Crossover tweaking question

[Bolghar]

Hello all,

I did search the net for the past few days but couldn`t get any information on this, hope you could share some information.

When using, lets say a 2nd or 3rd order passive and you have a rising driver acoustical response around the crossover region, the filtered slope should be the combination of the rise minus the real slope of the filter ( lets say 12db/oct ) which is less than 12db. If I need 12db, can I increase or decrease the value of the parallel component ( capacitor or inductor ) to achieve this? How is this going to affect the phase and how does it change the filter type from Butterworth to another?

Thanks!

[cjd]

Well... you already have a rising impedance on the driver so... yeah. You're asking how to build a crossover and that's a book or two.

But. Yes. Maybe. And the rest play into the difference between reality and a textbook, but really both answerable with that same textbook. Acoustic transfer function is not going to match acoustic slope.

[Wheels]

One possibility is to design the crossover electrically at a lower cutoff freq than your desired acoustical crossover point, and maybe (big on the maybe) you can flatten out the peak and still achieve the 12db/o slope with a butterworth design.

but CJD is right... it would take waayy too much information than can be adequately explained here to fully cover this. If you want to start reading, Loudspeaker design cookbook and Speaker building 201 is where I started and is very helpfull. I highly recommend just experimenting with different crossover designs in either Speaker Workshop or Passive Crossover Designer along with reading to help grasp all that is happening.

[Bolghar]

I did buy a book from madisound, it is written by David B. Weems. It was described as the best I could get to step into simple speaker design. A waste of money, unfortunately. The crossover section consists of 4 pages where two are dedicated to how to wind inductors and two ( including large size charts ) to what is a 1st order, 2nd order and etc. crossover and what is a capacitor/inductor and how to construct L-pads. It didn`t give me any clues on what happens when changing one value - lets say just the inductor to compensate for rising woofer response or the parallel capacitor to change the slope. Nothing was said even for what happens if different crossover frequencies for the two drivers are chosen, like 3.5Khz for the tweeter and 2.3Khz for the woofer... Anyways, I`ll just try to dig it out from somewhere.
Thanks!

[Wheels]

Well, to give you an idea of what you are attempting to tackle... basic filter design (crossovers on fixed impedance loads without really discussing linear phase tracking between loads) is typically the 2nd circuits class in an Electrical Engineering degree. Typically taken your sophomore year with a fairly involved lab. Not trying to discourage you from tackling this, Just saying there is a ton of information and concepts going on here. Most speaker books will only cover crossovers with matching cutoff points because they are by far the most common. There are way too many possibilities for them to cover them all. I'm not trying to discourage you, just trying to help you realize how much information there is to grasp it all, (and I don't yet) so you set a reasonable goal and don't get overwhelemed and give up on it.

but, in a very vauge nutshell... a capacitor has high low frequency impedance and low high freq impedance. so if it is in parallel with your speaker it acts as a lowpass filter as the high freqs are shorted through the cap. if it is in series it acts as a highpass filter because the added impedance to low freqs limits the amount of current flow through the speaker for the low freqs.

An inductor has the exact opposite effect of a capacitor

Now in a circuit, increasing capacitance moves its cutoff freq lower, in both types of filters. So in a lowpass filter, increasing capacitance will give you more attenuation at a given freqency. In a highpass circuit it will give you less attenuation at a given freq.

Changing inductor values change cutoff freqs the same direction as a capacitor (increasing, lower cutoff)

As for the effects on phase and how that makes one no longer a butterworth or approaching a different kind of filter... I don't know

Loudspeaker Design Cookbook by Vance Dickenson has 46 pages on crossovers, with lots of graphs. Speaker building 201 by Ray Alden has 20 pages on it with some pictures.

But in all honesty, it will help a lot if you start with a basic speaker and a single cap/inductor and play with the values in a crossover design simulator.

[Bolghar]

Thank you for the reply. I looked at my question and I think it is my mistake, it is not very clear, please excuse me.
I perfectly know what a capacitor or an inductor does and I managed to find the Cook book on a pdf file yesterday. Very detailed in crossovers, still doesn`t answer my question. I`ll add an example:
We have a 2nd order low-pass Butterworth done with the proper formulas. However, driver has a rising responce beyond the crossover point. When we apply this filter type, it provides 12db electrical but summed - lets say 8db acoustical due to 4db increase in speaker response. This is at a crossover point of 1.7Khz.
1. What happens with phase and filter type if we increase only the capacitance of the parallel capacitor in order to get 12db/oct acoustical as on tweeter ( 16db electrical )? I am sure this increases the Q of the filter as its pass band is in a tighter frequency frame so it should get closer to a Chebyshev lets say. But changing one value only, how does this affect the phase? What I wanted to ask is how is this combination of two different butterworth filters will work together? Theoretically phase is dependant on the crossover frequency, having a cascad of two butterworth centered on different frequencies, how is this going to affect the total phase response?
2. The question about the cascading butterworths at different frequencies should give me a hint, but what if we choose different crossover frequencies for the two or three drivers - lets say to cut off the woofer at 1.4Khz and the midrange at 2.3Khz and because of rising for woofer and peak in the midrange response at lets say 1.6Khz - to compensate and obtain final acoustical slopes of 18db at 1.8Khz for example( 3rd order ), how is this going to affect the phase?

[Wheels]

In all honesty, you could spend a long time figuring that out mathematically, or you could just model it. If you don't have the speakers yet use SPL tracer to get a freq response and impedance off the data sheet and have fun. Your acoustic centers won't be properly defined and the final summing of the speakers will be a little different, but you can see how the phase is affected individually. And also, I know Speaker workshop has the ability to let you define an acoustic response (butterworth, linear phase and some others, and order for each) you want to achieve and then it will tweak the values of the crossover to get as close as it can. Its not perfect, but it saves you the time of doing the math and gets pretty close most of the time.