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designing w/winisd need advice


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sedate 
Silver - Posts: 1,173
Silver spacespace
Joined: July 03, 2004
Location: Colorado, United States
Posted: May 07, 2007 at 3:43 AM / IP Logged  

dollarlongnecks wrote:
Can you give me a little info on the fiberglass resin you use for the box? Is this something you spread on the inside of the box?

Get it at HomeDepot along with ur MDF...  the smaller size will be more than adequeate... i think its like a quart for $12.  Get an extra tube of hardner.  Get a box of disposable latex gloves. VERY IMPORTANT.  If you get resin on your skin it will not only stick worse than super glue, it will sting and itch like 10 jellyfish... the resin will eat the latex too so change ur gloves if you get any on them.  Get a couple of disposable brushes.

Anyway, its a binary liquid.. a little tube of hardner a the big tub of resin.  You mix like 1oz of resin with 1 drop of hardner, and stir.  It only comes with enough hardner for like 1/2 the tub of resin so grab an extra one while ur there.

Then paint the resin on the inside of the box.  Wait about 20 minutes or so, apply a second coat.  Repeat 3 or 4 times.  This effectively paints very durable plastic onto and into the wood... MDF is a phenomenally good substrate for resin.. very porous...  Let it cure overnight and .. viola! ... ur box is smaller and 25% lighter than a comparable 3/4" MDF enclosure.

The resin will ruin everything you get it on and everything it touches.  Completely unremoveable with anything less than sandpaper.  Handle carefully with lotsa newspaper over your work area.

Oh yea.. it smells and the fumes make gas smell like perfume.  I heard a story about a guy that stuck his head inside his box to make sure he'd gotten the resin in all the corners... and woke up in the emergency room with his head shaved hooked up to an iron lung.

dollarlongnecks wrote:
And in the relation to the ports, when I look at the air velocity graph in WinISD, it looks like it is way above other recommendations I have read, which was to keep it below 110 fps. Should I try to bring this down

Hmm... I think you're talking about port noise. I actually use an older version of WinISD that calls port noise 'vent mach' and it didn't have any problems with that configuration...  I really need to start using WinISD alpha that graph you have up there is neato.

I had a 12 for sometime that was in an enclosure almost identical to the one we are talking about .. 1.25 cft.. I don't remember exactly what I was trying to tune it to... something around 30hz.  Anyway I used 3 1" ports that were 4" long and I thought it sounded quite good... no 'chuffing' or any other anomolies. 

Playing with the calculator I have on my computer right now, it tells me this configuration is perfectly acceptable.. actually calculated right about to your number of 110fps.  Generally I try to keep this number below about 150..   Can you point to something that says something different?  I'd take a look.  That graph you have up there isn't something I've seen before, but it does make sense. 

I'm not sure dude.  I tried several other configurations and get the same results.. two 2 inch ports gives me the same air velocity.... so does a single 3" port... but I don't have the nifty graph and it looks like you aren't going to have problems until about 50hz or so..

Point to whatever you've read, I'd like to read it too.  Try graphing a pair of 2 inch ports against that graph above and see what comes out.  Or a single 3" port.. that looks like it would work out to about 11inches.. not unworkable really.

"I'm finished!" - Daniel Plainview
dollarlongnecks 
Member - Posts: 9
Member spacespace
Joined: June 03, 2003
Posted: May 07, 2007 at 4:26 PM / IP Logged  

The information I read about the port air velocity came from stevdart's WinISD guide located <here> and copied below:

Vented enclosures, specifically

Possibly the greatest value of this program is that it will enable you to model a vented box.  Some woofers are best as sealed, others are best as vented.  The EBP tells you the story, although it's not always 100% (hint:  Solobaric wants a sealed, but everybody vents it).  The EBP, you remember, appears in the center of the 'New project' box when you first bring up the new project.  It is simply a calculation of Fs / Qes and a number is shown.  The Kappa sub we were looking at above had a 50.7 EBP, which is withing the range of a sealed enclosure.  Higher EBP's generally call for a vented box.  The program determines the best box using this EBP value by default.  But as a working tool, the program allows you to change to whichever you want.

Here is an ID sub from the program's database:  ventedwinisd.jpg  You see the picture of the sound;  there is no peak at all in the response and the low end extension is quite good, with F3 in the 20's.  Most of the reason this sub "looks" like it sounds so good is due to its engineering;  the rest is due to providing just the right mix of airspace and port airspace to the enclosure.  In the pic, you can see the vent diameter of 4" and vent length of 32". 

FAQ:  "WinISD tells me that the port has to be 32" long.  My box is only 18".  Now what?"

This is where you work the program to get the best response for what space you have available.  This box is also specified to be 2.5 cu ft.  If you don't have that much space available you have to make a change there as well.  By default, WinISD will calculate a vent opening at 4.02 inches, which in metric terms is 102 mm.  This is where almost every user of the program will have to make a change.  A larger diameter vent will equate to a longer length, and a smaller diameter vent will call for a shorter length.  Use the 'Rear port air velocity' chart that I mentioned in the beginning to see what port noise looks like:  velocitywinisd.jpg

I have my chart shown set to feet/second and specified to show RMS, not peak, output.  This pic shows a port noise of 55 ft/sec, which is great for car audio because it is 5% of the speed of sound.  You should always keep it at 10% (110 ft/sec) or less.  The vent opening area will determine this noise, so making the vent smaller in diameter will make more noise, which might become unacceptable.  If you are working along with this guide, you will have to give your sub some power in order to get a port noise response.  Select the 'Signal' tab on your working box and input the sub's RMS, or the power that it will be getting, in that block labelled 'Input power'.  Now your port noise chart will jump to attention.

So what do we do with this long port issue?  How do we get a 4" X 32" port in this box?  And what will it do to the overall size of the box?  You see that this program allows you to change to a slotted port...is this the best thing to do in this case?

The port, if inside the box, infringes on the air space.  It has to be calculated as displacement, so the box has to be that much bigger to compensate for this infringement.  I use the cylinder volume calculator on this site to figure volume for a round port tube.  If that port takes up .262 cu ft, that volume has to be added to the box volume, making it now a 2.8 cu ft box...and that's just the inside.  A round PVC port will take up less interior space than the equivalent slotted port because of port wall thickness.  Those 3/4" MDF walls normally used for slot port construction can really add up to some serious displacement.

You have to take all these things into consideration when you finally arrive at what will work for YOUR situation.  Fiddle, fiddle, fiddle.  Try to reduce the air volume and the port length grows, every time!  Make the port opening smaller and the noise grows.  In this example, if I were strapped for room, I would allow the port opening to get small enough so that the port noise chart showed me about 110 ft./sec.  Fiddle...if I change the opening to 3", the port length shrinks to 17 3/4", the net box air space is 2.5 cu ft, the port noise shows at 105 ft/sec.......and I'm happy with that.  The response looks just as great as it did in the first pic and I won't hear the port noise in the car.  Done.  Build it.

Thanks for the info on the resin, that should work out well. I'll continue to play with the port size and placement, I want to make sure I get this right the first time...

dollarlongnecks 
Member - Posts: 9
Member spacespace
Joined: June 03, 2003
Posted: May 14, 2007 at 10:02 PM / IP Logged  
Hey Sedate, I just wanted to update you on the box. I've been drawing it up for a few weeks and finally put it together Saturday afternoon. It sounds awesome... It's really quite a bit better than I thought it would be. I've actually been able to ture it properly and have finally found that "thing" that just wasn't right with my system. Thanks again for all the help..
sedate 
Silver - Posts: 1,173
Silver spacespace
Joined: July 03, 2004
Location: Colorado, United States
Posted: May 15, 2007 at 9:21 AM / IP Logged  

dollarlongnecks wrote:
It sounds awesome... It's really quite a bit better than I thought it would be.

Woot! 

I'm glad I could help..!

"I'm finished!" - Daniel Plainview
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