, but each ball joint was still pivoting about the axis normal to the clamping face. I tried to use some aluminum to increase friction on the ball on the right, but that didn’t work out very well. What I eventually did was to take a countersink bit and bevel the holes to provide a wider grip and more surface area for the friction. That got everything locked down well enough that it felt like it wasn’t going anywhere.

Today at practice I got to install it, and the new mount seemed to work out pretty well.

joystick mounted

I wrapped some foam and black tape around it, and it seems like it’s good to go.

Sound check, one two!

Sound check, one two!

They have been a long time coming, but I am finally making some headway. I have the laser cut acrylic end caps mated to the cardboard tubes, the DSP programmed, and the woofers in place. Here’s what it looks like so far:


She. I add the lithium battery pack to this, the entire thing weighs twelve pounds and is surprisingly loud, with a fairly rich bass sound. I’m still trying to work out some cabinet resonance issues and fine tuning of the DSP crossover, but it feels like a milestone worth posting. This thing can piss off the neighbors.

Goofy photo technique

Goofy photo technique

I recently read about how out facial features (like all things) take on a distorted perspective as we get closer or farther away. Getting closer will make your nose bigger, and anything farther from the lens smaller. It isn’t just your nose- it’s whatever is closest to the lens that looks bigger.(I think this technique may be used in certain magazines that promote unrealistic body images) Things might look interesting when a wide angle lens is used to allow you to get closer, but they don’t look normal.

I don’t usually use image editing programs like photoshop, but I had the idea, “what if I pasted a close-up head onto a more normally proportioned body?”

Here’s the result:


Another soccer guard (3)

Another soccer guard (3)

Here’s a picture of my setup for how I’m able to (relatively) quickly bang out some bars for the soccer guard.


When I set up the mill like this and use the vertical tube to orient and locate the horizontal tube, I just need to drive the X axis to the predetermined spot while squirting a little coolant, and all of the tubes I make are the same within a few hundredths of an inch. It’s not an accurate way to do it from setup to setup, but it’s good enough for what I need, and it gets the gaps tight enough for easy TIG welding.

It could be better, but with the tools I have at Techshop and the size of batches I do, this is working well enough for me right now.

Another soccer guard

Another soccer guard

I’ve been working on a design for the team that will be somewhat light weight, and not as expensive, time consuming, or fragile as the one I made for Kendra.
Aircraft Spruce sells some 1″ diameter tube for $2.80 per foot, compared to $3.50 per foot for the smaller diameter tubing I used before, and I get to use less of it. I also don’t use any of the square tubing that costs $10.75 per foot.

More important than the cost of the tubes, the old design had a lot of semi-precise milling required, and had almost 50 welded joints. This design has simpler tube preparation and less than half the welds. Access to the joints is better, so it’s a little easier to get the torch into all the required angles. The tubes are much stronger than the 1/2″ tubes, so I don’t expect this guard to get dented in from any normal hit.

The guard weighs about half a pound to a pound more than the one I designed for Kendra, and a lot less than the guard I posted on September 24th. It remains to be seen if it gives a good hit on the ball, but I’m optimistic.


Building Power Soccer Equipment

This month I’ve been working on power soccer equipment for Borp. Not to brag or anything, but I think it’s OK to toot your own horn every now and then, and now seems like a good time to do it. Here’s a list of what I’ve been up to:

  • Installed extra-large motors in one powerchair.
  • Created a faster anti-wheelie method and installed it on two chairs.
  • Re-balanced two chairs (plus the chair that received new motors)
  • Created a new method of attaching a guard to a midwheel drive chair and installed it on one chair, letting somebody play who hasn’t been able to play for months since his last chair broke.
  • Welded a new foot guard.
  • Fabricated wheel spacers for two chairs, installed them on one chair so far.
  • Reprogrammed and reconfigured a donated chair to work for somebody who’s daily chair isn’t cutting it for soccer.

All in all, I have made about 80 welds, machined 13 aluminum parts, six steel parts, and spliced six wires. This allowed one person to play who could not play before, and hopefully improved the competitiveness of five other players. It has been a good month, and I’m not done. Hopefully I’m just getting started.

I have big plans for new things to build, and ways to get more people playing at higher levels of competition. Let me know if you want to help!

Day 27 (2)

Day 27 (2)

I’m sitting in Sausalito having a sandwich, drinking a coke, and looking at the bay bridge. I made it!


It was such a culture shock when I made it into Marin/Saualito. I was suddenly surrounded by traffic, and had a choice of bike paths, bike lanes, and signs, and I needed to know which way to ride to make it to the bridge. I was at an intersection and tried to stop about ten cyclists to ask this simple thing, but they were all headphones and heart rate monitors, and I couldn’t get a glint of recognition through their silvered wraparound sunglasses. It’s totally different from the way people were just 30 miles ago.