TLDR: I stuck a 20'+ long 4" ABS pipe into the back of a 1.6cuft box with a Dayton 12" sub. What is going on that it resonates like an open pipe at 27-28Hz? Hi everybody, first post. Sorry it's a bit of a book, but it is going somewhere, and I've tried to break it into pieces so you can skim and skip. Never seen anyone else try this, but if this is a repeat link me to it, I'm happy to read what others have done! I've got so many questions that I'm not sure how to answer. History and reasoning: I've been into bass for about a decade and a half now, and transmission lines for about a decade. The first time I noticed the effect was taking the rear baffle off a 5' long "behind the couch" PartsExpress recipe that I made for my Titanic Mk IV. Check out the specs for the Mk IV here if you don't know about it. It got louder! I've had a few different physics classes so I knew about resonance and I discovered transmission lines with 1/4 wave designs. I've read the legend of El-pipe-o at least 10 times now and it never gets old. This sub is still working all these years later! Same sub, last winter, I upgrade to a 150Wx2 class D amp and put it into a PE knock down cabinet. The cabinet in question. I'm not a shill for PE, just a happy customer. While it was good on space it really lacked what the old box could do. I didn't want to build a transmission line box again without some more knowledge. I had always wondered how important the dimensions of the T-line are for resonance, and I know that pipe organs can make awesome bass, so that's where the ABS pipe idea crept in. Modular with slip fit joints and angles, relatively cheap, pre-formed, waterproof, useful for other house projects, and durable it was worth a try. I'll be the first to admit the SQ is horrible, you can tell it is coming from a plastic sewer pipe, especially in a punchy track or voice recordings. Methods and results: Other than a tone generator, ears, hands, and a sheet of newspaper or two I have no SPL equipment or measurement devices. Without the ability do measure SPL and only single Hz granularity on the tone generator these results are far from scientific. First I tried straight 10', it resonated around 58Hz. Then I added an elbow, the small amount of extra length was enough to create a non-node at the pipe and dropping the frequency 2Hz brought the node back. The driver was still making sound at the frequency of the node, so the pipe was chopped up and two 90 degree bends were introduced, dropping the frequency slightly again but putting the outlet of the pipe in the same plane and the same direction as the driver. This made a really big difference and being in the small hallway that housed the whole thing was a great experience. Next was adding a coupler and an extra 10'. As mentioned above, this dropped the pipe frequency to a feet tingling 27Hz. There seems to be a secondary peak around 54-56Hz and a big drop out between 30-40Hz where it's only the sub making sound, not the pipe. Also tried a tee at around the 12' mark instead of the coupler. It definitely makes a node at a frequency, but i'm not sure how it influences the other parts of the pipe or the driver. That's about where I'm at now, with about 3' of ABS sticking out my front door and the whole (small) house just starting to tremble when the right frequency is hit. Discussion: Length and area of pipe Physics wise it seems to be acting as an open pipe and I'm getting a half wave resonance. The bad part, the pipe has to be twice as long as a transmission line. The good part, it seems like the pipe can be much smaller. 4" ABS has an area of 12.76in^2, with MDF you could make a chamber that's about 3"x4" and have something similar. Clearly I'm not pushing a 5kW monster, but there are no signs that the pipe would need to be significantly bigger to handle more power. Another good part, if you want to change the tune you add or remove pipe. With MDF it might be more difficult, but you could always add an adapter for an ABS pipe to get the tune perfect. It seems that a larger pipe could allow you to tune for a frequency with a shorter pipe due to end length correction. https://en.wikipedia.org/wiki/End_correction I'm not really interested in buying more PVC to try this since it sounds so terrible. It may also be possible to "overblow" a pipe like this in the way a flute player can get higher notes. What is the smallest pipe that can be used? Can multiple pipe outlets have multiple resonance frequencies? They can, as discovered with the tee, but how does that work? Can that work for a transmission line? Is the pipe outlet size and shape important? I had considered chopping up a garden hose at different lengths and packing them into the end. Box and driver Really don't know much about how this is set up or where it should be peaking. When the tube is removed (open 4" hole in the back of the box) it seems like they are happy in the 52-55Hz range. If anyone knows more about what I'm doing I'd love to know what tuning the box, and potentially using a box with a tuned vent could do. I know there's no free lunch, but aligning the two resonating systems could give positive results. If the box and driver are a separate resonating system how many tubes could be made to vibrate from one box? What is going on? Why is it acting like an open pipe instead of a closed pipe? As box size is reduced and pipe size is increased it will eventually become a transmission line system. I even tried it with a 6" driver and it couldn't get enough of a seal to quarter the wave. It's unlikely you've heard a cheap door speaker hum like that though ^_^. Where and how does this transition happen and how can that knowledge be used to make either design more efficient in power and space? Speaking of efficiency, in terms of power how much is being sacrificed with a half wave open tube over the quarter wave closed tube or other box designs? The Future: In the next week or so I'd like to be able to close my door, it's starting to get cold. I think I could also put the end of the tube, the driver, and myself in the same room together. I've never experienced that, just the weird spot between them where the two waves meet. I'll take a picture if I get to that point. After that I would like to try some slightly larger square tubes made out of wood. A lot of the bad sound is a plastic tube issue. From there, figuring out a way to get a 16' tube curled up and put into the back of the car. Wow, you made it to the end.