figuring out that third dimension...but bracing_displacement is gettin on my nerves!

[sumone 10+ year member]

oh, I see. so the area from the upper right corner of the port (where it changes direction) is not included in the port displacement?

 

[sumone 10+ year member]

ahh, ok. I kept confusing the definition of internal depth as (external depth - 2 * MDF_thickness - port_width) when it's just without subtracting port_width.

man you's a killah! thanks a lot!

 

[OLD_SCHOOL 10+ year member]

and thats only when your first port wall buts against the back of the face and not to the side of the face sitting flush with the front of the box coreect ?

 

[sumone 10+ year member]

so, does the top view of this look like what you've been describing?

 

[Dogmeat]

**** sumone ...that's worth 4-12 bucks in my opinion. Send JMAC some money bro....And this should be pinned for the valuable info it offers. If I save this do I have to send money too.....or just if I use it? lol

 

[DBfan187 5,000+ posts]

Donate to the Jmac foundation for Subwoofer Enclosures.

 

[sumone 10+ year member]

I've written up something to help myself understand...cause I know I'm gonna forget all this stuff in a week. It's only right that I use the help that I got to help others

here

since the gallery doesn't work I can't show that picture cause geocities makes sure that the referrer is blank or from them; but here's the text:

Based on instructions from Jmac (email/PM him for a CHEAP, fully COMPLETE box plan!)

...An L-shaped port's displacement....

1. Figure out theoretical Lv

Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)

Lv = Port Length in inches

Av = Port Area in square inches

Vb = Net Internal Volume in cubic feet

Fb = Desired Tuning Frequency

2. End_Correction = Port_width / 2.

Physical Lv, Lv_p, is Lv_p = Lv - End_Correction

3. Choose a number smaller than Lv_p and bigger than 2 * MDF_Thickness as a test depth of box (D).

- As viewed from top, D = vertical (external) length from front of box to back of box

4. Port wall P1 is interior length of the depth (P1 = D - 2 * MDF_Thickness)

P1_MDF_Needed is Port wall P1 minus the Port_Width (P1_MDF_Needed = P1 - Port_Width)

5. Port wall P2 is physical port length minus depth of box (Lv_p - D)

P2_MDF_Needed is equal to Port Wall P2 (P2_MDF_Needed = P2)

(Port_Wall_2's right edge connects to the top-left edge of Port_Wall_1, not the top-right edge of Port_Wall_1)

Displacement of port is:

(P1 * Height_of_Box * (MDF_Thickness + Port_Width)) + (P2 * Height_of_Box * (MDF_Thickness + Port_Width))

or

(P1 + P2) * Height_of_Box * (MDF_Thickness + Port_Width)

 

[sumone 10+ year member]

I tried to go through some algebra to get the port area/displacement including mdf in one line but didn't check it cause then Jmac responded and he's the "man with the plan"...

just to prove that I wasn't just in search of just an ANSWER, but a method to producing the answer, here's what I got before Jmac responded on getting port displacement (mdf included):

p + q = port_length, where port_length is an L-shaped port, p is the vertical component & q is the horizontal component (as if there were a line running directly center of the port)

q = port_length - p

x = p + p_width/2

y = q - p_width/2

where x is the interior depth of the box, and y is the width of the horizontal mdf needed (along with the mdf_thickness from the vertical component)

area_mdf_height = (x - port_width) * mdf_thickness

area_mdf_width = (y - mdf_thickness) * mdf_thickness

total_mdf_area =

(x - port_width + y-mdf_thickness) mdf_thickness

((p+p_width/2) - p_width + (q - p_width/2) - mdf_thickness) * mdf_thickness

((p + p_width/2) - p_width + ((port_len-p) - p_width/2) - mdf_thickness) * mdf_thickness

p + p_width/2 - p_width + port_len - p - p_width/2 - mdf_thickness) * mdf_thickness

((p - p) + (p_width/2 - p_width/2) - p_width + port_len - mdfthicnkness) * mdf_thickness

(0 + 0 + port_len - p_width - mdf_thickness) * mdf_thickness

= (port_len - p_width - mdf_thickness) * mdf_thickness

total horizontal area (extending all the way to the right wall)

(port_width) * (Y + Port_Width)

total vertical area

(X - Port_Width) * (port_width)

therefore, port area excluding mdf is:

total port_area =

(port_width) * ((y + port_width) + (x - port_width))

((q - port_width/2) + port_width + (p + port_width/2) - port_width) * (port_width)

((port_len-p - port_width/2) + port_width + (p + port_width/2) - port_width) * port_width

(port_len - p - port_width/2 + port_width + p + port_width/2 - port_width) * port_width

(port_len + (-p + p) + (-port_width/2 + port_width/2) + (port_width - port_width)) * port_width

= port_len * port_width

total mdf area with port area:

(port_len - p_width - mdf_thickness) * mdf_thickness) + (port_len * port_width)

port_len*mdf_thick - p_width*mdf_thickness - mdf_thickness*mdf_thickness + port_len*port_width

= (port_len - p_width) * mdf_thickness - mdf_thickness^2 + port_len*port_width

port_len = 18.786

p_width = 1.75

mdf_thick = .75

= 17.036 * .75 - .5625 + 32.8755

= 45.09

45.09 multiplied by the height 15" = 45.09 * 15 = 676.35

port_len = 18

= 16.25 * .75 - .5625 + 31.5

= 43.125 multiplied by 15, = 646.875

compared with Jmac's 618.75, I'd say I was kinda close //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

This ain't over, I'm gonna figure out what it is I was doing wrong...

 

[sumone 10+ year member]

It's over: I got it:

going backwards, (knowing what the final displacement is, and solving for port_length),

((port_len - 1.75) * .75 - .5625 + port_len * 1.75) * 15 = 618.75

41.25 = (port_len - 1.75).75 - .5625 + port_len*1.75

41.8125 = (port_len-1.75).75 + port_len*1.75

= .75port_len - 1.3125 + 1.75*port_len

43.125 = (.75 + 1.75)port-len

43.125 = 2.5port_len

17.25 = port_len

this 17.25 is actually the distance of the port excluding the front opening of the box. the forumlae I came up with all had to do with the inside of the box.

therefore, with an Lv of Lv_Theoretical,

Port_length_For_my_stuff = RoundUp(Lv - port_width/2) - MDF_thickness

port_length_for_my_stuff = RoundUp(18.786 - 1.75/2) - 0.75

port_length_for_my_stuff = 18 - 0.75

= 17.25

so to sum it up, the displacement for an L-shaped port is:

((internal_port_len - p_width) * mdf_thickness - mdf_thickness^2 + internal_port_len*port_width) * internal_height_of_box

where:

+ internal_port_len = RoundUp(Lv - port_width/2) - MDF_thickness

+ Lv = theoretical port length (Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av))

+ p_width = port_width

+ mdf_thickness = thickness of mdf (.5, .75, 1)

+ internal_height_of_box = self explanatory

just to check,

internal_port_len = roundUp(18.786 - 1.75/2) - .75 = 18 - .75 = 17.25

p_width = 1.75

mdf_thickness = .75

internal height_of_box = 15

((internal_port_len - p_width) * mdf_thickness - mdf_thickness^2 + internal_port_len*port_width) * internal_height_of_box

((17.25 - 1.75) * .75 - .75^2 + 17.25*1.75) * 15

(15.5 *.75 - .5625 + 30.1875) * 15

(41.25) * 15

= 618.75 in^3

//content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

so now that I got the displacement, adding all displacements up & net_volume, then dividing by the already known two dimensions gives you the third internal dimension. Round this up.

Therefore, the port walls will be:

vertical port wall: (internal_depth_rounded_up - port_width) x MDF_Thickness x internal_box_height

horizontal port wall: ((internal_port_length + MDF_thickness) - external_depth) x MDF_Thickness x internal_box_height

Thanks Jmac!!! couldn't have did it w/o you!

****, and school ain't even in session //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif