[floresjc]

Ok, so I have a question about passive radiator systems. I talked to Jim last night about these type of sub/woofer sections and he mentioned 2 key requirements. One is that the total area of the passive radiators is greater than the area of the driver, and the other is that the xmax is also greater, so that full excursion of the driver won't bottom out the passives.

For example, the HT4 woofer uses a 12" driver with dual 12" passives I believe. What are the tradeoffs of using different size passive radiators? I'm looking to use a 15" Rythmik driver, and Jim mentioned that dual 12" passive radiators would probly work fine, or dual 15" passives as well. I've seen people build subs with 12" drivers and 18" radiators and so forth. What changes as play with the size of those passive radiators? Is there a key to being "optimum" for a given driver and cabinet size?

I wouldn't want to go out and have a 15" driver and dual 18's if it would perform the same as a 15" and dual 12's or any other combo. I'd simply like to buy the best setup.

[Jeff B.]

Ok, so I have a question about passive radiator systems. I talked to Jim last night about these type of sub/woofer sections and he mentioned 2 key requirements. One is that the total area of the passive radiators is greater than the area of the driver, and the other is that the xmax is also greater, so that full excursion of the driver won't bottom out the passives.

For example, the HT4 woofer uses a 12" driver with dual 12" passives I believe. What are the tradeoffs of using different size passive radiators? I'm looking to use a 15" Rythmik driver, and Jim mentioned that dual 12" passive radiators would probly work fine, or dual 15" passives as well. I've seen people build subs with 12" drivers and 18" radiators and so forth. What changes as play with the size of those passive radiators? Is there a key to being "optimum" for a given driver and cabinet size?

I wouldn't want to go out and have a 15" driver and dual 18's if it would perform the same as a 15" and dual 12's or any other combo. I'd simply like to buy the best setup.

As you go lower in frequency the displacement requirement increase significantly. For a vent this means it must be larger and larger in diameter in order to avoid excessive air velocity from the port. The larger diameter requires the port be larger in order to tune to the same frequency. Before long the size of the port is too big to be practical. These demands are proportional to the amount of displacement that the active driver possesses. This is the reason why passive radiators are used so often with high excursion woofers, like the ones Jim uses, if a vented alignment is being pursued. It is the only way to tune to a low frequency and still allow enough air displacement to be adequate to avoid high port noise and compression of low bass output.

At Fb (the cabinet tuning frequency) the excursion demands fall almost entirely on the passive radiator (or port in a vented speaker). The driver's displacement is reduced about 20dB at this frequency and it's travel is very low in the narrow frequency  range. Because of this, as a general rule, passive radiators need to move at least twice as much air by volume as the woofer is capable of moving. This can come from a combination of increased surface area and increased excursion, but the total volume needs to be about twice that of the woofer -as a minimum. The the best low bass reproduction without distortion or compression it is best to go with as much passive radiator displacement as is practical. The bigger you can go, the better, but I understand that this needs to stop somewhere.

With a standard 15" woofer with a typical higher excursion passive radiator using one 15" radiator would probably be acceptable. However, when using a woofer like the Rythmic 15" with its high excursion cababiliies I would not recommend less than two 15's to keep up with it, and even this may push the passive radiators if a lot of very low frequency demands are placed on the system (ie; a lot of signal below 20Hz).

The other question is - Is the Rythmic 15" an appropriate driver to use in a vented / passive radiator system.I have not modeled it to see. Perhaps Jim already has, but I don't personally know at this time. I also know there has been some talk of using the Rythmic servo system with passive radiators. I am not sure how Rythmic is doing this. If the woofer's cone motion is braked at Fb and reduced by 20dB then the sensing coil on the woofer signaling the servo would see this, and not the output from the passive radiator and this would cause the servo to make an incorrect correction. Maybe Brian Ding at Rythmic has worked out some circuitry to deal with this, but I am not sure how he is doing it accurately as this notch will change for different systems.

If it is not servo then it's just a matter of matching the woofer, box, and tuning.

Jeff B.

[Woolz]

Jeff, years ago I built in to the end wall of my listening room a pair of 24" Hartley woofers using the rest of the basement as an infinite baffle (I no longer live in that house).  In my discussions with Hartley about bass they said that the greater the surface area of the reproducer the more natural the reproduction.  This is also echoed by Magnepan and Legacy designers as well as others.  So is the use of passive radiators a good way to accomplish greater surface area in which case you would use as many and as large as practicable?  And is this one of the features of the HT4 that makes its bass response so effective?

[Nuance]

I too would not use less than two passive radiators.  And to ensure the most depth possible, I'd use dual 18's.  This will also help ensure the box doesn't move around when the woofer is slammin' away.  Of course, dual 15's would work, but it might not extend as low.  Although, room gain will help, so I guess it just all depends on your room and a few other factors.

How big of a cabinet do you want?  That will probably be your decision maker right there.

[floresjc]

Well I talked to Brian Ding today on the phone, and he was able to address my questions. Given the long work hours I had today and the highly complex nature of his answer to Jeff's question at the end, I can't repeat it here in any coherent manner. But I left the conversation assured that he has modeled and had systems built like what I was looking for with no problems. His recommendation was to go with a 15" servo driver and two 15" passives, and something like 4 to 5 cubic feet as a box size. I think that roughly comes out to 22" on each side for a perfect cube. He recommended a tuning of 18hz.

I'm sure I will get even more feedback from Jim when I place the order this weekend, this is going to be one of the best subs I've heard, I'm sure.

[floresjc]

Nuance -

I'm not tied to any particular cabinet size, although the box Brian recommended seemed plenty big for either dual 15's or dual 18's as the passives. I wouldn't be unhappy with a 22" cube, its quite a bit bigger than a SongSub, but its not enormous either. I'll probly talk to Jim and see how big a box either system would be, and if I can't live with the bigger size I'll stick to the 15's. I'm pretty flexible at this point, the wife will go for anything as long as its a cube or wider than it is tall.

[Nuance]

floresjc -

That sounds perfect dude.  22" with daul 15" passives and the main 15" driver will sound great!  An 18Hz tune sounds about right and will leave a little headroom.  Pair that will your room gain and you'll be flat probably to 10Hz.  Nice!

Ive got dual 20" ^3 using 15" woofers (not passive, though), and the size isn't too bad at all.  Another 2" won't be bad. 

I had the pleasure of hearing a 15" Acoutic Elegance woofer with dual 18" passives in a 24" enclosure last year at the WI subwoofer GTG, and it was my favorite DIY subwoofer along with the 18" LMS.  I think you'll be VERY pleased with the 22" 15.  If I had pay for a completed sub, I'd go the route you are and have Brian/Jim collaborate on a passive sub or two. 

Did they happen to give you a price estimate on what something like that would cost?  If you cannot share that publicity - no prob. 

[Jeff B.]

After posting, I modeled the driver today myself using my own sofware, which anyone can download, by the way. (Bragging here,  but I find my passive radiator model to be the most complete I have ever found. It will tell you exactly how much mass to add and recalculate all of the parameters with the added mass. It has proved to be very accurate, as Dennis' measurement of the HT4 match my software on the nose - OK, enough bragging  ) Having said that, I came up with almost exactly the same thing you have here. This woofer is a serious BASS driver. I would recommend 4-5 cu ft and would tune it to about 18 Hz as well. Two 15 Radiators will be perfect. My software predicts flat response to 21Hz with no servo, but a only a small boost of 3dB, so with a servo you should be able to get to whatever Brian programs it for. It will also get honkin' loud too, as this combo doesn't push linear Xmax until you are hitting about 114dB and with room gain will likely do this same level at 20Hz (with enought power, that is.) It should be an awesome Subwoofer. I would be glad to offer my assistance in the design process if it is needed in tuning the radiators.


I will also add that the same woofer in 3 cu ft sealed with the Oaudio 500 Watt amp set to the 16 Hz setting will yield response to 16Hz. It just won't play as loud ultimately as the larger PR unit does, but loud enough for most folks I might consider one of those for myself sometime. The bigger box won't fit well in my room.

Jeff B.

Nuance -

I'm not tied to any particular cabinet size, although the box Brian recommended seemed plenty big for either dual 15's or dual 18's as the passives. I wouldn't be unhappy with a 22" cube, its quite a bit bigger than a SongSub, but its not enormous either. I'll probly talk to Jim and see how big a box either system would be, and if I can't live with the bigger size I'll stick to the 15's. I'm pretty flexible at this point, the wife will go for anything as long as its a cube or wider than it is tall.

[Jeff B.]

Actually, if the 15's and the 18's are tuned to the same frequency then the response and the extension will be identical. The difference would be if the 15's don't have enough displacement at high output, then the radiators will begin to bottom out. However, using decent 15" radiators in my model that should not be problem. The 18's offer more ultimate output if the system can be pushed that hard, but under 500 Watts it won't make a difference.

Jeff B.

I too would not use less than two passive radiators.  And to ensure the most depth possible, I'd use dual 18's.  This will also help ensure the box doesn't move around when the woofer is slammin' away.  Of course, dual 15's would work, but it might not extend as low.  Although, room gain will help, so I guess it just all depends on your room and a few other factors.

How big of a cabinet do you want?  That will probably be your decision maker right there.

[Nuance]

Whoops - right you are, Jeff.  I said "extension" when I should have said "output."  But like you said, with an amp under 500 watts it won't matter.

Thanks for the correction.

[floresjc]

Jeff -

Thanks for the model info. I'm really glad to hear this could be one heck of a subwoofer for me. I had suspected as much, since Jim turned me onto a servo passive radiator system, as being one of the best all around subs you could find. I'm also grateful for your offer to help in the design process, its really great to have so many good system designers work on something for me. I'm going to call Jim tomorrow morning and place the order, I'll be sure to speak to him about what ideas were tossed around here so we can move into the build phase!

Nuance -

I'm not sure what this is all going to cost yet. I have provided Jim with veneer for a custom finish, and he quoted me $1500 for a 15" Rythmik system in a sealed configuration using that custom veneer. Whether that goes down if one would choose a standard veneer, similar to his other offerings/pricing, I do not know, this has been custom since the beginning. To move from that sealed configuration to the dual passive radiators, he had said that the additional cost would be the price of the passive drivers and the extra labor involved in mounting them into the system since a few more cuts would have to be made and finished. A ballpark range I was give was $1800-$2000, depending on the final passive drivers and so forth being selected. All told, I probly will have $2k in it, including the veneer I found.