If only it were this easy: From 3D model to reality. - AVS Forum
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post #1 of 11 Old 06-07-2012, 03:47 PM - Thread Starter
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I plan to embark on a new subwoofer build (or two) in the next few (several?) months. I did a similar drawing when I built my mains. It's comforting to me to know every joint before anything is cut. But it's also essential when using CNC. Sketchup is great for calculations and by-hand building, but it really doesn't do curves properly if you plan on deriving g-code from it.

The model is for the FTW 21. Net volume is 7.14 with the driver installed. 24" tall, 28" deep, and 30" at it's widest. All 1" mdf w/double thick baffle. Overkill? With the curved sides and all the bracing, probably. This may get revised, as the overall circumference is just over 8 feet - would be nice to get it under 8 to open up single-sheet veneer options. Then again, I'd have to trim the height to a little to cover two of these at 24" with a 4x8 sheet. Oakwood has some 5x9's, so that could work too.

I figured out how to animate the exploded view, so I thought I'd share. It sure would be nice if it went this quickly in the garage!

The rendering is a little poor - it would have taken about 7 minute per frame to run it at the high setting. The picture below represents the potential, but I rarely leave my laptop in one place for 15 hours.

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post #2 of 11 Old 06-07-2012, 05:53 PM
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Subscribed because your mains turned out great!
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post #3 of 11 Old 06-07-2012, 06:16 PM
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I'll be watching this one. Solidworks?

High Desert Theater - work in progress
Building Bass - Subs

Surrounds - Easy as Pi

Storage - unRAID unDELL

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post #4 of 11 Old 06-08-2012, 07:10 AM - Thread Starter
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Yes, Solidworks.

Wow. I suspected I might be fooling myself on the scale of these. This is how they compare to my mains. I might have to rethink the depth and make them taller.

Are there any considerations when choosing the depth in a sealed design?

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post #5 of 11 Old 06-08-2012, 09:37 AM
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Yes, reason.

Any extreme is a bad idea, so we all start with cubes. Subs are sometimes very deep drivers, so there are hard limits in some applications. However, the only panel of a speaker that can hurt response by being too close is the back. We put drivers next to side and end panels, sometimes sandwiching drivers in narrow columns, as your towers. None of those things have a bad effect, and the only thing left is too shallow, which does, but perhaps less so with subs given the wavelength-to-box size relationiship.

Have fun,
Frank
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post #6 of 11 Old 06-10-2012, 10:51 AM - Thread Starter
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Let's say I'm unable to muster any reason. What would be the technical guidelines for determining a "safe" depth? And what would be the consequence of going shallower than this in a sealed sub? With a 21" sub that is 10" deep from flange to the back of the magnet, what would be considered extreme?

My wife thinks the 28" won't be too deep, so I'm only asking for the sake of education.
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Yes, reason.
Any extreme is a bad idea, so we all start with cubes. Subs are sometimes very deep drivers, so there are hard limits in some applications. However, the only panel of a speaker that can hurt response by being too close is the back. We put drivers next to side and end panels, sometimes sandwiching drivers in narrow columns, as your towers. None of those things have a bad effect, and the only thing left is too shallow, which does, but perhaps less so with subs given the wavelength-to-box size relationiship.
Have fun,
Frank
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post #7 of 11 Old 09-02-2012, 06:58 PM
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At very low frequencies (where the wavelengths are larger than any box dimension) the only parameter that really matters is the volume - which is what your speaker modelling software is taking into account. At the higher frequencies, you start running into transmission line effects - which is basically the same thing as standing waves / room modes, but inside the enclosure. When choosing cabinet dimensions, it's a good goal to avoid resonances within the intended passband (which is usually easy to do on a subwoofer).

As a good rule of thumb, 1/4 wavelength of 90Hz is 3ft. Generally you can expect any transmission line effects to be small once you get shorter than 1/4 wavelength...so basically for 90Hz and below, it's good to avoid dimensions longer than 3ft.

Btw, that's not to say that it can't be addressed with internal damping (which you probably want to do anyway). It's just that internal damping starts to reduce the energy storage of the rear volume and will cost you a little bit of efficiency - that's probably more a design elegance thing instead of a dramatic audible artifact.

Also, you might want to consider adding a ton more bracing to your designs....

-Mike Bentz
~It's all about compromise~
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post #8 of 11 Old 09-02-2012, 07:03 PM - Thread Starter
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Is that a joke?
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Also, you might want to consider adding a ton more bracing to your designs....
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post #9 of 11 Old 09-02-2012, 11:44 PM
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Quote:
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Is that a joke?

Im about .1% sure he was being sarcastic
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post #10 of 11 Old 09-03-2012, 02:21 AM
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Great modelling. Yes, you have more than enough bracing so it's definitely a joke smile.gif I was just wondering why you changed the footprint and dont go for something that better matches the profile of your mains? Seems it would also mean easier to get the volume off the normal sheet size if it was more square. You wont have a problem with the shape given the low frequencies. Of course, curves are very nice to look at wink.gif

explore the music
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post #11 of 11 Old 09-03-2012, 06:34 AM - Thread Starter
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Well, I like to create problems to solve. I've never done curved cabs before.

I'm also not attached to making them a perfect match with the mains. I'll be using veneer vs fabric this time, and since I'm already diverging from the motif... Matching the granite is enough of a tie-in.
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Great modelling. Yes, you have more than enough bracing so it's definitely a joke smile.gif I was just wondering why you changed the footprint and dont go for something that better matches the profile of your mains? Seems it would also mean easier to get the volume off the normal sheet size if it was more square. You wont have a problem with the shape given the low frequencies. Of course, curves are very nice to look at wink.gif
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