Project Eburon - Step 1 - Advanced AkAbak modeling of Bassreflex and QB5 Alignments - diyAudio
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Old 21st December 2012, 08:07 PM   #1
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Default Project Eburon - Step 1 - Advanced AkAbak modeling of Bassreflex and QB5 Alignments

I'd like to post a series of threads about a project I'm going public with: the "Eburon". The eventual design will be a full-ranger in a QB5 alignment, with a cabineth optimized to reduce reflections/diffraction on both sides of the cone. This will be the 5.1 sattelite speaker to go with the bDEAP sub I already have.

The reason I post this in the subwoofer forum, is because I want to split up the design steps, and the first steps are beneficial for subwoofer designers. For the latter I'll go to the other appropriate forums. To the mods: feel free to move this thread to a more suitable place if you deem it nessesairly.

Ideally I can get some gears turning with the brighter minds and more Akabak experienced members across the DIYaudio forum, so the community can benefit from the condensed knowledge here. There have been some instances recently of people asking about support for some of the things I'll want to tacle in this endeavor, so this way they'll find the knowledge to do so without futilessly nudging the program developers for support of filters in their programs.


- Modeling a bassreflex enclosure with internal reflections in Akabak
- Calculating and then modeling a QB5 Class II enclosure with passive HP filter in Akabak.
- Model and assess baffle step/diffraction
- Model resonators to absorb peaks/reflections in the backwave.

In the next few days I'll try to type out the first step and post it here in this thread. Hopefully I'll get some feedback and this whole thing gets rolling. (And also it's a good motivator to get down and do some things to post the results instead of keep pushing it before me!)
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Old 22nd December 2012, 04:37 PM   #2
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Default Base model

OK, lets see. The driver I choose is the Monacor SPH-68XAD (I've used its -60X cousin in the past and I liked it.)
Once put in a 9,8L box with a 18x18mm square vent of 16mm long, the (flat) F3 is 60Hz. With a little over 1mm xmax, it barely holds itself at 2 watts input. In this configuration, a friend of mine still uses them for the last years next to his plasma screen, without the need for a sub.
And judging from the SPL, above 2 watts...

This can all be modeled in HornResp and then be exported to AkAbak. (Just make sure you get the T/S parameters right, or Mr. McBean makes you feel like a fool. )
You can use an "Enclosure" element in your script, but if you use a "Duct" element, you can input the width, height and depth of a rectangular box and you have the Viscosity parameter to play with to simulate stuffing. The internal reflections/resonances of the backwave, will also be visible in the plot.

Here is the script: (I have to admit, once I cleaned everything up to this simple script, the SPL plot smoothed out a lot, but the basic resonances are there. Increase the viscosity parameter to 4 to see the effect of stuffing.)

|DATA EXPORTED FROM HORNRESP - RESONANCES NOT MASKED

|COMMENT: Monacor SPH-68XAD

|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~

|REQUIRED AKABAK SETTINGS:

|File > Preferences > Physical system constants:

|Sound velocity c = 344m/s
|Medium density rho = 1.205kg/m3

|Sum > Acoustic power:

|Frequency range = 10Hz to 20kHz
|Points = 533
|Input voltage = 2.83V rms
|Integration = 2Pi-sr
|Integration steps = 1 degree ... 1 degree
|Integration method = Cross


|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~

Def_Driver 'Driver'

Sd=80.00cm2
Bl=5.70Tm
Cms=1.10E-03m/N
Rms=0.756Ns/m
fs=60Hz
Le=0.50mH
Re=6.50ohm


System 'System'

Driver Def='Driver''Driver'
Node=1=0=100=101


Duct 'Rear chamber'
Node=200=100
WD=0,20m
HD=0,33m
Len=0,148m
Visc=0

Radiator 'Front Driver'
Node=101
SD=80.00cm2


Duct 'Bassport'
Node=200=102
WD=0,018m
HD=0,018m
Len=0,016m
Visc=0

Radiator 'Bassport Exit'
Node=102
SD=3.24cm2
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Last edited by Cordraconis; 22nd December 2012 at 04:39 PM. Reason: improved layout
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Old 22nd December 2012, 04:53 PM   #3
tb46 is offline tb46  United States
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Hi Cordraconis,

I'm going to be watching your project "Eburon" with high interest. I think it would be great, if the parts could stay consolidated in one thread.

When messin' around with compound arrangements in Hornresp I split out the bass reflex portion, and it transferred just fine into AkAbak. Naturally, this does not give you a rectangular box, or duct, and no stuffing, but the basic method seems to work.

I renamed the file extension from .aks to .txt - to use it just rename it back to .aks.

Regards,
Attached Files
File Type: txt lab15brA.txt (2.6 KB, 21 views)
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Last edited by tb46; 22nd December 2012 at 05:02 PM.
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Old 22nd December 2012, 08:40 PM   #4
Xoc1 is offline Xoc1  United Kingdom
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Quote:
This can all be modeled in HornResp and then be exported to AkAbak. (Just make sure you get the T/S parameters right, or Mr. McBean makes you feel like a fool. )
You can use an "Enclosure" element in your script, but if you use a "Duct" element, you can input the width, height and depth of a rectangular box and you have the Viscosity parameter to play with to simulate stuffing. The internal reflections/resonances of the backwave, will also be visible in the plot.
Hi Cordraconis
A screenshot of the Hornresp input parameters used to create the Akabak script would be very useful at this stage.
Regards Xoc1
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Old 24th December 2012, 09:39 AM   #5
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Quote:
Originally Posted by Xoc1 View Post
Hi Cordraconis
A screenshot of the Hornresp input parameters used to create the Akabak script would be very useful at this stage.
Regards Xoc1
Should be done.
(Don't forget to choose HornResp "Combined output" option for the correct SPL plot.)

I've already modified/detailed the script above, but this should get you started.

Also I think the internal reflections are dependant on the position where the driver loads the cabinet, but that will be determined when I tacle the diffraction properties of the front baffle. I'm not sure if AkAbak has a way of inputting those point-of-loading information into a "Duct" Element. But that's for later. Next up is how to use the QB5 tables to bump-tune the cabineth.


Also, please keep in mind I'm not making this as a tutorial, more like a method for myself and hopefully when I make a mistake someone can step in and correct me. Eventually this wil benefit everyone.
As I have no woodworking capabilities nor space in my small house, I'll have a carpenter or woodworker make the cabinets, once the final design is finished.
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File Type: jpg BR-HornResp-inputparam.jpg (84.9 KB, 359 views)
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Old 24th December 2012, 11:22 AM   #6
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Akabak only models reflections in the "length" dimension. It does not measure reflections between the walls.
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Old 24th December 2012, 10:10 PM   #7
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Further to what I said above, Akabak does not, by default, model resonances in ducts where their areas change suddenly. So in the above model, Akabak will not model the reflections from the change in area where the "enclosure" duct joins the "port" duct. You need to add acoustic mass and resistance at the join. I suggest using an Enclosure rather than a Duct, and use the "Lb" and "Deep/Long" parameters. Also pay attention to the "box loss" parameter Qb/fo. It makes a significant difference to the calculated response. If you want to stuff the enclosure, add an AcouCompliance element.
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Old 25th December 2012, 04:25 PM   #8
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Quote:
Originally Posted by Don Hills View Post
Further to what I said above, Akabak does not, by default, model resonances in ducts where their areas change suddenly. So in the above model, Akabak will not model the reflections from the change in area where the "enclosure" duct joins the "port" duct. You need to add acoustic mass and resistance at the join. I suggest using an Enclosure rather than a Duct, and use the "Lb" and "Deep/Long" parameters. Also pay attention to the "box loss" parameter Qb/fo. It makes a significant difference to the calculated response. If you want to stuff the enclosure, add an AcouCompliance element.
Thanks for the tips so far!
With regards to your lengthwise-only reflections, I was thinking of using 5 ducts (up, down, left, right and depth) of a correct combined volume, to approximate the internal "eigenmodes". Somewhere in an AkAbak manual they also used multiple Ducts to model a bandpass enclosure, so it should be feasible.
I didn't know the change in area's didn't reflect, aldough I missed them in the SPL plot. Guess I'll hunt for them later and play with the acoustic mass and compliance elements then. Nice tip!

Right now I'm still in a row of working nights (yes, also at X-mas! ), but I'll get to my next post soon. Hang on!

Enjoy the hollidays ... if you have them!

Joris
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Old 26th December 2012, 11:13 AM   #9
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Quote:
Originally Posted by Cordraconis View Post
Thanks for the tips so far!
With regards to your lengthwise-only reflections, I was thinking of using 5 ducts (up, down, left, right and depth) of a correct combined volume, to approximate the internal "eigenmodes".
Interesting idea. Remember to leave the far ends of the ducts open circuit to tell AkAbak that the ends of the ducts are closed. I suggest comparing it with a fully specified Enclosure model. I compared your original single-duct model with an equivalent enclosure model and found that they were very similar but the enclosure model better showed the main front-back resonance.

Quote:
Originally Posted by Cordraconis View Post
Somewhere in an AkAbak manual they also used multiple Ducts to model a bandpass enclosure, so it should be feasible.
Page 64 of the main manual. Note that the multiple ducts are connected end to end with no changes in area.

I had good food and company today... I also managed to try some AkAbak ideas (see the threads on "Cornu" horns) so it has been a good day all around.
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Old 30th December 2012, 03:51 PM   #10
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The next step is to modify the "base system" to a QB5 Alignment.
This isn't as straightforward at first glance, since you have to condense out the basics.

First I'll give a brief summary of the Elliot Sound Article about compliance scaling: Loudspeaker Compliance Scaling
When looking at a driver T/S parameters, the Qt factor is not that important. There can be naturally occurring variations till 20% coming from the production line. The article shows some graphs about what happens when the Qt is changed on a given box. Basically there is more ripple in the passband, and when using a filter, there is even less ripple.
Then the article continues with instructions on how to transform the T/S parameters, so the altered parameters can be put into one of the box calc programs which then magically can give the filter assisted response. Winisd is given as an example.
Then some techniques are given on how to modify the amplifier output so the driver behaves as if it would have a different Qt or be differently
filtered.

What I remembered from this, is that if the Qt factor is not THAT important, and even less important when we are using a filtered alignment, the QB5 alignment can be chosen quite straigtforward: Satellites and Subwoofers

Lets look at the QB5 Group II alignment table in the above article. In particular the 3rd column.
I would like to have the -3dB point at 80Hz, and the chosen driver's free air resonance is 60Hz. F3/Fs = 80Hz/60Hz = 1,33. This falls between 2 rows in the table. The first colum gives a desired Qt of a little bit less than 0,4 while the manufacturer states a measured Qt of 0,43 - about 10% deviation from the tables. So likely there will be a bit of ripple in the end result, but it should be acceptable.
Vas/Vb should be around unity, so I'll take Vas = Vb = 9,7L box volume.
Fb/Fs = x Hz/60Hz = 1,28 so the box should be tuned to 77Hz.
Fa/Fs = x Hz/60Hz = 1,34 so the filter should be at 80Hz. With 1/Qa = 1,59 (the inverse of the Q-factor. I'll get back on that later.)
All this information can be fed into Eminence Designer/BassBox pro - which has recently been mentioned a few times on this forum - or in Akabak which can also model active filters. By juggling around the port tuning frequencies, box sizes or active filter tuning frequencies, one can reach the desired F3 frequency in a few steps.

However, since I wanted to make a self-contained system, I opted for a passive filter inside the cabinet. This has the problem that one needs a way of designing a passive filter with a variable Q-factor. I couldn't find such a way at first, and I tried to contact the author and also website owner of the QB5align article for help, but to no avail. Maybe my question was foolish, got lost in the heap or simply not enough time for it. As the saying here in Belgium goes: "A 'No' you have for sure, a 'yes' you can get." Meaning it never hurts to try.
If any of the author(s) or someone more knowledgeable than me happens to pass by this thread, please feel free to share any knowledge or insights you have.

I got my insight when I looked at paragraph 5.0 (or more correctly: the lines above paragraph 6.0) of the article on passive crossovers: Passive Crossover Network Design
There we see the full original formulae for finding the capacitor and inductor values of 2nd order passive filters, along with a short overview of ... the different 1/Q factors (=d factor) needed to get Linkwitz-Reiley, Bessel, etc... filters. When I saw 1/Q mentioned, I knew I had found what I was looking for!

Edit: Hmmmm... With what I have explained so far, one can try to model the passive highpass-filter himself. (Just don't forget to adjust the entrance nodes of the driver accordingly, like this:

Driver Def='Driver''Driver'
Node=2=0=100=101

Capacitor | In Series with driver
Node=1=2
C=139uF

Coil | In Parallell with driver
Node=2=0
L=28,5mH
Rs=1ohm



HOWEVER... In Eminence Designer/BassBox, a single 800F capacitor (=1st order filter) in series seemed to give a flat SPL response, with the total impendance of the system bottoming around 1,4 Ohms and staying there over a very long freq range (something I don't like to feed to my amplifier.) With the later discovery of the variable Q-formulae and the resulting 2nd-order filter, it nearly doubled till about 2,6 Ohms.
Now, when testing all this in AkAbak, I see none of the above? The driver and box tuning resonances are visible in the impendance plot (they were gone in ED/BB), and the SPL plot also doesn't look like it was before. Hmmm... I'll let it all rest for now and maybe check out SpeakerWorkshop
as I remember it to have a tool to optimize filter component values to a desired SPL curve. Right now I'm thinking about a calculation error, modeling/scripting error in AkAbak, maybe needing a Zobel Network for the box resonance peak, or more generally: severely lacking experience with crossover designing on my part.

Once I've figured out what went wrong (any pointers are welcome!), I'll post the crossover and discuss the power handling and IM distortion benefits of this QB5.

Service message: no more post from me till at least the 2nd of Januari, as my GF feels like a laptop widow right now...

Happy Newyear everyone!
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