Mr. Foxen Posted September 1, 2009 Share Posted September 1, 2009 Am I right in thinking bigger ports are better as there is more coupling with the air, but for a given tuning, they need to be longer, and take up core cab space? Or are bigger ports better cause the tuning band is wider? Trying to understand this whole cab design thing, and a sort of summary of what changes in response to different port shapes/sizes would be good to know. Link to comment Share on other sites More sharing options...
alexclaber Posted September 2, 2009 Share Posted September 2, 2009 The main reason why big ports are better is because the airspeed in the port is lower, so the frictional losses and associated noises are reduced. A larger port will couple better but the difference is marginal compared to the larger port's reduced compression at high SPL. Undersized ports work perfectly well at shop SPL but crank them up on the gig and not only will they compress and make chuffing noises, the tuning frequency will change and the phase response can shift to the point that they actually subtract lows instead of adding them. Alex Link to comment Share on other sites More sharing options...
Mr. Foxen Posted September 4, 2009 Author Share Posted September 4, 2009 Lots of small ports or one big one? Can see single port is easier to make, any advantage to many? Link to comment Share on other sites More sharing options...
alexclaber Posted September 7, 2009 Share Posted September 7, 2009 [quote name='Mr. Foxen' post='590306' date='Sep 5 2009, 12:46 AM']Lots of small ports or one big one? Can see single port is easier to make, any advantage to many?[/quote] Generally more ports equals more friction which equals earlier compression. But it's complicated because the key is to maintain laminar rather than turbulent flow, so shape matters. I've been trying to be diplomatic but sod it, here's two fine examples of how not to do ports: Alex Link to comment Share on other sites More sharing options...
Mr. Foxen Posted September 7, 2009 Author Share Posted September 7, 2009 So round ports are best for port purposes as have least internal surface and smooth airflow, but the shelf/slot jobs provide bracing, which is more significant in the real world. And my grandad's books on jet air intake flow and fluid dynamics is sort of applicable, but is has to work in both directions. Link to comment Share on other sites More sharing options...
alexclaber Posted September 7, 2009 Share Posted September 7, 2009 If you have lots of space then a single large flared round port is ideal. It's important though that you minimise the amount of midrange energy (see BagEnd S15-D for a great cab with a silly port) escaping through the port, which limits your location choices. I'm also convinced that unless your ports are huge that putting them behind a grill is a bad idea. There is an argument that a slot port is less likely than a large round port to go turbulent if it's sized correctly, due to the reduction in airspeed differences across the cross-section. I'm digging back into my degree-level fluid mechanics here, on which I'm somewhat rusty. One thing that is rather disappointing about ports is that wherever they're positioned they completely fail to act as a cooling mechanism for the enclosure. The only way you can get them to provide cooling is make them so small that the have very high speed turbulent air flow, which totally defeats the object! I guess you could put the port on top of a sub to encourage warm air to rise from the enclosure but that'll reduce the beneficial effects of floor coupling as the enclosure gets larger. Bass Gear Magazine has had impedance and response plots in its first three issues and I've learnt a lot about the competition's driver choices, cab alignments and port sizing. What's fascinating is how some cabs seem very well designed whilst others are clearly just a case of cross your fingers and hope for the best, and there's almost no correlation between what you pay and whether best practices have been followed. Bizarre! Alex Link to comment Share on other sites More sharing options...
alexclaber Posted September 7, 2009 Share Posted September 7, 2009 Note that the port in the Big One has similar csa to a 9" diameter round port, two 6" ports or three 5" ports. Alex Link to comment Share on other sites More sharing options...
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