Originally Posted by Schrodinger23
Another thing to take into account is that Audyssey can boost frequencies as much as 9 dB and cut them by as much as 20 dB in trying to hit the target curve. It will first determine how low the subwoofer can play by either the -3 dB or -6 dB point and then try to correct as low as 10 Hz. Here's the problem though: we have no way of knowing which frequencies and by how much Audyssey might be trying to boost by as much as 9 dB. Having multiple subwoofers can reduce this problem, since it should minimize standing wave nulls to some extent, but that doesn't mean that you still can't have a 5 to 10 dB null. So you might want to leave another 5 to 10 dB headroom, if you want to use Audyssey.
So even though I only listen at about -20dB usually, I like listening with about a 5 dB boost and want to leave about 9 dB headroom for Audyssey to boost and I end up with needing a subwoofer capable of about -5 dB or peaks of 110 dB.
I think it's always a good idea to leave ourselves a little extra headroom, as you have suggested. I think that having an extra 3 or 4db available is a good idea in order to prevent compression, or excessive THD, or port chuffing in the case of ported subwoofers, irrespective of whether someone has Audyssey.
But, for others reading along, I thought it might be helpful to clarify a couple of points about Audyssey. First, Audyssey is designed to stop EQing at the F3 point of any transducer, including subwoofers. It measures subwoofers all the way down to 10Hz, and it is capable of setting control points that low. But, once Audyssey determines that a subwoofer is rolling-off naturally by 3db, it will stop setting control points at that -3 point. It would be fairly rare for Audyssey to be setting any control points below ~15Hz unless a subwoofer had a very low port tune, and/or unless there was a lot of room gain.
The second thing that I think is sometimes overlooked in Audyssey discussions is the point that you made about the numerical difference between Audyssey's boosts and it's cuts. The ability to reduce peaks is deliberately greater than the ability to pull-up dips. The intent of the +9 boosts and the -20 cuts is to allow them to average-out over the sub's passband, without
reducing the overall headroom. Since Audyssey is typically not EQing below the -3 point on a subwoofer, there would rarely be any +9 scenarios that would consume excessive headroom, when balanced against the cuts Audyssey was making at other frequencies.
For instance, if Audyssey tried to boost two nulls by a total of +18db, and pulled down three peaks of -6db apiece, depending on the frequencies involved, and the width of the dips and peaks, the net effect might be neutral with respect to overall headroom. This would obviously be situation-dependent, and people would need to perform compression tests both with, and without Audyssey, to be sure about their specific rooms and systems. But, I would not expect Audyssey to routinely reduce subwoofer headroom to any significant degree.
I would never say never, with respect to any of this, but generally speaking, I don't believe that using Audyssey should consume excessive headroom. In fact, I more often see Audyssey trying to reduce the effects of room gain below 20Hz, than I see it adding boosts below 20Hz. And, the low-frequencies would typically be the most significant ones from the standpoint of overall headroom.