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Old 28th February 2005, 10:24 PM   #1
Rave is offline Rave  Latvia
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Default Rebuilding my old boxies.....

Hello!

Thought a while where to post my message- into full range speakers thread or subwoofers... I'm I kind of traditionalist and wanting to have subwoofer together with full range speaker. :-)

OK, actually up to now I had a little practise with closed and vented boxies. At present I have selfbuilt vented boxies. But as speakers are bad, week and detorated, I decided to change them to Peerless XLS 12" series. After browsing a lot in web, I understood that it is better to use these speakers in PR mode in my situation. Box size is a bit over 50 liters. If I'm right, PR offers more or less, but good compromize between closed box and vented.

So, for further use this picture- http://www.transalp.lv/audio/boxies.jpg to follow my thoughts. In (1) is present situation.

(2) is how I think would be best to realize this- using two PR's. The thing I'm concerned is- how stable speaker box will be, because on PR's are quite big moving weights- will it shake or not? Box is almost 80cm in height. Lowest PR is on same axis as active speaker.
Good thing must be, I think, that I usually keep speakers not far from wall with back side, so in this case will be something like vented box, because everyting from PRs wil be reflected from wall towards listener.

(3) the same, only with one PR. Is this worse?

(4) one PR in front to compensate others movements, both on same axis.

(5) same as (4), backside PR on same axis as active speaker.

(6) PRs on sides, on same axis also. Probably not very well, because I need to put boxies close with these sides to something, at least from one side, because do not have much space in my flat...

Waiting for some advise, suggestions.

Thank you for you time! :-)
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Old 28th February 2005, 10:32 PM   #2
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Quote:
If I'm right, PR offers more or less, but good compromize between closed box and vented.
You are wrong. A PR achieves exacty what a vent does, just without the wind noises and impossibly long vent in a small box.

I think any of your choices would be OK except for #4 as it would be top heavy. Whether you go for 1 or 2 PRs will depend on budget and how loud you want it to go. A single PR is probably OK.
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Old 1st March 2005, 12:26 PM   #3
Rave is offline Rave  Latvia
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Quote:


You are wrong. A PR achieves exacty what a vent does, just without the wind noises and impossibly long vent in a small box.
Well, I have red very different opinions even here in some threads... I think, that PR cannot achieve exactly same, because simply in case of vent active driver can move more freely as resistence for air in vent is less than in case of PR- active driver have to push also PR's which have more resistance than vent... That's why I think that PR's are compromise, because you can get closed box with expandable volume...
But vents are noisy, my present boxies makes noises under max power...

Quote:
I think any of your choices would be OK except for #4 as it would be top heavy. Whether you go for 1 or 2 PRs will depend on budget and how loud you want it to go. A single PR is probably OK.
Yes, a budget is another question, fortunately PR costs not so expensive comparing to active driver. Sad thing also is that Peerless drivers in USA costs almost 2.5 less than in Europe.

I'll try to go for 2 PR's for each boxies. Seems, that I will assemble some test box just for tests to get some practical experience. I also have access to acustics laboratory...
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Old 1st March 2005, 12:42 PM   #4
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You maybe getting confused with variovents and PRs. A variovent provides some acoustic resistance, lowering the Q of the tuning. A PR really does work exactly like a vent. A vent may seem like it's easy to push because it's just full of air, but that air resonates in anti-sympathy to the driver and thus becomes 'heavy' or hard to push.
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Old 2nd March 2005, 01:40 PM   #5
Rave is offline Rave  Latvia
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You maybe getting confused with variovents and PRs. A variovent provides some acoustic resistance, lowering the Q of the tuning. A PR really does work exactly like a vent.
As written in diysubwoofers.org:

"The response of a passive radiator system is similar to that of a ported system using the same driver. However, the cutoff (-3dB) frequency is slightly higher, and the cutoff slope is deeper, mostly due to the presence of a "notch" in the frequency response corresponding to the passive radiator's resonance frequency. However, this notch is normally located far outside of the passband of the system, and therefore usually of little audible significance. The larger the passive radiator, the lower the passive radiator's resonance frequency (for the same target Fb), and the further the notch is out of the passband."

So, vented is not exacty same as PR system. But similar.

I know about variavents also.

Quote:

A vent may seem like it's easy to push because it's just full of air, but that air resonates in anti-sympathy to the driver and thus becomes 'heavy' or hard to push.
Probably you are right. But anyway resistance against slower driver movements will be less in vented box than in PR. When speeds are getting higher, then air also have resistence, for sure.
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Old 2nd March 2005, 02:05 PM   #6
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Interesting claims on that site there. It's the first time I've come across something like that.
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Old 2nd March 2005, 08:43 PM   #7
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The self-resonance of a PR is often ignored, but important. If you can find the llambda list (maybe in the BASSlist too) archives there is quite a detailed thread on PRs... John at Stryke (and the Stryke forum possibly) are good PR info resources... typically you want to have more PR than driver -- i'd go with scheme 6.

dave
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Old 2nd March 2005, 10:10 PM   #8
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Of the options, number 6 would be the best. We almost always use a a pair of PR's on opposing sides. IT cancels net forces on the cabinet and non-linearities. This means your box won't move around much.

You also want to have your PR's displace 2-4x as much air as the active. So typically a 12" woofer would get a pair of 12" PR's or a pair of 15" PR's even. That is what we do in our bigger systems.

For more general PR info check here:

Passive Radiator FAQ

John
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Old 3rd March 2005, 12:37 AM   #9
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Quote:
But vents are noisy, my present boxies makes noises under max power...
That's due to a design flaw.

The ports just should have been made bigger.

Mvg Johan
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Old 4th March 2005, 06:10 AM   #10
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The problem is that ports can't always be made bigger. We do a 3 cubic foot enclosure with 12" woofer and dual 15" PR's. An equivalent port to move the same amount of air would be 18" diameter and something like 56 feet long. Even a single 6" diameter vent would be something around 70" long to get a 16hz tuning.

In a small box, PR's are the way to go. In larger boxes, they offer no benefit over a port, and a properly done port is actually better than a PR because it has no losses due to suspension. With the majority of the world wanting small subwoofers though, PR's are about the best option out there.

John
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