Jim, I had a question or two regarding the quote below from one of your posts. The Rythmik sub that you compared to the song sub was presumably crafted and altered in your shop and had different specs (size, amp etc..), please correct me if I'm wrong.
The Rythmik subs we have built to date use the same drivers and amps as the stock Rythmik product. The drivers have an extra voicecoil that is used for the feedback circuit in the Rythmik servo amp. And the amp itself contains the servo circuitry. So if we do a Rythmik design, we have to use their drivers and amps.
Rythmik subwoofer cabinets are made from well-braced 3/4" MDF. When we build them, we use either 1" or 1 1/2" MDF and similar bracing. So our cabinets would tend to have slightly less cabinet resonance (and would be quite a bit heavier, of course).
The most recent sub we built for Josh, uses two 15" passive radiators rather than a port. This allows the sub to play as deep as it would if ported, but there is no port noise. When you get down to 20Hz or lower, the air velocity out of a port is quite high (you could probably dry your hair with it
). You can get rather severe port "chuffing" from this type of set-up. Using passive radiators increases the parts and labor costs in building a sub, but eliminates the possibility of port noise.
Keep in mind here that there is nothing magic about subs. Take a great driver, give it a well-braced cabinet with the volume it requires, supply enough wattage and you're in business. When compared to stock Rythmik subs, we can certainly build more inert cabinets and we can employ passives. But I think the main reason people have us build their Rythmik subs is that they can have the exact same finish as their speakers.
Therefore, I assume that the modified "Rythmik sub" had been improved over the original design that one would purchase from rythmik. If that is the case, then I also would assume that if a stock rythmik sub was run head to head with your songsub, it might not compare as favorably?
I wouldn't say that. I would say that a stock 12" Rythmik sub and a SongSub would be very comparable. The Rythmik sub may have slightly lower distortion due to the servo circuitry. (That said, the SongSub is a very low distortion (VLD) design.) On the other hand, the SongSub has more power and uses a passive radiator, so there is no port noise. Performance of the two subs would be relatively similar, but the SongSub's added power, coupled with the use of the passive, might be an advantage to some users.
If you compare the SongSub to the 15" version of the Rythmik, that is another story. The 15" Rythmik will play deeper than the SongSub and with more authority. There is simply no substitute for the added displacement of a 15" driver. If we built a custom sub with a 15" driver and a pair of 15" passives, the playing field would be more even. In that case, it would come down to a trade-off between and servo circuitry in the Rythmik and the added power (and perhaps the use of passives) you could have in a custom 15" sub. (The 15" driver we would use in a custom sub would essentially be the same driver without the servo voicecoil.) Since we have never done a comparison, I don't know how they would stack up.
As I indicated above, I think Brian is working on a higher power servo amp which would eliminate the power disparity. In that case, I would tend to choose the Rythmik set-up since the cost would be similar and the set-up would take advantage of servo technology. And we could still use thicker walls and perhaps passive radiators resulting in the best of all worlds.
I hope this helps.