Baffle Step Compensation Adjustment

[wigginjs]

Originally posted by augerpro
*In the circuit posted* if you raise the inductor value the woofer's higher frequencies will drop further, no? So bass is higher in relation (disregard DCR loss for the moment), thus it has more BSC correction. If we were to lower the value the high frequencies would rise in relation to the bass, thus less BSC.

I really appreciate your insight, but I want to be clear about what you stated. I believe your instructions would be correct for raising and lowering the magnitude of the BSC, but I'm interested in lowering the FREQUENCY at which the BSC occurs. Are we talking about the same thing?

[marchel]

If I were the OP, I would change the 1.8mh inductor into 2.5mh And then tweak the tweeter L pad if necessary.

[augerpro]

Originally posted by m0tion



I really appreciate your insight, but I want to be clear about what you stated. I believe your instructions would be correct for raising and lowering the magnitude of the BSC, but I'm interested in lowering the FREQUENCY at which the BSC occurs. Are we talking about the same thing?

We would be IF the crossover posted were using a textbook BSC circuit, but it's not. If you have access to modeling software and real driver measurements, construct a crossover like the one posted. Then change the inductor value and see what happens. Again there is no resistor bypassing the inductor, this changes how each circuit behaves.

Originally posted by marchel
If I were the OP, I would change the 1.8mh inductor into 2.5mh And then tweak the tweeter L pad if necessary.

Just so we are understanding what this will result in, do you agree that going to a 2.5mh inductor will drop the higher frequencies of the woofer? This then requiring the tweeter to be dropped in level to match? If so, do you agree that you have now increased the amount of BSC, as the bass is now at a relativeley higher level than the midrange and treble?

Another mental example: if you placed a woofer on a baffle and made that baffle larger and larger, the inductor would get smaller and smaller until the point that the baffle became infinitely large, at which point the inductor would have 0 value. Do you agree?

I think perhaps the key here is understanding the difference between textbook BSC circuits and values and what we use in the real world. Anyone who's used crossover sim software like LSPcad or SE or SW knows intuitively what I'm talking about.
I'll sim some examples and show you. The main advantage to a textbook filter is that you can smooth out the response right at the baffle step, imposing a knee right at the baffle step, while keeping the higher frequency response flat because of the resistor. Using an ordinary inductor drops the response linearily, so you may end up with a small dip just before the baffle step, but I've only seen this effect on very small baffles. Larger ones tend to even it out.

BTW if you see diy designs on the net with a reduced BSC version, does this version's inductor usually use a larger or smaller value? I think you'll find that it is smaller with out exception.

[augerpro]

Ok here is a quick hack and slash sim using the RS150 and 810921 measurements from my bookshelf speaker i built, which use 7.5" baffle. The network layout is nearly identical to cjd's. The blue is the original near full BSC and uses a 2.5mh inductor on the woofer. The red is with the inductor dropped to 1.8mh, which required dropping the tweeter series resistor from 6 ohm to 3 ohm, and raising it's series cap about .8uf to 5uf total. Now comparing the bass response with the response higher in frequency, it should be clear the blue response has much more BSC by about 3-4 dB and uses the larger 2.5mh inductor. While the red has less BSC and uses a smaller 1.8mh inductor.





Now this only for a given baffle width, but you get the idea of BSC works just using an inductor and a typical baffle.

I see in that htguide thread that cjd means 4 dB when he says it is full BSC, which is probably realistic in most rooms. Given that, I would again suggest teh OP build the design as is and see if he feels it needs less BSC. My guess it will sound nuetral to slightly warmish, but otherwise fine. The main problem changing the baffle width I see isn't BSC, but phase integration of the tweeter and woofer adn smooth response under the crossover frequency. These are somewhat unpredictable without having measurements on the actual baffle.

[sandstorm33]

If you go to Martin J King's website you will find an excellent article on BSC along with ways of calculating the proper values for components. He is always very thorough and ,IMHO, always right on target.

[augerpro]

Originally posted by augerpro
Now this only for a given baffle width, but you get the idea of BSC works just using an inductor and a typical baffle.

I want to make sure this comment was not missed. Especially since I don't want to go off on a tangent myself! Which I probably did ...Changing baffle width is not corrected by simply changing baffle step correction, but you could raise the inductor value to lower the woofer response back down. But the response is slightly different around the crossover frequency, so again I think the concern here is not BSC changes, but maintaining smooth integration with the tweeter, whether or not the woofer's inductor is changed or not. The difference between 7.5" and 11" baffle is only about 1 dB or so under 800Hz.

Bleh, I need my morning coffee.

[augerpro]

Originally posted by augerpro
The difference between 7.5" and 11" baffle is only about 1 dB or so under 800Hz.

If you do raise the inductor I wouldn't go more than 2.0mh to keep from messing up the crossover. That should drop the response about 1 dB.

[Jay_WJ]

I'd take Brandon's advice. Go first with a 2.0 mH inductor with a bit less padding on the tweeter. If you need less BSC after extensive listening, you can easily modify it by unwinding the coil and adjusting the padding resistance. But this simple method will only work within a certain limit. I'd not go outside of 1.5 to 2.0 mH range. Within this range, the designed phase tracking won't be ruined. And I think this range should be sufficient for your purpose.

To unwind an inductor, an online calculator:

http://www.lalena.com/Audio/Calculator/Inductor/

is pretty accurate if you know how to use it. I've actually compared a result (i.e., number of turns or wire length that needs to be taken out to reach a certain value) obtained from the calculator to an actual result obtained using my LC meter, and found excellent accuracy.