Passive Radiator - A detriment when on the same plane as the driver? - AVS Forum
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post #1 of 5 Old 05-09-2013, 04:02 PM - Thread Starter
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I have no professional experience with sound, but have a great ear. Any time I would go into a store, and put my head between two speakers sitting on a shelf, I swear, about half the time, the store geeks set them up wrong, because I could very clearly and instantly tell that the speakers were out of phase - they wired one backwards - and half the bass is missing.

So because of my experience, I understand the importance of speaker "phasing" (+ and -) is just to make sure that the speakers are both popping the same direction at the same instant. With one wired backwards, the left speaker will push out while the right speaker is pulling in. This results in, if you are listening to some content with adequate bass, one speaker absorbing the bass that the other one is putting out, then a reversal at the change of the next frequency crest. Doesn't look like much on paper, but when your HEAD is smack dab in the middle of two such out of phase speakers, it's night and day. reverse the + and - of EITHER speaker, even if you end up having both wired backwards, and the sound will be good again, and you'll have ADDITIVE bass with both speakers, rather than HALF the bass (sounding hollow). That was the preface for my question: If you have a cabinet with two active 8 inch woofers, let's say, and phase them correctly, both will fire OUT at the same time, which is normal to get your speakers in phase and the correct bass sound. Now let's say this speaker uses one 8 inch woofer, and a passive radiator? If this is a sealed cabinet, then call me a liar - but won't the passive radiator be out of phase with the active woofer? Of course it will. As the active woofer pushes out, it is pulling air from inside the cabinet, thus PULLING in the passive radiator code while the active woofer is pushing. This to me is a total bogus design? Now you could fix that by putting the passive radiator in a sealed second area of the cabinet, but then it won't make ANY noise. So my guess on possible correctly phased passive radiators using a second sealed compartment is completely not happening either.

This hit me when I read in another thread that passive radiators act as tuned ports. I get this, as they are both a hole in the speaker, with the ability to change the frequency response heard outside the speaker cabinet when it is moving.

Now I know for certain, from my earlier confirmations of being able to hear out of phase speaker pairs with my head in the middle, that you definitely want both speakers PUSHING at the same time on the save crest of the same frequency, and that is WHY there's a + and - on the speakers. I don't get to constructively talk with anyone about my theories so I am glad I found this site, but one of my buddies said speakers are DC current and thus the + and - on them, didn't I know that? I laughed, and tried to explain they were for phasing them only, and that they are really alternating current, and it works just the same if both of them are wired backwards, just don't have one correct and one incorrect or you'll have sucky sound, but you won't BLOW any speakers. Unless you have more than 2 speakers, in which you need to phase all of the the same and correctly. So that is what I know for certain. What I am questioning is, since a passive radiator on the same plane as an active driver will result in one PUSHING and one PULLING and thus be out of phase, isn't this TABOO? But I know this design exists. I've also seen passive radiators on Subwoofers. Sounds OK, but again thinking of this, it occurred to me: with the above dilemma about passive radiators out of phase if located on the same plane as an active driver, if you put the passive radiator on the BACK (opposite) side, then sure, one is pulling while the other is pushing, but in this case, the air around the box is in unison – and the passive radiators reverse polarity of air movement makes sense. So it seems to me that ONLY if a passive radiator is located on the opposite side of a cube, then it can have a positive effect for sound. I think I’ve even seen passive radiators on the SIDE (90 degrees) of a cube opposite the active driver.

I am probably all washed up here, because in my head it sounds like I have made some sort of breakthough in thought and dismissed engineers who have made sealed enclosures with passive radiators on the same plane as the driver. I am guessing that while I may have some theoretical accuracy, maybe the harmonics/volume value of the passive radiator outweighs the fact that it is pulling when the driver is pushing? I dunno and why I am questioning this.

If anyone can set me straight on this, it would be great. I am just about to try my hand at building another speaker enclosure, this time a JBL 15 inch subwoofer with a tuned port in a big cube cabinet, with a power amp inside. I’ve built a few speaker enclosures in the past, with good results, after referring to my 25 year old radio shack book “building speaker enclosures”. I have fun with it, and am an enthusiast speaker maker. But I’ve always wondered about passive radiators and their effect on air displacement phasing.
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post #2 of 5 Old 05-09-2013, 04:26 PM
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Ported and PR designs are two different but closely related types of "bass reflex" designs. The key part you are missing is that at the tuning frequency of the port or PR, the driver is moving very little due to the reflexive resonance of the port or PR in the box. Since the driver is not moving much, it isn't making much sound to cancel with the port or PR.

Exactly what you are worried about happens BELOW the port tuning, and is the cause of the rapid roll off seen in bass reflex designs below the tuning frequency.

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post #3 of 5 Old 05-09-2013, 05:48 PM
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Quote:
Originally Posted by Steve Doane View Post

I have no professional experience with sound, but have a great ear. Any time I would go into a store, and put my head between two speakers sitting on a shelf, I swear, about half the time, the store geeks set them up wrong, because I could very clearly and instantly tell that the speakers were out of phase - they wired one backwards - and half the bass is missing.
When you put your head in the middle they'll be 180 degrees out of phase at one frequency and there will be a null, but only at that frequency in that spot. What counts is what happens at a normal listening position.
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Now let's say this speaker uses one 8 inch woofer, and a passive radiator? If this is a sealed cabinet, then call me a liar - but won't the passive radiator be out of phase with the active woofer?
No, because the air volume of the box acts as a capacitive load which delays the movement of the PR, keeping it in phase with the woofer down to the tuning frequency of the cab. This is exactly the same manner in which a bass reflex box works. The PR output, or port output, is only out of phase with the front wave below the tuning frequency.
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post #4 of 5 Old 05-09-2013, 07:00 PM
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Originally Posted by Bill Fitzmaurice View Post

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Now let's say this speaker uses one 8 inch woofer, and a passive radiator? If this is a sealed cabinet, then call me a liar - but won't the passive radiator be out of phase with the active woofer?
No, because the air volume of the box acts as a capacitive load which delays the movement of the PR, keeping it in phase with the woofer down to the tuning frequency of the cab. This is exactly the same manner in which a bass reflex box works. The PR output, or port output, is only out of phase with the front wave below the tuning frequency.

Sure you typed that as intended? Mostly right, as the delayed output of the PR is a 180 deg delay, which is why there is the added phase shift to a reflex design vs. a sealed design at the tuning frequency. They are always out of phase, they just aren't both moving at the same time until below the tuning frequency in the 24dB/oct roll off region.
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post #5 of 5 Old 05-10-2013, 08:25 AM - Thread Starter
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Thanks guys for your responses. Again having no professional experience, and my only background was reading an old radio shack book "building speaker enclosures". I wanted to be an audio engineer, and build or design speaker cabinets, but life took me in a different direction, so now (at age 50) I settle for this as a passionate hobby. It never dawned on me that there would be such a thing as capacitance delay, which would slow down in time, the point where the PR would pull in when the active pushed out. This seems then to be a critical factor when building either a tuned port or PR design cabinet, correct? As the internal cabinet air volume increases (starting from a theoretical zero), the time at which the PR would pull in vs the active driver pushes out will increase. So this means I think you would want to pick an idea cabinet size that causes both active driver and PR to move in or out at roughly the same time. What I don’t' know, and maybe you an answer, is if, in a practical size cabinet, in this case for a subwoofer only cabinet, whether 1 - it is possible to achieve this without having the cabinet be the size of a refrigerator. And 2 - If it is possible with a "normal " size cube design, then perhaps it is critical to design the amount of movable air inside the cabinet to be exact, since if you are even a little bigger than optimal, you start to go the other way, and make the active driver and PR out of phase again.

And all this is frequency based I gather. Meaning at 1.5 times the frequency (60 hz as opposed to 40 Hz) maybe it all acts different? What I am realizing is that there's an awful lot of theory that goes into the deisgn of a speaker cabinet, especially at the lower frequencies, and there's likely often a tradeoff between theory, and how it actually sounds, given the materials you are using, that might mean certain theoretical formulas can be overlooked or left out of the design because another design factor is 3-5 times more prominent that affects the final listening experience.

I am all amped up again recently, because I am about to upgrade my home theater sound system, and found that the most expensive and critical piece is the subwoofer. With my home theater and my desires, I need to buy a sub that goes for $1,000 minimum, but I don’t have that kind of money. It’s been a while since I built my last enclosure, and it dawned on me that I might build my own powered subwoofer. I looked on Amazon, and can buy a JBL dual voice coil 1400 watt peak subwoofer driver for only $150. I also can buy a matching barebones amp for about $100. It would appear one could make their own 15 inch 1,000 watt powered subwoofer with a honking JBL driver, for around $250 + wood + wires/connectors, that should roughly compare to subs costing $1,000 or more easily.

Can any of you point me to any reference material that will help me design a cube or rectangular cabinet for a tune port design for a 15 inch subwoofer driver. I’d like to take a stab at this for my next cabinet making project, and looking to start designing right away. I think at this point I can way too easily get lost on all the theory around individual design elements, and how each one can affect sound adversely. It would be nice if someone in a book somewhere spelled out, from most important, to least important, the considerations for making a bass cabinet sound good, as opposed to books where 100 different points in theory are described for your understanding, leaving you without a clue as to which elements are actually important to the final listening experience for your intended deliverable as a cabinet.

My other way of going about this would be to try to check out in person other 15 inch powered sub cabinets with tuned ports, and start with the port dimensions that someone already used for a retail design. Of course, I understand I have no way of knowing what the inside of the cabinet looks like, if they used filler wood as part of the capacitance and to tweak the air volume design, and I heard that tune ports often have “cones” rather than just an open box hole, that contribute to the tuning point (length of cone, shape, etc). While it seems it’s advanced calculus to exactly design ANY enclosure to theoretical perfection, my HOPE is that me as a proud individual enthusiast, and maybe my close family and friends, listening to my home theater home made mega subwoofer, might find that it sounds nothing less than spectacular, even if I miss a key element or two in the design. My other question is, are there any inexpensive TOOLS I can buy to analyze the sound, and give a visual representation on the tuning and frequency response, so that I can make tweaks to the original design even post construct?
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