I noticed and mentioned this strange problem over a week ago, after I had decided how to do the 3 way system time alignment on the living room system. I do not trust the Delay times computed by the Room EQ Wizard (REW) program for the subwoofer output. I found a different way of doing the time alignment using the summed response, but more about that in a future post. In this post I'm just going to show the problem with the REW computed delay time, which is a very serious issue in my opinion. I can imagine many people using the delay times to time align their system, as I almost did, and it would be wrong, at least if it's like mine. (I don't really know what is causing these weird results, perhaps it's something that mainly applies only to me. As I am writing this post I've completed about six different experiments to try to get REW to calculate the subwoofer delay correctly, but none have worked so far.)
Examining the (computed) step response REW is showing for the subwoofer shows the problem with REW's delay computation.
Notice how the step response begins (that's where it clearly rises above the noise) with a series of humps. The first hump is not as high as the second, and the second one is the tallest. FWIW, the black line, which represents the Schroeder Integral, seems to start pretty much where I would consider the bass step to have begun. But that is not where the computed delay time would lead you.
The computed delay time, which doesn't show up in this graph for no good reason, but rather in an information box for "the measurement" in the REW program, is 12.05 ms +/- 0.042 relative to loopback. (I am taking loopback at the line loop output of the subwoofer, so it is seeing the same signal as the subwoofer amplifier. The crossover inside the subwoofer is turned off, so this signal has already been low-passed with 24dB/octave Linkwitz Riley. I have tried taking full range loopback and it makes no difference.)
If you find 12.05ms on the graph, it is within the rise of the second larger hump, not the first hump.
Now the impulse shown above had a long line of predecessor measurements, which looked basically identical. I first did loopback using the full range output of my preamp, where it's the easiest to tap off the loopback for my 3-way system. If I take the loopback at the speaker inputs of the Acoustats, it doesn't include the bass or super tweeter signals--that didn't work at all when measuring the subwoofer by itself.
So then I got more sophisticated, and took the loopback at the subwoofer itself, which has a plate amplifier. This plate amplifier conveniently already has a balanced "loop output" which I routed bact to my Focusrite interface (it required the attenuator being straight up to not clip) which is also handling the microphone input and stimulus output.
Well then I suspected that the double hump might represent the EQ, of which I am applying 8 additional typically very narrow band and up to -12dB notch corrections to the subwoofer room response.
Not wanting to erase my evolved EQ settings by mistake, I waited until I had time to photograph all the settings in my 3 Behringer units, THEN I saved them to memory (not wanted to do that--which might cause overwriting the current settings by mistakenly selecting "load" rather than "save" until I took the pictures first), then I turned off all the EQ's except for the two HC's which represent the linkwitz riley 24dB crossover. Then I power cycled the Behringer DEQ (because sometimes it doesn't seem to immediately respond to changes in HC or LC filters, until power cycled) with the subwoofer turned off, then I turned the subs back on. That's what the picture above represents: No EQ except the crossover itself.
Disabling HC eq's in the Behringer is tricky. It doesn't want to turn HC or LC filters off when you simply turn them off with the controls. The "cut" EQ's are "sticky" and remain turned on. I tried power cycling and even that didn't always work either. This led to several days of confusing measurements.
Finally I figured out how to shut the EQ's off, AND how to verify the number of HC filters you have enabled (both are important!).
The verification is done by using the REW "generator" function to play a fixed 200 Hz sinewave signal at a convenient level, not too loud. I set it to -16dB. The actual subwoofer should be turned off, but even if it isn't, -16dB isn't bad.
The most foolproof way to ensure there are no HC filters enabled is to use the Memory function to reload the "Initial Data" of the EQ.
After that is done, I go to the Level display, choosing in particular the bargraph which has the greatest resolution. With no EQ active, the 200 Hz tone output level (analog or digital) should be the same dB level, within a few 0.1dB's, of the input level.
When one HC at 100 Hz is enabled, the level at 200 Hz drops about 12dB. When 2 HC's are enabled, the level at 200 Hz drops about 24dB.
Then, the second HC can usually be disabled by turning it off, then pressing the "reset EQ" button in the PEQ second page. If that doesn't work, power cycling might help.
When there is only one HC enabled, turning it off and pressing "reset EQ" doesn't seem to work. In that situation, the working solution is to reload the initial data again.
Until I developed these strategies, I was beginning to fear that that two HC's couldn't be run at the same time because the result did not seem to be a steeper slope than just one. (It turned out, I was not correctly turning off the second one, so I was comparing 2xHC with 2xHC.) Then I feared the subwoofer itself (Ultra PB13) had serious rolloff above 100 Hz even with no HC filters (and the crossover in the subwoofer itself has always been turned off). That resulted from not correctly getting both HC's turned off, which may require that loading the inital data step I just mentioned.
Anyway, when I finally figured out how to set the crossover correctly, and verify it, with all these variations, the subwoofer step response stayed fairly similar, and in every case the "delay time" seems incorrect in about the same way.
Here's the step response of the woofer measured with 24dB/octave crossover again, this time slightly differently but the same as in the next two graphs. Because of how I captured this image from the "Impulse" and not the "Filtered IR" tab of REW, the Schroder Integral is not available. The leftmost red dotted line is showing where the delay ends and the step proper begins (this info is shown in this unfiltered version, but not the "Filtered IR"). It is showing the beginning of the step at the beginning of the second hump, or 12.5ms in this measurement using loopback from the sub loop output.
With the 12dB/octave crossover, the humps look thinner somehow, but the beginning of the step is still being calculated to be at the beginning of the second hump at 12.5 ms.
With NO HC filters (no crossover!) the humps are even narrower, with the first hump turning into two humps, and now the computed delay falls at the beginning of the third hump, at 15.3ms.
In no case does REW compute the delay to begin at the beginning of the first hump, but that's where my other method seems to land for the time alignment with the beginning of the panels and the super tweeters. The step responses for the panels and supertweeters does not have this issue at all, the computed delay begins right where the first hump rises from the noise.
Examining the (computed) step response REW is showing for the subwoofer shows the problem with REW's delay computation.
Notice how the step response begins (that's where it clearly rises above the noise) with a series of humps. The first hump is not as high as the second, and the second one is the tallest. FWIW, the black line, which represents the Schroeder Integral, seems to start pretty much where I would consider the bass step to have begun. But that is not where the computed delay time would lead you.
The computed delay time, which doesn't show up in this graph for no good reason, but rather in an information box for "the measurement" in the REW program, is 12.05 ms +/- 0.042 relative to loopback. (I am taking loopback at the line loop output of the subwoofer, so it is seeing the same signal as the subwoofer amplifier. The crossover inside the subwoofer is turned off, so this signal has already been low-passed with 24dB/octave Linkwitz Riley. I have tried taking full range loopback and it makes no difference.)
If you find 12.05ms on the graph, it is within the rise of the second larger hump, not the first hump.
Now the impulse shown above had a long line of predecessor measurements, which looked basically identical. I first did loopback using the full range output of my preamp, where it's the easiest to tap off the loopback for my 3-way system. If I take the loopback at the speaker inputs of the Acoustats, it doesn't include the bass or super tweeter signals--that didn't work at all when measuring the subwoofer by itself.
So then I got more sophisticated, and took the loopback at the subwoofer itself, which has a plate amplifier. This plate amplifier conveniently already has a balanced "loop output" which I routed bact to my Focusrite interface (it required the attenuator being straight up to not clip) which is also handling the microphone input and stimulus output.
Well then I suspected that the double hump might represent the EQ, of which I am applying 8 additional typically very narrow band and up to -12dB notch corrections to the subwoofer room response.
Not wanting to erase my evolved EQ settings by mistake, I waited until I had time to photograph all the settings in my 3 Behringer units, THEN I saved them to memory (not wanted to do that--which might cause overwriting the current settings by mistakenly selecting "load" rather than "save" until I took the pictures first), then I turned off all the EQ's except for the two HC's which represent the linkwitz riley 24dB crossover. Then I power cycled the Behringer DEQ (because sometimes it doesn't seem to immediately respond to changes in HC or LC filters, until power cycled) with the subwoofer turned off, then I turned the subs back on. That's what the picture above represents: No EQ except the crossover itself.
Disabling HC eq's in the Behringer is tricky. It doesn't want to turn HC or LC filters off when you simply turn them off with the controls. The "cut" EQ's are "sticky" and remain turned on. I tried power cycling and even that didn't always work either. This led to several days of confusing measurements.
Finally I figured out how to shut the EQ's off, AND how to verify the number of HC filters you have enabled (both are important!).
The verification is done by using the REW "generator" function to play a fixed 200 Hz sinewave signal at a convenient level, not too loud. I set it to -16dB. The actual subwoofer should be turned off, but even if it isn't, -16dB isn't bad.
The most foolproof way to ensure there are no HC filters enabled is to use the Memory function to reload the "Initial Data" of the EQ.
After that is done, I go to the Level display, choosing in particular the bargraph which has the greatest resolution. With no EQ active, the 200 Hz tone output level (analog or digital) should be the same dB level, within a few 0.1dB's, of the input level.
When one HC at 100 Hz is enabled, the level at 200 Hz drops about 12dB. When 2 HC's are enabled, the level at 200 Hz drops about 24dB.
Then, the second HC can usually be disabled by turning it off, then pressing the "reset EQ" button in the PEQ second page. If that doesn't work, power cycling might help.
When there is only one HC enabled, turning it off and pressing "reset EQ" doesn't seem to work. In that situation, the working solution is to reload the initial data again.
Until I developed these strategies, I was beginning to fear that that two HC's couldn't be run at the same time because the result did not seem to be a steeper slope than just one. (It turned out, I was not correctly turning off the second one, so I was comparing 2xHC with 2xHC.) Then I feared the subwoofer itself (Ultra PB13) had serious rolloff above 100 Hz even with no HC filters (and the crossover in the subwoofer itself has always been turned off). That resulted from not correctly getting both HC's turned off, which may require that loading the inital data step I just mentioned.
Anyway, when I finally figured out how to set the crossover correctly, and verify it, with all these variations, the subwoofer step response stayed fairly similar, and in every case the "delay time" seems incorrect in about the same way.
Here's the step response of the woofer measured with 24dB/octave crossover again, this time slightly differently but the same as in the next two graphs. Because of how I captured this image from the "Impulse" and not the "Filtered IR" tab of REW, the Schroder Integral is not available. The leftmost red dotted line is showing where the delay ends and the step proper begins (this info is shown in this unfiltered version, but not the "Filtered IR"). It is showing the beginning of the step at the beginning of the second hump, or 12.5ms in this measurement using loopback from the sub loop output.
With the 12dB/octave crossover, the humps look thinner somehow, but the beginning of the step is still being calculated to be at the beginning of the second hump at 12.5 ms.
With NO HC filters (no crossover!) the humps are even narrower, with the first hump turning into two humps, and now the computed delay falls at the beginning of the third hump, at 15.3ms.
In no case does REW compute the delay to begin at the beginning of the first hump, but that's where my other method seems to land for the time alignment with the beginning of the panels and the super tweeters. The step responses for the panels and supertweeters does not have this issue at all, the computed delay begins right where the first hump rises from the noise.
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