How much port area for an 18?

[Rail]

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

Port noise is less noticeable in a large SUV, and it also depends on what frequency is being played and how hard the system is being pushed. There is a big difference if you are closer to the port and facing the port. If you are fairly close to how much you need then you may have minimal port noise that might not be audible from the front of the vehicle. Also Small defines xmax as coil overhang, which is less than the xmax that manufacturers typically give which is the xmax at 70% Bl. I still use the xmax at 70% Bl though just to be on the safe side, because after all it is a minimum, not the recommended.

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.

Small's formula takes the Vd in meters^3 and through the formula converts it to inches at the end, no extra conversions necessary, and T/S specs are given in the metric system so I don't see why you would go through the trouble to convert to inches. I worked through your formula since I hadn't seen it before and it works (close enough, error of about .3 square inches). But Vd is not (xmax*2*Sd). You don't multiply by 2 because it is not the travel of the cone, it is the volume of air that the cone displaces relative to its resting point. Here is an example of both formulas to prove this:

So then if we take a 15" Sundown Z v.4 with 35mm xmax and 810cm^2 Sd and tune it to 30Hz

Small's formula in its original form looks like this:

39.37 * sqrt(30(.035*.0810)) = minimum diameter of port in inches (which in this case is 11.48")

Then pi*(11.48/2)^2 = 103.536 square inches minimum area

You say:

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

So then in inches Sd = 125.55 in^2

And the xmax would be 1.378 in

Which gives a Vd of 173.002 in^3

Then your formula would be: 173.002*30*.02 = area of the port in inches^2

Which gives 103.801181 square inches.

You can even look at this wiki page which has the formulas for some of the other T/S parameters too.

http://en.wikipedia.org/wiki/Thiele/Small

[bkolfo4]

I have preached on this and many other things until it has driven me away from posting much anymore.

I will add, you can definitely get away with less than optimal port area in a car, but having the ideal amount of port area (will seem large to most people) is not reserved for SPL setups, does not cause the frequency response of the sub to be "narrower", and does not cause unloading issues as long as it is designed correctly. . .

[soundsystemaniac]

This is Great information and good reading! thanks for sharing! I would love to know more if anyone can go farther in depth

[Triticum Agricolam]

Small's formula takes the Vd in meters^3 and through the formula converts it to inches at the end, no extra conversions necessary, and T/S specs are given in the metric system so I don't see why you would go through the trouble to convert to inches. I worked through your formula since I hadn't seen it before and it works (close enough, error of about .3 square inches). But Vd is not (xmax*2*Sd). You don't multiply by 2 because it is not the travel of the cone, it is the volume of air that the cone displaces relative to its resting point. Here is an example of both formulas to prove this:

So then if we take a 15" Sundown Z v.4 with 35mm xmax and 810cm^2 Sd and tune it to 30Hz

Small's formula in its original form looks like this:

39.37 * sqrt(30(.035*.0810)) = minimum diameter of port in inches (which in this case is 11.48")

Then pi*(11.48/2)^2 = 103.536 square inches minimum area

You say:

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

So then in inches Sd = 125.55 in^2

And the xmax would be 1.378 in

Which gives a Vd of 173.002 in^3

Then your formula would be: 173.002*30*.02 = area of the port in inches^2

Which gives 103.801181 square inches.

You can even look at this wiki page which has the formulas for some of the other T/S parameters too.

http://en.wikipedia.org/wiki/Thiele/Small

Well I stand corrected on the Vd calculation, thanks for getting me straightened out. I know I read somewhere that it was Sd * Xmax * 2, but obviously they were wrong.

Calculating the Vd in inches is kinda a hassle, I agree, that's just how I learned it. In application, I use a spreadsheet where I have all the metric to imperial conversion built into the formula so I've never had to deal with it much. FYI, here is where I learned the formula:

https://www.trueaudio.com/st_ventq.htm Look at the bottom of the page.

[Rail]

This is Great information and good reading! thanks for sharing! I would love to know more if anyone can go farther in depth

Farther in depth on what in particular? If you want to get more technical, check out the Loudspeaker Design Cookbook by Vance Dickason. You can find it on Amazon, I think its in the 7th edition now. There is a lot of detailed information in there.

Well I stand corrected on the Vd calculation, thanks for getting me straightened out. I know I read somewhere that it was Sd * Xmax * 2, but obviously they were wrong.

Calculating the Vd in inches is kinda a hassle, I agree, that's just how I learned it. In application, I use a spreadsheet where I have all the metric to imperial conversion built into the formula so I've never had to deal with it much. FYI, here is where I learned the formula:

https://www.trueaudio.com/st_ventq.htm Look at the bottom of the page.

No problem, I learned something along the way too (the other form of that formula). And that page also has some good info on the placement of ports.

[LoudBimmer]

But doesn't higher port velocity yield a higher SPL score on the meter?

[Rail]

But doesn't higher port velocity yield a higher SPL score on the meter?

I'm pretty sure that it doesn't get you a higher score. Its just creating port noise and excessive turbulence in the port that can effect the performance of the sub negatively, it should be more efficient without port compression. The only benefit from a small port and high vent mach is that normally the exchange of air in and out of the box would be about the same, but if you have too little port then there will actually be a small exchange of air, which could possibly help with cooling the sub(s) by replacing hot air with the ambient air. However this is not a good excuse to intentionally make a small port, the system will be both more efficient and sound cleaner with the proper port size.

[soundsystemaniac]

But doesn't higher port velocity yield a higher SPL score on the meter?

I'm pretty sure that it doesn't get you a higher score. Its just creating port noise and excessive turbulence in the port that can effect the performance of the sub negatively, it should be more efficient without port compression. The only benefit from a small port and high vent mach is that normally the exchange of air in and out of the box would be about the same, but if you have too little port then there will actually be a small exchange of air, which could possibly help with cooling the sub(s) by replacing hot air with the ambient air. However this is not a good excuse to intentionally make a small port, the system will be both more efficient and sound cleaner with the proper port size.

Well, Im looking to design a port for our astro van. We already have a wall behind the 3rd row bench seat. the box is ~12.8 cubes gross. The box is setup for 2 18s. They are sundown zv3 18s. My problem is- i dont know the xmax of the subs.

[Jd1102]

But doesn't higher port velocity yield a higher SPL score on the meter?

I'm pretty sure that it doesn't get you a higher score. Its just creating port noise and excessive turbulence in the port that can effect the performance of the sub negatively, it should be more efficient without port compression. The only benefit from a small port and high vent mach is that normally the exchange of air in and out of the box would be about the same, but if you have too little port then there will actually be a small exchange of air, which could possibly help with cooling the sub(s) by replacing hot air with the ambient air. However this is not a good excuse to intentionally make a small port, the system will be both more efficient and sound cleaner with the proper port size.

Well, Im looking to design a port for our astro van. We already have a wall behind the 3rd row bench seat. the box is ~12.8 cubes gross. The box is setup for 2 18s. They are sundown zv3 18s. My problem is- i dont know the xmax of the subs.

ask In sundown thread?

[soundsystemaniac]

But doesn't higher port velocity yield a higher SPL score on the meter?

I'm pretty sure that it doesn't get you a higher score. Its just creating port noise and excessive turbulence in the port that can effect the performance of the sub negatively, it should be more efficient without port compression. The only benefit from a small port and high vent mach is that normally the exchange of air in and out of the box would be about the same, but if you have too little port then there will actually be a small exchange of air, which could possibly help with cooling the sub(s) by replacing hot air with the ambient air. However this is not a good excuse to intentionally make a small port, the system will be both more efficient and sound cleaner with the proper port size.

Well, Im looking to design a port for our astro van. We already have a wall behind the 3rd row bench seat. the box is ~12.8 cubes gross. The box is setup for 2 18s. They are sundown zv3 18s. My problem is- i dont know the xmax of the subs.

ask In sundown thread?

well, i didnt know if there was a method. I looked on sundowns site to find nothing. I will ask in one though