Calculating port area with xmax?

[alaskanzx5]

you can use these formula's:

http://www.carstereo.com/help/Articles.cfm?id=21

http://www.diysubwoofers.org/misc/portcal.htm

Or this calculator for quick references.

http://www.carstereo.com/help/Articles.cfm?id=31

so for my box, which is 2 CF, I will need 8" aero port which needs to be 56" long. That make no sense at all. (using the calculator)

That what i'm saying, for a box 5.5cuft and 2 15" subs according to that calculator would need a 14.8" port and would have to be about 60-70" long. A cargo van wouldn't work for that.

[LoudBimmer]

using other formula gave me 106" of length. LOL

[alaskanzx5]

Using that calculator if you have a single 8 with let say 10mm xmas and a 1cuft box you would need a 3.42" port at 17.7" in length for 35hz. Since there isn't a port that size.using a 4" port.it would have.to.be.roughly 24" long. Or 2 2" port at 12" or 3 2" Ports at 19" long. Have fun with fitting that in a small box in which is.gd Benefit of an 8" sub.

[alaskanzx5]

using other formula gave me 106" of length. LOL

And a 2cuft box is only good for a single 12.

[LoudBimmer]

using other formula gave me 106" of length. LOL

And a 2cuft box is only good for a single 12.

I have 2 10's.

[Krakin]

The vent of a vented enclosure must provide the necessary small-signal enclosure resonance frequency f_B; it must also provide the maximum required large-signal volume velocity without excessive losses or generation of spurious noise.

The second requirement can be satisfied by adjusting the vent area to a value which prevents the vent air velocity from exceeding a specified limit. An experimentally determined limit which avoids excessive noise generation is about 5% of the velocity of sound, provided that the inside of the vent is smooth and that the edges are rounded off with a reasonable radius. This velocity limitation generally ensures acceptable losses as well, provided that the vent is not unduly obstructed.

The formulas given from Richard Small's 2nd paper on Vented enclosures are:

Sv> 0.8 * fB * V_D

and

d_v> (f_BV_D)^1/2

Where S_v is the area of the vent in m² or d_v is the diameter of a circular vent in meters.

V_D must be expressed in m³ and f_B in Hz.

This formula should be regarded as a general guide only, not as a rule. As the formulas are only accurate +10% for the entire C4-B4-QB3 range of alignments.

[Rail]

The vent of a vented enclosure must provide the necessary small-signal enclosure resonance frequency f_B; it must also provide the maximum required large-signal volume velocity without excessive losses or generation of spurious noise.

The second requirement can be satisfied by adjusting the vent area to a value which prevents the vent air velocity from exceeding a specified limit. An experimentally determined limit which avoids excessive noise generation is about 5% of the velocity of sound, provided that the inside of the vent is smooth and that the edges are rounded off with a reasonable radius. This velocity limitation generally ensures acceptable losses as well, provided that the vent is not unduly obstructed.

This formula should be regarded as a general guide only, not as a rule. As the formulas are only accurate +10% for the entire C4-B4-QB3 range of alignments.

Exactly. The 12-16 sq in per cubic foot "rule" holds no weight, Small did in depth research. Also xmax is defined by Small as coil overhang, so the xmax used to calculate Vd is NOT the manufacturer's xmax spec which is usually rated at 70% Bl. If you use the xmax at 70% Bl then you will get a conservative answer on how much port you need.

I find this form of Small's formula to be the simplest.

39.37((Fb*Vd)^(1/2))

Fb = tuning frequency of the enclosure in Hz

Vd = (Sd*coil overhang) expressed in m³

This will get you the minimum diameter of the port in inches, so if you divide by 2 to get the minimum radius, you can then use pi*(r²) to find the minimum area of the port.

[alaskanzx5]

For a round port you need the area of the full port which is a cylinder and not a flat circle or am I missing.y something here?

Part of the flares on a port is counted towards the port area to.

[Rail]

It is the minimum area of the port, so with a round port the diameter would get larger, with a slot port it would get wider. The volume of the port will vary depending on what frequency you want to tune the enclosure to, a lower tuning frequency will need a longer port. And as for flanges/flares on a port, you count half of the flange length as port length, the other half belongs to either the outside air or the air inside the box, depending on which side of the port we are talking about.

[Krakin]

For a round port you need the area of the full port which is a cylinder and not a flat circle or am I missing.y something here?

Part of the flares on a port is counted towards the port area to.

For a circular port use either the second formula I stated or the formula Rail posted.

This is assuming correct vent terminations and correct calculations for the terminations. For tuning you need to take in account flanged ends, round-over radius's, or anything else that will effect vent air velocity.