Originally Posted by 2ms
Thanks for the extensive reply, I’ll have to review it again and process carefully. However, just quickly, did you consider using two opposing 6.5s rather than the 1 and 2 radiators? Just curious since I would intuitively think that’d potentially offer even greater power density and vibration elimination for placement off floor.
Be aware when you use two drivers the box size doubles. For my boombox, size/weight is critical so the light weight TangBand at 3.4 pounds at 4 ohms is about as large as I want to go. Also be aware that each TangBand takes TWO of those passive radiators so if you want to use two drivers--you need to use longer stroke 8" passive radiators ($$$ if available) or step up to 10" passive radiators (Dayton Audio has 10's for $25 each)
The other issue is impedanance, if you decide to go opposed driver or (optimally) Push-Pull or Push-Pull Slot Load the two drivers must be wired in parallel. The TangBands are 4 ohms which two in parallel are 2 ohms--not a problem if your amp is rated for 2 ohms (not a problem in car audio, big problem with home audio)
If I was going to hang a subwoofer on a wall--say I put a shelf piece on it like a bizarre bookshelf, I would do it this way.
The above "7 inch" woofers have 12.5mm of Xmax or a full one inch stroke. They also have a larger voice coil so can handle more power which is a good thing. The above are 8 ohm drivers so you can put two of them in parallel for a nominal 4 ohm load which can easily be driven by most plate amps. I have no idea what the box size required for those drivers, I'd need to model the T/S parameters in software but the TangBand does around 28 Hz in 0.65 cubic feet so I would not expect more than .75 cubic feet at around 30Hz. The Fs or resonant frequency is a bit higher than the TangBands but that driver (originally called "Anarchy") is used with some really narrow (but not small) tapped horns that tune to 25 to 27 Hz. Should get rather close.
I would model the Anarchy drivers tuned to around 29Hz and see how well it plays. Use a push-pull slot loading scheme so it won't look too weird and you have the ends for the two 10" passive radiators. This would mean the box will need to grow to 10.5" X 10.5" so the radiators will fit on each end. The "slot" or plenumn will be in the center of the box. You don't have to build it with such a cross section, build it to fit whatever fits your space better. Passive radiators should be opposed to each other, one on each end facing away from each other to cancel vibration.
Another option is to just build a push-pull sealed box with the Anarcy drivers. They run $66 each so $132 for the pair. You can model what they look like in a sealed box and figure out how much power they can handle. WinISD Pro will model that for you, just keep adding power until you blow past Xmax of the driver by a millimeter or two. This would allow a smaller box and much easier build--one driver firing from one end cone out as normal while the other driver has the magnet side out. Wire the magnet side out driver reverse polarity so it works properly. A very simple and smallest (and least expensive) option if hanging it on a wall is what you want. Sealed is great because as long as you get the cabinet internal air space correct, it allows you more flexibility with what shape you desire to use.
Later on, if you want to get maximum output and wish to run ported or use passive radiators, cut some more plywood and redesign the box (generally larger) to model well with either ports if you can handle the size increase or passive radiators if small as possible is key and the additional 50 bucks for 10" PRs and size demands are OK.
My boombox sub will use a single TangBand Neo 6.5" subwoofer and two Dayton Audio 8" aluminum PRs for size reasons etc. The passive radiators allow porting efficiency/tuning low but without ports, bugs/spiders etc. can't crawl in and they don't eat up space like a huge port length required for such a small box. Basically, the box size is 10" tall, 30" wide and only 7.5 inches deep with the two speakers laid on top (held in place with magnets) They will be driven with a 2.1 channel chip amp at either 24 VDC from a plug in power supply, 18V from a drill battery or plugged into a car at 12 to 14.4 volts DC. The chip amp puts out 64 real world watts into a 4 ohm load at 24 to 25 volts so I don't need or want the Anarchy because of weight concerns. Vibration is not an issue, the entire thing comes apart be it the main speakers or the chip amp and will be taken apart when plugged in. For portable use, my battery voltage is less so not an issue as sub power declines to around 20 watts when connected to a car battery. Total weight for the design is 30 to 32 pounds which is about the max I'd want to carry around--the monster boom boxes of the early 80's were about that weight loaded with 10 D cells and around the same size so I'm close.
For more information, look up push-pull slot loaded or PPSL subs on the DIY Audio site--more jibber-jabber to read to keep you occupided for a loooooong time!
I think the idea came out in the 80's from M&K subwoofers and works very well. Sure, almost 40 years later some loser companies imply it takes a genius (GE cough, cough) but they have been around for a very long time. You might have seen some subwoofers used in PA systems that had one sub cone facing out while the other sub cone was magnet side out--that is done to eliminate vibration which is a good thing when "flying" subwoofers. Vibration from subwoofers flying over people's heads is not good--things tend to vibrate out and most people get deeply offended when a 150 pound sub slams them in the head and messes up the hairdo. My dual 15" PPSL subs in the garage will shake the house but not vibrate--it is only magic until you know the trick. Never tried to hang them on my wall, they weigh about 175 pounds each so on terra-firma they stay.
How come you don't see many (any?) push-pull or PPSL subs in consumer audio? They are weird looking, two drivers cost more than one and they are harder to build or hide the external magnet out driver. Be aware if you want to build such a thing, get drivers that are known to be very quiet during their full stroke--the Anarchy is one such driver and works well. You can get vent noise (airflow out of the vent hole in the magnet of subwoofers) this makes a mess if you start cranking and the driver with the magnet side out starts making noise. Generally not a problem when inside a box but when the magnet is outside the box the noise will not be a good thing. Very long stroke drivers tend to get noisy from the vent so if you want push-pull, make an extra effort to find very quiet drivers.
If you want to start building sub boxes--a fun hobby that offers you different performance with the same drivers. The Anarchy can be sealed, ported, bandpass or tapped horn loaded so in the future--if you love cutting wood--two or four of those 25Hz tapped horns will fit under your bed.
First things first, if you want to roll your own and hang them on the wall--best to educate yourself on push-pull alignments, how passive raditors work (think tuning forks for bass!) Get a good idea how different drivers model in software, model them sealed and ported to gain a feel how they work. Not sure how much power you can throw at the Anarchy drivers, quite a bit considering they are so small so they just might be able to run sealed well enough (over 100dB at one meter at 27.5 Hz) Sure, you will be wasting power but as long as they can handle it you should be good. I would expect a pair of Anarchy woofers sealed should be able to handle at least 150 to 200 watts so might do it. Download WinISD Pro (it's free!) load up the T/S parameters and play around with it sealed. You can also model it with EQ to get an idea of what it will do with some plate amps that have a 6dB boost in bass say at 30Hz. Depending on how the box is tuned, that might get you there. Takes a bit of time but you'll learn alot about how subwoofers work in different boxes and alignements. I tend to be curious how things work with my hobbies, others just use hobbies as a way to collect things--all depends on what angle your audio hobby is.
In case you wondered--I am a very fast typist... enjoy!