# Cheap DIY Passive Radiators



## Patrick Bateman (Sep 11, 2006)

Can any of you think of a reason to use expensive passive radiators?










Here's the reason that I ask:

A passive radiator is a really attractive alternative to a port. The efficiency of any radiator is related to the size of the radiator. For instance, everything else being equal, *an 8" port will be more efficient than a 4" port.*

This is NOT a small difference. Programs like hornresp demonstrate that the efficiency difference is 1-3dB. *So using a bigger port is virtually the same as increasing your power by 25-100%.*

The main reason that I've only built one speaker with a passive radiator is because they're expensive, and there aren't many options out there.


















So why not do what Logitech does with their subs? The passive radiator is a simple piece of cardboard bonded to a surround.

There's no basket and no voice coil, and I can't figure out why we need one for a PR. In a regular loudspeaker we need the voice coil because it's driven by the motor, but there's no motor in a PR, so why include a voice coil?









It should be possible to buy the surrounds for about $5-$10, maybe less. The surrounds above are $7 at PE.


So basically, why not build a cheap PR like this:

step 1 - make a hole in your sub that's about 100% larger than the woofer. (IE, if you have a 8" woofer use a 12" PR.)
step 2 - Glue a surround to the box that's appropriate for a 12" woofer
step 3 - Glue cardboard to the surround with polyurethane glue
step 4 - Add weight to the passive radiator until the resonant frequency is where you need it. (The same tuning frequency as a vented box, but with a PR instead of a port.) Fiberglass or steel washers could be used to add weight, or clay if you want to tweak the frequency up or down.


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## minbari (Mar 3, 2011)

how do you weight it? without decent xmax on the PR and a way to weight and tune the PR, it is pretty useless.


****edit lol, you edited it and answered my question. if it works well. then why not?


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## supermaxx123 (Apr 3, 2010)

I bought some pr's that are outter mounting ring, surround and wooden cone with a weight in the middle. They are 10" and i've never used them! I jut felt like the xmax on the is weak. Would this be a problem as well? using a much lower xmax pr than your sub has?


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## Patrick Bateman (Sep 11, 2006)

supermaxx123 said:


> I bought some pr's that are outter mounting ring, surround and wooden cone with a weight in the middle. They are 10" and i've never used them! I jut felt like the xmax on the is weak. Would this be a problem as well? using a much lower xmax pr than your sub has?


Where did you get them?

If the PR is significantly bigger than the active radiator, than the xmax shouldn't be a big issue

Taken to the extreme, I imagine you could use an entire side of the sub box as a PR. (IE, make it super thin and flexy, then tune it like a PR.)

But that would be tough to tune


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## bassace (Oct 31, 2011)

Patrick Bateman said:


> So basically, why not build a cheap PR like this:
> 
> step 1 - make a hole in your sub that's about 100% larger than the woofer. (IE, if you have a 8" woofer use a 12" PR.)
> step 2 - Glue a surround to the box that's appropriate for a 12" woofer
> ...




I really think creating your own PR is a great idea, but is it necessary to find the right surround that is not too stiff or loose, how would I figure the Vas, Qms, and Fs? 

Also, is it possible to use a cone with just a surround, will the weight from the rear of the cone deform the surround if mounted upright? 

I would think the best material to use in this DIY PR is a flat piece of Balsa Wood, I am not sure how stiff of a cardboard (and where to purchase) is needed to ensure that it does not deform from the pressure of the driver. I would really like to know your thoughts.


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## t3sn4f2 (Jan 3, 2007)

How would the car's acceleration and braking affect performance in PR designs? Assuming a front/rear facing vertical installation.


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## subwoofery (Nov 9, 2008)

t3sn4f2 said:


> How would the car's acceleration and braking affect performance in PR designs? Assuming a front/rear facing vertical installation.


On accelerations, your midbass would be able to play 20Hz notes with ease  

Kelvin


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## Patrick Bateman (Sep 11, 2006)

bassace said:


> I really think creating your own PR is a great idea, but is it necessary to find the right surround that is not too stiff or loose, how would I figure the Vas, Qms, and Fs?
> 
> Also, is it possible to use a cone with just a surround, will the weight from the rear of the cone deform the surround if mounted upright?
> 
> I would think the best material to use in this DIY PR is a flat piece of Balsa Wood, I am not sure how stiff of a cardboard (and where to purchase) is needed to ensure that it does not deform from the pressure of the driver. I would really like to know your thoughts.


Balsa might be tricky bcuz it might flex. In a conventional loudspeaker we want a relatively light and strong cone, because a lighter cone increases high frequency response. In a PR, we don't want *any* high frequency response, so a heavy well damped cone works well. Hence, the use of wood or aluminum passive radiators.

I *do* think that a cone shape might sag a bit, but a disc shouldn't.

I'd guess that mounting the PR horizontally would cause it to be offset, and this would limit excursion but only in one direction, which might create some odd harmonics.

I'm thinking a flat disc with a surround would work pretty good, basically the very thing that logitech is doing.

I also wonder if a VERY large passive radiator could ditch the surround entirely. This might be impractical, it might have to be so large that it wouldn't fit in the box. But since efficiency is tied to radiator area, it would seem to me that a 15" passive radiator that weighs ten grams should function the same as a 7.5" passive radiator that weights ten grams, only with higher efficiency due to the larger surface area.

And of course, more surface area = less excursion. It's the same reason boom boxes have those ridiculous massively flared ports.


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## onebadmonte (Sep 4, 2008)

These home made passive radiators remind me of the ones earthquake use to make to go along with their magma subs. The pr's they made were essentially a disk of a particular weight with a surround on the outside of the disk and an opposing surround on the backside of the disk. The theory behind the opposed surrounds is to keep the disk moving in a linear/pistonic motion. From my experience that didn't work. When pushed hard the pr's would wobble rather than move in a linear motion. Pr's with surrounds and spiders don't seem to have this issue when pushed to the limits. I guess this home made pr will work if not pushed to the limits, but then if you're not gonna push it to the limits then what's the point.


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## chad (Jun 30, 2005)

t3sn4f2 said:


> How would the car's acceleration and braking affect performance in PR designs? Assuming a front/rear facing vertical installation.


that has GOT to be a joke.


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## Victor_inox (Apr 27, 2012)

chad said:


> that has GOT to be a joke.


You think?


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## chad (Jun 30, 2005)

Victor_inox said:


> You think?


You never know here


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## t3sn4f2 (Jan 3, 2007)

chad said:


> that has GOT to be a joke.


:blush:


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## Hillbilly SQ (Jan 26, 2007)

Would a cheap sub work as a pr?


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## sqshoestring (Jun 19, 2007)

I recall Stillwater boxes had a cardboard radiator and they glued a layer of foam on the front with the name on it. All it had otherwise was a typical surround. I replaced some back in the day. They would wobble but were overpowered by then. Maybe the foam dampened it and was the weight, the cardboard was stiff part. Anyway the foam looked cool lol.

A sub can work except you need a way to add weight to the cone, and it needs some xmax. But really a sub has more stuff than you need, if you don't need a VC/motor/spider then you don't even need a basket. Just like a port full of air volume, its the weight that tunes a PR. You would need the spider and basket to support weight on the VC area of a sub, because the cone shape.


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## Nismo (Jan 10, 2010)

I think you guys are on the right track. To limit the wobble effect, I would think that some depth would be necessary. If the surround on the inner part of the PR was to be mounted near the outer, you would have more wobble...and the further you spread them out, the less wobble you could have.

Use a large tube...or spacers for your depth. A small sonotube might work well. Attach surround to both ends, then do a cone on each, and attach them in the middle. Should work wonders, and would be less likely to have issues when adding weight.

Eric


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## geolemon (Aug 15, 2005)

Not to raise a thread from the dead frivolously, but I'm building a box for a pair of 8's now, and had sold a car before I could have implemented the fiberglass rear deck and pair of 10" flat-panel passive radiators that I had purchased... would have been less than half a cube total in that flat space - so now, dying of curiosity, I'm going to probably do the same enclosure, less fancy, just a box for the trunk of the new car, to see how it would have turned out.

Specifically on topic here, with regard to the original poster's idea (and possibly building on what the post immediately above was trying to state, which wasn't clear to me) -

You could presumably build a 3/4" thick cardboard disc (maybe using cardboard honeycomb, or a couple panels of cardboard spaced apart by other similar means, like 1/2" slices of paper towel rolls or foam or similar to yield a 3/4" thick disc) as the radiator membrane.

And you could presumably use two surrounds - one glued inside your enclosure and to one side of the disc, and one glued outside your enclosure, and to the other side of your disc. That would be cheap, still pretty easy, and provide both a front and rear surround.

It's also very similar to how Earthquake made passive radiators - I was trying to hunt those down but had no luck. I did get to handle them at a CES show years back, and I recall that they had drilled a few big holes in the surround on the back side, presumably to let air out that might otherwise restrict motion - as the disc moves in and out, the air trapped between the two surrounds must change. Just something that might be good to note if you were going to build a PR like this to play with... just include a way to vent that air out - on one side only of course - one surround must stay fully intact without holes.

Also - did anyone in this thread ever end up building anything with PR's? How did they come out?
I'm noting some people commenting about the acceleration/deceleration (and cornering) - but realistically PR's are relatively high mass / low compliance devices - a theoretically "ideal" one (with a low resonant frequency) is a heavy disk without much compliance to hold it in place, other than box air compliance... which won't get you very far with gravity, if mounted facing up. Might seriously be a reason to consider a spider or just knocking the whole motor off a cheap sub instead.

Just curious if these worked.

Also, I recall reading somewhere about using a standard sub with a potentiometer wired across the voice coil terminals - presumably, adding resistance would increase Fs and compliance [in motion] as it would cause the motor to generate current across the potentiometer, yielding a tunable PR. Maybe those cheap Chinese subwoofers could be worth something after all...


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## cubdenno (Nov 10, 2007)

geolemon said:


> Not to raise a thread from the dead frivolously, but I'm building a box for a pair of 8's now, and had sold a car before I could have implemented the fiberglass rear deck and pair of 10" flat-panel passive radiators that I had purchased... would have been less than half a cube total in that flat space - so now, dying of curiosity, I'm going to probably do the same enclosure, less fancy, just a box for the trunk of the new car, to see how it would have turned out.
> 
> Specifically on topic here, with regard to the original poster's idea (and possibly building on what the post immediately above was trying to state, which wasn't clear to me) -
> 
> ...




I used the APR15's in a design for a 15" sub. I loved them. I am passing them down to my son for his build with a couple of 12's. I wanted the ability to alter tuning as i wanted to get the FR that matched the enclosure/woofer/vehicle and my goals. A passive radiator that allows the addition or subtraction weight easily... It can be a game changer. it just add expense. 

Now making your own PR's... I think it could be a fun project but I myself wanted something tried and true that I would not have to design, engineer,make and modify Version 1-100 on design changes :blush:

I like cheap as well as anyone, but anymore my time is worth something as well. But if it's fun for you.. Do it!


Try a PR though. it makes bass reflex so much easier. And the sound... OH yeah!!

edit:here is what I have
http://www.creativesound.ca/details.php?model=APR15

They are not the prettiest... no where near the good looks of the TC Sounds version but when I bought them... They were 1/4 the price


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## 94VG30DE (Nov 28, 2007)

A 10" PR new is $26. There is no way I would dump any time into making something custom when a pre-built solution is available for that cheap. Passive Radiators in the Speaker Components Department at Parts Express | 26 

A "blown" sub for cheap would be great too, as most people basically give those away.


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## HondAudio (Oct 19, 2006)

Would really small passive radiators like this one work for small wideband drivers? Could you put a ~3" wideband driver in a short length of pipe, with the passive radiator on the other end, and get any improvement in bass response when mounted on the dash?


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## 94VG30DE (Nov 28, 2007)

HondAudio said:


> Would really small passive radiators like this one work for small wideband drivers? Could you put a ~3" wideband driver in a short length of pipe, with the passive radiator on the other end, and get any improvement in bass response when mounted on the dash?


The general rule is that if porting makes it better, PR makes it better (in less space).


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## Patrick Bateman (Sep 11, 2006)

HondAudio said:


> Would really small passive radiators like this one work for small wideband drivers? Could you put a ~3" wideband driver in a short length of pipe, with the passive radiator on the other end, and get any improvement in bass response when mounted on the dash?


If you have a B&O store nearby, you can listen for yourself:










Those Peerless PRs are used in one of their speakers I believe.


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## HondAudio (Oct 19, 2006)

Patrick Bateman said:


> If you have a B&O store nearby, you can listen for yourself:
> 
> 
> 
> ...


I was referring specifically to the Dayton RS75s or RS100s


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## HondAudio (Oct 19, 2006)

cubdenno said:


> edit:here is what I have
> Creative Sound - Product Details
> 
> They are not the prettiest... no where near the good looks of the TC Sounds version but when I bought them... They were 1/4 the price


That's not a _terrible_ price, considering it's a 15" model and comes with all those weight discs. It just seems like a lot of money considering the cost of the materials


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## geolemon (Aug 15, 2005)

94VG30DE said:


> The general rule is that if porting makes it better, PR makes it better (in less space).


This is an interesting point worth exploring.

My original thought was that the space savings would be due only to the fact that you don't have a port (of potentially large volume) existing in the enclosure.

In modeling my PR enclosure - granted, it's for a pair of 8's - was that the space requirement was even lower.
...which to me still means this has to adhere to the iron law. As you go smaller, you trade off either efficiency or extension (or both). If you are maintaining extension, you have to be trading efficiency.

Also, if you make the box too large, you start to hit the limits of the PR suspension - excursion increases with efficiency of the system. So "better" still should have those qualifiers and considerations... and is a key reason WHY you go larger with the PR - or to multiple PR's, as then excursion on the individual PR's will be less.

How much less space would you generally expect (factoring out any efficiency loss) for a PR system to replicate, let's say, the same frequency response curve? Just as a general rule of thumb to note, would be good as a reference when making those early decisions to go vented vs. PR... 

especially since power is pretty cheap these days. Back in the early 90's when I got started class D didn't even exist... :laugh:


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## 94VG30DE (Nov 28, 2007)

I was just referring to the volume saved from not needing internal enclosure port volume. For exact calculations I would still be using software of some sort. If I was any more specific with my statement it wouldn't have worked as a generalization


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## t3sn4f2 (Jan 3, 2007)

94VG30DE said:


> I was just referring to the volume saved from not needing internal enclosure port volume. For exact calculations I would still be using software of some sort. If I was any more specific with my statement it wouldn't have worked as a generalization


I ran into this thread and software the other day. Looks pretty good.

Designing with the New Creative Sound Passive Radiators - Part 1

Loudspeaker Design Software


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## HondAudio (Oct 19, 2006)

What would be the effects of using a passive radiator if it was venting into a dashboard, completely separated from the front wave?

i.e. - instead of having some midranges on the dashboard running IB, or even in small aperiodic enclosures, their backwaves were venting into the dashboard via the PRs? Would port-like benefits of the PR be nullified because the PR's output would be "trapped" behind the dashboard?


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## minbari (Mar 3, 2011)

PR and ported behave almost exactly the same. If you can use ported, you can use PR


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## mitchyz250f (May 14, 2005)

I know this thread is old but I thought I would post this anyway a "Passive Radiator Repair Kit" from Speaker Works. $15.00!

12" Passive Radiator Speaker Repair Kit


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## GWijaya (Dec 8, 2011)

i remember did post on this forum about building Home Made Bandpass Passive Radiator enclosure for a 1964 Lincoln.i have 2 Bandpass Passive Radiator installed in the 1964 Lincoln and they do sound loud.Driver used are just our local(Indonesia) made.Any advice please?Thanks before.


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## GWijaya (Dec 8, 2011)

http://www.diymobileaudio.com/forum...-passive-radiator-enclosure-1964-lincoln.html


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