I just bought a MOFO-12 and the manual says that enclosure is to be 2.4 cubes with a tuning frequency of 40 hz, with 2 - 3"x 6.75" ports... what exactly is tuning frequency and how do you calculate it?
Replies (4)
swez on 02/7/2006 08:05:54
Basically, a tuned port is pretty much an acoustical resonator. The box is the air chamber (Vb). The port (pipe) length and diameter determine the resonant frequency of the air chamber.
Example: Ever take a beer/soda bottle and blow across the bottle opening? When we do this, we get a resonant tone. The frequency of the tone can be changed by adding or subtracting liquid from the bottle. However, the opening of the bottle is fixed. (D) The air volume in the bottle, (Vb) can be adjusted by adding or removing liquid from the container.
To determine the port length (for 1 round port), the diameter of the port (D = 6"), box volume (Vb = 2.4ft^3) in cubic feet, and the target tuning frequency (Fb = 40 Hz.). The formula for a single port used is:
((14630000*(D/2)*(D/2))/((Fb*Fb*Vb*1728)-(1463*(D/2))))
Port Calculator: http://www.the12volt.com/caraudio/boxcalcs.asp#por
====================================================
FYI: Assuming Vb is the same in all cases, we can increase the diameter of the port for less restrictive air flow and turbulance. However, when we do this, the net port length is longer as well.
If you wish to use a single 6" port, your new port length (PL) will be 15.45" long, for a Vb of 2.4 ft^3 box. (40 Hz.)
If using 2 ports, D = (6"/2 = 3") and the net length of each port is:
PL= (15.45"/1.144/2) = 6.75". (40 Hz.)
If you wish to use a single 4" port, your new port length (PL) will be 5.9" long, for a Vb of 2.4 ft^3 box. (40 Hz.) This is a common port diameter for a 12" sub.
If you wish to use a single 3" port, your new port length (PL) will be 2.77" long, for a Vb of 2.4 ft^3 box. (40 Hz.) Here, air velocity may be too high and we may get port noise.
Hope that helps,
Swez
MrBrownstone on 02/7/2006 12:16:35
In a nutshell, the tuning frequency is the end result of adding the little tube or slot to your box.
You'll hear an added boost about 1/2 octave above that frequency or around 60Hz. Basically, they've calculated it for you....they're just telling you what frequency 2 - 3"x 6.75" ports will result in.
Harvester on 02/7/2006 19:58:37
Oh I see.. so I dont have to worry about the tuning frequency cuz they already done the work....cool thanks guys
swez on 02/8/2006 13:04:09
Right, they supplied the details, you supply the materials and assembly process. It's kinda like those home & office furniture kits we buy and put together.
These kits include everything... well, not always. (some leave out key parts that leave us scrambling) But the sub enclosure notes will give you overall dimensions and such. However, we can adjust parameters as needed to fit our applications.
Good luck,
Swez
Replies (4)
swez on 02/7/2006 08:05:54
Basically, a tuned port is pretty much an acoustical resonator. The box is the air chamber (Vb). The port (pipe) length and diameter determine the resonant frequency of the air chamber.
Example: Ever take a beer/soda bottle and blow across the bottle opening? When we do this, we get a resonant tone. The frequency of the tone can be changed by adding or subtracting liquid from the bottle. However, the opening of the bottle is fixed. (D) The air volume in the bottle, (Vb) can be adjusted by adding or removing liquid from the container.
To determine the port length (for 1 round port), the diameter of the port (D = 6"), box volume (Vb = 2.4ft^3) in cubic feet, and the target tuning frequency (Fb = 40 Hz.). The formula for a single port used is:
((14630000*(D/2)*(D/2))/((Fb*Fb*Vb*1728)-(1463*(D/2))))
Port Calculator: http://www.the12volt.com/caraudio/boxcalcs.asp#por
====================================================
FYI: Assuming Vb is the same in all cases, we can increase the diameter of the port for less restrictive air flow and turbulance. However, when we do this, the net port length is longer as well.
If you wish to use a single 6" port, your new port length (PL) will be 15.45" long, for a Vb of 2.4 ft^3 box. (40 Hz.)
If using 2 ports, D = (6"/2 = 3") and the net length of each port is:
PL= (15.45"/1.144/2) = 6.75". (40 Hz.)
If you wish to use a single 4" port, your new port length (PL) will be 5.9" long, for a Vb of 2.4 ft^3 box. (40 Hz.) This is a common port diameter for a 12" sub.
If you wish to use a single 3" port, your new port length (PL) will be 2.77" long, for a Vb of 2.4 ft^3 box. (40 Hz.) Here, air velocity may be too high and we may get port noise.
Hope that helps,
Swez
MrBrownstone on 02/7/2006 12:16:35
In a nutshell, the tuning frequency is the end result of adding the little tube or slot to your box.
You'll hear an added boost about 1/2 octave above that frequency or around 60Hz. Basically, they've calculated it for you....they're just telling you what frequency 2 - 3"x 6.75" ports will result in.
Harvester on 02/7/2006 19:58:37
Oh I see.. so I dont have to worry about the tuning frequency cuz they already done the work....cool thanks guys
swez on 02/8/2006 13:04:09
Right, they supplied the details, you supply the materials and assembly process. It's kinda like those home & office furniture kits we buy and put together.
These kits include everything... well, not always. (some leave out key parts that leave us scrambling) But the sub enclosure notes will give you overall dimensions and such. However, we can adjust parameters as needed to fit our applications.
Good luck,
Swez
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