Active Crossovers & Baffle Step Compensation

Why do designers of crossovers ignore this? OK, I can understand it's a crossover.
But then builders using these on real world boxes need an extra stage to compensate for this. Why do they choose to ignore this after they have gone to the trouble of multi-amps?

Who offers a ready built PCB that can add this?
Baffle step is normally in the 3-4dB range due to room effects. Although you are correct that a 4th-order transition won't be correct, as long as there is a filter of about the right order in the right spot the ripple that results will probably be no worse than the ripples due to diffraction, driver FR variation, etc. I don't think that many people are using 4th order LR crossovers -- most people that are going for 4th order slopes use 2nd or 3rd order crossovers.

You are correct about the limitations of state variable crossovers, but that is really irrelevant -- there is no need for a asymmetric crossover to correct for baffle step.

Nothing you say is wrong, but when designing real world speakers you have to keep things in perspective. Nothing is flat, and the transition points and magnitude are going to have to be determined experimentally if you want them to be really accurate. If at the end of the day you can get a response that is +-3dB, you are doing great. There is no need to sweat an error of a dB or two, unless of course it is compounding another error. By the same token, an "error" in your baffle step filter may even serendipitously correct for another anomaly! This is why designing loudspeakers is an iterative process no matter how much modeling you do.
State variable designs do allow for separation of crossover points in a two way design if you plan ahead. The full SV circuit (as opposed to the abridged version) gives low pass, band pass, and high pass. The transitions from low to band and from band to high pass are independently variable. In a two way, if you ignore the band pass tap and use only the low and high pass taps then you can independently set the crossover point of each. It is true however that many SV designs use a somewhat consolidated topology that effectively has no band pass (the output tap is there in spirit, but its bandwidth is zero I believe) and the high/band and low/band transition points are locked in step. I use this simplified version myself. There is also an even simpler SV version that only provides either high or low pass output (can't remember which), which is nice when you need only one side of an XO to be continuously variable, but otherwise isn't of that much usefulness.
What feature is on the Marchand? There is a the variable resistor to vary how the two bands sum, but I don't see anything that can shelve a bit.

If you implement the correction via staggering the crossover point, in practice that is difficult because in a two way, it would require that the tweeter cross at a very low point wouldn't it?


2005-09-09 7:11 pm
The XM9 can be easily modded to include first order baffle step compensation by adding a series RC in parallel with the feedback resistors R22 and R23 in the output buffers of the xover. This creates a simple low-pass shelf filter which is all baffle step correction needs to be. Details for doing this are included in the manual (can be downloaded from their website).

Alternatively, the XM1 can be even more easily modiffied by adding the RC to the feedback resistor of the input buffer (R2). In many ways this is a superior option because the Xover is applied after the equalisation stage (any phase shift will be unifoirmly applied to high and low pass outputs). The manual for this Xover is also posted on the excellent Marchand website.

Personly, I am going to use for the XM9 pcbs since they are more flexible (allow simple adjustment of LP/HP output levels) and appear to be built on a better PCB (double sided with almost solid GND plain).


2002-10-18 11:49 am
catapult said:
The Marchand XM44 will accept plug-in cards for baffle step and most of the other circuits on the Linkwitz filter page. It will also do asymmetric slopes. It's a nice box and very flexible but it's also pretty spendy.

I have built the XM44. Marchand ones told me he can provide an expansion card for BSt compensation. Have not purchased it yet (cause my mid/high boxes are not ready yet).
It might be ignored because you CAN ignore it -- if you set the crossover frequency at the baffle step transition. Then you need only adjust the level to correct for baffle step, something which is rather easier to do actively.

I absolutely agree with you. According to the baffle step theory the woofer would have a 6db loss compared to the tweeter (or about 9db if you take into account that tweeters are usually more sensitive than woofers) which can easily be corrected by increasing the output level of the low pass filter by the amount required. One more reason for why active crossovers are the way to go.
DSP crossovers are now incorporating a low or high pass shelf filter that can be activated around 135 hertz and customised.

Unfortunately for tall boy enclosures of say 450 mm width or less the baffle step frequency is about 290 hertz - Home

The size of the indictors and insertion losses make passive baffle step networks impractical for lower frequencies.

Assuming an active filter can accommodate baffle step for a 500 mm baffle width there is no guarantee that you will have an even response from baffle step f3 to the onset of room gain which is un predicable.

In additional the most recent contemporary design practise that incorporates optimisation of total system power response suggests a lift below 135 of some 5 db of shelve filtering.

In other words, depending on the configuration, a clinically flat response (in the zone below 135hertz may not be optimal in terms of subjective evaluation.

Tailoring the system optimisation per the link below has been done with dsp.
with 5 db lift and progressively linear slit in response)

JBL Master Reference Monitor - Page 34