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  1. #1

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    Question I dont understand Passive Radiators...

    I just dont understand how those huge passive radiators on those SDA towers produce tight bass? Or do they? From reading this page the idea of how they work and why more power helps make the passive radiator more effective is somewhat beyond me. This was my reference reading that didnt quite explain it in language I completely understood.

    What causes the tuning fork-like motion? Blastmyspacebarisdying...

  2. #2

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    yupthespringisdefinitelydeadhahaha:D

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    Since you didn't understand what you read, I suggest you try to get your listen on. That will be all you need to know.
    'Political Correctness'.........defined

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    From the DIYsubwoofer.org page:

    "Passive radiator systems are very similar in operation to ported systems. However, instead of a port, the passive radiator system uses a passive radiator to extend the system's low frequency response."
    HT/2-channel Rig: Sony 50 LCD TV; Toshiba HD-A2 DVD player; Emotiva LMC-1 pre/pro; Rogue Audio M-120 monoblocks (modded); Placette RVC; Emotiva LPA-1 amp; Bada HD-22 tube CDP (modded); VMPS Tower II SE (fronts); DIY Clearwave Dynamic 4CC (center); Wharfedale Opus Tri-Surrounds (rear); and VMPS 215 sub

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  5. #5

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    Okay. Lemme explain. No, lemme sum up...

    You have your active drivers. Depending on what speaker you own, you may have more of less active drivers. In my monitor 10's, there's two active. In Monitor 7's and 5's there's one.

    Depending on the number of active drivers, your passive radiator can be a number of sizes. On my monitor 10's, it's a 10" radiator. the 7's have an 8", i believe. SDAs, you're looking at 12" or 15". The move active drivers, the larger the PR you can use and get away with.

    The passive radiator IS NOT POWERED. It has no wires hooked up to it, no voice coil, no magnet assembly, nothing. It's just a suspended cone w/ a spider to return it to resting position like a normal speaker.

    This is where the term "fluid-coupled" comes into play. All passive radiator designed cabinets are sealed. This is the only way they will work. The term "fluid-coupled" is used because, technically, air is a fluid (remember from HS science :)). When the active drivers move in a passive radiator system, they apply either a vacuum (if they're moving out) or pressure (if they're moving in) on the air inside the cabinet.

    This, in turn, moves the passive radiator (the the opposing direction of the active drivers). Now, we can all agree that a speaker is simply a device for moving air. The faster it moves the air, the higher the pitch. So, by moving the cone of the PR, you create sound. It can reach impressively low, lower than the limits of the active drivers themselves.

    It still amazes me every time I listen to my Monitor 10s and how much sound can be produced out of only 3 active drivers (1 tweeter and two 6.5" mid drivers).

    If anyone would like to correct anything in this post, go right ahead. :D
    Last edited by nadams; 05-17-2005 at 08:38 AM.
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    nadams, excellent reply. very basic and understandable. that's appreciated by many. this though presents a question from me.

    It can reach impressively low, lower than the limits of the active drivers themselves.
    it would be wonderful if you could explain why this is possible. i too have always wondered how.

    Thanks.
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    Well, you have a larger cone surface with the PR. The combined power of the active coupled drivers can move that radiator to achieve lower frequencies than the smaller cones would be able to alone. To a point, the larger the cone surface of the driver, the lower it can go before distorting. Distortion isn't really a problem with PR's themselves, but if the active drivers are driven to distortion, things obviously won't sound very good :)
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    Oh, okay. Thanks for the explanation. I misinterpreted your original explanation. For some reason I was thinking..."reach impressively low, lower than the limits of active drivers..." rather than..."reach impressively low, lower than the limits of the active drivers..."

    In other words, I was thinking that given the same size, yada, yada, yada, passives can lower than actives. This is not the case. Simply the larger passives, in a given speaker, can often go lower than its own actives.

    My mistake. Kinda feel stupid now ;)
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    so it seems to me cabinets with PR(s) in it MUST be sealed air tight completely.. right?

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    That is correct. If the air is getting out of the cabinet, it will not move the PR. Air, like any liquid, will take the path of least resistance. If you have a port, it will go out that port and will not move the PR, at least not as much. This is why people recaulk their cabinets and replace the rubber seals that are behind the drivers on older PR designed speakers.
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  11. #11

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    Originally posted by nadams
    This is where the term "fluid-coupled" comes into play. All passive radiator designed cabinets are sealed. This is the only way they will work. The term "fluid-coupled" is used because, technically, air is a fluid (remember from HS science :)). When the active drivers move in a passive radiator system, they apply either a vacuum (if they're moving out) or pressure (if they're moving in) on the air inside the cabinet.

    This, in turn, moves the passive radiator (the the opposing direction of the active drivers). Now, we can all agree that a speaker is simply a device for moving air. The faster it moves the air, the higher the pitch. So, by moving the cone of the PR, you create sound. It can reach impressively low, lower than the limits of the active drivers themselves.

    It still amazes me every time I listen to my Monitor 10s and how much sound can be produced out of only 3 active drivers (1 tweeter and two 6.5" mid drivers).

    If anyone would like to correct anything in this post, go right ahead. :D

    See, that is what I've always understood, but it just seems to me that this sort of system would just create bass that was slightly late since other drivers have to move first.

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    Not late... the PR is moving at the same time as the other drivers, just in the opposite direction :)
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    Ah I guess I was just thinking that the sound wouldnt be radiated properly unless the motion was toward the listener.

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    Since the passive driver is moving in the opposite direction as the active drivers, is it essentially "out-of-phase" with respect to the active drivers? If so, does this create a more diffused sound?
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  15. #15
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    Lets not also forget about the "compressability" of air. This makes passive radiators naturally sluggish. But, you know what, my SDA's and Monitor 10's still sound damn good. So, who cares.

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    I heard somewhere that a PR creates the same low notes of the active drivers one octave lower and at half the volume... I could be wrong but that's just what somone told me.

    I myself am pretty amazed at the bass that comes out of my Montor 7s. When I have people over they are amazed that the only drivers actually "playing" (active) are the single 6 1/2" drivers and the tweeter. The 10" pr really helps but at first I did have problems with the cabinets not being sealed.

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    Default Re: I dont understand Passive Radiators...

    yupper. we've all seen speaker frequency response curves, so we know the mid bass driver reproduces frequencies lower than the -3db limit and that the levels are rolled off relative to above the -3 db limit.

    the pr is weighted to change the rate of its roll off to where the sum of the roll offs of both the pr and mb drivers add up to something approaching a flatter frequency response.

    as far as phase goes, i believe the pr can't keep up with the mb, so it varies, and at some times they are in phase too.

    the drivers oppose each other, both pushing and pulling, so a more powerful amp helps by keeping a stronger grip on the drivers.

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  18. #18

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    This whole thing makes me want to buy a pair of monitor 10s hehe. Who has some? :D

  19. #19

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    PR's, sluggish??? I never noticed that. Port bloat, now that's something I do notice.
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    What if the PR's are the same size as the drivers themselves, such as my pair of 5jr+'s? Two questions:

    1. This design, will it let these speakers reach a lower note, even if they are the same size?

    2. What about midrange? I noticed really good midrange in these speakers. Is this why?
    Last edited by boltaction; 05-22-2005 at 02:20 AM.

  21. #21

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    I love my RTA-11T's. The bass is just phenomenial. Yes, a larger amp with a sufficient damping factor will defiently help tighten up the bass. The only thing I see missing from this discussion is the volume of the cabinets the drivers are mounted in. This plays a critical roll in reproducing smooth accurate controlled bass in a PR type system. Many PR systems are simply limited by cabinet volume. That's one of the reasons the big SRS's have such deep articulate bass. Look at the volume in those 6 foot cabinets. It's kinda like an 8 cylinder vs 4 cylinder in a car. There is no substuite for displacement!

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