I like the 18-Sound driver a lot, you probably can tell by now :p

I like that there are so many direct drop in's which can be used on this horn.

Hey, here's an off-topic thing that caught my interest. I hope maybe you'll have some input.

I'm wondering where the onset of non-linear distortion happens from compression/rarefaction asymmetry. Do you happen to know of any references to reliable research data on the subject?

I'm guessing it happens when the compression cycle of sinusoidal pressure reaches 1.5 to 2 atmospheres, 'cause that would put the rarefaction cycle between 0.5 and 0.0. This assumes (1.0) atmospheric pressure is the baseline, 'cause I'm wondering where the start of trapped-air distortion and other air pressure related acoustic nonlinearity begins. Since you can't go below 0.0, but you can go above 2.0, somewhere in the 1.5 - 2.0 range must start the onset of asymmetry. I just don't know where it is.

Check this out. Let me know if you know of any good data on the subject.

lab12 it is!

The non-linearity you speak of (called throat distortion in horns) is caused by the fact that the speed of sound is temperature dependant (with speed increasing with temp).
Imagine a sound wave, the "pressure" half of the wave is actually warmer than the rarified side. AS a result, the "hot" side travels slightly faster than the cool side and so after enough distance (and sufficiently high SPL) the nice sine wave progresses into a saw tooth wave. If I recall, at about 1100 degrees C, the speed has doubled.
This was a real problem in the acoustic Levitators I used to work on as once one had a saw tooth (or sufficiently distorted sine wave) then one also produced acoustic pumping (an effect like a fan). These operated in the range of 155 - 175 dB SPL @ 21Khz generally.

This effect is also seen in a sonic boom, here, on the leading edges, the air is compressed over 1atm at supersonic flight. Here, because of the same temperature/speed relationship, what finally reaches the ground as radiated sound is also a saw tooth (on the front edge), over all, the waveshape of a sonic boom is like a capitol letter N.

Throat distortion is pretty much a non-issue in bass horns though, cone and coil drivers generally cannot produce enough acoustic pressure to cause this.
While dozens of bass horns I have measured had sometimes even gross distortion, the driver is the primary source.
This distortion is the sum of the VC motor and mechanical system's non linearity.
The "bad" effects of the latter can be minimized by choosing a driver with a low Fs which then requires a small Vb to end with the right compliance.
Depending on the sealed volume more means that a more linear spring is dominating over the drivers relatively non-linear suspension (by comparison) spring.
Generally, stay away from High Fs driver for bass horns if low distortion is a goal.

The BT-7's for example of relitively high throat pressure, are the same horn (essentially) as the LAB sub but at full rated power have typically less than 1 ½ % distortion. This on a system having a higher compression ratio than the Lab sub (as the SDL has 2, 15 inch radiators and a much stronger motor). If air non-linearity were a problem for bass horns, it would be here.

Even with the LAB12 driver, the LAB sub has measured to have less distortion than any other box tested, even the other low distortion horn, the Bassmaxx.
I found the comment of one of your posters to be humorous in that he thought the lab sounded un-musical and preferred the sound of his vented boxes.
All of the vented boxes tested so far were VASTLY higher in distortion and less musical when listened to side by side though according to those doing the testing.
Your suggestion of a shorted turn being added to the LAB 12 is timely too, this was not possible at Eminance back when I gave Jerry my original request for the lab 12 driver.

On the other hand, reducing the distortion a little bit more is not going to make any horn system made with it sound MORE like a vented box or what people are used to given how badly most systems measure at "normal" levels..
We use a version of the LAB 12 in several of our products and I should have a sample pair of them (with the Shorted turn) in a week or so.
While I never built a LAB sub myself (I was confident enough in the computer program) I do have a similar horn (acoustically) I can compare the differences in.
I'll let you know what the difference is if you want.

I don't know if McBeans program can predict the Throat pressure but for a bass horn one is safe if you stay under 150-160dB MAX pressure. To put that in atmospheric terms, 132 dB is about 2 pounds per sq foot and 161 dB is about .1 psi if I recall.
Cheers,

Tom Danley

About motor distortion, I've always been impressed with the performance of flux stabilized motors. They provide about -15dB improvement in 2HD, and it's not just a figure - I find it immediately obvious. You can walk into a room with a sound system and immediately hear whether the woofers are flux stabilized or not. It just sounds cleaner.

Back when the LAB12 was first being made, I wondered why it wasn't built with flux stabilization. I think you and I discussed this back then. I got the impression you weren't interested in a better magnetic structure, but maybe I misunderstood you. In any case, that's the point I wanted to make. The motor is responsible for most of the 2HD in the entire system, so to me, it makes sense to make flux as symmetrical as possible if high-output and high-quality are the goals. Since the improvement from flux stabilization is 10dB to 15dB reduction in 2HD, it amounts to a significant improvement around the same magnitude as horn loading. That's like 10x to 35x and sometimes more - Certainly nothing to sneeze at.

So I questioned the decision not to use shorting rings in the original LAB12. The Magnum was already in the works, so I wondered why the LAB12 wasn't made with flux stabilization. And here lately, I just thought I'd ask Chris at Eminence about it. In hindsight, I don't know why it hadn't been done before now. But I guess there were cost concerns.

Jerry tells me that the flux stabilized woofer is being readied now for testing. If you are expecting such a woofer in a week, then I can only assume that the woofers we've been discussing here are going to be sent to you. Perhaps you will be able to take advantage of them in your BDEAP device as well then.

• Flux stabilized LAB12 thread

EV does, EV discovered this on there MTL-4 high cabinets, the 4-10's that were driving the mid-bass horn were developing to much air-compression causing non-linearity in the throat section of the horn and causeing the harmonic distortion to rise dramatically when driven near full power.

Bill W.

This is all in response to the flux-stabilized LAB12 project I asked Eminence to work on. Jerry McNutt said that he and John Sheerin modeled the magnetic structure and found that there were two solutions for putting a flux stabilization ring on the motor. One involves making the vent hole smaller, which might increase power compression and introduce non-linear distortion from the trapped air. The other option doesn't require a reduction in cooling vent size, but it will take a couple months more to develop.

If the vent is made smaller, the compression ratio between the cap and the vent will change. The concern is whether or not this feature will adversely affect power handling, thermal compression or introduce distortion. Tom Danley is going to test the flux stabilized LAB12 next week, so we can know the distortion of this version of the woofer. If it works out, perhaps Eminence will make a better low-distortion version of the LAB12 available fairly quickly.

If not, the second option is to install shorting rings in the plates, outside the voice coil. It has the advantage of allowing the vent to be made larger. This option will take 6-8 weeks though, so production would have to wait a little longer.

I think this is more of a problem pushing low frequencies, that compression ratios come more into play. On Tom Danley's BT-7 I think they have like a 3:1 ratio and when they were in development they had problems tearing cones apart at first, and I've heard other people talk about it when they start going over 3:1 ratios. On Ev's MTL's they crossover at 160Hz but I'm not sure what the compression ratio is on these, but they weren't ripping cones, just causing distortion to rise. After they discovered this they came out with mod kits for them to address the issue which I think was a 1/4" spacer between the 10's and horn's throat to reduce the distortion at the cost of 2-3db reduction in SPL's.

I've been following the threads on the new lab 12, but at first I thought you were talking about non-linearity in horns rather than the speakers vent hole.

Bill W.

Are you gonna be at Jim Denton's house on Saturday?