Question on cabinet design including port hole

[ACDCG400]

 8) Hey guys i made a cab a little while ago and now ive decided to make it closed back, since it was open back.  it is like 18 in (height). 12 3/4 in (width), 9 in (depth). does anyone know what the port hole size should be? =]  i am trying to get iit to sound as fukll as as loud/4X12ish as possible. because i am terrible at this kind of thing. =/ technically speaking, i know nothing about speaker design. =]  :D ;D 8) ::) :-[

[JN]

It depends entirely on the speaker you use. The port design is based on both the speaker and the box size. There’s a program called WinISD (google it, it’s free) that can then help you calculate the right size and length (length is important too) of port to tune the box properly.

Or just cut a 2” hole and hope for the best. It’ll probably work out fine.

[ACDCG400]

the speaker will be a celestion gt-75 or a hellatone (broken in celection) g-30h.  cool. ill use the program.  thatnks guys! =D

[ACDCG400]

omg how do you use this thing?!?!  :D ;D ::) :-* :o

[tubesornothing]

Click HELP->Getting Started.  You require the Thiele-Small parameters for your speaker (aka driver).  You **might** find these on the speaker manufacturers web site.

If this puts you off, you might want to try JN's second solution.

[ACDCG400]

I will try eminence speakers data for the closest thing to celestion.  like the wizard for the gh30 so lets see how this goes.  now what do i need to know in order to make this cab sound fuller?

[PRR]

> i am trying to get iit to sound as fukll as as loud/4X12ish as possible.

Open/closed/vented will ONLY affect the guitar's bottom octave, 160Hz to 80Hz. While "fukll" is partly about how that bottom-octave supports the mids-highs, there's no way one Twelve is going to BLAST the way a 4x12" array can.

> does anyone know what the port hole size should be?

"No", in the sense that I've never been able to compute a hole even if I knew what I want.

And for guitar-scale systems with good-size cones, there is NOT a "good" vented-box tuning. Vent-box can lever a small cone in a large box to low Hz. In guitar we -need- a large light cone and favor a small box (if your Econoline had room for a big box, you'd build the 4x12 of your dreams), and we don't have any use for very low Hz.

I do not believe classic filter theory is much use here. Use the time-honored method. Open-back plus cleats. "Close" the back with 1x2 slats. Leave one out, try, take another out, try, repeat until the bass goes away or you learn that open-back projection and midbass efficiency can trump fancy math.

> There’s a program called WinISD

http://www.linearteam.dk/

Such tools are overwhelming, even to me.

Try the basic on-line box-hole calculator:
http://www.linearteam.dk/default.aspx?pageid=ventcalculator

Measure the actual inside volume of your box, deducting a quart for magnet etc. It does not have to be so exact, but don't wild-guess.

Start a tuning at 75Hz to 90Hz. With a typical gitar speaker, this sets speaker, box, and bottom-string resonance all the same, and is liable to be boomy. But if box Fb is much lower, then it may as well be sealed. And if much higher, all your bottom-notes cancel-away. These may be valid, but start with Fb near 80Hz.

Put in a vent diameter, yes 2" is popular.

For a 2 cubic foot box, 2" vent, 80Hz, I get -0.8 inches. Yes, a negative length! That's physically impossible. I actually need to be told a length of 0.75" or greater.

Trying 1 cubic foot, 62Hz, 2" diameter, gives 0.76" length, good-enuff. However 62Hz box resonance will not be a lot of help for 82Hz+ guitar tone (drop-tuned axes may benefit).

1 cubic foot, 125Hz, 7" diameter, gives 0.753" length. This is in the zone of many smaller tweeds. The big hole allows ventilation of the tubes. And output may rise in the mid-bass (need too much data to calculate, test is best). Depending on speaker, it could be phatt or it tubby/muddy. But output below ~~110Hz, your bottom five notes, is gone-away. Great little amp tone, but won't have the gut-punch of some other (mostly larger) schemes.

Oh, and if you have a big speaker working hard (is there any other way?), a small vent will whistle (what they call "wisthling"). You really want the vent like 1/4 to 1/2 the diameter of the cone. Whistle is a tricky calculation and you need some realistic assumptions; I'm not sure the online calculator handles it right. Of course we don't need extreme clean on loud deep guitar tones.