Ported box question (stupid one actually)

[soundbit]

When figuring port tuning frequency and length what box volume is used? Is it the NET volume or GROSS volume. WinISD appears to use a Gross Volume before any displacement but some other port calculators appear to use a net volume. Please can someone shed some light on this?

[audiophyle_247]

Port tuning is based off of NET internal volume, it always is on any calculator.
The only times a gross volume is used is typically where values are published and its in the manufacturers best interest to assume people suck at math and they do all of the calculations for ya.

The volume number in WinISD does NOT include material, bracing, sub, or port displacements, those need to be accounted for in the actual design/build of the enclosure.
WinISD ignores all of that because there are WAY too many variables involved in any enclosure, it works on just the actual internal airspace value and its up to you to calculate everything else accurately.

& not a stupid question, I think this is actually the BIGGEST mistake people make when trying to design ported enclosures, not just first timers either Ive seen a few installers continually make this mistake.

[soundbit]

This makes sense but answer me this question. If a manufacture give me 1.6 cuft for a NET volume for a ported box and 16sq in of port and a port length of 18.5 inches to tune the box to 31Hz (all these number are from the manufacture mind you). should I be able to plug them numbers into WINISD and get the same thing? Cause cannot. If I plug in 1.6 cu ft. and a 31hz tuning freq. and 16sq port area then I get like a 201" port (can't remember exact number). It is only when I goto a 2 cu. ft. volume that the numbers match up. 2 cu ft would be about Gross for this senserio. Am I doing something wrong with winISD?

[shawn k]

Hmm.. I'm getting: 1.6 cuft net with a tuning of 31hz the port would be 24.89" long (16 square inches of port surface area)

16 sq in of surface area equates to a 4"x4" square port for example. Not 16"x16".... which for the box and tuning previoulsly mentioned would be like 433" long

[soundbit]

Shawn that is what I was getting too yet manufacturer says a port length of 18.5" with 16sq in of port area.

Well I think i found my problem. Correct me if i am wrong. Manufacturer give an F3 for tuning (which if I recall is not the tuning of the port but the freq at which it is 3db down.
Well i plugged the numbers back into winISD and did not specify a tuning freq. but used the 18.5" as the vent length. this gave me a tuning of around 35hz and low and behold a 3db down point of 31hz.

Does this sound right? Am I right in the F3 being a 3db down frequency NOT a tuning freqency.

[shawn k]

Yep.. Sounds right!

[soundbit]

It is amazing what you forget in 8 years. I have not built a ported box since 2004 and forgot what numbers to use. I feel like a clown.

[audiophyle_247]

Manufacturer give an F3 for tuning (which if I recall is not the tuning of the port but the freq at which it is 3db down.

That was going to be my next question, following what sub you are actually working around.
Alpine lists an Fb & F3 value for their recommended ported enclosures, which is helpful but the box cannot sound good if you model it. Port velocity is horrible and the low end is still weak.
I would use that manufacturer spec box as a starting point, but work out a better solution in WinISD.

[zeropoint0.5]

mr soundbit,

to be really really sure, you could make the effort to place the subwoofer,even the port at the outside of the box and making

a listening test this way. if it sound much better or makes a big difference, you can know if you went wrong or not.....

if you would doubt.....just a suggestion......

[soundbit]

Very good thought.

[zeropoint0.5]

last year i bought an old skool xmax 12 inch V2 from somebody in a self made calculated vented box.....

i let the box play and heard "no, something is wrong this doesn't sound good" so i measured out the box inside

and look on the old papers, and i saw it was to small. so i let it play again but with the sub mounted at the outside

of the box and and changed + and -. then it played very good...... so this box was made without calculating the driver

displacement into the volume..... and the guy said it was build and calculated by an official dealer back in the days.....

the calculation is very important, especially with a vented box......

my only thought with programs like winisd and everything is, will you get a better result then the "recommanded enclosure"

given by the brand itselfs ?

[audiophyle_247]

my only thought with programs like winisd and everything is, will you get a better result then the "recommanded enclosure"

given by the brand itselfs ?

That depends on several factors. First, you need to know what it is you want, and WinISD will show you what can be possible so you can fine tune to your liking. There is an almost infinite variety of options when it comes to box tuning, more so when talking ported but sealed has some options too.
If you know exactly what you want, and what you will give up to get that, modeling gives you the guide to achieve that, and takes you to factor 2.
Factor 2 is your ability to build, measure, and understand how to measure. WinISD is only as accurate as your building capacity (& the accuracy of any numbers you enter), the box has to be built accurately to realize the performance it was designed to achieve and the right numbers used in WinISD so it can calculate accurately.

If you build an equally inaccurate box to WinISD spec or manufacturer spec, either will most likely sound less than ideal, except the WinISD one you will know its not as designed because you know how it should have sounded. Unless manufacturers published the freq curves of their spec'd box, I bet it's more than likely the sub would get the blame because the performance has not been clearly shown/understood.

Manufacturer boxes are also designed around max output with "acceptable" sound quality, which you know they too modeled on the computer. The only real difference is they did most of the math for ya, and all the decisions about how the system should sound.