# Crossover slopes



## geopango (Jun 24, 2012)

Okay I have a slightly unusual 3 way design incorporating the Midrange positioned at the top, tweeter in the middle and woofer at the bottom. The 5"mids are at face level at the seated listening position and are approximately 50cm above the 12" woofer - dust cap to dust cap.
On My Behringer DCX I have been playing around with Bessel, LR and Butterworth slopes in an effort to achieve better subjective integration between woofer and mid which are physically quite far apart.
By the way the speakers are time aligned via REW which I find is more consistent and precise than the Behringers auto align.
Normally crossed over at 300hz, using 8th order filters I find that a Bessel slope produces the best transient response and integration between bass and mid, however is not quite as clinical as LR or Butterworth.
Any thoughts on crossover slopes for drivers that are far apart?
Thanks


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## gazoink (Apr 17, 2013)

Have you actually done a step response on that 8th order filter? If you're worried about transient response, 8th order aint really the way to do it.


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## geopango (Jun 24, 2012)

Only used Impluse response at this stage for time alignment at the listening position. Haven't done any measuring at the crossover points.


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## gazoink (Apr 17, 2013)

Impulse response and step response are two different things. A step response is basically one cycle of a square wave with the output voltage plotted against time. Higher order filters ring more that lower order, and different filter tunings affect ringing a lot, Butterworth vs Bessel vs Chebyshev, etc. You'll also want to look at phase response around the break points. All if it determines how things work out in the crossover region. 

Filter optimization is not trivial, there are experts that make a living out of it.


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## jtalden (Mar 12, 2009)

geopango said:


> Okay I have a slightly unusual 3 way design incorporating the Midrange positioned at the top, tweeter in the middle and woofer at the bottom. The 5"mids are at face level at the seated listening position and are approximately 50cm above the 12" woofer - dust cap to dust cap.
> On My Behringer DCX I have been playing around with Bessel, LR and Butterworth slopes in an effort to achieve better subjective integration between woofer and mid which are physically quite far apart.
> By the way the speakers are time aligned via REW which I find is more consistent and precise than the Behringers auto align.
> Normally crossed over at 300hz, using 8th order filters I find that a Bessel slope produces the best transient response and integration between bass and mid, however is not quite as clinical as LR or Butterworth.
> ...


There is no driver spacing issue with that setup. The spacing is considered acoustically small at 300Hz (less than 1/2 wavelength). 

The DCX alignment may not be reliable so it is best to it manually if you know how. A proper IR alignment (initial rise of the IRs, not aligned peaks) is a good starting point for the process. There are several methods to then fine tune the delay if needed. 

The only significant measureable difference between the filter functions is a small impact on the SPL through the XO range. The phase difference is trivial for speaker XOs. 8th order filters are usually considered a little extreme for IIR filters. Most recommendations for active setups are for LR24. When setup optimally the filters can be selected to best fit with the drivers phase characteristics to create close phase tracking throughout the XO range. To best meet that objective the 2 filters may actually be different orders. Using 8th order for the LPF is usually not an issue for phase tracking. It can be difficult to get good tracking when using 8th order on the HPF however. 

If you want help to confirm your current settings, I can do that if you are willing to take and post the needed measurements. Review *This* thread for background info on the most intensive/complicated method for driver alignment. It is possible to simplify the process if you are looking to DIY. That thread does explain some of the considerations and tradeoffs. Again, good results can be achieved using a much simpler process however.


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## geopango (Jun 24, 2012)

Here is an image of the three IR readings all aligned as close as possible to the peak.
I'm actually preferring the fuller sound of LR24 slopes. Fourth order Bessel is also an interesting contrast with seemingly better transition than eight order slopes and far better transient response.


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## jtalden (Mar 12, 2009)

As I stated above aligning the peaks is definitely not the proper alignment for an conventional IIR (DCX) type of XO. There is an alternate alignments that may also give good SPL results where the peaks are a little closer together, but it would be pure chance if you achieved one of those. The graph you posted is not helpful to determine the quality of your alignment. To do, it requires looking at the SPL and phase graphs and adjusting the graphs settings appropriately for the purpose. 

If your measurements were taken full range with the mic placed >1.5m on the listening axis (LP location is best) then I can probably use that file data to show you how well the drivers are aligned.


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## geopango (Jun 24, 2012)

Here are three graphs for phase and spl. The phase overlays are are focused around the crossover points.
I adjusted the mic level during testing so ignore the difference between spl's.
Drivers are as fullrange as possible. I had to highpass the tweeter at 2000hz for fear of destroying it with the sweep.
As recommended by numerous credible sources I have taken the measurements from the normal listening position which in this case is three meters away on axis, single channel only.
Crossover points are normally 300hz and 2.7khz but during testing I expanded the range of each driver.


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## jtalden (Mar 12, 2009)

Sorry, those graphs are not helpful.

One of the requirements is that the chosen XO filters must be turned on. They are turned on for the MR and TW, but not at the chosen XO freq. The SW LPF is not active so that measurement is not helpful. [With the chosen XO filters active there is no risk of damage when measuring at reasonable levels; maybe 75-80 dB.]

The 2 phase graphs are not setup properly to actually read the phase tracking even if the measurements were okay. 

I see 3 choices here.
1. Subjective approach: Leave the setup as is or adjust it to your preference and then continue to enjoy your system.

2. DIY approach: Measure as noted in the other thread and follow the steps to set the delays so the initial rise of the IRs are aligned. Fine tuning is then optional. The SPL chart is the only one needed for the fine tuning process. The steps in that post are not well defined, but I can help you through the steps by clarifying them and answering questions as needed.

3. Expert advice approach: Measure as noted below and post the .mdat file with 6 measurements. I will determine the best setting for the delays and show you the calculated results of that setup.

Measure each of the 6 drivers (L-TW, L-MR, L-W, R-TW, R-MR, R-W) with the following conditions applied:
> Activate the desired final DCX XO filters. [LR24 is a good option unless you have a particular reason to use something else.]
> Record all delay settings active for these measurements. [It may be easiest to just set all the delays at 0ms for the measurements so the delay values I determine can be entered directly rather than added to the current values - your choice]
> All measurements should be 20-20k Hz sweeps.
> Disabled EQ is recommended. [If old EQ is active that is okay, but EQ filters within the XO range are best disabled for this process.]
> Use REW Loopback Timing for these measurements.
> Speakers in normal room position*
> Mic at LP*

* If this speaker or mic location is a problem we can find another option.


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## geopango (Jun 24, 2012)

Thank you so very much for your advice and I will perform the tests when I have time.
In the mean time from the phase information in the overlays I have observed that at two very suitable crossover points - 260HZ and 2.68khz - phase looks close to aligned. It appears that a very simple 0.2ms nudge of the mid seems to line phase up at these two points. Audibly it sounds like everything just ties up with this increment at these crossover points.
Whatever you say time alignment via IR is not perfect , but very close and at least an essential starting place for attaining phase alignment
Thanks


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