At the end of my last week-long session of Time Alignment using ARTA, I made a list of future improvements to put ahead of expriments with linear phase crossovers.
During that "Time Alignment" I also made some adjustments to the bass in the left channel, using only ARTA as a tool. Now ARTA is very good in many ways, but relying only on one kind of measurement to make important adjustments is like trying to map a mountain from one picture window view.
It disturbed me since than that I had probably reduced the bass too much to make it look flat in the ARTA measurement. This seemed to be confirmed when I did my first new iPhone measurements on Sunday July 5. But then I realized I had the weighting switched to A. I switched it to C to make this measurement (though it's actually better if I use "Flat" which is an option in this app, and I remembered to use Flat for all measurements after this one):
The bass below 63 Hz was rolled off more than it appears here. I brought out my oscillator and resumed doing hand sweeps. It was clear the left channel was audibly lower at 30 Hz and below especially, which seemed to almost disappear.
That was relatively easy to correct simply by dialing back a little the cuts I had made when using ARTA. I experimented with changing the position of the wide 3/4 octave cut, there really seem to be two separate resonances involved in addition to a broad area of excess (without EQ). But I finally left the low EQ frequency more-or-less unchanged, because if I set it precisely to the center of the lower resonance, it tends to cut the lowest frequencies too much. So it's at a compromise frequency that works for both resonances AND the whole area pretty well, as currently adjusted.
But I became more and more bothered by the apparent notch at 100 Hz in the 6th octave RTA. I'd "worked" that a little bit before and it seemed fairly intractible. I tried, tried, and tried some more.
I reduced and even eliminated the numerous EQ cuts below and above 100 Hz. That didn't remove the problem, though with a bunch of EQ changes it seemed to be a bit better. Here I was using both hand sweeping the oscillator, AND the RTA app.) Sometimes. It was sounding nearly OK on the hand sweeps. But not really enough better on the RTA, in fact making the surrounding regions more level with more finely tuned EQ seemed to make the notch at 100 Hz look worse.
Clearly something bad is going on right around 100 Hz, which is also the crossover frequency. And clearly it has nothing to do with the EQ's I am applying nearby.
I first tried to get a handle on this by looking just at the bass response, which I figured was a factor:
There is a very little tiny dip around 100Hz, though it's hard to pay much attention to that when other things are not looking good. (BTW, the I left the supertweeters on for these measurements.) For one thing, the crossover doesn't seem to be really crossing over at 100 Hz, more like 140 Hz, and that's with two big notches dialed in to the subwoofer response above 100 Hz already.
I tried to do some more adjustments to fix the 100 Hz dip anyway, but nothing did much good. I began to determine that the 100 Hz dip in the sub response is caused by a room mode. When I've dialed in the frequency at maximum notch cancellation, I can walk around the room and the response varies enormously, going from way too much to just the little dip shown here at the listening position.
That's significant, but really not that bad. A bigger problem must be elsewhere, notably in the Acoustat response (as rolled off by my crossovers and EQ adjusted, but with the supertweeter left on as before):
This doesn't look like it rolls off as much as it should, but the electrical signal as shown by the RTA on my Behringer 2496 DEQ shows the bass rolloff exactly as you would expect from 24dB/octave Linkwitz Riley. The rolloff is flattened acoustically mainly by the high level of ambient noise. So just ignore that. The main feature here is the very large notch at both 100 Hz and 112 Hz. Those notches would look even worse if the ambient noise (from air conditioner and stuff) were removed.
Interestingly enough, I had dialed in a 100 Hz cut in the Acoustats, which I determined necessary from the Right channel on which I spent more time EQ'ing in the recent past. I had that same cut in the Left channel where it makes things worse, primarily because I had only done "stereo link" EQ's for the Acoustats previously. Right here I decided to change that, and removed the 100 Hz cut from the Left channel, but it made little difference (IIRC, I had removed the 100Hz cut just before I took the above measurment. Prior to that, 100 Hz and 112 Hz were more level.)
After playing around with this for far longer than should have been necessary, it finally began to occur to me that this 100-112 Hz notch was being caused by rear wall reflection and cancellation.
Somehow, my intuition had always been that the rear wall reflections would be a factor only at much higher frequencies, such as 220 Hz, or 160 Hz, where they had troubled me long ago.
After continuing to think wrongly about this problem, I finally figured out the right answer. The rear side emission of a dipole radiator is already out of polarity with the front side. Because of that, it has to make an entire wavelength (or none at all) of travel to have a cancellation effect. So the distance from the wall is 1/2 of the wavelength of the cancelled frequency. And not 1/4 of the wavelength, as it would be for a box speaker.
But calculating from the wall distance of the closest point of the speaker doesn't suggest 112 Hz at all. The minimum distance perpendicular to the wall is 40 inches, and at the center of the 2+2 panel it rises to 45 inches. Taking 45 inches as the distance, that would cause cancellation at 146 Hz.
Wavelength = Speed-of-Sound / Frequency
Frequency = Speed-of-Sound / Wavelength
(1100 * 12) / 90
146
But what's happening here is that not all the sound travels that minimum distance. The dipole radiates in the back as the front across a 90 degree arc. Nearly all of the reflected rear wave travels considerably more than 90 inches. 90 inches in the bare minimum, most of the reflected rear wave travels more like 120 inches before it reaches the plane of the speaker, and that is what corresponds to the cancellation at roughly 112 Hz.
Several times last year I moved my electrostats further from the wall to mitigate the cancellation notches. When I reached the current distance, the notch seemed to completely go away in the right channel. In fact I had added an additional cut around 100 Hz because there was too much energy there. I hadn't paid much attention to the left speaker.
I should have paid more attention to the left speaker, which shows the cancellation notch much more clearly, and NO amount of EQ can get rid of it. Even increasing the subwoofer output at 100 Hz doesn't help much because it is also a room mode for the monopole subwoofer.
I further confirmed the cancellation nature of the 100-112 notch in the panel response by measuring all around the room. Unlike the room mode caused by the subwoofer, the larger notch in the panel response is only a little affected in magnitude by listening position. There is no place where it gets louder instead of softer. The notch pretty well sucks out all the signal everywhere. But the center frequency of the notch shifts a bit, possibly because it also interacts with the room mode, OR because the angle from the speaker determines the rear wave travel distance. At the listening position, the notch seems centered around 106 Hz, and on the side further back, it seems to center around 120 Hz.
So I had two problems not amenable to solution with EQ: a room mode (for monopole subwoofer), AND a rear wall cancellation notch. But both are amenable to other solutions.
I was shocked at how much difference moving the listening position a mere 10 inches forwards made in the bass response from the subwoofer only. There is no shortage of 100 Hz from the sub at that position, which I've decided to make permanent at least temporarily. But this may mean way more readjustments of things like the speaker angles. But I've decided to take on that challenge anyway. Actually, the closer position might be better for stereo width anyway.
Fixing the panel response is done by opening another can of worms. But it only makes sense that if a speaker has problems in the 100 Hz range, I should not be using that frequency as my crossover frequency. I should cross over the panels higher then 100 Hz to push the rear wall cancellation effect way below the crossover frequency for the dipoles, not just above it!
Actually, however, I first though of this solution just from the fact that even with the existing crossovers and EQ's dialed in, the subwoofer response seems to naturally roll off around 140 Hz. So, I decided that would be a good place to start, and I then adjusted the panel crossover to have a 140 Hz acoustic crossover frequency, or what looks like one on my RTA.
All my fine-tunings now need to be fine tuned all over again, given that I've changed both the listening position and the crossover frequency. But it looks now like this is the only way to go forwards. What I was doing before doesn't make sense.
Even without additional fine tuning, just crudely readjusting the listening position and the crossover frequency has made the sound much punchier and yet clearer at the same time.
During that "Time Alignment" I also made some adjustments to the bass in the left channel, using only ARTA as a tool. Now ARTA is very good in many ways, but relying only on one kind of measurement to make important adjustments is like trying to map a mountain from one picture window view.
It disturbed me since than that I had probably reduced the bass too much to make it look flat in the ARTA measurement. This seemed to be confirmed when I did my first new iPhone measurements on Sunday July 5. But then I realized I had the weighting switched to A. I switched it to C to make this measurement (though it's actually better if I use "Flat" which is an option in this app, and I remembered to use Flat for all measurements after this one):
 |
| Left Channel, as last adjusted using ARTA |
The bass below 63 Hz was rolled off more than it appears here. I brought out my oscillator and resumed doing hand sweeps. It was clear the left channel was audibly lower at 30 Hz and below especially, which seemed to almost disappear.
That was relatively easy to correct simply by dialing back a little the cuts I had made when using ARTA. I experimented with changing the position of the wide 3/4 octave cut, there really seem to be two separate resonances involved in addition to a broad area of excess (without EQ). But I finally left the low EQ frequency more-or-less unchanged, because if I set it precisely to the center of the lower resonance, it tends to cut the lowest frequencies too much. So it's at a compromise frequency that works for both resonances AND the whole area pretty well, as currently adjusted.
But I became more and more bothered by the apparent notch at 100 Hz in the 6th octave RTA. I'd "worked" that a little bit before and it seemed fairly intractible. I tried, tried, and tried some more.
I reduced and even eliminated the numerous EQ cuts below and above 100 Hz. That didn't remove the problem, though with a bunch of EQ changes it seemed to be a bit better. Here I was using both hand sweeping the oscillator, AND the RTA app.) Sometimes. It was sounding nearly OK on the hand sweeps. But not really enough better on the RTA, in fact making the surrounding regions more level with more finely tuned EQ seemed to make the notch at 100 Hz look worse.
 |
| Left Channel, after raising low bass and fine tuning EQ's around 100 Hz |
Clearly something bad is going on right around 100 Hz, which is also the crossover frequency. And clearly it has nothing to do with the EQ's I am applying nearby.
I first tried to get a handle on this by looking just at the bass response, which I figured was a factor:
There is a very little tiny dip around 100Hz, though it's hard to pay much attention to that when other things are not looking good. (BTW, the I left the supertweeters on for these measurements.) For one thing, the crossover doesn't seem to be really crossing over at 100 Hz, more like 140 Hz, and that's with two big notches dialed in to the subwoofer response above 100 Hz already.
I tried to do some more adjustments to fix the 100 Hz dip anyway, but nothing did much good. I began to determine that the 100 Hz dip in the sub response is caused by a room mode. When I've dialed in the frequency at maximum notch cancellation, I can walk around the room and the response varies enormously, going from way too much to just the little dip shown here at the listening position.
That's significant, but really not that bad. A bigger problem must be elsewhere, notably in the Acoustat response (as rolled off by my crossovers and EQ adjusted, but with the supertweeter left on as before):
This doesn't look like it rolls off as much as it should, but the electrical signal as shown by the RTA on my Behringer 2496 DEQ shows the bass rolloff exactly as you would expect from 24dB/octave Linkwitz Riley. The rolloff is flattened acoustically mainly by the high level of ambient noise. So just ignore that. The main feature here is the very large notch at both 100 Hz and 112 Hz. Those notches would look even worse if the ambient noise (from air conditioner and stuff) were removed.
Interestingly enough, I had dialed in a 100 Hz cut in the Acoustats, which I determined necessary from the Right channel on which I spent more time EQ'ing in the recent past. I had that same cut in the Left channel where it makes things worse, primarily because I had only done "stereo link" EQ's for the Acoustats previously. Right here I decided to change that, and removed the 100 Hz cut from the Left channel, but it made little difference (IIRC, I had removed the 100Hz cut just before I took the above measurment. Prior to that, 100 Hz and 112 Hz were more level.)
After playing around with this for far longer than should have been necessary, it finally began to occur to me that this 100-112 Hz notch was being caused by rear wall reflection and cancellation.
Somehow, my intuition had always been that the rear wall reflections would be a factor only at much higher frequencies, such as 220 Hz, or 160 Hz, where they had troubled me long ago.
After continuing to think wrongly about this problem, I finally figured out the right answer. The rear side emission of a dipole radiator is already out of polarity with the front side. Because of that, it has to make an entire wavelength (or none at all) of travel to have a cancellation effect. So the distance from the wall is 1/2 of the wavelength of the cancelled frequency. And not 1/4 of the wavelength, as it would be for a box speaker.
But calculating from the wall distance of the closest point of the speaker doesn't suggest 112 Hz at all. The minimum distance perpendicular to the wall is 40 inches, and at the center of the 2+2 panel it rises to 45 inches. Taking 45 inches as the distance, that would cause cancellation at 146 Hz.
Wavelength = Speed-of-Sound / Frequency
Frequency = Speed-of-Sound / Wavelength
(1100 * 12) / 90
146
But what's happening here is that not all the sound travels that minimum distance. The dipole radiates in the back as the front across a 90 degree arc. Nearly all of the reflected rear wave travels considerably more than 90 inches. 90 inches in the bare minimum, most of the reflected rear wave travels more like 120 inches before it reaches the plane of the speaker, and that is what corresponds to the cancellation at roughly 112 Hz.
Several times last year I moved my electrostats further from the wall to mitigate the cancellation notches. When I reached the current distance, the notch seemed to completely go away in the right channel. In fact I had added an additional cut around 100 Hz because there was too much energy there. I hadn't paid much attention to the left speaker.
I should have paid more attention to the left speaker, which shows the cancellation notch much more clearly, and NO amount of EQ can get rid of it. Even increasing the subwoofer output at 100 Hz doesn't help much because it is also a room mode for the monopole subwoofer.
I further confirmed the cancellation nature of the 100-112 notch in the panel response by measuring all around the room. Unlike the room mode caused by the subwoofer, the larger notch in the panel response is only a little affected in magnitude by listening position. There is no place where it gets louder instead of softer. The notch pretty well sucks out all the signal everywhere. But the center frequency of the notch shifts a bit, possibly because it also interacts with the room mode, OR because the angle from the speaker determines the rear wave travel distance. At the listening position, the notch seems centered around 106 Hz, and on the side further back, it seems to center around 120 Hz.
So I had two problems not amenable to solution with EQ: a room mode (for monopole subwoofer), AND a rear wall cancellation notch. But both are amenable to other solutions.
I was shocked at how much difference moving the listening position a mere 10 inches forwards made in the bass response from the subwoofer only. There is no shortage of 100 Hz from the sub at that position, which I've decided to make permanent at least temporarily. But this may mean way more readjustments of things like the speaker angles. But I've decided to take on that challenge anyway. Actually, the closer position might be better for stereo width anyway.
Fixing the panel response is done by opening another can of worms. But it only makes sense that if a speaker has problems in the 100 Hz range, I should not be using that frequency as my crossover frequency. I should cross over the panels higher then 100 Hz to push the rear wall cancellation effect way below the crossover frequency for the dipoles, not just above it!
Actually, however, I first though of this solution just from the fact that even with the existing crossovers and EQ's dialed in, the subwoofer response seems to naturally roll off around 140 Hz. So, I decided that would be a good place to start, and I then adjusted the panel crossover to have a 140 Hz acoustic crossover frequency, or what looks like one on my RTA.
All my fine-tunings now need to be fine tuned all over again, given that I've changed both the listening position and the crossover frequency. But it looks now like this is the only way to go forwards. What I was doing before doesn't make sense.
Even without additional fine tuning, just crudely readjusting the listening position and the crossover frequency has made the sound much punchier and yet clearer at the same time.
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