School me on box design

[fogged306 0]

I recently opened up a tint shop and have had a lot of customers wanting audio, so I just picked up the Sundown line. I've been an installer for 14 yrs now, but all the shops I've worked at did cookie cutter systems with pre-fab boxes so I never really learned how to make a box and match it to the subs. I've built plenty of boxes, so the actual building is not what I'm looking for it's just getting the correct size and port length I need. I see a lot of threads recommending box calculators like torres ect. which I've downloaded, but I don't see how you're converting the sub specs to box design. I found the AJ vented designer program which does this (I'm looking to built mostly vented enclosures) but I'm getting huge difference in box sizes between the 3 options it lists for alignment. SBB4 is giving me .9 cu ft and qb3 is giving me 3.7 per sub and SC4 gives me 1.83. The port size and length are excessive as well when I put in the xmax for the sub like 10" round by 170" long for SC4 for example. What am I doing wrong here? I'm inputting the specs right from the sheet for the SA-12's I'm using. Qts .52 Qes .58 Fs 33.71 Vas 21.21L Diameter 12"(or 30.48cm) xmax 38mm and I've been playing with port diameter but it always seems the smallest number before it warns you the port is too small also gives you the shortest port length. I'm also looking to convert this to a slot port. So basically what I'm looking for is some direction on where to find box software that I can use or if this one is good, how to use it, and how to convert to slot port sizes. Any other knowledge you'd like to share with a rookie designer would be appreciated also. Thanks.

Also wouldn't mind some advice or links to some good reading material for this. I've been to this website and done some reading http://www.bcae1.com/spboxnew2.htm, but I'm always looking for new ways to learn and new stuff to read.

Edited May 16, 2013 by fogged306

[Impious 1,476]

WinISD is free.

 

Most box calculators aren't enclosure modeling software that predicts response, they simply tell you the measurements of the panels for designing an enclosure.  What you need is an enclosure modeling software, like WinISD.

 

Enclosure design 101: For a given enclosure volume & tuning frequency.....As port area goes up, port length increases.  As tuning frequency goes down, port length increases.  Which is why port length is decreasing as you decrease port area.  10" round is excessive for a SA-12.  And the SA-12 doesn't have 38mm Xmax.  It's 19mm if I recall correctly, 38mm peak-to-peak...but Xmax is one-way, so it's 19mm

 

When you are converting round to slot ports, all you need is to arrive at equivalent port area. 

 

What you need is to pick up a book on enclosure design.  The topic is WAY too large for a single thread or post to cover all necessary aspects, and it doesn't sound like you have enough of a foundation to simply "fill in the holes" for you.   

 

Pick up the Loudspeaker Design Cookbook by Vance Dickason.  

[ParrishElectronics 6]

If you haven't already, try looking over www.the12volt.com.  There is quite a good bit of info on there, including calculators, T/S parameter explanation, etc.

[Quentin Jarrell 671]

No software is gonna be any help at all if you don't know what you are doing.

Buy a copy of loudspeaker cookbook and learn why what to do and you'll end up be much more helpful to your customers.

[DanP 19]

 

Watch some of his videos, really helpful.

[fogged306 0]

Thanks for the replies so far. I'll pick up a copy of that loudspeaker design cookbook and I'm going to check out that video right now. I guess I was looking for just a simple program where I put in the T/S parameters and it spit out a volume and port area. Looks like it's not going to be all that simple though.. I should say that I'm not looking for absolute perfection, but just something that will outperform a pre-fab for any given speaker.

 

And thanks Impious, I was going to ask if xmax was peak to peak or one way but forgot to in my first post.

[Impious 1,476]

The problem is there is no "one" enclosure volume, tuning and port area that will work for a given driver.  It all depends on goals, and since those goals will vary person to person and install to install then the appropriate enclosure will also vary person to person and install to install.  Enclosure modeling software is a huge aid in designing an enclosure, but you still have to understand how to interpret the information and determine what is appropriate for a given situation. 

[///M5 2,833]

DanP, on 16 May 2013 - 20:06, said:

Watch some of his videos, really helpful.

While I love his exuberance and desire to make videos that regularly are helpful, don't follow it as the gospel as he makes plenty of little mistakes. I've never watched a full video of his but did part way on this and wasn't so impressed. It does give you a good idea of the steps though.

[fogged306 0]

So I've been reading and watching videos and have learned the basics of winISD I think. I've got the loudspeaker design cookbook on it's way, but in the meantime, I have some more questions. I'm going by Sundown's suggestions and using 2.0cu ft per driver and tuning the port to 35Hz. Maybe I'm not playing around with dimensions enough, but to get the required port area in that size box with one driver I'm going to have to either double the port back on itself a hair, or the interior end of the port will be very close to the wall. What are the rule of thumbs for how close you can get to the wall. This is mostly because I'm trying to keep the fps of the port to under 70 so the opening has to be around 2" with my box dimensions, which are 17.375W x 18.5H x 14.35D internal with a 2 x 18.5 x 30.xx length port. This is also based on using 750W RMS for a single driver. I'm using a SAZ 1500D at 1ohm on two of these subs in the back of an MK4 2 dr Golf.

 

My main question is this, do I need to separate the subs? I'm basically designing these boxes as individual boxes so there's a divider inbetween them and each sub has it's own port. I noticed that if I make it so they both share the same volume, the port length doesn't get much longer if I double the width of the port, in which case I can easily fit one single slot port for the two woofers and still satisfy the total port area required for 35Hz. What's the drawbacks to having 2 drivers share the same enclosure space? The goal to this set-up is to be loud on music, mainly rap and bass cd's so I can use this as a demo car to sell product.

[Impious 1,476]

The port area (width and height) needs to remain constant.  So if you have a 2" wide port, the port end needs to be atleast 2" from the nearest wall.  Now, to be technical, if you ended the port 2" from the side wall (I'm assuming a side wall is what you are talking about) then the effective port length would actually "curve" around with that side wall a bit, plus you would need to add the port end correction factor to your effective port length calculation (I think WinISD might do the correction factor, I don't mess with ported enclosures in there much to know off hand).  However, you are talking about changing the port tuning by a couple hertz at worst (if the port is a decent length) which really isn't going to matter much in a daily driving situation.  People sweat the small stuff too much IMO.  If you can't hear it, it doesn't matter.  Using WinISD model the same enclosure with a port 3" longer and see what the difference in response is.  I don't know the details of your enclosure but I wouldn't be surprised if the change was small enough to not audibly affect the response.

 

The downsides to having 2 subs share a common chamber is that 1) you lose a center enclosure brace in the enclosure created by the separating divider wall, and 2) with a common chamber if one driver fails you increase the possibility of damaging the 2nd driver as well because it now effectively "sees" twice the enclosure volume which will affect it's mechanical power handling capabilities. 

[fogged306 0]

If I add or subtract 3" from the port the frequency changes by less than 2Hz and about 1dB in either direction from where it is now. I'm assuming this is one of those scenarios where it's not something I would be able to notice, right?

 

Also, I have 71.92 fps port velocity if I design the box the way I would like it to be with 2 chambers and the exact fit I would like, is that something I would hear at all in the hatchback? This isn't a kerf port, it's just going to be a slot port with the inside edge router'd and a double router edge on the inside beginning of the port. I always make my boxes with a lip around the front and back edge mainly because I like the look. Here's a picture of one I built a while ago, and I would like to keep this look, but with a higher FPS on the port, should I not do the lip and router the port exit instead, or does it not matter?

 

Thanks for the replies so far.

 

[Quentin Jarrell 671]

Don't design the port based on velocity.  All of these programs design based on small parameters.  when a properly designed driver reaches xmax it's cms changes 50%.  This change(stiffening) of the suspension affects piston velocity which is directly related to the velocity of air going through the port.  Because these programs can not account for this they are inaccurate.

 

Instead use Small's minimum port area formula:

 

dv=39.37(fb x Vd)^.5

 

dv= minimum port diameter

fb=  tuning frequency in hz

Vd= cone displacement volume in cubic meters:

    = sd x xmax

       NOTE:   xmax is NOT the rating most manufacturers use which will be stated as 70% BL rating.  It is defined as coil over hang.  which means measure how much the coil seen above      the top plate.

Edited May 22, 2013 by Quentin Jarrell

[Quentin Jarrell 671]

and as brad said about considering the end port lenght.  Most formulas already account for this.  For example the folowing formula:

 

Lv= (1.463 x 10^7 x R^2) / (fb^2 x Vb) -1.463R

 

 

The red part of the formula accounts for end correction.

[Impious 1,476]

I would definitely not worry about a difference of 2hz and less than 1db