I knew I had at least one more day of sweeping and adjusting ahead of me, as when I last left off I had discovered the apparent notch I believed to be at 110 Hz or so in the stereo-correlated pink noise response, was actually being caused by the right channel. I knew that after I fixed the 90 Hz notch in the left channel, after which it was quite flat in bass, the subject of my last post. So any remaining notch had to be in the right channel, even though it was not fully apparent in the right channel response itself.
I went through a series of false theories before zeroing in on the apparent cause, fixing it, and measuring the results.
My first theory was that the notch at 110 Hz was being caused by rear wall reflection and cancellation. Such is my tendency to view results through the prism of my own theories, that my first sweeping 50-150 Hz on the subs seemed to verify this. The sub seemed quite flat. Actually I hadn't turned up the level enough to be very sensitive to changes.
So figured I should fix the problem by boosting the subs to fix the panel response, and I tried that. Sweeping had shown the suckout to be at 120 Hz, somewhat above the crossover point, but still permitting at least 4dB of correction by boosting the subs with a high q resonance at that point.
Indeed, it seemed to help. Only after much further investigation I decided to listen to the panel response without the subs (what I should have immediately done to fully test the backwave cancellation theory). Only then I discovered there was no "hole" in the panel response around 120 Hz. They were smoothly rolling off down to the -6dB at 100 Hz, and that was about all.
Redoing the bass sweep, I finally heard the suckout at 120 Hz. Now I noticed that this frequency was quite modal, and the listening position was indeed reduced about 6dB from the backwall position. I tried fixing this by tuning my eTrap to 120 Hz, which is technically out of it's range, but I tried and got a barely measureable reduction right in front of the eTrap. But it made at most 0.1dB difference at the listening position, so I decided to abandon this correction.
It was sometime after this I noticed something. I had previously dialed in a 140 Hz notch into the subwoofer response. As this was above the crossover frequency, I felt quite cavalier in a huge reduction, possibly not very well tested.
So I tuned out this 140 Hz notch, and it turned out to be entirely unnecessary. There was no large peak in the sub response around 140 Hz, so it was hardly deserving of a 12dB notch.
Removing this pre-existing 140 Hz notch, with a width of 1/3 octave, fixed the problem at 120 Hz and allowed for much flatter response in the right channel and the elimination of the apparent 110 Hz notch in the stereo-correlated response.
I made a new correction at 80 Hz which seemed to be useful and some other fine adjustments. I held to the process of initial adjustment with oscillator sweeping, which seems to help avoid invalid adjustments. Adding in new filters on the basis of pink noise measurement is asking for trouble.
Bottom line: the problem was caused by pre-existing currently (and/or previously) invalid adjustments which are now fixed. The bass is now quite sonically flat in both channels, confirmed by measurements and playing Rebecca Pidgeon's Spanish Harlem.
I've never noticed this before...now there does appear to be a suckout at exactly 1kHz! Worthy of more investigation. I doubt any of today's changes contributed to that.
The stereo uncorrelated bass looks as flat as ever, with just a wiggle around 78Hz I tried to reduce a bit today.
That wiggle around 78Hz, with gradual dips on either side, is simply exaggerated a bit in the stereo-correlated response. That fact and this graph looks excellent.
Here are the Right Channel EQ's that make this possible. I didn't even remember the 60 Hz EQ, but when I removed it sure enough things got worse (and the dip around 55 didn't go away).
I went through a series of false theories before zeroing in on the apparent cause, fixing it, and measuring the results.
My first theory was that the notch at 110 Hz was being caused by rear wall reflection and cancellation. Such is my tendency to view results through the prism of my own theories, that my first sweeping 50-150 Hz on the subs seemed to verify this. The sub seemed quite flat. Actually I hadn't turned up the level enough to be very sensitive to changes.
So figured I should fix the problem by boosting the subs to fix the panel response, and I tried that. Sweeping had shown the suckout to be at 120 Hz, somewhat above the crossover point, but still permitting at least 4dB of correction by boosting the subs with a high q resonance at that point.
Indeed, it seemed to help. Only after much further investigation I decided to listen to the panel response without the subs (what I should have immediately done to fully test the backwave cancellation theory). Only then I discovered there was no "hole" in the panel response around 120 Hz. They were smoothly rolling off down to the -6dB at 100 Hz, and that was about all.
Redoing the bass sweep, I finally heard the suckout at 120 Hz. Now I noticed that this frequency was quite modal, and the listening position was indeed reduced about 6dB from the backwall position. I tried fixing this by tuning my eTrap to 120 Hz, which is technically out of it's range, but I tried and got a barely measureable reduction right in front of the eTrap. But it made at most 0.1dB difference at the listening position, so I decided to abandon this correction.
It was sometime after this I noticed something. I had previously dialed in a 140 Hz notch into the subwoofer response. As this was above the crossover frequency, I felt quite cavalier in a huge reduction, possibly not very well tested.
So I tuned out this 140 Hz notch, and it turned out to be entirely unnecessary. There was no large peak in the sub response around 140 Hz, so it was hardly deserving of a 12dB notch.
Removing this pre-existing 140 Hz notch, with a width of 1/3 octave, fixed the problem at 120 Hz and allowed for much flatter response in the right channel and the elimination of the apparent 110 Hz notch in the stereo-correlated response.
I made a new correction at 80 Hz which seemed to be useful and some other fine adjustments. I held to the process of initial adjustment with oscillator sweeping, which seems to help avoid invalid adjustments. Adding in new filters on the basis of pink noise measurement is asking for trouble.
Bottom line: the problem was caused by pre-existing currently (and/or previously) invalid adjustments which are now fixed. The bass is now quite sonically flat in both channels, confirmed by measurements and playing Rebecca Pidgeon's Spanish Harlem.
 |
| Right Channel |
I've never noticed this before...now there does appear to be a suckout at exactly 1kHz! Worthy of more investigation. I doubt any of today's changes contributed to that.
The stereo uncorrelated bass looks as flat as ever, with just a wiggle around 78Hz I tried to reduce a bit today.
Here are the Right Channel EQ's that make this possible. I didn't even remember the 60 Hz EQ, but when I removed it sure enough things got worse (and the dip around 55 didn't go away).
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