Originally Posted by adam2434
On the subject of L/R and Sub phase alignment at the crossover and the sub distance tweak, I wondering if there are known and predictable speaker/sub(s)/room factors that contribute to better or worse phase alignment at the crossover (larger or smaller dip at the crossover).
I’ve read the following, and the different speaker models tested certainly show differences in magnitude of the dip at the crossover, with the sealed models having a minimal dip.
So, are ported vs. sealed for L/R and the L/R port tuning frequency vs. L/R/Sub(s) crossover frequency perhaps some critical factors for predicting whether there will be a large dip at the crossover?
For example (with Audyssey distances set), with ported L/R speakers, if the crossover is set to a frequency that corresponds to a lot of L/R port output, would the L/R port output be out of phase with the sub(s) output, causing a significant dip in the combined L/R/Sub output in the crossover region?
And, with the same ported L/R speakers, if the crossover were set to a higher frequency, where L/R port output is less, would the combined L/R/Sub(s) output have less of a dip (or maybe no dip) in the crossover region?
I'm sure it is more complicated than this, but just trying to get some fundamental understanding on this topic.
This is pretty deep water for me, but I will try to give you a response. I read what Brian wrote, and it makes sense that ported front speakers could exhibit more cancellation, at a crossover to a sub, than might be the case if they were sealed speakers. But, as you suggested, it is more complicated than that. First, room modes can influence the frequency response in a way that confounds our expectations. We don't always get cancellation where we might expect to have it, and we sometimes do get it where we wouldn't have predicted it.
Second, in an audio system that emphasizes music, we will typically be measuring the frequency response of the front speakers, in combination with the subwoofer(s). But, in a system that emphasizes movies, we will typically use the center channel for that measurement, and any phase adjustment will be based on that speaker/sub combination. So, that becomes another variable to consider.
Then, there is the fact that if we have multiple subwoofers (especially on opposing walls, but sometimes even if they are not) we may experience cancellation between, or among, the subwoofers themselves. In that case, we may actually want to make phase (or distance) adjustments in order to move the area of cancellation up into a frequency played by the front speakers, or by the center channel, so that the other speakers can compensate for the cancellation between subwoofers. Confused yet? I certainly am!
I think that we can only determine for sure whether we have cancellation by measuring our frequency response. The good news is that if the area of cancellation is relatively narrow, the overall effect on our audio will be inaudible, as our brains will compensate for the missing information. Complex sounds will also typically contain both fundamentals and harmonics of specific frequencies, which will help to fill-in missing frequencies in a way that we can't detect.
I probably wouldn't hesitate to select either ported front speakers, or a ported center channel, solely on the basis of potential cancellation, and I probably wouldn't worry too much about predicting potential cancellation, with the subwoofer(s), solely on that basis.
As for whether a higher crossover might reduce cancellation, I think someone would just have to experiment to find out. It very well might, but it would probably depend on how close the original crossover were to the speakers' port tune. And, remember that boundary gain and room modes would influence the speakers' actual roll-off. So, the specified port tune might not precisely correspond to the in-room response.
It's an interesting analysis, and one that might have real-world consequences in a specific situation. But, there are so many other variables involved that I think we would still need to treat the issue of potential cancellation on a case-by-case basis. I don't know if this was exactly what you were looking for, but it's about the best I can do with this complicated subject.