Are my dimensions/displacements correct?

[mlopez7353]

Hey Guys,

Not my first system, but my first system for my BMW and I want it to be right. I've never bought a prefab, always built my own, but typically they were sealed, or I never took the time to account for sub and port displacement in slot ported boxes .

I'm going to be fire my sub and aeroport through the rear ski port, so I want it to be right.

I'm running a Sundown e8V3 off a JBL MS-A5001 at 4ohms.

Its going to be a wedged box to accommodate the pitch of rear seat.
Dimensions are as follows:
W:21 H:15.375 D(Bottom):10 D(Top):5
This equates to 0.93945 cubes.

Now this is where I get lost.
Sundown states the sub displacement is .05, So i'll go with that.

That brings my area to .88945

Now port displacement, the real confusing part for me when figuring out the actual displacement.

I'm going to be running a single 3" aeroport. Port calculator says it should be at just under 18" to reach 35hz. So my math is as follows.
3.14*4.5*18=254.34
254.34/1728=.1471875 Displacement total

So with all displacements considered, this leaves me at .7422525 cubes. Which ideally where I want to be around the recommended .50-.75 cubes for the e8v3.

Does this math sound correct? Especially the port displacement.

Thanks for dealing with a redundant question!

[Joe X]

I get 0.798 net @35Hz with those external dimensions (0.75 wood thickness) and a 17.25 in port 3" in diameter, a couple things to bear in mind is that your port area is rather tight and that you should check the driver motor clearance inside the box.

[mlopez7353]

I get 0.798 net @35Hz with those external dimensions (0.75 wood thickness) and a 17.25 in port 3" in diameter, a couple things to bear in mind is that your port area is rather tight and that you should check the driver motor clearance inside the box.

Thanks, Joe. I appreciate your time.

Do you happen to know where I was going wrong in my calculations...?

So after thinking about your statement regarding clearance...I did realize I would be cutting it quite close, so I adjusted my dimensions a bit to hopefully accommodate more clearance.

Obviously my algorithm previously used was skewed so if you could maybe see what you get with the following...

Dimensions are as follows:

W:14.25 H:15.375 D(Bottom):13 D(Top):8

This equates to 0.92139 cubes. (Using 12volt - Wedge Enclosure 2 Calc)

Sundown states the sub displacement is .05, So i'll go with that.

That brings my area to .87139

I'm going to be running a single 3" aeroport. Port calculator says it should be at just under 15.7082" to reach 35hz. So my math is as follows. (Using Precision Sound Port Calc - When I enter the box volume im using the full .92 not including the sub displacement...is that the right or wrong way?)

3.14*4.5*15.7082=221.956866

221.956866/1728=.1284472604 Displacement total

NET=.742xxxxxx

[Joe X]

Remember that 0,75" of the port length displaces no internal volume, also, the port calculator requires net volume that is all displacements subtracted, lastly you need to keep 1 diameter clearance from the end of the round port to any nearby wall, as a result the port will not fit unless it's partially external.

[mlopez7353]

So here goes another question..I order a supposed xscorpion 3" flare port from fleabay. When I received the actual ID is 2.75". Is this the number I should be using to calculate my port length?

[Triticum Agricolam]

So here goes another question..I order a supposed xscorpion 3" flare port from fleabay. When I received the actual ID is 2.75". Is this the number I should be using to calculate my port length?

Yes, the 2.75" is the number you should use.

Please note that port is pretty significantly undersized for your application and its fairly likely you will have port issues.

[mlopez7353]

Port issues being? What size port would you recommend? Considering it a .75cu box, space is an issue for sure

[mlopez7353]

anything larger requires a tremendous amount of length

[Triticum Agricolam]

Port issues being? What size port would you recommend? Considering it a .75cu box, space is an issue for sure

We unfortunately I don't really have much good news for you. Sundown recommends 8-12 sq in of port area for this sub. The amount of port area you need depends on how large you make the box (bigger net volume needs more port area), how much power you are going to run (more power = more port area) and your tuning frequency (higher tune = more area). The sub is rated for 300 watts RMS and if you were going to put that much power to it and use a .75 cube box, which is the upper end of their recommendation, then 12 sq in of port area would be appropriate. With a smaller box or using less power, you could get away with less port area.

Now in your case, with a 2.75" diameter port, you only have about 6 sq in of port area. WIth a .75 cube box, by my estimate, that's good for about 130 watts before you get into the port velocity sad-face zone. :-( You really want to have at least 12 sq in of port area and 14 would be even better because your sub is probably going to see more than 300 watts. Its hard to say exactly how much power you sub actually will get since impedance is almost never as low as the nominal rating.

Boxes for these single 8" subs are always tough to get enough port area and still keep the tuning where you want it to be. 30-40" long ports are usually what you end up with. Unfortunately there aren't any easy fixes, its just something you have to deal with. If you don't have the space for a properly sized port, you will have to make a design compromise somewhere.