View Full Version : Calculation verifications please!

Damaso87

12-31-2004, 08:06 PM

Could anyone check out my vent calculations out real quick. I just need the multiple verification to make sure ive got it all right. Id hate to build a box then find out its tuned to something i dont want.

Materials: 3/4" MDF

Exterior Height: 17"

Exterior Width: 37"

Exterior Depth: 18"

Driver Displacement: 305 cubic inches

Interior Volume: 5.07 cubes

Interior Port Height: 3"

Interior Port Width: 17"

Port Length: 12"

If anyone could double-check the port calculations and make sure theyre accurate, id really appreciate it :)

Damaso87

01-01-2005, 06:15 AM

Bada-Bump. I just need one person, come on now!

Termlab

01-01-2005, 06:27 AM

i got 5.07 also but you have to subtract for port displacement also ;)

Damaso87

01-01-2005, 08:44 AM

So i subtract the port displacement from the total internal volume, then the end? Or do i have to recalculate the port length...?

(BTW, i pulled an all nighter, hence the odd post times.)

Damaso87

01-01-2005, 09:51 PM

Sigh...bump...

bcarpenterfhl

01-01-2005, 10:29 PM

So i subtract the port displacement from the total internal volume, then the end? Or do i have to recalculate the port length...?

(BTW, i pulled an all nighter, hence the odd post times.)

both, its a trial and error process. you increase/decrease the port, it affects the total internal volume (Vb), which affects the overall tuning. someone should extend on this, but im too lazy.

sumone

01-02-2005, 09:28 PM

Materials: 3/4" MDF

Exterior Height: 17"

Exterior Width: 37"

Exterior Depth: 18"

Driver Displacement: 305 cubic inches

Interior Volume: 5.07 cubes

Interior Port Height: 3"

Interior Port Width: 17"

Port Length: 12"

Port Displacement:

17 * (3+.75) * (12 - 3/2) = 669.375ci

+.75 for the mdf, minus 3 divided by 2 for end correction

Driver Displacement:

305ci

Displacements = 305 + 669.375 = 974.375ci

Internal Volume:

(17-1.5) * (37-1.5) * (18-1.5) = 9079.125ci

Net Volume = Internal Volume - Displacements: 9079.125 - 974.375 = 8104.75ci

= 4.69ft^3 => Vb

Port Area = 3*17 = 51 => Av

Lv = 12

With that, I get a frequency of 40.4651hz...

my mathematica stuff:

inputs 1 & 3 are the definitions of the Port length function (Lv) & then the solving for Fb based on Lv, Av, & Vb.

the first inputs are my box (listed in my sig) just to verify the results.

In[1]:=

Lv[Av_,Fb_,Vb_] := (Av*1.84*10^8)/(Vb*1728*(Fb/.159)^2) - 0.823*Sqrt[Av]

In[2]:=

Lv[26.25, 32, 3.0]

Out[2]=

18.786

In[3]:=

TunFreq[Av_,Vb_, PLen_] := Sqrt[1/(((PLen + 0.823*Sqrt[Av] )/(Av*1.84*10^8))*(Vb * 1728)) \

* .159^2]

In[4]:=

TunFreq[26.25, 3.0, 18.786]

Out[4]=

32.

In[5]:=

TunFreq[51, 4.69, 12]

Out[5]=

40.4651

In[6]:=

Lv[51, 40.4651, 4.69]

Out[6]=

12.