how to find the amount of air a subwoofer can displace?

[alaskanzx5]

I've thought about but never found anything on it. I know that it is needed info to help determine port area and port velocity.

I can't seem to find a formula for it or a calc online.

A question for the pros with out a doubt.

[Jessica]

i am 90% sure that most "displacement" has nothing to do with the space the sub takes up. It is more about xmax.

From what i have read, unless your sub has a really huge motor, dont worry about it when figuring box volume.

I may be wrong though

[Triticum Agricolam]

Driver displacement is usually referred to as Vd. Its generally measured in cubic centimeter (cc). To get it you take Sd (cone area) x Xmax (in mm) and divide by 10.

So your average 12" sub with 480 sq cm of cone area and 18 mm of xmax will have a Vd of 862.2 cc.

[Jeff4661]

i am 90% sure that most "displacement" has nothing to do with the space the sub takes up. It is more about xmax.

From what i have read, unless your sub has a really huge motor, dont worry about it when figuring box volume.

I may be wrong though

I disagree with us in my opinion all subs take up volume inside of enclosures..

Depending on how big your sub is could make or break your tuning point...

Example?

4 hdc3 15" subs take up .2 cubes per sub so if I built a 10 cubic foot box tuned to 35hz with out considering that volume those take up In an enclosure.. I would actually end up at only being a 9.2 cubic foot box bringing Ur tuning up to around 37hz

[alaskanzx5]

So what I could find is

Vd = sd x Xmax. Didn't see anything about divide by 10.

So when doing this from what I can see sd comes in cm squared and Xmax in mm obviously.

When doing it I'm assuming you use one way Xmax as that is the standard for Xmax.

However are you suppose to convert any of the measurements?

If not awesome this keeps it simple.

[Triticum Agricolam]

So what I could find is

Vd = sd x Xmax. Didn't see anything about divide by 10.

So when doing this from what I can see sd comes in cm squared and Xmax in mm obviously.

When doing it I'm assuming you use one way Xmax as that is the standard for Xmax.

However are you suppose to convert any of the measurements?

If not awesome this keeps it simple.

The divide by 10 part comes in because your final unit is cubic centimeters and you are inputting square centimeters (Sd) and millimeters (Xmax). One cubic centimeter = one square centimeter x 10 mm (or 1 cm). If you converted the xmax from mm to cm first you wouldn't have to divide by 10. Either way you end up at the same place.

The xmax input is the standard one way measurement.

[alaskanzx5]

Ok, I got this down. That was easy enough.

Now part 2 of this question.

How do you use vd as a determination factor of port size?

I know there is more to it then this. I also realize that this is the amount of air the sub can move if pushed to full Xmax which is box and power it seeing dependant.

I found 2 different formulas and I feel as if I'm doing it wrong.

Wish someone would create a calc where you could enter in you vd, desired tuning and what ever else was needed to figure in port velocity. Them again knowing how to do it by hand is nice to.

[Cj Kumz]

Yah this is a very complex topic. And involves a lot of calculations.. I have also tried and ended up with dead ends lol. Obviously the rate at which the cone is moving combined with the time and distance the air covers will need to be factored in too. Hopefully someone will chime in soon.

[kirill007]

Ok, I got this down. That was easy enough.

Now part 2 of this question.

How do you use vd as a determination factor of port size?

I know there is more to it then this. I also realize that this is the amount of air the sub can move if pushed to full Xmax which is box and power it seeing dependant.

I found 2 different formulas and I feel as if I'm doing it wrong.

Wish someone would create a calc where you could enter in you vd, desired tuning and what ever else was needed to figure in port velocity. Them again knowing how to do it by hand is nice to.

Why not just use WinIsd? You model the port velocity for your sub in your enclosure and much more

[Triticum Agricolam]

Ok, I got this down. That was easy enough.

Now part 2 of this question.

How do you use vd as a determination factor of port size?

I know there is more to it then this. I also realize that this is the amount of air the sub can move if pushed to full Xmax which is box and power it seeing dependant.

I found 2 different formulas and I feel as if I'm doing it wrong.

Wish someone would create a calc where you could enter in you vd, desired tuning and what ever else was needed to figure in port velocity. Them again knowing how to do it by hand is nice to.

Once you have Vd you can determine port size using Small's formula, which is: dv = 39.37 * (Fb x Vd)^.5

dv = diameter of the port in inches

Fb = tuning frequency in herz

Vd = driver displacement in cubic meters

Since this formula uses Vd in cubic meters you have to take your Vd calculation in cc and divide it by 1,000,000 to get cubic meters. Also the number is gives you back is the diameter of a round port, so to get sq in of port area you have to take dv and divide by two to get the radius and then use your normal area of a circle formula (Pi r^2).

I put all of the above math into a spreadsheet so all I have to do is enter in cone area, xmax, and tuning frequency and I get my answer in square inches.

The problem with all of this is it is based on the assumption that you are going to push the sub all the way to Xmax. A lot of today's subs have such huge Xmax that you are going to run out of power handling capacity long before you get anywhere near Xmax. This is one of the reasons why determining optimal port area is no easy task. I've put a lot of mental energy into this problem and I'm yet to come up with a system that I'm 100% satisfied with. These days I usually start out with Small's formula and then put that area into WinISD like kirill007 suggested and see what port velocities it gets me.

Of course once you get your data back from WinISD you have the question of how much port velocity is too much? I'm of the belief that lower is better, so I like to keep things below 20 m/sec, but often that isn't practical, 30 m/sec is my hard limit, but even that number is situation dependent.

Assuming you do determine what the optimal amount of port area is, if you are going to be using a slot port, they are less efficient than round ports of equal area so you have to take that into consideration as well. It all adds up to "how much port area?" being a very complex question.

The good news is if you have access to an AMM-1 you have the ability to find out if your port is undersized. Using an AMM-1 you can check to see if you are getting port compression. If you play a tone through your system at your box tuning frequency and watch the impedance as you turn the power up, if you are getting port compression the impedance will start to climb as power increases. I just got my AMM-1 and this is something I've been experimenting with. The only caveat to this technique is power compression will also increase the impedance as power increases so you have to test for that separately.