Dual passive AV15 sub


  • Member
    Woody on #67

    I’ve read a lot of outstanding reviews on commercial dual passive radiator subs lately, and would like to try to achieve the claimed benefits in a DIY project. I am waiting for delivery of two AV15’s any day now, and will be building two custom cabinets that will house my Ohm F drivers with their sealed compartments on top, and a down firing AV15, 500 watt plate amp, and two opposing 15″ passives in the base of each one. I haven’t yet found any definitive “how to” information on the subject of dual passives, and hope someone on this forum can direct me to a book, software, or a site that will give details and formulas to select the right diafragm weight to achieve the flattest 20 Hz or lower response I can get in a 4 cubes or less cabinet. I emailed John about this twice after the sale, but I guess he’s a busy boy right now. So, any help greatly appreciated!

    Woody


    Member
    Admin on #1399

    Hi Woody,

    We used to do a dual PR enclosure for the AV15. IT was a 24″ external cube made of 2 layers of 3/4″ thick MDF. This gives right around 5 cubic foot internal volume. We used a pair of the 18″ PR’s with 1600grams each. This gives a tuning of right around 20hz and an F3 point of 18hz.

    The AV15 would go on the front of the enclosure and the 18″ PR’s are on the sides opposing each other. This is pretty much a requirement to mount the PR’s on opposing sides to cancel all the net forces on the cabinet. Otherwise even a 150 lb cabinet can hop around the room due to the high mass moving back and forth. With 500W you’re really going to want the 18″ PR’s. The AV15 moves a lot of air and you will drive a pair of 15″ PR’s to their limits and risk damaging them. If at all possible the 18″ PR’s are better.

    In 4 cubic foot if you needed to use the 15″ Pr’s instead of the 18’s, a pair of 750gram Pr’s would tune to about 24hz with an F3 of just under 22hz. The 1050gram PR’s would get you tuned right around 20hz with F3 of about 20hz. With the 1050gram PR’s you’re tuned lower and have less risk of overexcursion on the PR’s. With the 18’s you’d never have to worry about it.

    John


    Member
    Woody on #1400

    Thanks, John,

    Just received tha AV15’s today… excellent workmanship!

    I suppose I could live with 5 cuft, though I’m afraid I wiould have to shrink the footprint to 20″ X 20″ and raise the height to compensate. That is mainly why I had hoped to use the 15″ PR’s so they wouldn’t come so close to the corners. The original Ohm F cabinets were tapered to 17″ sq, and I wanted to continue that theme to the floor at about 20″ sq. with the AV15 mounted facing down like the F driver under the F enclosure in a space separating the F enclosure from the sub enclosure. The space would be created with four aluminum angle corner standoffs 1/2″ or more higher than the AV15 to allow free air movement through the magnet vent, with a wrap around transparent grille. This mounting arrangement will allow me to shrink the cabinet volume by .25 to .5 cuft for the basket /magnet volume being removed and the cone volume being inverted, and to display the magnificent drivers like the F’s.

    If this arrangement isn’t totally rediculous, how about a third 15″ PR mounted downfiring under the sub enclosure with 3″ spikes for floor clearance? It would oppose the AV15 to help cancel shake much like the two opposing PR’s, which should be a good thing, right? It also would create 25% more surface area than two 18″ PR’s. I have no idea how to model this, but there was some positive discussion about 3 passives in another forum in an effort to reach 10 hz. flat at high SPL.

    This is an ambitous project, but I love a challenge and have the ability and equipment to try anything. Besides, I’ll be entertained twice, listening to incredible music, and watching the drivers dance creating the magic. German Physiks makes a line similar to this idea, but the ground floor is $12000.00!

    I can certainly go with 5 cubes, front driver and two 18″ PR’s to keep it simple, but is there any good possibility for my above 3 PR idea?

    Woody


    Member
    TeeCee on #1401

    Coincidentally, I’m in the process of biulding my first PR design and I designed it around one 15″ sub in the front and two opposing 15″ PRs on the sides in a 3.9 cu.ft. box with a 1000 watt plate amp.

    I built the box with just the sub and the amp and it sounded OK although I could hear the sub bottoming out at lower frequencies. I then ported it with two 3″ x 16″ PVC ports and the sound improved.

    I can’t wait to get the PRs in and listen to the difference. This sub already shakes the house.


    Member
    R. Buszka on #1402

    I wonder how the AV series woofers would do in an open baffle configuration? I know those guys are always looking for a quiet, high-displacement driver to do the dipole thing with.


    Member
    paulspencer on #1403

    Woody, the only reason not to use two 15 PRs rather than a pair of 18s is cost. If you have more total SD then that is better. You don’t want the PRs to have to move too far, as the suspension system is not linear, and hence you introduce non linearities into the system the higher the excursion of the PRs. Keep in mind that the advantage of PRs is that you avoid vent noise. If your box is large, then its easier to just do a vented box if you have the means to put a decent flare in and either a number of 100mm vents or at least a single 150mm vent.


    Member
    dB on #1404

    Woody,

    I would strongly suggest you give John’s suggestion a try. PR-15’s have an x-max of 25mm, PR 18’s have an X-max of 30mm one way. Two 18″ PR’s have a linear volume displacement of 18Liters, and three 15″ PR’s displace 12.45 Liters. You will have more low frequency output capability, and a more robust system.

    Down firing the A.V. 15, and ANY of the passive radiators is NOT recommended. The A.V. has over three quarters of a pound moving mass, and the suspension is NOT biased to combat the offset effects of gravity. The 750 gram PR’s with over 1.5lbs of moving mass and 1600 gram PR’s at over 3.5 lbs likewise are not biased. IF used in any orientation other than horizontal – firing forward, the driver and or PR’s will be severely offset which will eat into their linear travel and can cause serious distortion and if driven hard may cause physical damage to the unit.

    The shape you suggest is perfectly acceptable. You have my word that used as John suggested you will have a SERIOUS subwoofer – capable of out-performing virtually ALL commercially available systems under 5k. 🙂


    Member
    dB on #1405

    @teecee wrote:

    I built the box with just the sub and the amp and it sounded OK although I could hear the sub bottoming out at lower frequencies. I then ported it with two 3″ x 16″ PVC ports and the sound improved.

    I can’t wait to get the PRs in and listen to the difference. This sub already shakes the house.

    TeeCee,

    The differences in ‘bottoming’ were due to differences in enclosure design sealed v.s. ported.

    At and around Fb (enclosure tuning) the actives excursion is much reduced do to the displacement of the radiator. The active unit, and air slug come into phase and both operate to increase and decrease internal box pressure, while the resonator is excited by the active.

    This increase in internal pressure reduces the drivers con-motion – and as long as frequencies well below tuning aren’t introduced, effectively protects the driver from over excursion.


    Member
    dB on #1406

    @R. Buszka wrote:

    I wonder how the AV series woofers would do in an open baffle configuration? I know those guys are always looking for a quiet, high-displacement driver to do the dipole thing with.

    R. Buszka,

    We are close to re-opening shop, and will be loading new driver specifications shortly. We will be carrying an I.B. specific variant of the vaunted A.V. series – watch the sight for further details.


    Member
    dB on #1407

    @paulspencer wrote:

    You don’t want the PRs to have to move too far, as the suspension system is not linear, and hence you introduce non linearities into the system the higher the excursion of the PRs. Keep in mind that the advantage of PRs is that you avoid vent noise. If your box is large, then its easier to just do a vented box if you have the means to put a decent flare in and either a number of 100mm vents or at least a single 150mm vent.

    Paul Spencer,

    Boy do I wish life were that simple!! :):)

    S.D. is not the only determining factor, you need to look at total volume displacement.

    The suspensions on these PR’s is extremely linear through the ranges we specify for excursion.

    Correct you don’t want to severely over excurt the passives. Mechanical damage will occur. (I’ve launched mass slugs YARDS from the enclosure doing this :))

    Large vented boxes with large diameter long ports introduce response anomalies called port self resonance. These resonances can severely muddy the systems sound. This is part of the reason I use Passives so often. Yes a large box will allow ports of adequate surface area to keep port air velocity manageable, and low tuning to low frequencies


    Member
    paulspencer on #1408

    Boy do I wish life were that simple!!

    I thought we were talking about subwoofers here 😉

    S.D. is not the only determining factor, you need to look at total volume displacement.

    This is true. Actually, not having looked much into these PRs I was not aware that the excursion was greater with the larger units.

    I would strongly suggest you give John’s suggestion a try. PR-15’s have an x-max of 25mm, PR 18’s have an X-max of 30mm one way.

    Actually, the PR18 is quoted as 75mm p-p excursion limit, although I doubt that xmax is really the correct parameter in this case. I suspect it should be xsus. Looking at the surround, I doubt the suspension system could go further, and xmax would normally imply that the suspension system can go further.

    Correct you don’t want to severely over excurt the passives. Mechanical damage will occur.

    Well before there is a risk of mechanical damage, there will be power compression and an increase in distortion. If there is a risk of mechanical damage, then there is also some serious distortion going on.

    The suspensions on these PR’s is extremely linear through the ranges we specify for excursion.

    Who else in included in “we”?

    How exactly do you know how linear they are? What would you consider to be the ultimate in linearity for a PR? What measure would you use?

    Large vented boxes with large diameter long ports introduce response anomalies called port self resonance. These resonances can severely muddy the systems sound. This is part of the reason I use Passives so often. Yes a large box will allow ports of adequate surface area to keep port air velocity manageable, and low tuning to low frequencies

    A vented box can be designed so that the resonances are above the range reproduced by the sub. For a given output target and budget, I’d expect better performance. IMO a PR sub can only be better where more money is spent, not a fair comparison. I’d rather spend the money on drivers, but each to their own.

    I haven’t seen measurements on this, but I’d expect a properly engineered vent to be more linear than a passive radiator. With a vented box you are dealing with aerodynamics, but with a PR you substitute a mechanical system. The vent can be made to be what I’d expect to be a more perfect device at very little cost. The real value of a PR is IMO primarily that you can tune low in a box that would not fit a vent that won’t chuff.


    Member
    dB on #1409

    Edit:

    Since this was so far off topic – I started a different thread with the majority of this post moved there..


    Member
    dB on #1410

    We would be John J, and I, my name being Deon, just call me dB.

    I guess that doesn’t really answer your questions – so..

    @paulspencer wrote:

    Actually, the PR18 is quoted as 75mm p-p

    60mm linear – 75mm P-P.

    @paulspencer wrote:

    Well before there is a risk of mechanical damage, there will be power compression and an increase in distortion. If there is a risk of mechanical damage, then there is also some serious distortion going on.

    True enough. Unfortunately many end users will continue pushing their systems well after the onset of significant levels of distortion. The human ear is fairly insensitive to L.F. distortion products (by virtue of being relatively insensitive to L.F. period) AND most peoples ears aren’t trained to detect significant L.F. distortion

    @paulspencer wrote:

    How exactly do you know how linear they are? What would you consider to be the ultimate in linearity for a PR? What measure would you use?

    Now there are a few secrets I won’t divulge. 🙂

    @paulspencer wrote:

    A vented box can be designed so that the resonances are above the range reproduced by the sub. For a given output target and budget, I’d expect better performance. IMO a PR sub can only be better where more money is spent, not a fair comparison. I’d rather spend the money on drivers, but each to their own.

    True enough, and working as a DIY under a constrained budget – vents are very attractive.

    @paulspencer wrote:

    I haven’t seen measurements on this, but I’d expect a properly engineered vent to be more linear than a passive radiator. With a vented box you are dealing with aerodynamics, but with a PR you substitute a mechanical system. The vent can be made to be what I’d expect to be a more perfect device at very little cost. The real value of a PR is IMO primarily that you can tune low in a box that would not fit a vent that won’t chuff.

    100% of ALL vents compress. Most compress significantly well before the user is aware they are. I’ve measured significant port compression with vents as large as 1/2 the diameter of the driver at levels you would be surprised to see.


    Member
    R. Buszka on #1411

    Oops – I decided a PM to dB would be a better option. dB, check your PMs.


    Member
    TeeCee on #1412

    @db wrote:

    TeeCee,

    The differences in ‘bottoming’ were due to differences in enclosure design sealed v.s. ported.

    At and around Fb (enclosure tuning) the actives excursion is much reduced do to the displacement of the radiator. The active unit, and air slug come into phase and both operate to increase and decrease internal box pressure, while the resonator is excited by the active.

    This increase in internal pressure reduces the drivers con-motion – and as long as frequencies well below tuning aren’t introduced, effectively protects the driver from over excursion.

    dB, thanks, I understand how the passive reduces the active’s excursion.

    How do you technically explain the passive?

    This is the first time I’ve made a box sealed, ported it, and then put in the PRs. It’s good to witness the changes.

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