BSC for my Focal Towers - diyAudio
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Old 24th June 2008, 07:12 PM   #1
pjpoes is offline pjpoes  United States
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Default BSC for my Focal Towers

Hi guys, so I've done some more work on my Focal towers, finaly got the test enclosures together, and test crossovers. At the moment, they have no BSC at all. The Baffles are 9" wide, and the RE of the midbass is around 7.5 ohms. I calculated the fs to be around 506hz and the necessary values to be 2.3mh and 7.5 ohms for 6db's of baffle step compensation. Now I will attach a graph I made using an RS meter, test tones, and a Microsoft table. I know its crude and scaled wrong, but its all I have for now until I can replace the equipment damaged by the storm. The table already has the correction factors applied for the meter. It was measured at 1 meter in room, so I imagine the high frequency roll off is room absorption. I see more than 6db's of baffle step roll off, but I don't want to apply too much BSC, even 6 is more than I would like.

Anyone know if there is a program I can use that would allow me to manually type in the numbers and make a proper graph with the correct scale? What about just manually typing a .frd file and exporting the graph from Speaker Workshop? I don't have phase data, does it care?

Any recommendations on the Baffle Step. It seems like too much roll off to just be baffle step, but since measurements taken on an Infinite baffle don't match my finished product measurements, I am thinking it has to be the effects of the narrow 9" baffle. The efficiency of this setup is going to be horrific, I'm thinking 85 db's at best.
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Old 24th June 2008, 08:11 PM   #2
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Ok so I tried a 3.3 mh inductor and a 6 ohm resistor, only resistor I have right now, and that didn't work. Then I tried a 1.8mh inductor and the same 6ohm resistor, and it seems to be the sweet spot values. I don't have enough different values to try any others, so this will have to do for now. The only problem area now is that right around 80-100hz, in room, there is a slight null, about 6 decibles down from every other area. It's still a little light in the midbass area, but much better than it was. I'm measuring efficiency right around 80db's, so worse than I thought, and not very good by any standard, but oh well.
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Old 25th June 2008, 02:28 PM   #3
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I'm really starting to have a love hate relationship with baffle step. Does anyone else here feel it is the devil? It just kills efficiency, making simple 2-way designs almost pointless. I'm really thinking at this point that a 2.5 way will be better. I can't get another one of those midbass drivers though, so I have think about this some more.

Anyone have any comments or thoughts for using a different midbass. In the area of a simple, inexpensive option, I could go with the PE Dayton Reference midbass. In staying with Focal, I can get the newer version of this midbass driver, which has similar, but not identical specs. Problem is, the newer one doesn't like such a big enclosure.
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Old 25th June 2008, 03:45 PM   #4
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2-way with BSC is not pointless - there are a million good designs out there with it. That aside, a 2.5 with Dayton Reference is a good idea - they are about the best you can buy at their price point.

2.5-way or 2-way with small integrated active sub (stereo) is my preference.
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Old 25th June 2008, 06:09 PM   #5
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Quote:
Originally posted by pjpoes
I'm really starting to have a love hate relationship with baffle step. Does anyone else here feel it is the devil? It just kills efficiency, making simple 2-way designs almost pointless. I'm really thinking at this point that a 2.5 way will be better. I can't get another one of those midbass drivers though, so I have think about this some more.

Anyone have any comments or thoughts for using a different midbass. In the area of a simple, inexpensive option, I could go with the PE Dayton Reference midbass. In staying with Focal, I can get the newer version of this midbass driver, which has similar, but not identical specs. Problem is, the newer one doesn't like such a big enclosure.

There's just no where to go with BSC on a 2 way without losing lots of efficiency. I don't know what mid/bass model you are using but the lower response of the woofer is not going to be the same in the box as on that test board and that is what you are referencing the BSC curve from. You have to measure the drivers in the final enclosure to get the lower frequency response curve semi accurate.

I would look for a good quality but inexpensive woofer that is about 3- 4 db more sensitive in the lower regions and make a three way crossed over at the BSC.

Or if you have high current amplifiers you can do the 2.5 way with some kind of woofer the same sensitivity as the focal.
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Old 27th June 2008, 07:42 PM   #6
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Thanks for the input, my thoughts at the moment are a metal cone midbass from Dayton Reference or possibly the Seas in around a 7" or 8" size. One option for dealing with the sensitivity issue would be to use the 4ohm instead of 8 ohm version, which would give an increased voltage sensitivity.

I've never had this much problem with BSC, but the problem is that in the midst of a thunder storm, right in the middle of this project, Lightning struck my house causing significant damage to my measuring system, main speakers, main amplifier, two lcd tv's, and three dvd players. As a result, I lost my ability to take measurements of the speakers after the enclosures were done, and had to design the crossovers based on IEC infinite baffle style. I didn't properly account for the baffle step loss for one thing, but also had more baffle step loss than I had hoped.
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