SVS vs. VPMS (1 Viewer)

[Brian Bunge]

Eric,
A simple test disk with a 20Hz sine wave will suffice. No need for a super bass disk or anything like that. That's why I said "test disk" in my original post. You could even use the free tone generator software from NCH if you don't have a disk.
I think all involved would be much more interested in knowing the SPL AND THD at 20Hz though.
Brian

 

[Dennis XYZ]

I think Cheney has Clio. Get him to run a 10-300 sweep. It will show SPL and THD. Of course it wouldn't hurt to take a test disk and an SPL meter just in case it's "inconvenient" for him to set up Clio.

 

[Dennis XYZ]

This is just a for what it's worth....
I'm no PR designer but it only took me about 5 minutes to model a sub with 2 15" drivers and a single 15" PR that hits about 115dB at 20Hz and doesn't defy the laws of physics as we know them.

 

[Dustin B]

What values for the PR did you use to generate that graph and what size of box?

 

[Dennis XYZ]

Hmmm, I didn't save the file but maybe I can remember the steps. As I recall, I started with one of the stock Adire files for a Tempest PR SBB4. Then I:

Changed the drivers to 2 and the PR's to 1.
Did a "quick box" optimization.
Changed the box size to 400 liters.
Changed the PR mass to 400 grams.
Turned off the crossover.
Doubled the power so I was getting about 115dB.
I didn't change any of Dan's room settings.

Obviously this has little relationship to the VMPS sub. He's using a smaller box and completely different drivers. Rather it was just a quick exercise in going against the conventional wisdom that you need two PR's for every driver.

 

[Brian Bunge]

Conventional wisdom does not say you have to have 2 PR's per driver, but rather that the PR's should, as a minimum requirement, have twice the volume displacement as the driver, with 4x the displacement being even better.

In this case, the Tempest has around 2.55L displacement and the PR's that Adire sells have 5.1L displacement. So by using 2 drivers and 1 PR you end up with a 1:1 ratio.

Brian

 

[Dennis XYZ]

Conventional wisdom does not say you have to have 2 PR's per driver, but rather that the PR's should, as a minimum requirement, have twice the volume displacement as the driver, with 4x the displacement being even better.
In this case, the Tempest has around 2.55L displacement and the PR's that Adire sells have 5.1L displacement. So by using 2 drivers and 1 PR you end up with a 1:1 ratio.

Well, I've never paid much attention to conventional wisdom. Looks like my PR is somewhere between 1/4 and 1/2 as big as it "should" be.
I think the interesting thing with this design I pulled out of thin air is it meets our design requirement of 115db/1m/20Hz/in-room while only using 1/3 of the drivers' Xmax and 1/2 the PR's Xmax at 20Hz. It should be very low distortion at 20Hz although the distortion would be much higher at 15Hz where the driver excursion goes up to Xmax. Still, you should be able to drive it with that much power, without a highpass, and not come close to breaking anything. It looks like good performance above 17 or 18 Hz, especially with a highpass. But then, like I said, I'm no PR designer. Sealed boxes are my thing.

 

[Michael R Price]

Dennis, I can re-create your results. However, the LspCAD room gain simulation hasn't been very accurate in my experience. If I have 1 PR-15 tuned to 20Hz and I increase power indefinitely, the output does not go over 105db anechoic (due to compression), which realistically is about 110-112db in room. That's the *absolute limit* of the system and there's no way it can be producing 2.5% THD at that level.

Also, we don't know what the PR is tuned to. If the PR really has Mms of 120 grams, then its tuning would be in the 30Hz range. This would make any claims of 20Hz performance even more invalid.

 

[Dennis XYZ]

Hi Michael,
You've got me there. I can't comment on the accuracy of LspCad and I don't know where that 120 gram figure came from. It's not in the specs on the website.
What has troubled me is the ASSUMPTION by some people, without ever having seen or heard one of Cheney's subs, that it can't possibly work because the PR is too small. I think my little model proves that the rule of thumb about the PR needing twice the Vd of the drivers is just that, a rule of thumb; it's not set in stone. Beyond that, there is little point in trying to model Cheney's subs without knowing the T/S parameters of his drivers. We'll just have to wait until somebody measures one. Then we'll know if he is telling the truth or lying out his ***.

 

[Jack Gilvey]

What has troubled me is the ASSUMPTION by some people, without ever having seen or heard one of Cheney's subs, that it can't possibly work because the PR is too small.

All I want to know is what the sub can actually do when tested by the same methodology as others have been tested by, such as Nousaine's. Real-world subs, like that dual 18" Stryke, can't get those numbers you came up with...so I'm curious as to that magical single 15" PR. I've been asking what the PR can do...why is this so difficult to understand? Proof of claims are the burden of the claimant. I think that I'm the only one not ASSUMING anything.
Yeah, sims are fun when they assume ideal room gain. I'll take an anechoic result, thanks...less room for "optimistic" predictions.

 

[Eric Wood]

Jack, my first statement that should have jumped out at you was getting hold of an owner in New Jersey, duh. Do your own investigation before flinging insults and doubts.

 

[Dennis XYZ]

I think that I'm the only one not ASSUMING anything.

Of course you are. You are assuming Brian Cheney is a liar. Your sarcastic postings reek of that assumption. With that I think I'll bail on this thread lest I set some kind of record for the shortest stay on a forum before getting the boot.

 

[Jack Gilvey]

Of course you are. You are assuming Brian Cheney is a liar.

No, I want to know the conditions of the claim (are your assumptions correct?), and the properties of this unconventional PR. You are the only one in this thread to use the terms "liar", "con-man", and "fraudulent", not I. I don't know anything about they guy.
Like I said, the hardware forum is great fun. Anyway, enjoy. Don't think I'll get an answer here.

 

[Eric Wood]

Professionals rate them decent
If you cannot trust your ears, as I look down to my hands that only holds a remote, which are the only things that I can subjectively tell me what are the lowest and precise tones I have heard. Remember that our hearing recall is the 2nd strongest of all. Far above sight and experience. Very well documented, look it up if you need #'s.

 

[Dennis XYZ]

Peace, Jack.

 

[Jack Gilvey]

Eric, you've missed my point entirely, which might have been a bit technical. Thanks for the article, I don't know what I was supposed to find there. I trust my ears, but the claim I asked about was a specific decibel level at a specific frequency, which can only be backed up my actual testing.
If the claim was "Sounds great! Shakes the house!", then that's more easily verified. Two different things. See?

 

[Eric Wood]

Sorry Jack, I was not trying to give you any measurements, just that it would appear by impartial judges that Mr. Cheney manufactures award winning speakers, that presumably would equal his proclamations.
Have a great day!
It was the best I could manage (at this time):b

 

[Ryan Schnacke]

Seems like the whole slot loading and passive radiator thing confuses the matter. Is it possible that the 15" and 12" active drivers are in seperate chambers? Then the 12" driver could be loaded exclusively by the PR and the 15" driver could be loaded exclusively by the port. Things would make a little more sense - a 15" PR with decent excursion would be adequate for a 12" driver.

Or maybe its something more exotic. We know that multiple ports can be used to tune an enclosure to one frequency. And we know that PR's can do essentially the same thing. Is there any reason why we couldn't replace one port in a multi-port system with a PR? Then the single PR would not be responsible for the entire venting job. I know vents and PR's basically do the same job - the mass (of air or driver), surface area and box size are used to determine the tune. But the PR creates a 6th order system while the vent creates a 4th order ...

Or maybe the PR is at a different tuning point than the vent. A normal vented system's response will look like a sealed system's response until you get close (I use "close" loosely here) to the tuning frequency. So is it unnreasonable to think that two seperate tunings could be achieved if they are far enough apart from one another? The PR could be tuned higher so that less excursion is required near its tuning point. It still seems like it would have problems unloading below tuning. Still, the fact that its mounted horizontally suggests that it can't be tuned very low.

Now my head's starting to hurt.

 

[Tim_S]

Guys, I think you need to look at what Jack is saying instead of putting words into his mouth. FWIW, I own VMPS speakers, think they are fabulous, and think Brian is an excellent speaker designer and likely to be pretty trustworthy. Reading through this thread, I don't recall Jack or anyone else asking for verification of claims saying that
a: the VMPS subs don't sound good or that
b: Brian is lieing about his numbers.

They are, in my interpreation which they are free to correct, saying that
a: "sounding good" to people does not imply good SPL performance down at 20 Hz. In fact, they are pointing out that "sounding good" in whis manner typically involves the sub rolling off low end performance. Several points have been made that most of the DVD activity is greater than 30 HZ so when someone claims their VMPS sub is louder than subwoofer y on DVD x, this in no way supports or verifies the claim of 20 Hz performance. It is non-responsive to the question. So are all the claims of "just listen to the sub and you'll hear."

b: While Brian's numbers may be accurate, they might be in a highly specialized situation that makes the numbers he is claiming not comparable to TN numbers or other well known sets of measurements. Thus saying the VMPS subs perform better based on these claims is not necessarily correct as we don't know how the other subs would measure in the same circumstances.

c: Subwoofer design in terms of output is a well known science in which certain charactaristics are known to produce certain output levels. The publicly known details of the VMPS subs don't seem to lead to the output levels claimed.

Not being a subwoofer designer myself, I have no idea if these three claims are accurate, though I suspect they are. You should at least understand what they are saying before trying to start an argument with them telling them how wrong they are.

Personally I think it would be great to have proof of VMPS's performance. If indeed Brian is doing something with his subs to get this performance level others don't know how to do, I would tend to understand his reluctance to share how he did it. Why give away trade secrets? But proving the perfomance numbers should be good for everyone.

Tim

 

[Michael R Price]

Ryan, I believe the PR itself is slot loaded. I don't know what this accomplishes, but it may act as a low pass to filter out distortion. This "vent" is not the same kind of vent used to tune a bass-reflex enclosure.

And how is the PR mounted horizontally? I thought you weren't supposed to do that (this is why there are no PR sonotubes)?

I don't know how the active drivers work either... does one driver have a separate, sealed chamber? If not, how do they both work together as one PR system?

The point is, Brian Cheney would have had to measure this sub in a closet to get 115db at 1m, 20Hz with 2.5% THD. We have proved this by showing a Stryke sub with at least three times the PR displacement is not capable of this, and by showing through LspCAD that an Adire PR-15 (a serious 15" PR) is not capable of more than 105db anechoic at 20Hz and will begin compressing and causing distortion before that point.