yondergo Posted April 3, 2015 Share Posted April 3, 2015 (edited) Hey guys, hopefully someone a little more experienced in speaker design will be able to shed a little light on this for me, i'm going through the process of adding a Kappalite 3015 to an old Peavey BW 115 cab i have, it's one of the older, shallower ones with a 1502 driver inside and a 4" hole (lets not call it a port) in the front. I've been modelling it with the 3015 installed in WinISD and i'm coming up against some really quite high port velocities and i'm pretty stumped about how i can get them down to a reasonable level, I've calculated the internal volume as 75 litres or 2.65 cubic feet when taking into account the foam damping i've added, and i'm shooting for around 50hz for the tuning frequency. I'm running the model at 450w power (the stated upper RMS given for the 3015)| and my issue is that when examining the port velocity it spikes at about 28m/s around the 35hz point and i don't seem to be able to get it to come down to a sensible level without the length of the ports greatly exceeding the depth of the cab which is a fairly pokey 9" (22.5cm). It seems to show broadly similar results with 2x4" ports or 1x6" port, the 6" being a touch shorter, does anyone have any creative suggestions for ways i might be able to get the velocity down, or am i worrying too much about it? I'm hoping to avoid installing a shelf port if possible to keep the woodworking down to a minimum, but i'm expecting to have to enlarge or add holes to the baffle for the ports. thanks! Dave Edited April 3, 2015 by yondergo Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted April 3, 2015 Share Posted April 3, 2015 I'm not an expert on this, but I suspect you might be looking at the port velocity at a higher power than you're likely to use. I've just opened up WinISD and modelled the 3015 in 75 litres, tuned to 50Hz. Below about 42Hz, the maximum power is limited by the Xmax of the driver rather than the thermal handling. At 35Hz it can handle around 107 watts before exceeding Xmax, so you'd be farting out the driver long before you heard any port chuffing. Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted April 3, 2015 Share Posted April 3, 2015 Port velocity at 35Hz is moot with electric bass. You'll never put 450w at 35Hz into that cab. What happens between 50 and 60Hz is what's significant. That said, a 4'' round port is too small for a 15" long excursion driver. BTW, foam damping does not reduce the cab volume. If you're lining the cab with foam you can predict its influence in the Box window, using the advanced tab, changing the Qa value from the default 100, which is a bare box, to 50. Quote Link to comment Share on other sites More sharing options...
yondergo Posted April 3, 2015 Author Share Posted April 3, 2015 thanks for the replies, apologies i wasn't mentioning the 35hz peak in port velocity as an expectation of playing with such low frequencies, just as the point at which it showed the highest m/s speed on the graph. thanks for the heads up on the foam not reducing internal volume Bill, really appreciate your input - i'm remodelling accordingly. I was hoping to use 2 4" ports, would your point about 4" ports not being suitable apply to using multiple ports or is it 4" ports in general? Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted April 3, 2015 Share Posted April 3, 2015 [quote name='yondergo' timestamp='1428095685' post='2738133'] I was hoping to use 2 4" ports, would your point about 4" ports not being suitable apply to using multiple ports or is it 4" ports in general? [/quote]There's nothing wrong with 4" ports, but that's insufficient area to use with a 15" driver. Two would be OK. Quote Link to comment Share on other sites More sharing options...
yondergo Posted April 4, 2015 Author Share Posted April 4, 2015 Thanks Bill, I think I'll go for the 2 4" ports, they seemed to model with slightly lower velocity than a single 6" port. They still seems to be pretty fast, 23m/s at 50hz, but that's running at a fairly unrealistic 450w so I guess I'll just have to try it out and hope I don't play loud enough for port noise to become an issue. One other concern was how much clearance I would need inside the cab where the end of the port approaches the back of the cabinet, it's looking like i'll have probably an inch and a half between the port end and the foam damping, is there any negative effect to be expected from this? Thanks, really appreciate the guidance on these matters, I've begun to wonder if selling my 2x12 was such a good idea now. 😃 Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted April 4, 2015 Share Posted April 4, 2015 [quote name='yondergo' timestamp='1428132343' post='2738273'] One other concern was how much clearance I would need inside the cab where the end of the port approaches the back of the cabinet, it's looking like i'll have probably an inch and a half between the port end and the foam damping, is there any negative effect to be expected from this? [/quote]You need approximately the port diameter distance to the back of the cab, otherwise the air mass between the cab back and port entrance can add to the air mass in the port, lowering the tuning. Quote Link to comment Share on other sites More sharing options...
yondergo Posted April 5, 2015 Author Share Posted April 5, 2015 Thanks again Bill, that's made a big difference to the plan, I'm going to look at sealing the existing 4" baffle hole and porting the sides of the cab now, once I subtracted the volume of the ports and winISD recalculated the length of the ports I was almost butting up to the back of the cab. Should have done this work earlier, probably would have decided to buy a different cab and scrap the peavey enclosure. Oh well, we soldier on. Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted April 5, 2015 Share Posted April 5, 2015 [quote name='yondergo' timestamp='1428242799' post='2739481'] Thanks again Bill, that's made a big difference to the plan, I'm going to look at sealing the existing 4" baffle hole and porting the sides of the cab now, once I subtracted the volume of the ports and winISD recalculated the length of the ports I was almost butting up to the back of the cab. Should have done this work earlier, probably would have decided to buy a different cab and scrap the peavey enclosure. Oh well, we soldier on. [/quote]You can keep the ports behind the grill, just add 90 degree elbows to the ducts. The effective duct length is measured on the centerline. Quote Link to comment Share on other sites More sharing options...
yondergo Posted May 4, 2015 Author Share Posted May 4, 2015 Just to close this out and offer a big thank you to everyone, especially BFM, for their advice on this. Finally got the ports in and tuned this weekend and used it at practice tonight, I ran it last week without the proper ports and was pretty underwhelmed, but expectedly so. With the ports tuned to around 50-55hz it is an absolute beast, kicks out more perceptible low end than an ampeg 6x10 we have in our room and has loads of definition in the mids. Sounds more old school than I was expecting, possibly due to the lack of damping just now, I might introduce damping bit by bit to tame it a little but not too much. Really grateful for the support people offered, very pleased with how it worked out! Cheers! Dave Quote Link to comment Share on other sites More sharing options...
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