Spreadsheet for Folded Horn Layouts...

[Brian Steele]

To avoid any further thread-jacking of another thread, I'm starting a new thread to cover this idea: an Excel spreadsheet that can be used to quickly determine panel dimensions for certain folded bass horns. To use it, you basically enter the relevant parameters for the chosen bass horn you'd like to build, and then determine the panel dimensions via the built-in "Goal Seek" function. The spreadsheet also includes a graph that shows the cross-section of the folded horn.

If there are any similar spreadsheets available on the Internet, please let me know, so wheel-inventing can be avoided .

At the moment, only one "fold" is included in the attached spreadsheet, but it should be possible to include other folding schemes, once they can be described mathematically.

Some notes on the attached spreadsheet:

1. Figures in bold blue are the ones you get to play with.

2. Enter the TH's basic parameters into the corresponding cells - S1 (throat, cm^2), S4 (mouth, cm^2) and path length L (cm).

3. For "p", enter the thickness of the wood or ply (cm) that you plan to use for this design.

4. Choose an initial internal depth and height for the box that you'd like to use to implement your chosen TH and enter under "d" and "h". Adjust until the value for "L-check" is close or equal to "L". Easiest way to do this is to use Excel's "Goal Seek" function to find a value for "d" or "h" that sets "L-check" to the same value as "L". I recommend keeping a square profile for the box (e.g. set "h" equal to "d").

5. Examine "L1-Check" and "dL". These indicate the possible error that your chosen geometry (ratio of height to depth) might introduce. If it's too high, then repeat step 4 with different values for "d" and "h".

6. Use Excel's goal-seek function to set the value for "balance" to zero by changing the value for "w". This result will be the required internal width for the box to achieve the target TH alignment with the folding scheme illustrated in the spreadsheet.

7. Once you've completed steps 2 - 6, the cross-section of your TH will be illustrated in the right-hand graph. If you've completed the steps right, the graph should illustrate a cross-section consisting of panels of equal thickness. If you haven't completed the steps correctly, the cross-section will not look right.

8. The dimensions for each required panel are given in the green table at the bottom of the spreadsheet. All dimensions are in cm. Note - the bracing panels are not included in this list of panels, but you should be able to figure these out on your own. The calculations don't take the effect of bracing into effect, but once the the braces are kept small, their effect on the horn's response shouldn't be significant. If you're concerned about the effect of any bracing, try starting with a slightly larger value for S1 than suggested by the HornResp sim, then design your bracing to bring the cross-sections at different points at the horn back into close alignment with the HornResp sim.

9. The panel dimensions given assume that butt-joints are used throughout.

[Brian Steele]

One issue that's come up is calculated horn length.

I used the "centerline" method to determine horn length. I've also not included any "end effects" in the calculation. I used the spreadsheet to produce the layout for my POC2 TH. It predicted a horn length of 245cm (as required by HornResp). However, when I measured the TH's lowest resonance frequency, it turned out to be more like 43 Hz, not 40 Hz as predicted by HornResp. Reverse-engineering this in HornResp (i.e. adjusting the path length until the predicted resonance frequency increased to 43 Hz) results in an effective path length of 231 cm, which is much shorter than that given by either the centerline method or the square-root method. Any ideas?

[mwmkravchenko]

You are worried about 3 hz?

For one thing Fs usually goes up when a driver is actually in a horn. The air mass loading inside the box is different than outside. Especially in a tapped horn as compact as yours. Another thing is how well broken in is the driver? That 3 hz should even out After a bit oh exercise.

Playing with the spreadsheet is pretty cool. Still have to figure out the auto function but it seems to do what you designed it for. I'll mess with it some more tonight. I'll punch in a couple other horns.

Now many I have designed are up to S5 in the sections. I have taken for granted that you are simply looking for beginning area, path length and mouth area. Is this correct?

One question is if you could set it up with sections as Hornresp could you then calculate horns with a partial negative flare rate? Lots of my designs seem to function better with a section that acts more like a resonator than a true horn flare.

Mark

[Brian Steele]

Originally Posted by mwmkravchenko

You are worried about 3 hz?

Yup . More correctly, I'm worried about a measured result that's 7.5% off the predicted result.

Originally Posted by mwmkravchenko

For one thing Fs usually goes up when a driver is actually in a horn. The air mass loading inside the box is different than outside. Especially in a tapped horn as compact as yours. Another thing is how well broken in is the driver? That 3 hz should even out After a bit oh exercise.

I'm concernined about the resonance frequency of the horn, not the driver. It's represented by the first impedance minimum and it's independent of driver characteristics. It's solely dependent on the characteristics of the horn.

Originally Posted by mwmkravchenko

Playing with the spreadsheet is pretty cool. Still have to figure out the auto function but it seems to do what you designed it for. I'll mess with it some more tonight. I'll punch in a couple other horns.

Now many I have designed are up to S5 in the sections. I have taken for granted that you are simply looking for beginning area, path length and mouth area. Is this correct?

Not quite. I found a way to mathematically describe the size and shape of the tapered and stepped horn illustrated in the graph and relate those to beginning area, path length and mouth area. The technique can be theoretically expanded to include other parameters such as varying flare rate, but the important thing is to be able to mathematically describe how the folds in the horn are developed to fit into the cross-section of the box that needs to be built to hold it.

[BP1Fanatic]

Is the different panel colors an option...example: red for incorrect length & blue for correct length?

[Brian Steele]

Originally Posted by BP1Fanatic

Is the different panel colors an option...example: red for incorrect length & blue for correct length?

I've attached an updated version of the spreadsheet that does this. When the graph is red, the panel dimensions have not been optimized. When optimized, the graph turns blue.

[BP1Fanatic]

Nice!!! How about swapping the locations of the Horn Expansion and Cross-Section charts for us blind folks?

[BP1Fanatic]

How was I able to obtain a 132.82L blue enclosure when my inputs are for a 210L enclosure?





L = 274.32 108.00 balance = 0.00
S1 = 516.13 L-check = 172.2
hmin = 6.42 L'-check = 167.8
S4 = 1032.26 dL = 4.4
hmax = 12.84 2.5%
p = 1.91 0.75
d = 40.64 16.00 17.50
h = 40.64 16.00 17.50
w = 80.42 31.66 33.16
delta = 0.02

Vol (1) = 212.38 l. 7.50 cu.ft.
Vol (2) = 166.42 l. 5.88 cu.ft.

[Brian Steele]

Originally Posted by BP1Fanatic

How was I able to obtain a 132.82L blue enclosure when my inputs are for a 210L enclosure?

From your images, it looks like you haven't optimized L. Your horn's length should be 274.3 cm, based on the HornResp input screen. However the value for L-check in your image is much less than 274.3 cm. Another sign that something's wrong is that the net volume (Vol 1) is more than the gross volume (Vol 2).

It looks like you have to complete steps 4 and 6 about three times to get the correct results (the chart plotting component that swaps the colours in the graph only checks the "balance" figure, not the "L-check" figure).

[soho54]

Originally Posted by Brian Steele

One issue that's come up is calculated horn length.

I used the "centerline" method to determine horn length.

When you say "centerline" do you mean right angles through the corners? If so, this will always be a little long, and the horn will play shorter.

I haven't looked at your sheet yet. Is it unlocked?

You will have better luck if you can get the program to use SQRT(Inside path*Outside path) as the length of each section. This is the easiest way to get the average path length.