Port end correction for slot port enclosure - Home Theater Forum and Systems - HomeTheaterShack.com

Old 12-14-10, 02:09 PM Thread Starter
Shackster

Join Date: Dec 2010
Location: England
Posts: 11
Port end correction for slot port enclosure

Can someone please explain or link me to a simple explanation of port end correction, I have found a diagram show different values for different positions. I have tried entering these values into Winisd but it does not seem to alter the length of the port. Thanks
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Old 12-14-10, 02:41 PM
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Collo

Join Date: Dec 2006
Location: Newcastle, Australia
Posts: 284
Re: Port end correction for slot port enclosure

When a port is close to a wall, the wall extends the zone of influence of the port, making the effective length longer. The same applies to a port that is flanged, for example, when a port is flush with the front face of an encolsure. The end correction facor is a way of quantifying this effect. WinISD can calculate the correction for flanged and unflanged, but for slot ports that are against walls or corners, you will have to enter the value manually.

JL Audio notes
Flaring slot ports
WinISD page with drawing of flanged vs flared

Ports Rule ...
collo is offline
Old 12-14-10, 04:25 PM Thread Starter
Shackster

Join Date: Dec 2010
Location: England
Posts: 11
Re: Port end correction for slot port enclosure

Thanks, I read in one place to add half the slot port height to the length of the port for end correction, will this be accurate enough compared with using the formula?

I am trying to design a slot ported box of about 9cuft net, the port will be 14"x4" and 41.75" long for the box tuning of 20Hz. There for my plan will be to have the port mouth on the front face below the driver and the slot port to run along the bottom edge of the box and then turn 90degrees and go up the back edge of the box. The slot will be narrower than the box itself as I guess the box will be around 24" wide. But this port/box dimensions are flexible.

Can anyone tell me if this will be ok? I realise the length has to be adjusted if the ends are flared, but what adjustment would I need to make for end correction and is it possible to calculate this in winisd? Or am I just worrying unecessarily not do most people do when building enclosures?

Thanks.

Last edited by Ianbuckwell; 12-14-10 at 04:32 PM.
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Old 12-14-10, 07:42 PM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Port end correction for slot port enclosure

Assuming your cabinet is exactly 9 cu.ft., allowing for end correction or not isn't going to make much difference. If the plan is for a 20 hz tuning and the end result is a 21.5 hz tuning if won't make any noticeable difference in the performance of the sub. Just my opinion.
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Old 12-14-10, 07:42 PM
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lsiberian

Join Date: Mar 2009
Location: Fort Worth, TX
Posts: 2,625
Re: Port end correction for slot port enclosure

Quote:
Ianbuckwell wrote: View Post
Thanks, I read in one place to add half the slot port height to the length of the port for end correction, will this be accurate enough compared with using the formula?

I am trying to design a slot ported box of about 9cuft net, the port will be 14"x4" and 41.75" long for the box tuning of 20Hz. There for my plan will be to have the port mouth on the front face below the driver and the slot port to run along the bottom edge of the box and then turn 90degrees and go up the back edge of the box. The slot will be narrower than the box itself as I guess the box will be around 24" wide. But this port/box dimensions are flexible.

Can anyone tell me if this will be ok? I realise the length has to be adjusted if the ends are flared, but what adjustment would I need to make for end correction and is it possible to calculate this in winisd? Or am I just worrying unecessarily not do most people do when building enclosures?

Thanks.
With a large slot port normally port end correction is unnecessary, but I do recommend using a roundover bit on the exit to smooth off the corners.
lsiberian is offline
Old 12-15-10, 07:17 AM Thread Starter
Shackster

Join Date: Dec 2010
Location: England
Posts: 11
Re: Port end correction for slot port enclosure

So should I just use the option of "one flanged end" in Winisd which I believe is a value of 0.732 and base my enclosure on the port that it calculates with that?
Ianbuckwell is offline
Old 12-15-10, 01:07 PM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Port end correction for slot port enclosure

The values listed in WinISD is for round ports, an article by J.L. Audio states to add one half the height for the end correction on slot ports.

http://mobile.jlaudio.com/support_pages.php?page_id=165
Mike P. is offline
Old 12-15-10, 02:03 PM Thread Starter
Shackster

Join Date: Dec 2010
Location: England
Posts: 11
Re: Port end correction for slot port enclosure

Ok, thanks, i saw that, but i also was shown a link to another way of calculating it by using a rather complex looking formula. So for my planned enclosure the port is 4*14*41.75 (h*w*d) so what you are saying is to add 2" to the 41.75" giving a total length of 43.75", so if this works it will retain the 20Hz tuning, but if not lengthening the port will lower the tuning to 19.54Hz, so I guess just as Mike P. said it will not make a noticeable difference either way.

Can I just say thanks to all you experts who are helping me with sorting my design and answering all my novice questions, I am learning alot and it makes this project less daunting as having answers helps reassure me I am heading in the right direction, thankyou.
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Old 12-15-10, 07:38 PM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Port end correction for slot port enclosure

Just to be clear you don't physically add anything to the port, you build the port 4x14x41.75. The port acts like it's 4x14x43.75. So for modeling purposes you would enter a length of 43.75 and build the port 41.75.
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Old 12-16-10, 05:01 AM Thread Starter
Shackster

Join Date: Dec 2010
Location: England
Posts: 11
Re: Port end correction for slot port enclosure

Ok, thanks, glad you said that as I would have added it on the end, making it even worse. I see now, that as the port is along the side of the enclosure and that side carries on past the end of the port it therefore is effectively lengthening the port.

If, I choose to have my sub down firing i read that sag can be an issue, I have found a formula online and entering my subs specs it calculates the sag as 0.971%, I read that it should be below 5%, does this seem correct?

When looking at the graph of the model in Winisd on the "transfer function magnitude" the line gradually tapers from 0dB at about 100Hz down to -3dB at around 20Hz is this a problem or OK? Thanks

Last edited by Ianbuckwell; 12-16-10 at 07:11 AM.
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