Square ports

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   Hi, 

If winISD calculates a square port depth of say 150mm and I measure that from the front of the baffle, what do you do about the 18mm lip/overhang which surrounds 3 sides of the port? how much affect will this have on the tuning frequency?

Or would it be better to include the edge in your port depth and put a bar across like you can see in this pic.

Which is the best approach?
Thanks for the help

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herke Members Profile Send Private Message Find Members Posts Add to Buddy List Registered User Joined: 04 November 2009 Location: Huddersfield Status: Offline Points: 171	Post Options Post Reply Quote herke Report Post Thanks(0) Quote Reply Topic: Square ports Posted: 24 August 2016 at 2:34pm
	Hi, If winISD calculates a square port depth of say 150mm and I measure that from the front of the baffle, what do you do about the 18mm lip/overhang which surrounds 3 sides of the port? how much affect will this have on the tuning frequency? Or would it be better to include the edge in your port depth and put a bar across like you can see in this pic. Which is the best approach? Thanks for the help

ceharden Members Profile Send Private Message Find Members Posts Add to Buddy List The 10,000 Points Club Joined: 05 June 2005 Location: Southampton Status: Offline Points: 11776	Post Options Post Reply Quote ceharden Report Post Thanks(0) Quote Reply Posted: 24 August 2016 at 2:52pm
	So called 'shelf' ports such as this will actually appear acoustically much longer due to their position in the cabinet. The effect is quite significant. On the internal end you can expect the effective length to be extended by about the height of the port. However there are no definitive rules and in most cases, prototyping is necessary to get it spot on.

herke Members Profile Send Private Message Find Members Posts Add to Buddy List Registered User Joined: 04 November 2009 Location: Huddersfield Status: Offline Points: 171	Post Options Post Reply Quote herke Report Post Thanks(0) Quote Reply Posted: 25 August 2016 at 9:42am
	Ok thanks for the info. So would it be safe to say that winISDs prediction would be the shortest the shelf will be? So I could build to that and add to the shelf rather than building it too long and hacking it out somehow?

ceharden Members Profile Send Private Message Find Members Posts Add to Buddy List The 10,000 Points Club Joined: 05 June 2005 Location: Southampton Status: Offline Points: 11776	Post Options Post Reply Quote ceharden Report Post Thanks(0) Quote Reply Posted: 25 August 2016 at 11:05am
	Other way round. WinISD will tend to design the ports too long. What I find tends to work as a very rough rule of thumb is to design in WinISD for the tuning to be about 5Hz or so higher than you actually want it to be. The effective length of the ports will get you down towards your target.

herke Members Profile Send Private Message Find Members Posts Add to Buddy List Registered User Joined: 04 November 2009 Location: Huddersfield Status: Offline Points: 171	Post Options Post Reply Quote herke Report Post Thanks(0) Quote Reply Posted: 25 August 2016 at 11:47am
	Ah right thanks for the advice. I'll give it a go. Cheers

SamV Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 21 October 2008 Location: London Status: Offline Points: 8711	Post Options Post Reply Quote SamV Report Post Thanks(0) Quote Reply Posted: 25 August 2016 at 1:29pm
	It's BassBoxPro that handles shelf ports the best in my experience. Always been near spot on. Edited by SamV - 25 August 2016 at 1:30pm

_djk_ Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002	Post Options Post Reply Quote _djk_ Report Post Thanks(0) Quote Reply Posted: 26 August 2016 at 1:20am
	"It's BassBoxPro that handles shelf ports the best in my experience. Always been near spot on. " +1
	djk