Does port location matter? - AVS Forum
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post #1 of 9 Old 07-22-2012, 02:40 AM - Thread Starter
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I want to do a slot port and WiniSD is telling me to have a 6.5" deep port. Can I place the port on the bottom of the cabinet? Or does it need to have a top and bottom thing? Like does it have to be off the bottom or can part of the port depth be part of the bottom of the cabinet?


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post #2 of 9 Old 07-22-2012, 06:12 AM
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Quote:
Originally Posted by ifor View Post

can part of the port depth be part of the bottom of the cabinet?
That configuration is common.
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post #3 of 9 Old 07-22-2012, 07:57 AM - Thread Starter
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Thanks Bill. I appreciate the confirmation.


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post #4 of 9 Old 07-22-2012, 12:48 PM
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Kinda like this build.


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post #5 of 9 Old 07-22-2012, 02:38 PM
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The question then is, what is the effective port length for a given length of the top piece.

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post #6 of 9 Old 07-22-2012, 03:22 PM
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Quote:
Originally Posted by noah katz View Post

The question then is, what is the effective port length for a given length of the top piece.

Take the sqaure root of your cross sectional area of port and multply it by 13.3%, then add that number to top piece length to get the total effective port length?
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post #7 of 9 Old 07-22-2012, 03:53 PM
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Originally Posted by mtg90 View Post

Take the sqaure root of your cross sectional area of port and multply it by 13.3%, then add that number to top piece length to get the total effective port length?
Ducts with very high width to height ratios will give a lower resonance than those with moderate ratios, but there's no precise method that I know of to predict it, as the length reduction varies with that ratio. I've measured triangular corner mounted ducts and they were identical in result to baffle mounted round ducts of the same length and area, indicating that the presence of the sidewalls has no effect in and of itself. To be prudent with a shelf duct size it 10% or so shorter than a square duct, it's easy to add more duct length if Fb measures high, not so easy to make it shorter if Fb measures low.

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post #8 of 9 Old 07-22-2012, 04:05 PM
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Quote:
Originally Posted by noah katz View Post

The question then is, what is the effective port length for a given length of the top piece.
Same way I tune all ported speakers - measure Z and adjust port length until correct.
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post #9 of 9 Old 07-22-2012, 04:20 PM
 
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Assuring adequate clearance (in order to prevent loading of the port), the placement of driver and port relative to one another matters "to a degree".

In a prated enclosure all of the acoustic a output is from the woofer above the tuned frequency.
At lower frequencies, specifically for a narrow bandpass centered about the tuned frequency, the acoustic output s from the port.

Thus the total acoustic output is the sum of the output from the woofer and from the port.
And it thus become important that the two outputs sum in phase.

In order to sum in phase, they must be within 1/4 wavelength of each other.
There for the highest frequency corresponding to the pass band shared by both the woofer and port must be LESS than the distance corresponding to the highest shared frequency.

There fore, if the tuned frequency is, say 27 Hz, and the highest common frequency is , say, 30 Hz, then the greatest separation between the acoustic origins of the port and woofer would be .25(450 Inches) or 112 inches, or 9.38 feet...generally not a problem....

Similar considerations apply to the spacing of drivers reproducing the same pass band...
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