Dual opposed vs line array - Page 2

Claims are cheap. Show me the data. But what this post has to do with the one you quoted is beyond me.

Build a dual opposed cabinet with no bracing and a dual front firing cabinet with no bracing and you will quickly find out why.

The dual front firing cabinet will need more mass to keep it from physically moving when both 20mm plus subs are firing in the same direction at high spl's. The dual opposed cabinet will keep still, you can put a glass of water on top of it if it is designed correctly and the water will hardly move. I have had high x-max drivers like the LMS Ultra and XXX 18 physically move enclosures when they were firing from only one baffle, this would not happen in a properly designed dual opposed enclosure.

It just takes a little more work to make a proper sub enclosure with both subs firing on the same baffle if they are of the high excursion variety, a dual opposed is just easier.

I really didn't notice much movement either until I went to disconnect the sub and noticed the cabinet had moved back so far the binding posts were right against the wall. I had to scoot the sub back so I could remove the speaker cables. This was with a LMS Ultra 18" and only one day of testing so I know it was the sub that physically moved the enclosure which was sitting on rubber feet on a tile floor. Not other 18" sub did that besides the XXX but I am guessing 3 SI 18" with 23.5mm xmax each will do some serious pushing. The XXX 18 moved the enclosure as well but it was sitting on carpet with no feet, maybe if I would have really cranked it the XXX could have taken the enclosure on a ride like a toboggan

Not when his cabinets weigh 150 to 200 lbs each with another 170lbs in weight from the 2 LMS Ultras, that's probably about 350 lbs a cabinet Like I mentioned earlier, you just need more mass when all the drivers are firing in the same direction.

You had me worried there for a minute, I was trying to recall the last time I had seen Notnyt post

I was just responding to the reason for a vibration canceling opposed design compared to all front firing, enclosure is easier to construct if having an inert cabinet is what your after.

I would just recommend you make the cabinet well braced and heavy if having all 3 on the same baffle, light weight will only be your friend when you move the enclosure, not when your subs do

Heck, you can balance a penny on it while the sub is playing full SPL at low frequencies.

I have line source speakers (GR-Research LS-6) that have bass extension to 20 Hz in my room. Like Bill said, the drivers become mutually coupled point sources at some point. I also have infinite baffle manifolds located just outside and a little behind each speaker. From around 120 Hz and down, both the speaker and IB measure the same due to the room modes even though one is firing up from the floor and the other is in a line array extending over 5' above the floor. I think this illustrates Bill's point.

What do you guys mean by "coupling"?

If two bass drivers are within a 1/4 wavelength of each other, they essentially don't "fight" against each other and seemingly play "as-one" While also getting the benefits of added output for the exact same reasons.


Are you using the LS6's all the way down to 20hz usually? or are you crossing them higher for movies/music and letting the IB handle the rest?

You used the same drivers (and heavier units, still) and experienced movement. And I seriously doubt his cabs alone weigh anywhere near 200 lbs, but much more likely, very similar in total mass to your own.

That all said, IIRC, I believe he (NOTNYT) DID say he experienced some movement at extreme levels.

I built a pair dual Dayton 18's with rubber feet on a hardwood floor and neither have moved anywhere at 125+db levels...even before my wides were stacked atop them.



I believe with enough output you could move a cabinet on a bare floor with less-than-ideal feet. But with spikes on carpet, or "proper" rubber feet (yes, some are MUCH more susceptible to sliding than others) on a hardwood or tile floor, you'd be hard-pressed.

I'm actually delighted to learn that I didn't really sacrifice anything in the sonics dept, as I simply did not have the sq footage to construct a DO cabinet.

James
Edited by mastermaybe - 1/3/13 at 8:30am

This is where it gets confusing for the layman. When you say 'vibration cancelling' that is true of opposed as far as the vibration of the entire cab fore and aft, in obeyance of Sir Isaac's Third Law. It does not reduce vibration of the cabinet panels, other than that vibration reduction of the baffle that results from its being smaller.

^ That's how I understand things. Then, so long as my cabinet's feet are not moving, the actual top/sides of the cabinet are not much more prone to vibrating than in a DO (outside of your noted smaller baffle point)? Not that it matters, but my cabs seem to vibrate very little, even while producing some pretty extreme (IMO) output.

James

Not one bit.

^ Ok...I used the "not much" to account for the larger baffle you would inevitably use for a line array that I thought may induce a bit more panel vibration, but I'd be happy to be told I'm in error, there.

thanks,
James

As far as that goes a brace connecting the center of the baffle and back of the cab will give the same vibration reduction as halving the baffle size by going dual opposed. And while that adds one level of complexity to the build, with dual opposed the cab would have to be deeper to achieve the same internal volume, so the top. bottom and sides would be larger, and then they'd need bracing to be as stiff as the smaller panels in a cab with two drivers sharing a baffle. As always, there's no such thing as a free lunch.
And what you have there isn't a line array. You'd need a dozen or so more drivers to make that claim.

Interesting. I would never guessed this could happen.

If I understood Bill correctly, coupling in multi driver subs is not the same as point source summing. With coupling, the output is increased but it does not mean that all drivers share single acoustic center.

Whenever I am doing (RTA guided) sub crawl, I see that a few feet can make a difference, sometime big difference, in frequency response at the MLP. I explained this by the differences in boundary reinforcement and what room modes get excited at any given position.
So, intuitively, I thought that while a multi driver subwoofer and a hypothetical single driver one may have the same SPL, they will have different FR due to the room interactions.
Apparently this is not the case at least in your set up. I still do not understand why though. For example, a wall-to-wall multi driver unit would load two corners, something that a single driver sub cannot do. That's why I believed that Bosso's 4 stacked units could provide some modal smoothing though probably not as effective as properly executed mutisub with individual units decorrelated from each other


Edited by zheka - 1/3/13 at 11:45am

Yeah, I was being a bit liberal with the definition of line array, lol.


James

I use an 80hz crossover with very shallow slopes on the speaker and subwoofer. Because of the shallow slopes, I am only using the left sub with the left speaker and right sub with the right speaker for music. Each speaker/sub combo is then playing from around 20 Hz up to 320 Hz with output contributing from both. This has the best bass, midbass, and soundstage of any options I've tried so far.

For music, the IB bass by itself (100 Hz crossover and steeper slopes) sounds better than the LS-6 bass (no subs). However, combining them like I did sounds even better.

The left sub and left speaker may measure similar, but they are different than the right sub/speaker. Moving the line array further away from the sub location would probably make a greater difference. Also, a single sub placed 4 ft above the IB might have a more different measurement.

I did say 150 to 200 lbs, he did use 2 sheets to construct each cabinet and if I remember correctly they were in the 9cuft range each. So lets say 150ish lbs for the cabs and 170lbs in LMS Ultra's = 300+ lb cabs each one.

I have no doubt of that, I never said it would be completely still, I was just stating that more mass would equal less movement, not no movement.

It all comes down to building a good enclosure to get good results, you can just save a little time with dual opposed. Your not missing anything with the fine enclosures you built.

The acoustic center will be the center of the grouping, as the drivers will in effect sum together to become one large driver. But they can only do so over a limited bandwidth; note that his example is from 120Hz down.

It does: http://www.bossobass.com/Bossobass.com/Technical.html

Commercial, but that's a good link for the subject at hand.

there is no end to my confusion. i appreciate your patience.

so will a wall-to-wall multidriver subwoofer produce the same frequency response curve at the listening position as an equally powerful single driver unit placed at the wall's midpoint?

No, because of the presence of boundaries. I think the term 'acoustic center' has you a bit confused. What's pertinent in this case is the radiating plane, one that's, say, thirteen inches wide versus thirteen feet wide. One will react quite differently with respect to the radiation pattern than the other, and that matters more than the response curve in this scenario.
Edited by Bill Fitzmaurice - 1/3/13 at 5:33pm