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Old 3rd December 2012, 01:42 AM   #31
Djim is offline Djim  Netherlands
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Hi Forsman,

Sorry I didn't reply earlier but (enough) time is sometimes difficult to find...

To be honest I think (just my opinion) that using Resonators/absorbing techniques only increases losses. I prefer to optimise the design rather than creating more losses. To me your 'dip problem' is the result of the difference between sim and design, like I stated earlier. I think it would be helpful to have a more accurate model to see what’s going on. It also help me to explain what I meant with the cone volume correction.

The volume within the cone is usually the main reason why the dip is (usually) larger in measurements compared to the sim. In other words the volume at S2 in the original model doesn’t represent the volume in reality. In my post#23 of the "SS15 or Tham 15?" thread I showed an example of an alternative way of simming the THAM. The alternative model shows the dip pretty accurate and looks similar to what one can measure from a standard Tham.

To make such alternative sim for your Thorn F1, I used your measurements to go from. First, I implied your measurement within the graphic ratio of HornResp. This gives a much easier comparison tool between model and measurement. After that I made some small changes in the HornResp input parameters to correct the volume at S2 and S4 and some small changes for the correct position of the point source ( lays within the cone). When you increase the volume at S2 one needs to compensate at S4 in order to keep the correct total internal volume of the enclosure. By using the exact frequency location of the peaks and dips of your measurements I tried to improve the model further. In the next picture you see the result (black) in combination with your actual measurement (red), all in the same plot ratio.

Click the image to open in full size.

The input page looks like this:

Click the image to open in full size.



Now you have an more accurate model you can see that by making the S2 area smaller (Cone Volume Correction) you can optimise the model to what you have modelled in the first place. I did so by decreasing the volume of S2:

Click the image to open in full size.

This example of the cone volume correction should bring it back to what you have simmed (freqeuncy response) in your original model. What this Cone Volume Correction will do is increase the compression at S2. Therefore the pressure minimums at S2, those of the 1/2 wavelength and full wavelength, will rise.

If you decide to experiment with the cone correction I made a quick sketch of how to start. You can experiment with smaller and larger shapes until you find the best solution.

Click the image to open in full size.

Last edited by Djim; 3rd December 2012 at 02:10 AM.
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Old 3rd December 2012, 06:01 PM   #32
Forsman is offline Forsman  Sweden
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Thank you very much Djim!

Donít apologize for not having time. This is hobby and that is something we do for the fun of it and when we have time.

I really appreciate that you are taking you time to explain in such pedagogic way. I must confess that I didnít really get it the first timeÖ Your simulations show your point in a excellent way.

I must do some experiments with this Cone Volume Compensation.

Regards
/Fredrik
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Old 11th December 2012, 11:14 PM   #33
Forsman is offline Forsman  Sweden
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- No I am not trying to cook rice pudding in the driver. The powerrating is close to a kilowatt so there is potential to heat things up. The problem is to get the food not to fly all over the kitchen...

Click the image to open in full size.

Well, now I know that the cone can hold exactly 1,5 l of rice. And the hole cut out in the baffle can hold anther 0,99 l.

/Forsman
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Old 12th December 2012, 02:29 AM   #34
Djim is offline Djim  Netherlands
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Hi Forsman,

When you make a 'perfect' (afterwards) cone volume correction it will cause a narrowing passage at the edge of the baffle cut out. The example I gave you in my post (last picture) has about a 2 Litre correction but should be at the correct height at the source point (see help-lines within the sketch). It should be enough for correction but without causing obstruction of the horn-path.
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Old 12th December 2012, 10:55 PM   #35
Forsman is offline Forsman  Sweden
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Hi Djim

I would be pretty easy to do a cast in gypsum of the cone volume. Just cover the driver with plastic and pour the gypsum in it. When it has hardened it could be cut in half and placed with the cut towards the side walls and thus creating a funnel right under the driver with perfect cone volume compensation.

It is much easier though to do it your way Djim. Maybe I should start there. I just wish the temperature could raise a couple of degrees, minus 15 C this morning and the workshop isn’t insulated.

edit:

I don’t know if this can be another way of simulationg the effects of the cone volume. I took my 2,5 l and added 2500 cc to Vtc (front chamber volume) and 531 cm2 to Atc (front chamber average cross section). That gives me this result:

Click the image to open in full size.
(grey: original, black: with frontchamber/cone volume)

Not as close to the real measurements as your sim Djim but closer than the original sim.

/Forsman
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Last edited by Forsman; 12th December 2012 at 11:07 PM. Reason: adding info
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Old 12th December 2012, 11:36 PM   #36
Forsman is offline Forsman  Sweden
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I have to ponder your first post on this page some more Djim. I relize that there is a lot of thinking behind it. Maybe more then I can grasp at the moment.

/Forsman
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Old 12th December 2012, 11:39 PM   #37
Djim is offline Djim  Netherlands
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Hi Forsman,

I’ll send you a nice Atlantic south-west depression this weekend to get rid off that cold.

The acoustic air within a horn isn’t affected by relative small sized shapes compared to the relative large soundwaves in question. It’s all about the volume per section and not the shapes. In other words, it doesn't matter how the Cone Volume Correction looks like as long it sits in the correct position on the horn-path.

So you can use Cone Volume Correction also sideways like in this example picture;

Click the image to open in full size.

I am not sure why the Atc/Vtc function gives different results but I guess it's because sims use a 'flat disk' instead of cone shape for simulating the driver's cone.
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Old 19th December 2012, 12:21 AM   #38
Forsman is offline Forsman  Sweden
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Look what I have bought today.

Click the image to open in full size.

I had never imagined that I ever should build loudspeakers out of Styrofoam but that I shall.

/Forsman
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Old 19th December 2012, 03:26 AM   #39
Djim is offline Djim  Netherlands
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Quote:
Originally Posted by Forsman View Post
I had never imagined that I ever should build loudspeakers out of Styrofoam but that I shall.
Do I think what you think? In that case I'm not sure it will work. The density/stiffness of the styrofoam doesn't look 'experimental correction' enough....
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Old 19th December 2012, 05:42 AM   #40
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Is it expanded or extruded polystyrene?
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