How much port area for an 18?

[JesseyMclame]

Okay so I've got An M1 XL 18 with psi soft parts, ittl be getting right around 1800-2000 watts, I'm gonna do 5.5 cubes after all displacement, (came from rusty), I'm gonna do aero ports, I just have no idea how much port area to give it. I'm thinkin like 10-12 SQ inches per foot ? Any input would be awesome, thanks guys!

[Rail]

Do yourself a favor and forget about the sq in per cube "rule", because it is a complete myth. Port area does not relate directly to volume of the enclosure. Factors that do matter are the tuning frequency of the enclosure (Fb), the xmax of the sub, cone area of the sub (Sd), and of course the number of subs. You know the T/S parameters of a speaker? Created by A.N. Thiele and Richard H. Small? Small derived a formula to calculate the minimum port area required to avoid port noise and compression.

Following his formula you get MINIMUMS of:

77 sq inches at a tuning of 20Hz

115 sq inches at a tuning of 30Hz

134 sq inches at a tuning of 35Hz

With your "rule", 5.5 cu ft with 12 sq in per cubic foot would only get you 66 sq inches of port. This is far too small for an 18" subwoofer, its not even enough area if you were to tune to 20Hz which I doubt you are planning.

[CleanSierra]

You're a new member, but I like you already^^^

Great info and I agree we need to throw the "per cubic foot" rule out the window.

[JesseyMclame]

Okay so pretty much, no matter the net volume of the box, those are the minimums I need to make the speaker sound it's best at that tuning frequency?

[JesseyMclame]

Or more so, let the woofer work proper in said enclosure.

[Triticum Agricolam]

^{To expand on what Rail said, the formula I use is:}

^{Vd (Volume of the driver in cubic inches) * Tuning Frequency (In Hertz) * .02 = Area of the port (In square inches)}

^{This formula is pretty conservative and should keep your port velocity under 10 meters/sec.}

^{If you don't know the Vd of your driver you can calculate it, its Sd (Cone area in square inches) * Xmax (In inches) * 2.}

[john253a]

^{To expand on what Rail said, the formula I use is:}

^{Vd (Volume of the driver in cubic inches) * Tuning Frequency (In Hertz) * .02 = Area of the port (In square inches)}

^{This formula is pretty conservative and should keep your port velocity under 10 meters/sec.}

^{If you don't know the Vd of your driver you can calculate it, its Sd (Cone area in square inches) * Xmax (In inches) * 2.}

using sd you'll be looking around 130sq" and thats just crazy in 5.5 net

3x 6" at 17 long (psp) is what idd use and block 1 off for daily

edit:

that is if you don't want low 30's tune,

you will get better performance form you equipment with a larger port, just you'll be hard to get 3x 6" 41" long @30hz in a 5.5net box

[Rail]

You're a new member, but I like you already^^^

Thanks! New member, but not new to car audio

Okay so pretty much, no matter the net volume of the box, those are the minimums I need to make the speaker sound it's best at that tuning frequency?

Yes, in order to avoid port noise and port compression due to high air velocity.

^{To expand on what Rail said, the formula I use is:}

^{Vd (Volume of the driver in cubic inches) * Tuning Frequency (In Hertz) * .02 = Area of the port (In square inches)}

^{This formula is pretty conservative and should keep your port velocity under 10 meters/sec.}

^{If you don't know the Vd of your driver you can calculate it, its Sd (Cone area in square inches) * Xmax (In inches) * 2.}

Small's formula is actually:

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

Fb is the tuning frequency of the enclosure

Vd is the amount of air that the sub can displace (which is cone area times the xmax, so Vd = Sd*xmax)

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*r^2 to find the minimum area of the port.

Edit: To make this more clear, you must convert both the Sd and Xmax to meters, so that for the Vd you get cubic meters.

[Brandonbkd]

The setup in my sig only has 86 sq in of port and it sounds great. No port noise whatsoever.

[Triticum Agricolam]

Small's formula is actually:

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

Fb is the tuning frequency of the enclosure

Vd is the amount of air that the sub can displace (which is cone area times the xmax, so Vd = Sd*xmax)

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*r^2 to find the minimum area of the port.

The formula I gave is Small's formula, its just been worked around to use Vd in cubic inches instead of cubic meters and to give you your result in square inches of areas instead of the diameter of a round port.

Also to calculate Vd you have to multiply the Xmax by 2 since it is a measurement of travel each direction.