Passive Radiator vs Open Baffle Bass

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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 18” driver in OB for each of the front mains, but there isn’t 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 don’t 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.
ok thanks badman, if I use them in the traditional way do you think they would be usable in the configuration I mentioned?

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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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.
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
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! :D 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...
@Michael: I have done some experimenting with extending the baffle through a rough U box, but it really isn’t 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 isn’t 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 don’t 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) and this is the driver I want to use it with I already own a pair of the drivers. The video says the PRs have a 4” excursion which should be plenty since I don’t want to tune them mega low, because I have this driver 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.

An 18" bass + 2x18" PR's to be used above a subwoofer is seriously
OTT stuff, probably pointless and not thought through seems to me.

Just converting the the OB to a sealed box (about half the volume
of the PR box) would give you oodles more bass and low bass SPL.

I cannot see any scenario for PR's above a lower bass subwoofer,
transient response and integration will be poor. Parallel with the
PR's tuned deep and tight is a different matter. That could work
well with music with the sub off and then add the sub for AV.

rgds, sreten.
Here is the thinking behind it. Geddes advocates the use of a three sub mixture with all subs not necessarily being the same, or covering the same frequencies for movies to create a fog of sub frequencies.

The left and right Pro 5100 subs / lower bass (with 2 x PR in each) have a higher efficiency then the Ultra LMS 5400 lower frequency sub, which means I could for the biggest part use only the Pro 5100s off a smaller amp for both music and movies. When the movie requires seriously low bass fire up the LMS 5400 sub. If the Pro 5100s were tuned for higher frequencies (i.e. not adding a lot of weight to the PR) I thought it might integrate better into an active 4 way system.

Should that fail I could tune the Pro 5100s lower and add some cheaper OB woofers back into the mix, that are better suited to OB, while the TC Sounds are great drivers, and sound fantastic, they are not really well suited to OB with their low QTS.

Also buying just the two PRs at this point, if the Pro 5100 don't work well in the desired arrangement, I can merely slip the Ultra LMS 5400 into the box I build and use it for deep sub. So either way I am going to wind up using them.
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