Originally Posted by JeffChap
I see this posted a lot, and I’m not questioning it, but I would like to understand it better. Granted, a ported sub is going to have a different frequency response versus one that is sealed. But wouldn’t this be true of any dissimilar subs, regardless of whether one, both or neither are ported? Sound waves are sound waves, and have no knowledge of whether they originated from a ported or sealed sub? The only way to insure a uniform frequency response would be to employ two identical, or at least very similar subs. I don’t see where the fact of being ported or sealed is as much an issue as simply the frequency response of each sub.
Maybe I’m nitpicking. But it seems that there’s a general consensus that you shouldn’t ever mix ported and sealed subs. I’m contending that whether the sub is ported or not is irrelevant, and that what matters is the overall response curve. Theoretically you could have a ported and a sealed sub with much more similar response curves than two different ported subs, could you not?
Am I missing something? I look forward to the experts to educate me.
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I think that it's mostly the phase shift near the port tune, that Alan referred to, but even at higher frequencies, the slope of the frequency response would be different. I think that at low listening levels, both sealed and ported subwoofers would be pretty linear in their responses. But, as you increased the volume of both subs, the sealed sub would start to roll-off much earlier--perhaps around 50Hz with SVS subs. At that point, they would no longer be producing the same SPL, at the same frequencies, and the sound would become somewhat unbalanced.
It's to try to maintain that SPL balance that it is typically recommended to position a less powerful ported (or sealed) sub closer to a listening position, and a more powerful sub further way, as SPL declines with distance. Placement can help to equalize SPL, where one sub is more powerful than the other. But, the shape of the frequency response needs to be the same. And, even then, there could be some unexpected interactions with room modes that might produce unexpected peaks or cancellations, unless the nearfield sub were very
close to the listening position. I believe that explanation is essentially correct, and if not, a real expert like Ed Mullen can correct it.
Another factor is the automated room EQ that most of us use when we calibrate our systems. Room EQ systems, such as Audyssey, attempt to remove peaks and valleys in the frequency response by setting control points to add and subtract SPL at certain frequencies. But, they deliberately don't EQ subs below the combined F3 point of the subs in an audio system. So, if a sealed sub starts to roll-off by 3db at 50Hz, compared to a ported sub, Audyssey will simply stop EQing at that point, and the listener will lose the benefit of EQ below 50Hz where it can typically still be very helpful.
All things considered, it is much better not to mix ported and sealed subs unless you can measure the frequency response and make independent adjustments to it with some outboard form of EQ. And, even then, the results are usually a kludge as Bill said.