First - import the starting point file I have attached to the first post. Download the text file, place it in your hornresp import directory (usually c:\\hornresp\\import), then import it into hornresp.
This will provide the thiele small parameters of a single MCM 55-2421 8" woofer and the default horn record.
Here is the annotated diagram again (no sketchup yet - sorry...) of the relationships between the areas and the lengths.
To make a tapped horn model, you need three horn segments and no rear chamber. Remember the numbers above?
Double click S1, then enter S1 = 100, S2 = 100, L12 = 32, press Calculate, then press Save.
Notice that the next segment is has exp rather than con, so double click on exp to change to con.
Double click on S2, then enter S2 = 100, S3 = 800, L23 = 250, press Calculate, then press Save.
Double click on S3, select conical, then enter S3 = 800, S4 = 800, L34 = 32, press Calculate, then press Save.
OK, so we have our three horn segments and our driver parameters. Things should look like this:
We want to use two drivers though.
Select the Tools menu item, then select driver arrangement. We want to use two drivers in series, and we want them in a tapped horn.
A tapped horn can not have a rear chamber, enter zeroes in Vrc and Lrc. We are not using a throat chamber either, so enter zeroes in Vtc and Atc as well.
OK, we're done. Press calculate.
Not bad, but the response is a little jagged, and the cutoff is too high.
Go back to your Input Parameters (Window - Input Parameters).
Now - the cool part. Select the tools menu, then select the tapped horn wizard. (Loudspeaker Wizard in V. 26)
We want to see the frequency response, so click on the arrow next to "Schematic" and select "Response". We also want S2 to vary, so select "Horn - S2 Variable" from the second menu box.
Move the sliders and watch the magic.
(No - McBean will not be implementing wizards or sliders in any of the other horn calculators any time soon - please don't ask him) - Scratch that!
As of V. 26, Hornesp has implemented sliders for many of the other horn calculations under the Loudspeaker Wizard, so please thank Mr. McBean for this amazing piece of software!
We want a sub that will play an honest 30 Hz. Drag the L23 slider to the right.
I stopped at 350 cm.
Hmm, that response is a little too lumpy. What can we do with that? Similar to a front-loaded horn, ripples in the response are affected by the area at the throat of the horn. Let's reduce S1 and see what we get.
Setting S1 smaller certainly reduces the ripples, but it also moves the corner frequency up a bit. I set it at 53.
Press Save, that takes you back to your input parameters.
What about compression ratio? Hover your mouse pointer over S2, then look in the bottom bar. 5.27:1 is too high. Time to compromise.
Get back into the Tapped Horn wizard and set S2 a little bit bigger. Note - you will have to set the view and the S2 - Variable options each time. I increased S1 to 80. Sure there is a little bit more ripple, but it is not that bad.
Press calculate. The simulation predicts an SPL response that is pretty respectable. 105 dB? Wow - oh - wait - Ang is still set to 0.5 X pi (corner-loaded).
Let's look at 2pi and compare the responses (press F4). While the peaks and valleys get a little more pronounced, the low-end response does not droop.
Looking a little more closely - we're predicting 94 dB +/- 3 dB from 30 Hz to 100 Hz. Sure, the response gets ugly above that, use the crossover to eliminate it.
Seems to be in line with our goals that we set earlier.
How about excursion? Displacement looks fine at 1 watt. What happens when we feed it some power? These drivers are rated at 120 W RMS, and have an Xmax of 8 mm one way.
Double click Eg, enter 240 W and 8 ohms. Over 115 dB, 2pi, from 2 8 inch drivers and 240 watts (or over 125 dB when placed in a corner).
Excursion is OK but only ABOVE 25 HZ!!! Subsonic filters are not optional. I can't emphasize this enough - Do not drive these enclosures out of their pass-band unless you like replacing drivers!
Oh - yeah - how big is this thing? Click Schematic Diagram - the net internal volume is 156 Liters.
So - can 2 $25 dollar 8-inch drivers and a 6 cubic foot box really get you an honest 30 Hz at 115 dB?
No. But it will get really close. Hornresp models a truly non-resonant enclosure. In reality, there will be some losses due to panel vibrations. Actual response will typically be a dB or two lower, but it will also be flatter than predicted as a result of the panel resonances.
Remember jbell's big 3015 horn? Hornresp predicted 123 dB, he measured 122 with the prototype (unbraced). My quad Tang Band W6-1139 tapped horn was predicted to hit 95 dB @ 2.83 V, I measured 92 dB under sub-optimal measurement conditions. I also added some poly batting that significantly smoothed the response. Most importantly - the shape of the curve was as hornresp predicted, but smoother. The magnitude was certainly within the error bounds of my measurement equipment.
Do they really work? Yeah, they really do. The only failures I have experienced so far were implementation errors on my part.
If you measure the driver you model, model the driver you measured, and build the model you simulated, your results should measure very close to what you modeled.
If not - do not blame the model.
I have learned to trust Hornresp, my Woofer Tester 2, and my SPL meter more than my cabinetry skills or my ears. No magic here - It is all math and physics, and those rules are pretty solid.
So - how the heck do I fold one up and build one? That's another post. (Coming soon - I promise.)