file 1x18 versus 2x18, mini scoops?

12 years 1 month ago #3491 by simonr
1x18 versus 2x18, mini scoops? was created by simonr
Hi guys, yet another thread has prompted a few questions about accepted rules, and audio lore, and I seem to be at odds with the physical universe again!. As I said when I first posted a few months ago, my investigations into the question of bass cabinets have been going on for nearly two years now, and have not been solely confined to horn loaded systems. The first few attempts were made using bass reflex cabinets, and the rest of this post is the results.
When considering the question of subs, and accepted standards for bass reflex, the natural choice for comparison purposes was the Gsub. The good old 186 and one of RM's designs again. Some people might get the impression that my designs have been targeted in some way, but I have to point out that this is only because the standards set in these designs is high enough to be worthy of emulation, and if equalled, it is then of some significance.
I had a good look round and decided on the PD18br40. On paper this driver seemed to be significantly
more suited to the application. Next was the task of producing a design that was not so large as to be impractical. When playing around with the figures, it seemed that the 40 hz mark could be obtained in a dual 18 cabinet, with a volume of around 280 litres and a larger port area and length than would normally be considered. This is where the PD18br40 came into its' own (on paper anyway). After playing some more with the volumes etc. I noticed something that struck me as odd, because the dual concept was firmly lodged in my brain, and I had resigned myself to the rule that the larger cabinet was better for low end bass.
Having designed the cabinet, it was rather large at over 400 litres, and I was wondering if there was any way that this could be reduced, or if it was possible to reproduce the large cabinet response in two singles. Having halved everything, and extended the port length by about 25mm to get the tuning back to where it should be, I got a big surprise!. Why this was true, I had to investigate, because everything I had learnt up to that point said it wasn't possible. The only way I can explain it is that the rule for the 2x cabinet does not hold for port areas in excess of half the sd of the drivers. I think that something like the 'mini scoop' seems to come into play, and I was surprised to see the same topic appear on a thread over on SP, a few months later!.
So the prototypes were made. As I was very dubious about the results on paper, I designed the cabinets so that the singles could be coupled using a removable panel in the 'top' of each so that I could test the theory in the real world, and prove it, one way or the other.
The tests were actually more conclusive and gratifying than I had any right to expect. Not only were the singles slightly louder at 40hz, the upper kick frequencies sounded better. The cabinets sounded good, and were nearly as loud as I had hoped for. The only problem turned out to be the drivers. After some extended testing and careful 'playing in', one of the PD's coils burnt out. Another exhibited some signs of rubbing, indicating that this was on the way out as well. This was a severe dent to the project, as a lot was based on the response of that particular driver. If this was due to the fact that production had been moved and these drivers were part of the problems they experienced, I do not know, but I decided that once let down I could not risk further problems. So another driver was needed, and as this will not be too much of a surprise to most people, I tried the Celestion FTR18-4080FD. (let's just throw away the rule books). After an initial disappointing test or two, I decided just to let the drivers 'play in' properly. They were used at a few gigs, and the sound kept on improving. Now they've clocked up at least a years worth of abuse, and are still going!. They sound absolutely brilliant. Punchy and tight on kick, and extremely controlled low end. No muddy waffle on the lows. When 4 are placed at the bottom of the stack with ports in a line across the width, these cabinets are very impressive. Plots and drawings to follow.

Gsub in magenta, original dual br40 in green both at 1000watts 4 ohms parallel.

Original BR40 magenta, FTR18fd dual cyan, single green. Notice roll-off plot exactly parallel for single and dual, 3 db down at same wattage.

Here's an approximation of all 4 singles driven 2000 watts 8 ohms series-parallel.

Gsub versus 2 single ftr's, parallel into 4 ohms 1000 watts.

Plan Drawing of cabinet, step in port to counter back-box volume for handles.

I would not recommend using more than about 500 watts per driver for the ftr's, they will be hard pushed beyond this point, but I have been looking into the later versions of the voids, for a better power handling
figure, and a slightly larger cabinet and porting using these. I think it might give some of the larger scoops a bit of a challenge.
If anyone else cares to shed a bit more light on this exception to the rules, I would be glad of their opinion.
Possibly a bit more food for thought.
Happy Building!.

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12 years 1 month ago #3496 by deadbeat
Replied by deadbeat on topic 1x18 versus 2x18, mini scoops?
Is that vertical or horizontal simon? The plans look a bit funny. I see two ports on a single cab. [img]smileys/smiley5.gif

You might want to talk to csg, he's got cabs with port areas bigger than half the sd.

About the 'miniscoop' effect.
The miniscoops aren't really reflex boxes imo, and if they did, their ports would be much longer than your cab. Once you have a very long port like that, I'd think that they start acting a bit like tapered tlines. Then at higher frequencies, if there was a constant expansion, I would think they start exhibiting a sort of horn loading up to where the acoustic length and mouth area of the design couldn't handle it.

Your miniscoop effect wouldn't be a miniscoop effect, you don't have the rear baffle etc i guess.

i don't think you'd be able to challenge a scoop though, there's no efficiency like direct radiator + horn (well, some exceptions....)

Edited by: Deadbeat

[url=\"\"]Beranek\'s law[/url]
\'bits of ply round a driver\'

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