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Discussion Starter #1
So I'm already confident enough that I am fixing to try this anyway... But I do have one slight pause. I have read and heard from the Dayton tech guy, that if your using a passive radiator, for say, a 15" sub, you should have twice the surface area in passive radiators ? So in other words, I should be using two 15" PR's for my one 15" sub {oh, remember, I'm turning my two 15 Dayton HO's, into one, by mounting them face to face}. But here's the thing, I can't understand why surface area should matter ? My 15" Dayton has a peak to peak excursion of 26 mm, while the Dayton PR has a peak to peak excursion of 60 mm's ! Shouldn't it be the surface area X the excursion (displacement volume) that matters ? Because my PR actually has more than double the displacement as my subwoofer does. I would think this would be the important thing. No ?

If I have to buy another 15" PR, and slap it on the other side of my cube, so be it. But even as it is, the one PR that I have will only be traveling less than half of what it is designed for...

This is going to be one TINY cube for two 15"s... only 16 x 16 x 16 ! And I already know my two 15"s mounted face to face isobarically, in a tiny enclosure sound fantastic (from my recent building experiments. The only thing I don't know about, is how well I'm going to be able to duplicate a big, long port (it would be a 6" diameter by 48" long for a tuning of 32 hz) with a PR ? Fixing to find out though. The PR should be here next week :)
 

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Discussion Starter #2
Okay.... I think I answered my own question... I just found this on the same page as my PR >

Tip: As a general rule, the passive radiator(s) should have at least double the displacement of air (Vd) as the active woofer(s)/subwoofer(s) in the system.
To calculate Vd: Sd x Xmax = Vd

Just as I suspected, surface area has nothing to do with it. It's all about "Displacement {Vd}". So yea, my PR has a little more than double the Vd as my sub does :) I'm good :)
 

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I would start with 1...

But at the tuning freq the sub will not really be moving much

If you are really demanding a lot, I think you could get the PR to over excursion... but then realisitically you have cabin gain, and at some point the window will be flexing and there is only so much a person can take.
So it think if it is a low enough tune, then you should be OK... but I would not swear to it.
 

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Discussion Starter #4
Well yeah, I'm thinking of a tuning of 30-32 hz, which will be super easy to adjust, as I'm going to mount the PR inverted (at least to start, even if i flip it back later)
But like I say, my PR will have more than double the peak to peak X-max as my sub. I think if I can get that PR flexing even close to the 60 mm X-max, that will be rediculous(ly) cool 😀

BTW, for those that don't know, 60 mm's is equal to 2.36" !
 

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Discussion Starter #6
WinIsd will model both the driver and passive excursion.
Only problem is, I downloaded WinIsd and could not make that program work to save my life.

If you or anyone else who is good with WinIsd and who finds it fun to model this stuff, wants to do that, that would be totally appreciated.
 

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Hey Chris - I'm trying this out because I've been looking into modeling some passive radiator systems in winisd. I know you're running 15" HOs clamshelled. I can't remember exactly where you ended up with size and tuning so I put them in 3 cubes. If you want me to rerun the models, just let me know the specific of your current net box size and port configuration.

  • Blue: Ported box with 3 cubes net tuned to 30 Hz (6" diameter by 24" long port)
  • Green: Passive box with 3 cubes net using a Dayton designer series with 150 g added mass (two washers)




This is as close as I can get it to matching the output of your ported box (both on 1000W).

Cone and PR excursion is within spec.
Subs are pretty similar to the ported box and MAYBE could use a 20 Hz subsonic if you're really pushing em (14 mm @ 20 Hz).
PR maxes at 22 mm @ 27 Hz.

I'm no PR expert, but I do like how they sound. According to WinISD, they don't quite have the output potential of a ported box in this build. Maybe someone can help with ideas as to why we're losing 1-6 DBs.
 

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Discussion Starter #8
TYSM JMike, but a few things I want to discuss here... First, I see that it says "Dayton 15's ISO" on the top left.... but in the little diagram picture at the bottom, its only showing a single sub, not two in a clamshell ?

Another thing that I recently learned from experience, is that 3 cubes is WAY too many cubes, when two of these subs are mounted face to face (Isobarically) .....and here's the thing, even the tech guys at Parts Express were telling me... "Oh, you can go with 4.4 cubes... or 4 cubes"..... However, based on what I had researched from other sources, I decided to try 2.7 cubes (which BTW, is what is recommended for one of these subs in a standard tuned, ported enclosure) and guess what ? That was too much ! I totally experienced over excursion ! So I put a space taking block inside ofg the enclosure, dropping it to 2.2 cubes.... And it was STILL too much ! I mean, it helped, but on certain notes, really beating on it, I could still bottom the subs out. So finally, I took a saw to my box and chopped it down to 1.5 cubes. Bam ! :) No more bottoming out, and OMG, Ive never heard 15"s snap so tightly ! Id swear I was listening to four 8"s :) Yet it digs crazy deep when it needs too. Maybe not the loudest (not quite as loud as these same two 15"s used to be in a massive 7 cu ft standard ported design, but WAY more musical :)

What I learned is, a pair of subs mounted isobarically do not use "half" as much space, as I have read multiple times, but 1/4 ! I mean, yes, 1/2 as much space as the sub mounted "in the enclosure" but zero space for the inverted sub outside. So I'd call that, 1/4....

Anyway, now that the box is a hacked up mess, I'm rebuilding a new, clean one.... But this time, with the 15" passive radiator, instead of a port.
 

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Firstly, I double-checked that the drivers are configured as isobaric. It just doesn't reflect it on that specific icon.

Secondly, I think you are 100% right that the box was just way too big for this build. After modeling with 1.5 cubes, we're getting a much more normal-looking response and the PR box seems to line up better to the ported box.

Blue is always 1.5 cube tuned to 31 Hz which is what I get when I configure a 6" x 48" long port.

#1 Green is a 1.5 cube with 375g added weight to me which looked like the closest fit:


#2 Green has only 225g added for a little punchier response:


#3 Green has 450g add for a deeper tuning:


Still lose a little output, but at least now we're in the ballpark and you do get the ability to tune it without playing around with that massive port.
 

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Discussion Starter #10
Bro ! This is exactly what I needed to see ! Again, TYSM !

Btw, is there anyway way to determine the excursion of the PR ? I mean I'm confident I will not have any over-excursion issues... but at the same time I'm hoping I can get that sucker flexing 😉

Oh also, I think you are going to get a kick out of this.... but read the thing I'm getting ready to post about my OCD 😀 lol
 

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Awesome! Yeah I hope you get this project exactly where you want it!

WinISD models excursion for both driven subs and the PR. In each case, your max is going to be reached low, but be careful because you're doubling xmax of the HOs if you feed them full power at 10 Hz for example. Usually, though, modeling falls all to hell at lower frequencies in a car anyway. Once you get down below about 20 Hz, the car interior space is as much a part of the box as the MDF cube it's sitting in, and that affects the excursion as well.

Anyway, your subs are spec'd for 12 mm xmax, and it sounds like based on your previous testing that you don't want to go over that. According the modeling, that means the most you're going to get the PR swinging is about 19 mm each way. I know it's not the 2+ inches you were hoping for, but that's still 1.5" peak to peak. If you want the PR to "go big", you're going to have to make the box big again or add more driver displacement and power.

Here's the driver and (below that) the PR excursion in the same 1.5 cube box with 1000 W and 450 g added weight (lighter weight PRs are pretty similar except they have a bit less excursion in the deeper frequencies than this one, but get going a little higher up like 11 mm @ 40 Hz for the 225 g added weight):


 

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Discussion Starter #12
Yea, I'm okay with this. And like I say, just from my current test box, I'm super confident. I know it won't be the loudest pair of 15"s but I'm telling you, they are so freaking musical. Best I've ever heard from 15"s. Certainly from mine ☺
 

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At some point, when the vision gets blurry, one has to declare some amount of success.

It will be interesting to see what frequency (weight) you end up tuning them to.
 

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Discussion Starter #14
Agreed. On the one hand, I think 35 hz or so makes more use (and loudness) from a subwoofer, but especially with an 18" sub, it will move more air tuned to 30 hz or slightly below. Their is always tradeoffs.
 

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Depends if there is music with content under 30-Hz, then a 35Hz port would be unloaded???

I am afraid that you will be moving the weights though the finger like rosary beads to figure it out... (but that is easier than changing ports, so I am not too afraid.)

And then is there a limit to the loudness needed?
 

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So a little more food for thought - if you look at the min excursion of the driver as the tuning frequency of the system (which makes sense to me) 350 g mass is about 31.7 Hz.
 
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