Speaker Projects – Part 2, aiming higher

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

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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.

 

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