First - a disclaimer:
The following is presented based on my own empirical observations, by someone with no formal training in acoustics. I'm a hobbyist, I don't do this as a day job.
OK, that being said, let me see if I can help a bit here.
First off - it is great that you're taking panel utilization into mind. My experience suggests that it's a good idea to give yourself a bit more room to work though, that driver will be a tight fit. That being said - why are you wasting all that space inside? Make the horn bigger by increasing the rate of flare, there is no reason to block the space off.
Re the model:
Model with PAR flares, because that's what you'll be building.
A simulation at 2.00 volts is a fair 1 watt comparison, 2.83 is 2 watts with a 4-ohm driver.
Subtracting 3 dB from your plot, it looks like your modeled efficiency is somewhere near 94 dB at a watt, The driver is 89, so you're seeing some efficiency gains, that's good.
L12 is far too long. The impulse response can be cleaned up a lot.
Enter 0.00 for both APT1 and LPT. I don't think they matter in a tapped horn model, but it's a housekeeping thing.
Calculate your Vtc and Atc, then enter those values. The recent versions have a wizard that makes this easy to do.
Finally - look at your acoustic reactance plot when you calculate the model. Like all the other data presented in Hornresp, it matters. In a nutshell, big tall spikes are bad, we want the heights to be similar.
Re the driver (assuming I get the math right this time):
BL = 20.57
re = 3.41
Sd = 1090
BL^2/re/Sd = my "magic number"
In this case, my "magic number" is 0.1138. This means that based on my experience and observations, this driver falls on the under-motored side of the spectrum for use in a tapped horn.
That said, will it work? More than likely, but just cause you can doesn't always mean you should. To be totally honest, nearly anything can be made to make noise in a tapped horn, but there are reasons I choose the drivers I do.
Is it optimal? No, not quite, but it really isn't that far off. Qes is a touch high, and I'd like to see a BL nearer 25. Still - it will work, and if the horn is right for the driver, it should work quite closely to the model's predictions. Qes is not that high, so with a good amp, and the driver in a proper horn, things should be fine.
I've built horns using drivers that were undermotored, I've built several that were overmotored, and I have a few that used motor to cone ratios that fall in the sweet spot. I much prefer the sound of the horns with enough (or even too much) motor, and I much prefer the sound of a proper amp. I will definitely explore motor force further, I don't have a lot of spare cash for driver purchases at the moment, nor do I have any shop time for the builds and testing.
I really look for the magic number to be higher than 0.15, but no higher than 0.25. A lot depends on what I am trying to do with the horn though, and there is a lot more that needs to be considered.
Also - keep in mind that the small signal parameters we're modeling with really aren't valid at anything other than small signals. I know we have to start somewhere, but do keep in mind that several of the parameters will shift significantly as power is applied.
Regarding compression ratio:
It's a grossly oversimplified way of looking at things, because the aspect ratio of the horn matters a lot too. The losses in the horn are something that Hornresp can't account for. Losses result from friction, and with more side wall relative to the sectional area of the horn, losses increase. The more squished the aspect ratio is, the lossier the horn becomes. These losses are non-linear, and will also vary with the power applied.
Remember, air can be compressed and rarefied. If the losses in the horn you build are high enough to where it becomes easier for the driver to compress and rarefy the air in the throat rather than resonate the entire air column, the horn ceases to function as a horn. This will not be evident in the model, but it will be painfully obvious in the results. I've built several of these, I don't mention them much other than referring to them as learning experiences. This is not just a result of too much compression at the throat - the entire horn has to be "big enough" for the driver. That's where the rules of thumb I mention in my tutorial came from.
Don't think that I am trying to sink your project, quite the contrary. If I were doing it, the model would look something more like the attached. The fold could be similar too, just use the space inside a bit differently.
rss460.txt 0k .txt file
Considering the excursion limits Josh identified, the driver should be fine at power levels up to the driver's RMS limit, provided you use a ~15 Hz highpass. Sure - it takes things past xmax, but the moving parts shouldn't slam into the stationary ones.
@ LTD02 I believe you're correct, that notch appears to be a result of the long L12.
edit: Completed my thought...oops.
edit 2: Added equation for clarityEdited by lilmike - 7/7/13 at 10:03am