# How the heck do you tune passive radiators!?!?!



## IDGAF (Dec 27, 2009)

This is annoying me. One search tells me to do it a certain way and by my math, I come up wit 5lbs of weight. A calculator I found is telling 25 flippin pounds!! WTF?

I used to have a Boston SPG555 with the matching PR. And in 1cf, the 30hz tune weight was tiny. I don't get it.

I mean, I am trying to tune a relatively small box; 1cf for a single 12 with (2) 12" PR's but this isn't adding up. I mean, my HT sub uses a PR and I know that thing isn't 10lbs.

Can anybody shed some light here?


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## qwertydude (Dec 22, 2008)

The problem is a passive radiator still acts like a ported box and just like ported box set ups you need a larger box.

The weight a passive radiator needs is like port length. If you try to go with a smaller box the port length will dramatically increase or you end up with a very high tuned subwoofer system. Same as with a passive radiator you end up needing too much weight. At the point where you need 25 pounds of weight you're basically making it a sealed box, and considering how small a box you have there's not enough volume to resonate and control the woofer and reinforce the sound. The box is simply undersized, in fact it's undersized for even a sealed box. It's no wonder you need an impossible amount of weight. The only way to get a decent response out of a box that small is to make it sealed.

Passive radiators are not a cure-all for an improper box design. It has the exact same limitations as a ported box. Your home theater sub can get away with using a lightweight radiator because the box volume is larger so for the same tuning frequency you don't need as much weight and you get good efficiency, it's much larger than your cars subwoofer and it's also likely tuned higher.


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## IDGAF (Dec 27, 2009)

Well, I'm not one to just let stuff slide because of math. So, I managed to get ~ 3lbs of weight on each one and gave it a listen. I can't tell you where it's tuned because the small signal sweeps are either lying or don't have enough signal to overcome the weight of the PR's because it looks just like a sealed box on the sweeps. Actually, once I got past ~ 100 grams or so, the graphs didn't change no matter how much weight I added.

I should have mentioned this was originally for an iso pair. That's why it's 1 cube.










I also should have mentioned that efficiency isn't much of a concern since I have up to 10kw of power available. All I'm worried about at this point is small box and low end extension.

This monster is the one I'm trying to replace. Pretty typical. 2 cubes. 33hz.









With the iso pair pictured above, I made this. It's right at 1cf after displacements.









At a "sane" volume setting, the standard ported box was [email protected] up to a peak of [email protected] and back down to [email protected]

I had relatively very little weight to begin with, so the original tune for the iso pair was ~ 55hz? I couldn't get to the chosen volume setting at the 30hz tune without slapping the PR's to death, but even so it was [email protected] up to a peak of [email protected] and (not so) obviously louder with [email protected] Intersting that the PR alignment was almost as loud but at a lower volume. Although, the amp was at a lower nominal load. So, maybe it's not so surprising.

Anyway, I wasn't convinced I needed the iso pair so that's what led me to the OP. I just tried a single. It looks like this. Not pictured is the other PR opposite the one shown.










The picture makes it look much bigger than it actually is. It's relatively tiny. Well, for me anyway.

Keep in mind, I have a total of 6+ pounds of moving mass on these things. Lol. Listening impressions... it's got the output. It can really get moving. Kick drums are snappy. Upper sub bass is smoothed compared to the ported box which tended to be pretty peaky ~ 45hz. The rub is that the lower registers are slow and sloppy. "Muddy" and sound very far away. That's not good.

This is a 750w sub and my contention is that perhaps if I had a beefier sub with more motor strength, I might be able to overcome the sheer mass of the PR's. Or maybe not. We'll see. I'll try a 1,500w sub today. Then a 2,500w sub. But at that point, I start adding so much weight that my goal becomes counterproductive.


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## IDGAF (Dec 27, 2009)

cajunner said:


> with 2 passives, maybe you're just putting on too much weight.
> 
> I don't know where a 3 pound passive makes sense, that just boggles.
> 
> I'd try a re-dux, starting with box resonance indicated with no added mass and go from there in an incremental fashion, checking with tones to see that the resonant frequency corresponds to the additional weight.


Well, I'm trying to adhere to the whole 1.5-2x displacement rule for passives. The sub and PR's have the same soft parts so the xmax is similar. I went with double the cone. I suppose I could try just one. But you're right; a 3lb PR is retarded.

For the re-dux, do you mean in-car resonance? The stupid little, useless DAT woofer tester is proving rather unreliable for the purposes of this evolution.

So, for example... no weight - peak on a sweep -perhaps 70hz
add weight - 65hz
add - 55hz
and so on?


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## Oliver (Jun 25, 2007)

qwertydude said:


> The problem is a passive radiator still acts like a ported box and just like ported box set ups you need a larger box.
> 
> At the point where you need 25 pounds of weight you're basically making it a sealed box, and considering how small a box you have there's not enough volume to resonate and control the woofer and reinforce the sound. The box is simply undersized, in fact it's undersized for even a sealed box. It's no wonder you need an impossible amount of weight.


Bingo

IDGAF, find a tutorial or 2 on PR'S ( how the hell do use these? )


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## Hanatsu (Nov 9, 2010)

Tried modelling with WinISD? Pretty straight-forward. Made a PR enclosure once and WinISD was pretty spot on.


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