Jump to content

15" Jubilee Design


infurno

Recommended Posts

Yes, for the effects of actual horn enclosure geometries, the BEM analysis codes are probably required.  Hornresp really isn't geared for that.

Link to comment
Share on other sites

I've played a little with the demo version of ABEC3.  You're going to see that the learning curve is steep in terms of defining horn geometries, but they have some demo data files that can help you get started.

 

Since I was spending my efforts on a non-folded horn design with known/measured properties, I didn't need ABEC3.  But if I decide to use a folded or curved horn, or a horn expansion that's not "in the books", then ABEC3 is what I'd use.

 

From what I can tell, developing the skill to use ABEC is like woodworking: it's a nice skill to have and will also be something that others would want to tap into.  For me, I just haven't gotten to that point yet.  I will soon after the MEH thing is complete.  It's a nice capability to have.  I think that most of the people that have tried it and failed aren't terribly serious about using it.  I found that finite element methods (ANSYS, NASTRAN) were a lot easier to use than doing cut-and-try methods.  YMMV.

 

Some threads:

 

http://www.diyaudio.com/forums/multi-way/88237-suitable-midrange-cone-bandpass-mid-unity-horn-187.html#post4152484

 

https://en.wikipedia.org/wiki/Boundary_element_method

 

http://www.diyaudio.com/forums/multi-way/247717-free-bem-solver.html

 

Chris

Link to comment
Share on other sites

On 4/17/2017 at 6:34 AM, jwc said:

What are the limitations known to that software ....if there are any.

Hornresp is fairly accurate if you maintain the assumptions behind its equations. I use it as a best case first order sanity check. It's usually not too hard to calculate by hand when you're deviating from the ideal assumptions - I'll often manipulate the inputs to the tool to see the results of those deviations. You kind of have to put the full story together in your head after the fact....or just build it and measure.

 

Hornresp is modelling the performance of a circular cross-section with the defined area expansion. Once you deviate from that, then hornresp is totally invalid. It's also assuming the wavefront follows the shape assumed by the exponential/conical/tractrix equations, which isn't reality either - even for circular horns. So from that perspective it's probably better to say that it's inaccurate for most systems.

 

FEA (or BEM) is the way to go, but you're not going to get a nice interface like winISD or anything like that....and you can really muck things up if you don't get your meshes right. That or it will take weeks to solve the model.

Link to comment
Share on other sites

""""Hornresp is modelling the performance of a circular cross-section with the defined area expansion. Once you deviate from that, then hornresp is totally invalid.""""

 

Yes...I lose my faith in it.  I use head prediction.....for lack of better words.

 

So Doc....how do you calculate compressing ratio and how do you make horn loaded efficiency gain from it.

 

Jc

Link to comment
Share on other sites

1 hour ago, jwc said:

.how do you calculate compressing ratio and how do you make horn loaded efficiency gain from it.

 

I found a closed form solution many moons ago in Beranek's Acoustics, chapter nine.  Compression ratio affects the efficiency of the horn-loading in the "PAE" calculations.  Generally, the lower the compression ratio, the higher the efficiency...until you get to 1:1. 

 

However, is efficiency is the design factor that you're really wanting to maximize? Or rather that you're trying to maintain resistive loading on the driver in order to extend the frequency response of the horn/driver and that efficiency can be traded?  Also, most bass horns don't have full-sized mouths.  That design parameter also strongly affects both polar control at lower frequencies and, surprisingly, extending frequency response.

 

Chris

Link to comment
Share on other sites

I'm not looking for a formula here.  I was asking how Bentz "head guesses" or gross estimates the compression ratio then from that...... make a plausible explanation of what happens to efficiency from the compression ratio.

 

I've seen these kinds of estimates before where it doesn't appear somebody truly relied on a formula or software.   So I'm picking the Doc's brain on this.

Link to comment
Share on other sites

20log(Sd/St)

Where

Sd = driver area

St = throat area

 

Conceptually, the driver moves one unit volume of air for a given excursion. When that volume of air moves through a smaller opening, then the velocity increases proportionally (because the same total volume gets displaced).

 

The job of the horn flare that comes afterwards is to maintain as much of that higher velocity all the way to the mouth - where the surface area is even greater, and you're displacing even more air.

 

In order for this to work, the horn needs to be long enough to allow the pressure to build up into a proper wave. If the horn flares too fast, or is too short, then the air "rushes around the corner" of the mouth / horn wall and yields a large pressure change. That pressure change creates an opposite polarity wave that travels in all directions, including back into the horn. That's why the quarter wavelength horn length is such a magic number.

 

This is a horrible explanation for the concept of impedance matching, but I think it's helpful from an empirical perspective. The compression ratio basically sets your starting point.

  • Like 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...