Passive Radiator vs Open Baffle Bass - diyAudio
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Old 13th May 2013, 01:13 AM   #1
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Default Passive Radiator vs Open Baffle Bass

Just wanted to put an idea out there and see what everyone thinks.
Typically with passive radiators you have the active drive in the front and the two passive radiators on the sides, what if you were to put one of the passive radiators on the rear and one on top (or just one at the back)?
That would give you a similar driver dispersion to an open baffle with sound coming off the front and the back.

Currently I have an 18Ē driver in OB for each of the front mains, but there isnít a great deal of bass coming off it due to the roll off inherent with OB.
To make the solution a bit more robust I was wondering if adding a passive radiator or two to each drivers, and building a sealed box might help the situation.

With a dedicated sub for low frequencies, I donít need it to be tuned super low or need it to go massively high in the frequency range either.
Just thinking that I might be able to dual purpose the drivers, crisp enough to use them for music, and a flick of the switch on the active crossover and have the second map give them to give extra oomph for movies, effectively turning them into extra subs.
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Old 13th May 2013, 01:40 AM   #2
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Simply put, no.

It doesn't matter much where you put the passives with the exception that they're best when 2 are used on opposite walls for force cancellation. You will not affect the dispersion in any way by moving passives around on the box.
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Old 13th May 2013, 01:42 AM   #3
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ok thanks badman, if I use them in the traditional way do you think they would be usable in the configuration I mentioned?

Edit:
I should mention that previously I did have a Sonus Faber Gravis PR sub which didn't seem to work all that well with main speakers, but I don't want to throw the baby out with the bathwater. Just because it didn't work before doesn't mean it cant work.

Perhaps with external active crossover, using 18" drivers and larger volume it might seem less boomy.
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Last edited by Silent Screamer; 13th May 2013 at 01:47 AM.
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Old 13th May 2013, 04:02 AM   #4
badman is offline badman  United States
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passives have high moving mass and can transfer a lot of energy into a cabinet, which is why they're always best used in pairs in a force-cancelling arrangement. I did a pair of subs with 15" passives, only one per box, but the box weighed >100lbs with extensive networked bracing (interlinked bracing) and very thick walls. Mine was a budget project and I used mostly scrap wood and repurposed dressers, but the smarter way is to cancel the force from the get go rather than trying to do an over-the-top box.
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Old 13th May 2013, 04:29 AM   #5
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Hi there SS: Try converting your OB bass section to a U frame by adding an open box section behind the front mounting board. I have used a 6.76 inch deep "box" section and produced acceptable bass response for C-minor organ symphony. My box section is a pentagon, where most H or U frames are rectangles or square section. Search for H or U frames in this forum. ...regards, Michael
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Old 13th May 2013, 04:30 AM   #6
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In my view one should look at PR loading as an improvement over the typical ported enclosure, IF it is done properly. Optimized. But using 18" high performance drivers you have a problem. The lower you tune, the more excursion is required from the PR and the higher the mass for a given diameter PR. The simple version is that you run out of excursion in the PR for equal sized PR vs. Woofer. So you can double the number of PR, but do the math to see just how much more surface area you get as compared to going up say two standard diameter sizes.

Of course in the case of the 18" woofer, there are no two standard diameter sizes to go up to! But in the case of a 12" the 18" would be the two sizes up size. Then too it makes for a larger wall on the box - but the yin/yang there is that you will likely need a fair amount of Vb for a high output/high performance speaker since the VAS will require it in order to tune as low as you would like...

The "force cancelling" (opposing) PRs has some merit for systems tuned low low low, since the moving mass is high high high, and you'd kinda like the box to sit where you put it.

Keep in mind that a 12" driver with a pair of 18" PRs might seem excessive but the excursion per PR will be substantially lower than with one, and the lower the excursion the lower the distortion and the better controlled the PR.

The sizing of the main driver is mostly dependent upon your maximum SPL required at freqs other than in the center of the PR's passband, since there the driver is not experiencing substantial excursions, but the PR is... but the lower you tune, the more the active driver has to do at lower and lower frequencies until the coupling to the PR becomes substantial.

Otoh, there are limits to the motor assemblies of speakers, especially high excursion, low frequency woofers. That is to say, that one could fit the maximum efficiency and flux motor assembly on either a 12" cone or an 18" cone. Beyond a certain point you can't get any more turns or flux in a gap. So which is better, a large 18" cone or a 12" cone for this sort of application?

Not sure if this jabbering is helping to explain anything or not...
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Old 13th May 2013, 05:38 AM   #7
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@Michael: I have done some experimenting with extending the baffle through a rough U box, but it really isnít cutting the mustard. StigEric said it takes something 8 drivers to achieve the same result in OB.
I like the sound I get from the OB arrangement there just really isnít enough of it, by going PR with the same driver I think I can get a bit more bang for buck out of the driver (metaphorical not literal as I would need to purchase the PRs).
What I am concerned about is getting a boomy sound out of the box and losing the nice crisp sound I get in OB. I donít know if this sound is characteristic of the sealed with PR box, or because the Gravis was a purpose built sub concentrating on lower frequencies.
What I want is a more relaxed version of a PR, so it can extend up higher in the frequencies and act as a bass driver, and with a flick of the active cross over switch act more like a sub.

@bear: These are the PRs I would use (2x) http://www.parts-express.com/pe/showdetl.cfm?partnumber=293-684 and this is the driver I want to use it with http://www.parts-express.com/pe/showdetl.cfm?partnumber=293-674 I already own a pair of the drivers. The video says the PRs have a 4Ē excursion which should be plenty since I donít want to tune them mega low, because I have this driver http://www.parts-express.com/pe/showdetl.cfm?partnumber=293-666 for that purpose.

I guess the question I am asking is will the PR box colour the sound too much? I have read where people on the forum are conflicted if box colouration even exists, but to me I think it does because I can hear a very different sound between a driver in a box and one that is in free air.
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Old 13th May 2013, 06:38 AM   #8
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Curiosity question… Since I am going to be using a separate external amp to drive these speakers and don’t need room for a plate amp, would I be better off putting a third passive radiator in the box opposite the active driver?
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Old 13th May 2013, 11:44 PM   #9
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Decided to bite the bullet and buy two 18" VMP passive radiators, and if all goes well order some more...

What would the difference be in sound / distortion if I run 3 PR vs 2 PR?
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Old 14th May 2013, 11:27 PM   #10
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Just placed an order for 2 x TC Sounds 18" VMP PRs, so shall see how it goes shortly. Providing I brace it properly, I think a PR box is probably the least risky of all the designs.
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