Martin J King's Ported Sheets-Vent Height Question

[kelticwizard]

Martin J King's Ported sheets, as well as some other sheets, have a provision for height of vent from the top.

Below I have put some crude drawings to illustrate my question.

Since there is no provision to measure how far the port extends inside the enclosure, I believe what Martin's sheets measure is an enclosure such as A. The straight port starts from the enclosure side and extends outward. Martin's sheets measure the effects of how far the port is from the top of the enclosure. and indeed, there are differences in the response as you move the port up and down.

Most ported enclosures look like B. A short port runs inside, not outside, the enclosure. However, since most ports are short and straight, for all practical purposes B will give pretty equal response to A, so there should be little problem.

With C, however, there is an issue. The exit of the port is 42" from the top, just like A and B. But the opening of the port is 15" from the top.

I would assume that C is modelled as the vent being 15" from the top. But I don't know for certain.

Any help in this direction is appreciated. Should I model C as having the port 15" from the top or 42" from the top?

[MJK]

I agree that pictures A and B will produce essentially the same result, I would not worry about the small volume taken by the port inside the enclosure. If you are at that level of modeling and are worrying about internal or external port volume I think you are well beyond the level of accuracy in the calculations.

In the old worksheets, or Part 1 of the new worksheets, you would model the port as being 15 inches from the top. Then in Part 2 of the new models you would place the port opening at 42 inches from the top. That would be the most accurate calculation.

Hope that helps,

[kelticwizard]

Martin:

Thank you very much for your answer. That clears it up.

I look forward to using the new worksheets, with their additional features.

[Elam]

Talking about accuracy ... I found out that the simulations give different results for a straight line and port positions at 25% and 75% of line length when the driver is at 50% of line length. Very strange since the problem is symmetric.

[Elam]

Example

[MJK]

The driver position has to be closer to the top then the port position. In your second case, you have modeled a different system and are double book keeping the air between the driver and the port, hence the lower tuning frequency. Take a look at the application note to see the assumed positions of the driver and port.

[Elam]

Thank's Martin. I almost never read manuals because I think I'm too clever. Bad habit.

Elam

[MJK]

And I always assume the user/reader understands exactly what I am trying to convey. Both assumptions are just human nature.