Originally Posted by Mark Seaton
Actually I've been working on 5 separate custom subwoofer designs over the course of the weekend while trying to pretend I have a social life. I'd wager you will be staring at various models for quite a while before making any sawdust or having parts in hand. The laws of physics aren't going to change between then and now. Chill out.
Thanks so much for responding. To be clear, I didn't think you were going to, that's why I was not chilled.
Only in you more recent posts did you clarify that your hope/intent is to in essence not expect the sub to get worked much at the 2Hz tuning, and to take advantage of some excursion reduction above tuning where the port velocity is still low.
Ok, I'm sorry I didn't mention this earlier but I thought it was more than obvious by looking at the graphs in post 1, combined with the fact that there is no useful content at 2 hz in any media, and even if there was it would probably have to be 140 db to even perceive it and you would need a specialized signal chain to pass that signal along to the sub.
That can work so long as the port airflow isn't pushed high at ANY frequency. Just like driver excursion, just because it might only take 4mm of xmax in the middle of the bandwidth to produce high SPL, if a 16Hz signal is asking 40mm at the same time, both get distorted. In other words, any signal which pushes too much airflow through the port will affect the entire range of operation of the port. The limited electrical throughput at 2Hz greatly reduces this risk...
Yes, of course this is all true. If for example, a massive 1 hz blast did somehow come through the system it would mess up the port output through the port's entire passband. That is exactly why I tuned it so low that this will never ever happen because it simply isn't pssible.
but it brings us back to how great is the realized advantage vs. the same driver in a sealed box?
Probably the same as ANY driver compared in a sealed vs ported box - except for the fact that I'm willingly and purposefully throwing out the entire lower half of the port output.
I'd argue you will see much more variation in the port linearity than the amount of compression at the power and excursion levels you are concerned about.
Again, all the 3 softwares I've shown show my port velocity is lower than 10 m/s above 5 hz, and that the core velocity of a 2 inch pipe is 10 m/s. That says to me this design is good to go with no linearity problems above 5 hz. But if you don't agree with that please explain why. I'm not saying you are not wrong, I just don't see it from your perspective and if there's something to learn here I want to learn it. What point exactly do you think the model breaks down?
For most any overly large or particularly complex design, I start with a quick sanity check to determine what the idea offers vs. some reasonably comparable solution such as a simple sealed system with EQ. IMO, the real world behavior of a long port won't be worth the added construction hassle. Instead build something simpler, possibly smaller, and add save some money on the project to add a second driver at a later date. You don't need any more power to see a benefit in a pair of sealed drivers by comparison, and you leave yourself some room to upgrade whenever you can justify a little more power down the road.
There could be some cases you might find where a huge box volume and very low tuned port could benefit much more over a sealed alternate, but I'm not seeing the benefit for this comparison.
Finally, if you really want to test what will happen, build a box scaled by 10x higher frequency with a similar response shape. You can use electronic filters in a MiniDSP or similar to see what the real world operation will be at 10x the frequency and in a much more manageable package.
This is all very good advice and I've thought about a scaled test model (I have a Tang Band 6 inch driver with 13 mm xmax) but I'm not sure where to find a port material small enough in diameter and rigid enough to make a true scale model. I don't have anything like that at home
At least with this post we are gaining some common ground on this subject. This response clearly tells me that you think this idea has merit even if reality might throw a small wrench into the works at some point. I'm not sure if that was your point, but I'm not feeling so cranky anymore at least.
Please understand though, (everyone) I've tried to provide as much evidence and context as possible, I posted a bunch of pics of different software that all agree with each other. I've tried to answer questions that weren't even asked (although they were implied by saying I was wrong). I've tried to keep this on topic (THIS IS NOT A CONVERSATION ABOUT TUNING PREFERENCES, it's a conversation about a single proposed design and the validity of the simulation software. So please forgive me if I seem a bit cranky. It's only because I am.Edited by diy speaker guy - 12/10/12 at 1:38pm