In general, EQ can fix anything as far as frequency response and summation are concerned, as long as (1) there is only one listening position and (2) one is not trying to pull deep nulls. The thing is, some problems have to do with spatial impressions of sound (e.g. stereo image), not just frequency response. I suspect that weird non-minimal-phase acoustic issues such as these tend to not only alter the frequency response, they also tend to mess up spatial attributes (presumably because the brain is confused by the direct sound vs. reflection behavior). EQ can fix the former, but not the latter. This is why I'm trying to find an acoustic solution to replace my EQ solution. (Also I'm hopeful acoustic solutions might also help with the large nulls in the same frequency range, which may have the same root cause.)
Good question. If I flip the allpass filter on and off, I do hear a difference. With the allpass off the bass becomes "thinner" and some bass notes seem to disappear. It's quite content dependent though. This is exactly what one would expect given the measurements. A -6 to -10 dB dip between 80-110 Hz on coherent bass signals is not something that will make a night and day difference on everything, but it's definitely worth fixing.
Well you need to know precisely what you're doing. Allpass filters are a highly specialized tool meant to solve very specific problems and you wouldn't reach for one unless you have to. This is the first time I had to reach for an allpass filter in ~13 years experience calibrating various systems.
Many people would argue that phase response by itself is not audible. I agree with this statement. Phase itself is not the problem… until you end up trying to sum signals that have different phase, in which case it
does become a real problem. Not because of the phase mind you, but because of the resulting cancellation in the magnitude response.
If you apply the same allpass filter on all channels simultaneously, I'd expect you would be completely unable to hear any difference. If you apply an allpass filter on a single channel without a very good reason, you could indeed make it sound much worse. Basically you'd end up taking a system that was (presumably) already summing correctly and destroying its summed response, thus introducing problems where none existed. It's very much a "if it ain't broke, don't fix it" kind of thing.
Yes, this is fully expected of small rooms. However in practice I don't move enough in my chair that this is a significant problem. And even if it was, I'd still want the best average response.
Some people have suggested listening to the system nearfield. In fact I was using these speakers nearfield for ~12 years before I decided to switch to a far-field (well, relatively speaking) system. Part of the reason was precisely to reduce location dependency. Nearfield systems have more consistent bass due to the sheer dominance of the direct sound. But the spatial experience is pretty bad because of the lack of reverb and, more importantly, even small movements of the head significantly affect listener-speaker distances, resulting in a collapse of the stereo image.
Obviously the best solution is to listen to the far field in a large room, which takes care of both problems. But it's a luxury most people don't have when living in central London. Sorry
@restorer-john.
Right. People in this thread might have noticed I quickly tried some of the proposed solutions and ignored others. I'm not picking the suggestions at random. It's a matter of (1) how much hassle/cost to even try the solution, (2) how practical it is as a long-term solution and (3) how likely it is to work.
For example, it was proposed several times to rotate the system by 180°. Rear speaker measurements show this is quite likely to work so it's fine as far as (3) is concerned, but rearranging the room would take me many hours so it's not great with (1) and more importantly it would block my view out the window so it goes against (2) as well. That doesn't mean I won't do it. It just means it's not high on my list right now.
This is why I want to do some simulation work. It would be a fun project and it would greatly help with (1) because it would allow me to try stuff without actually having to do it. For example it might make it possible to determine if tuned bass absorbers would help without having to actually order a whole bunch of them.
If you have multiple subwoofers you should definitely take a look at
MSO. It's a fantastic piece of software that makes it very easy to compute IIR filters for individual subs for optimal summation in a multi-sub setup. I use it even though I only have one sub just for the frequency response optimization algorithm because it's so damn good. It is this algorithm that
automatically optimized my allpass parameters.
This is precisely what this thread is for: to fix the acoustics so that I don't have to do play heroics with DSP which is a giant hack.
The problem is still there with the speakers positioned on the floor. Also a purely vertical mode would affect both speakers equally and would therefore not alter their phase in relation to one another.