Pencil and Paper 101

[Used2Know]

After seeing yet another monotonous thread on this subject, I am posting this surely simple way to aid in ported enclosure construction.

1. figure out your workable cubes (space you have to fit your enclosure, the max dimensions minus 1.5 inches from external measurements (hxwxd), giving you your gross internal volume assuming you are using 3/4 mdf)
2. choose an alignment given your speakers parameters or use an accepted real world tested enclosure size and tuning that will fit via your workable cubes
3. use the first port length formula to figure the first unadjusted port length
4. calculate port(s) displacement and any other internal displacements i.e. speaker(s) or any bracing and then subtract that from gross volume
5. use the other port length formula to figure out tuning
6. adjust length, thusly affecting your volume obviously, until you are satisfied with end result

You most likely will need to only recalculate twice or sometimes trice.



Formulas


Cubic Feet

• height * width * depth / 1728 = Ft3

Port Displacement

• (outside diameter/2)^2 * 3.14 *(length of port inside said enclosure)= in^3
• then obviously convert that to ft^3
• don’t worry about the internal flanges of an aero as they are minuscule in the whole scheme of things

Multiple Port Diameters

• SQRT(A^2 + B^2) = total combined diameter

Slot Port Displacement

• calculate the internal volume inside the port as well as the volume of the wood making the vent wall(s) for the port and then subtract that from the internal volume of the enclosure
• length * width * height / 1728 = Ft3

Slot Length to Width Ratio

• length/width = ratio
• keep it under 9:1

Speaker Displacement

• most manufacturers include this in their parameter specs, but if not use this following formula, not 100% accurate but works none the less
• 4 * 0.33 * 3.14 * (depth from back of basket to rear of magnet)^3 * 0.5 * 0.6

Port Area

• shoot for 5% of speed of sound (</ 17 m/s) and you wont have any problems

• 12-16 square inches of port area per cubic foot of volume is the generally accepted guideline
• area of a circle is pi*r^2
• area of a rectangle is h*w
  
  or
  
  
• dV > 39.37*(Fb*Vd)^0.5
• dV is minimum useable port diameter
• Fb is enclosures tuning frequency
• Vd is the volume displaced by the Speaker at its full excursion (peak-to-peak)
• convert min useable diameter to square inches if going slotted

Port Length

• Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2]-0.823* SQRT (Av)
• Fb = 0.159*SQRT[(Av*(1.84*10^8)]/(Vb*1728*[Lv+(.823*SQRT(Av))])

• Vb is true net volume (all displacements taken into account)
• Fb is the enclosures resonant frequency (tuning)
• Av is the cross-sectional area of the port (port area)
  
  1. (area of a circle is pi*r^2)
  2. (area of a rectangle is h*w)
• Lv is the length of the port

[Corey Anderson]

sticky'ed

[momentarylapse7]

Great post, Used2Know.

[clampjaw25]

i've seen you answer with almost everything in this thread multiple times.
great enclosure sticky

love the avatar btw

[TheDukey42]

Wow! great sticky!

[IVIike13]

i get all that but how to you find out your port tuning?

[carguy6702]

thanx finally found what i was looking for

[weeznur32]

In thefomula what is SQRT? Square root?

[xhilr8n]

does someone know where there is a good terms dictionary or guide.


thanks.....guess i am just an idiot.

[Coolwazabi]

um... you have the english version ?

good stuff.. just little confused on aero ports..

[Used2Know]

Originally Posted by Coolwazabi

um... you have the english version ?

good stuff.. just little confused on aero ports..

check out subwoofer builder...

[kensaudio99]

What does this mean? * ^

[kensaudio99]

Nevermind! I got it.

[kensaudio99]

Hi! Is this the formula for calculating the port displacement?
length * width * height / 1728 = Ft3


Slot Port Displacement

• calculate the internal volume inside the port as well as the volume of the wood making the vent wall(s) for the port and then subtract that from the internal volume of the enclosure
• length * width * height / 1728 = Ft3

[2400watts]

i am so lost im trying to tune my box to 35hz w/ a 6 cu ft box
any help from anybody would be fine... its just so confusing

[Used2Know]

Originally Posted by 2400watts

i am so lost im trying to tune my box to 35hz w/ a 6 cu ft box
any help from anybody would be fine... its just so confusing

Shoot me a PM.

[fosg8freak]

I finally got off my lazy ass (hypothetically speaking) and entered that tuning frequency formula into Microsoft Excel, then I did all the formula's for volume, port area recomendation, external dimension calc, ect.

You just feel good about yoruself when you make the formula's yoruself then connect everything up in excel

[Grizzleyadams69]

shoot for 5% of speed of sound (</ 17 m/s) and you wont have any problems

this guide doesnt state HOW to make a port 5% of the speed of sound. i have no idea how adjusting port size, width, length, etc would affect the speed of the port. can someone explain this for me please? is there a formula?

[Used2Know]

Originally Posted by Grizzleyadams69

this guide doesnt state HOW to make a port 5% of the speed of sound. i have no idea how adjusting port size, width, length, etc would affect the speed of the port. can someone explain this for me please? is there a formula?

The formula provided via this sticky does give you the "best" minimum port diameter... which you can covert to cubic inches to get a general idea. Model the vent velocity in an acceptable program such as Bass Box Pro or even WinISD. You can go a little higher up to mid 20s m/s without a “noticeable” amount of port noise, even up to 35 m/s max (10% mach aka speed of sound) if space of the enclosure size is a major concern. Although disregarding aero ports (you can get away with even a tad more), obviously the lower the vent velocity the better... I stated via the thread the most opportune vent velocity which is roughly 5% mach, but can often be hard to accomplish in mobile audio. Those programs I mentioned will show vent velocity given your port area, Power applied and such.

[Used2Know]

Originally Posted by fosg8freak

I finally got off my lazy ass (hypothetically speaking) and entered that tuning frequency formula into Microsoft Excel, then I did all the formula's for volume, port area recomendation, external dimension calc, ect.

You just feel good about yoruself when you make the formula's yoruself then connect everything up in excel

Nice job! Mind pming me that worksheet?

[fosg8freak]

Originally Posted by Used2Know

Nice job! Mind pming me that worksheet?

Im not extremely confident in the accuracy of it, but Ill send you what I got. I spent a few hours putting things in, putting in real life numbers and comparing to make sure everything was ok.. and then on the ones that were not simple math I used bbp6 to compare and they seemed very close. Ill go get the file and send you a pm.

[fosg8freak]

Originally Posted by Used2Know

Nice job! Mind pming me that worksheet?

I attached it, via a zip file...

[zlibby76]

Thanks.

[-0nes-]

every sounds great if only there wasnt so much math in volved but this should come in handy tomorrow thanks

[03protege]

Originally Posted by -0nes-

every sounds great if only there wasnt so much math in volved but this should come in handy tomorrow thanks

Subwoofer Enclosure Calculators, Fraction to Decimal, Parallel, Series, Port Length and Volume Calculators

this can help check some of the math for you