Calculating woofer displacement

[eRiCdWoNg]

....any easy way of calculating woofer displacement? Tech support for the woofer is closed right now and I'm in the process of making the box for my car. I thought about doing the cylinder of the magnet, but I was not sure about the other part. The driver in question is a Kove Audio Z15D/ Partsexpress close out 15" model 299-665 . This thing is deeper then the average driver so maybe I'll just use the specs from another woofer of the same caliber. But is there a way I can be a little more accurate then this?

 

[Ron E]

Calculate the volume of a truncated cone (called a frustum - do a google search) with the piston diameter of the woofer and the tip diameter of the voice coil. Then add the volume of the magnet system.....and if you are picky, the frame - and don't forget to compensate for the fact that the driver is pushed outward by a cabinet thickness if surface mounted.

This isn't rocket science.....a 10% difference in net box volume causes almost no change in most cases.

 

[kelticwizard]

This isn't rocket science.....a 10% difference in net box volume causes almost no change in most cases.

Absolutely true. This is one area where even being off by 50% wouldn't even mean much.

I once calculated the displacement a Peerless XLS 10 incher as being less than 3 liters. Since a 15 incher is twice as big as a 10 incher, (conewise at least), and that Kove seems to be somewhat bigger than many 15 inchers, I would call it 10 liters and relax.

I am sure that for whatever size box you plan to stick that 15 incher into, the difference between the 10 liters and the real number will be utterly insignificant. Just as long as you figure in something.

 

[eRiCdWoNg]

OK sounds good. The woofer enclosure isnt exactly huge either, I calculated it roughly out to 2.5 cubic foot (70 L) before driver and port displacement. The top 1/2 was easy cause its just a right triangle, the bottom is sort of a right triangle too but its fiberglass and molded to the bottom of the car so its got contours in every which direction.

Other questions- what about 'polyfill'? I heard it works well in sealed but in the vented do I have to change any parameters?

#2, I've always been sorta unclear on the port length. Do you include the 3/4" or so that sticks through the baffle or do you count it seperate? In other words, if a port is calculated to be 10 inches long, do I make it 10" or 10.75" if it were going thru a 3/4" MDF baffle?

 

[kelticwizard]

Port length is calculated from where the air enters the port to where it emerges into the free air. In other words, 10.75".

I think if you check out WinSID or any box calculation program, though, the difference between a 10" port and a 10.75" port is essentially negligible-probably not even 1 Hz.

The volume of the port is deducted from the box volume as well. With most ports, it is so small as to be negligible.

 

[Stephen D]

#2, I've always been sorta unclear on the port length. Do you include the 3/4" or so that sticks through the baffle or do you count it seperate? In other words, if a port is calculated to be 10 inches long, do I make it 10" or 10.75" if it were going thru a 3/4" MDF baffle?

N0.. you include the 3/4" baffle as part of the 10" calculated port length. In other words the length of port that extends inside the box is only 9 1/4". If you were to stick a port tube through the hole cut in the baffle so that it extends to flush with the outside wall of the baffle, you would simply still just use a 10" port tube. However, if you were to butt the port tube up against the inside wall of the baffle (meeting the cut port hole), you would use a 9 1/4" port tube.