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Old 14th April 2006, 06:26 PM   #1
mathman is offline mathman  United States
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Default Port Correction Factor???

OK, I can't seem to find much info on the port correction factor, and how accurate it really is. This link has some interesting work, indicating that the 'typical' 0.73 is not really that close:

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Does anyone have any other articles, data, or anecdotal evidence regarding this quantity? I'm midway through box building, and thought I would leave my ports unglued so I could experiment a little bit with different lengths. Maybe use some straws inserted into the port, as George Short recommends with his North Creek stuff?
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Old 14th April 2006, 07:54 PM   #2
mathman is offline mathman  United States
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Oops, I almost forgot... as a corollary to this question, is there a general rule for minimum port length? WinISD is showing a 2.5" diameter port about 3" long. Would such a short port cause problems? I have a 7" MTM with the Seas W18 woofers, so I think 2.5" diameter is sufficient, but I could go larger if a 3" port is too short.
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Old 16th April 2006, 12:27 AM   #3
Ron E is offline Ron E  United States
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Port correction factor is discussed very briefly in L.L. Beranek's "Acoustics" text in a chapter about acoustic masses.

He gives the 0.85 and 0.613 values for flanged and free ends.

The end effects depend somewhat on diameter, but the diameter would have to approach a fraction of a wavelength before much of an effect came here.

One thing to question in the webpage study you cite is whether or ot the actual resonant frequency was measured properly. It does not necessarily coincide with the minimum in impedance. The diaphragm SPL minimum is a more accurate indicator.

If the traditional formulas result in lengths that are too long, that just means one can cut off the excess....
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Old 17th April 2006, 12:28 AM   #4
mathman is offline mathman  United States
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Quote:
Originally posted by Ron E
Port correction factor is discussed very briefly in L.L. Beranek's "Acoustics" text in a chapter about acoustic masses.

He gives the 0.85 and 0.613 values for flanged and free ends.

The end effects depend somewhat on diameter, but the diameter would have to approach a fraction of a wavelength before much of an effect came here.

One thing to question in the webpage study you cite is whether or ot the actual resonant frequency was measured properly. It does not necessarily coincide with the minimum in impedance. The diaphragm SPL minimum is a more accurate indicator.

If the traditional formulas result in lengths that are too long, that just means one can cut off the excess....
Thanks for the info Ron!

I hadn't thought about the impedance curve not being an accurate indicator of tuning, as I've seen it used before. It does make sense though, so I'll be a bit more cautious in the future.

Do you know if I'm going too short on my port length? I've never heard of a short port causing problems, but I thought I would check. In reviewing my model, the port length is closer to 4", which seems fine, but is there a practical lower limit?
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Old 17th April 2006, 02:22 AM   #5
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There are several variables that will affect actual port tuning frequency. In my experience, it seems that the effects of different cabinet deadening or dampening products are hard to predict.

Checking your tuning frequency is straighforward and cheap to do- all you need is an A/C voltmeter (any standard $10 multimeter will measure this), a 10W wirewound resistor and a laptop computer or some sines waves burnt to CD-R (suggest 1Hz steps from 10-100Hz


1.02 and 1.03 of:
http://www.diysubwoofers.org/faq.htm


I have been able to confirm the tuning of my vented boxes using this technique, and yes, often the actual tuning frequency is different from the predicted frequency.

I used Troel's correction factor to build a port. But with my deadening products, I hit a tuned frequency of 45Hz instead of 42Hz. This is still very good (better than no correction factor), but to hit the exact frequency I would want to take measurements.

At the end of the day a difference of 3Hz is not a big deal. Of course the question is, after we measure it and it turns out wrong, how do we extend or reduce of a port that is already fixed to the cabinet?

I have been told that Unibox predicts port dimensions rather accurately, but I am not sure of the conditions of the cabinet or the parameters entered into the software...
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Old 17th April 2006, 03:53 PM   #6
mathman is offline mathman  United States
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Quote:
Originally posted by tktran303
There are several variables that will affect actual port tuning frequency. In my experience, it seems that the effects of different cabinet deadening or dampening products are hard to predict.

Checking your tuning frequency is straighforward and cheap to do- all you need is an A/C voltmeter (any standard $10 multimeter will measure this), a 10W wirewound resistor and a laptop computer or some sines waves burnt to CD-R (suggest 1Hz steps from 10-100Hz


1.02 and 1.03 of:
http://www.diysubwoofers.org/faq.htm


I have been able to confirm the tuning of my vented boxes using this technique, and yes, often the actual tuning frequency is different from the predicted frequency.

I used Troel's correction factor to build a port. But with my deadening products, I hit a tuned frequency of 45Hz instead of 42Hz. This is still very good (better than no correction factor), but to hit the exact frequency I would want to take measurements.

At the end of the day a difference of 3Hz is not a big deal. Of course the question is, after we measure it and it turns out wrong, how do we extend or reduce of a port that is already fixed to the cabinet?

I have been told that Unibox predicts port dimensions rather accurately, but I am not sure of the conditions of the cabinet or the parameters entered into the software...
Thanks for the input. I'll give the multimeter method a shot when I'm done building. For test purposes, the port will be press-fit into place. Once I'm happy, it will get glued in for good.

And I forgot about Unibox! I'll have to download that and see what it has to say.
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