Porting Gurus...Please help me design a port for this box.

[valin 10+ year member]

I have built my box as big as I possibly can for my vehicle. The box has an inner volume of 6.95 cubic feet empty.

I am going to run two Memphis M3 15" subs off of a Memphis MC-2500D Mojo amplifier, which produces roughly 2400 watts or so at 2 ohms.

I am not sure what to do with a port because I do not understand the difference in output vs. port size. How big should my port be for this box? I used to be running these subs in a 6.0 cubic foot enclosure with 36 sq. inches of port size, 12 inches deep. I was able to achieve 149.7 db, and the box was tuned to just over 30 Hz. I would like more output, but still maintain some SQ. Somebody on here recommended 50 sq. inches of port size. How long should it be? I noticed on WinISD that you can achieve the same results for tuning with a small port, and short length, and with a large port, long length. Is it better to go bigger and longer?

The subs will be facing the rear of the vehicle (Toyota FJ Cruiser), and so will the port. I will probably put the port centered on the bottom of the box.

Click to read more...

 

[mlstrass 5,000+ posts]

General rule of thumb is 16^2 per cubic foot of box, so 36^2 seems pretty small. Even 50^2 that was suggested.

You could probably get away with 2 6" aeros. Go to precision's site and punch in the numbers on the calculator.

 

[valin 10+ year member]

I am going to do a slot port in the box. Is there a ratio that I should stick under in terms of length vs. width of the port? ie. is it ok to run a 40" x 2" port for 80 sq. inches, or better to run a 10" x 8"?

 

[PV Audio 5,000+ posts]

Port area efficiency isn't just dependent on the open area of the port, but on the dimensions as well. A longer but narrower port is not usually desired as you can still get lots of compression inside the port.

 

[PV Audio 5,000+ posts]

And the only ratios that I use are the Thiele ratios for enclosure size, not tuning. However, a 1:9 ratio is good.

 

[valin 10+ year member]

Ok. I checked out the RE site, and they're box building / port design page. Their slot port heads the entire depth of the box, then the measurements are from there, out. My old port in my box designed by memphis was simply 12" long......not an L shaped that heads in the depth, then 12". Why would you do this, and why is that not considered the 12" + the depth.....so in my case over 24"?

 

[valin 10+ year member]

Or, if I do decide to go with aeroports, will i benefit as much as going with a slot port? Also, what would I need for length and size?

 

[Trixter 10+ year member]

Ok. I checked out the RE site, and they're box building / port design page. Their slot port heads the entire depth of the box, then the measurements are from there, out. My old port in my box designed by memphis was simply 12" long......not an L shaped that heads in the depth, then 12". Why would you do this, and why is that not considered the 12" + the depth.....so in my case over 24"?

the port's length is determined by it's area and the box's volume. if you port has a large area, it may need to go longer then just the depth of the box, it may need to "L" around the back of the box. some port also zig-zag instead of folowing the outside of the box.

didn't quite get your Q, but i think this is what your getting at.

 

[Trixter 10+ year member]

Or, if I do decide to go with aeroports, will i benefit as much as going with a slot port? Also, what would I need for length and size?

a slot or PP's...it's up to you. if your using the PP's use the length calculator on thier site. for you box size, divide your volume by the number of ports your going to use.

if your box is already made, kinda sounds like it is, then you need to figure twice. once to get the port length and a second to account for the port's displacement. first find the port length. find the port's volume (Pi x R^2 x L). subtract this from the box size you used the first time. now re-calculate using your new volume. make your port this long. this would have to be done if you were using a slot as well. don't forget to subtract the sub displacement from your original volume.

 

[valin 10+ year member]

Ok, but depicted in the image on the RE site, when you select a 2" long port, it shows the port going the entire depth of the box, then 2" sideways. If the box is 12" deep, why would this not be a 12" long port then?

 

[Trixter 10+ year member]

Ok, but depicted in the image on the RE site, when you select a 2" long port, it shows the port going the entire depth of the box, then 2" sideways. If the box is 12" deep, why would this not be a 12" long port then?

the port isn't going to be 12" long just because the box is 12" deep. the length of the port is what tunes the box. the longer the port the lower the tuning, the shorter the port the higher the tuning.

a port is measured down the center. if you have a 2" wide port that need to be 14" long, and the box is 12" deep, the port will go back 11" and turn 3". the walls of this port will be 10" and 2" long.

 

[valin 10+ year member]

Ok. I finished the box today. I went with 88 sq. inches of port, 16" long. Should be tuned to around 40Hz.

I hooked up the system today for a test. It hits hard. I'm sure it's in the very low 150's, and the SQ is excellent. I'm sure the Memphis M3 subs won't hold up to this much power though. I'll be looking for a pair of subs that will handle the 2400 watts I'm giving them, but will also work in my 6 cu. ft. enclosure.

 

[valin 10+ year member]

btw, thanks Trixter for the info. I appreciate it.

 

[PV Audio 5,000+ posts]

Ok. I finished the box today. I went with 88 sq. inches of port, 16" long. Should be tuned to around 40Hz.
I hooked up the system today for a test. It hits hard. I'm sure it's in the very low 150's, and the SQ is excellent. I'm sure the Memphis M3 subs won't hold up to this much power though. I'll be looking for a pair of subs that will handle the 2400 watts I'm giving them, but will also work in my 6 cu. ft. enclosure.

I highly doubt it. If you were easily hitting in the 150db range, you wouldn't be on here asking how to design a port for your own enclosure. Not saying that it isn't true, but I'm skeptical (also, what mic was your first score registered on?).

 

[valin 10+ year member]

The meter is an RTA, that's all I know. With my old box, 36 Sq. in. of port size, 6 cubic feet internal, I achieved 149.7db. This box produces more pressure, but is not nearly as responsive.