# Filtered impulse response



## geopango (Jun 24, 2012)

I've only just discovered the benefits of applying filtering to IR measurements. 
Setting the filter point as close as possible to the preferred crossover point you can very quickly nudge the waveforms into alignment. 
In the phase plots this filtered IR alignment at the crossover point will also result in the closest phase tracking.
Cheers
G


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## jtalden (Mar 12, 2009)

Interesting! I hadn't noticed that before. Well Done. :T

It does work reasonably well. It adds another way to look at and adjust the XO timing. 

It seems to add an extra 360° phase rotation at the XO, but that does not impact its utility in aligning the phase tracking. We just need to keep in mind that the actual phase rotation is distorted by the filter, but that tracking between the 2 drivers is still representative. 

Thanks for sharing!

If you have the time an example may be helpful to members to understand the process.


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## subterFUSE (May 10, 2014)

Please explain how this is done. I'm planning to tune my new DSP in my car later this week, and would like to incorporate this if possible.


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## geopango (Jun 24, 2012)

Ill post graphs and instructions for you when I get home this evening.


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## geopango (Jun 24, 2012)

Hi there. Hope some of the information below can help.
I have performed multiple sweeps from a distance of 1meter, in increments of 0.1ms.
We're looking at 12" bass and 4.5" bass midrange here crossed over at 400 hz with LR8 filters. My processor is the Behringer DXC.
The overlays below include impulse response, wrapped, unwrapped phase and SPL.









As you can see, the impulse response of the bass driver exhibits a very broad wave form making it somewhat confusing as to where exactly the mid should line up.
The two phase plots give a better picture and show that I'm getting close to alignment.
















In order to render a filtered IR view go to the Filtered IR tab and apply filtering to each impulse to value of your crossover point.
Now go to your overlays view and examine how the IR waveforms have changed. As you can see from the graph the filtered Impulses now align more symmetrically. 










From this point adjust your time delay settings so as to achieve the closest possible alignment.









This alignment will result in the closest possible phase tracking at the cross over point as is demonstrated in the phase plots.
















Now remove the filtering on the IR and go back to the normal IR view. You will note that the mids properly align with the bass waveform at the very beginning of the impulse, not at the peak.









I've attached an spl overlay








Take note of the following points.

1- Any adjustment to your crossover point will require you to re-measure and realign. Changing your crossover point changes the delay on the driver. The same can also be said for slope types which add or reduce delay as adjusted.
2- Always choose a crossover point with good spl support either side of the band pass.


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## jtalden (Mar 12, 2009)

Thanks geopango! Nicely done. :sn:

I will just add some additional comments that may be helpful in some circumstances. 

The phase tracking data can become much more difficult to read than this example if the IR offset delay is greater, or the XO freq is higher, or the mic is at the LP rather than near field. The overlay of the IRs will still be reasonably similar to read however so the phase chart is not really required.

If we do need to read the phase more clearly in those conditions it is helpful to:
> Manually shift the IRs back to near 0ms using the same offset value for all IRs. It's important to shift both driver IRs the same amount as this keeps the relative phase relationship of the 2 drivers.
> Use no smoothing or 1/48 smoothing for the best readability of the phase.

Below is a difficult example using my FL speaker MW/TW 1700Hz XO measured at my 13 ft LP. IRs delays are significant at roughly 94.15ms.









Below, when the IRs are filtered with 1600Hz, 1/3 octave, the close IR overlay is apparent. This to enough to confirm the close phase tracking through the XO.









Below, the phase chart is unreadable however.









Below, the IRs are now both shifted exactly 94.15ms to place them near 0ms.









Below, the close phase tracking can now be seen reasonably well when no smoothing is applied. That is, it's barely readable if we are experienced with reading phase charts with lots of reflections that tend to hide/smear the direct sound phase.









Below, the IRs are windowed as shown in order to eliminate as much of the late arriving reflections as possible while keeping enough of the early IR response as possible to still see some detail. The window settings were adjusted via trial and error. We can see the phase path is similar to that in the unwindowed phase chart above. Had the measurements been done with the mic at 1m, the chart would be very smooth.









Just additional hints for those dealing with reading phase tracking charts in difficult circumstances.


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## subterFUSE (May 10, 2014)

This is brilliant! Thanks so much for the explanations. I look forward to delving into this on Thursday-Saturday with my new Helix DSP Pro. It has time alignment in 3.5mm steps and Phase Angle adjustment in 11.5 degree steps.

I do have a question about measuring mic location. You mention the mic at 1m yields cleaner phase plots due to reduced reflectional influence. But isn't it more important to measure at the LP to properly dial in the time alignment?


Sent from my iPhone using Tapatalk


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## jtalden (Mar 12, 2009)

In a car there may be no other good options. The LP is a safe choice.

For a small home "bookshelf" size speaker as I measured above, both drivers are closely stacked on the baffle in the traditional way. In that case we could have moved the mic closer while keeping it on the listening axis. At 1m or even a little closer the relative delay timing would still be accurate. We are just removing some of the excess distance/delay without changing the relative delay needed between the 2 drivers. The closer mic position reduces the magnitude of the reflections and results in a much cleaner phase chart. The closer the mic is placed though, the harder it is to be sure the mic is still on the listening axis. 

Often SWs are placed away from the mains and thus the LP is the only option for the mic placement. That is why I added the hints about how to clean up phase charts when needed. 

Just choose the mic position appropriately for the situation.


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## AudiocRaver (Jun 6, 2012)

Wizards at work!

You guys knock me out!

geopango and jtalden, great work, another reference thread for sure.

Edit:
For those who are following along, here are some interesting links:

jtalden's Alignment Driver Phase reference guide

similar threads:
Time Alignment
Understanding Phase Graphs


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## subterFUSE (May 10, 2014)

geopango said:


> Take note of the following points.
> 
> 1- Any adjustment to your crossover point will require you to re-measure and realign. Changing your crossover point changes the delay on the driver. The same can also be said for slope types which add or reduce delay as adjusted.
> 2- Always choose a crossover point with good spl support either side of the band pass.



Just to be clear, we need to *pick a crossover point first* and then measure as above to determine the delay?

Or should we get a ballpark delay, and then set crossover and fine tune the delay as above?



My situation is a little different because it's a car, with listening position in the driver seat. So I need some large delays on the left midbass and the subwoofer in particular. Furthest speaker is my right compression horn under the passenger side dash.


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## Talley (Dec 8, 2010)

this is way over my head.... i wish i knew what all this meant ha


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## geopango (Jun 24, 2012)

subterFUSE said:


> Just to be clear, we need to pick a crossover point first and then measure as above to determine the delay? Or should we get a ballpark delay, and then set crossover and fine tune the delay as above? My situation is a little different because it's a car, with listening position in the driver seat. So I need some large delays on the left midbass and the subwoofer in particular. Furthest speaker is my right compression horn under the passenger side dash.


The selection of the correct crossover point based on the most linear frequency response of the two drivers combined is paramount. Ensure that both drivers exhibit excellent linearity before and after the crossover point. This attention to spl will allow the best phase matching and well matched filtering after the pass band.
My bass and mid drivers are very flat between between 250 and 500 Hz. I've chosen 345hz as crossover point providing excellent linearity above and below the crossover point.
Cheers


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## subterFUSE (May 10, 2014)

When you say you took multiple measurements at .1ms increments, do you mean you measured... then adjusted the DSP delay for a driver by .1ms, and measured again, then repeat, etc..?


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## geopango (Jun 24, 2012)

subterFUSE said:


> When you say you took multiple measurements at .1ms increments, do you mean you measured... then adjusted the DSP delay for a driver by .1ms, and measured again, then repeat, etc..?


You just keep measuring and changing the delay incrementally till the phase align at the crossover point. You can theoretically do this just once. You can make a copy of that measurement , nudge it into alignment in REW, then subtract the time difference between the two measurements. That delay amount can then be applied on your DSP. It works.


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## subterFUSE (May 10, 2014)

When we are trying to do this at a higher crossover point from the LP, and we get a phase chart that looks like this:












Is it OK to use the Unwrap Phase button to view it that way?


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## geopango (Jun 24, 2012)

You really have to zoom right into the crossover point and examine how the wrapped and unwrapped phase aligns. You can then apply filtered IR for each measurement approximately at the crossover point. In overlays, view the alignment of IR and adjust delay accordingly till the wave forms sit perfectly on top of each other .
Good luck.


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## subterFUSE (May 10, 2014)

So I definitely understand that the crossover point must be set first because that will affect the timing.

What about EQ? If there will be a lot of channel-independent EQ applied, should the EQ be done before or after using this approach to adjusting time alignment/phase?


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## jtalden (Mar 12, 2009)

subterFUSE said:


> When we are trying to do this at a higher crossover point from the LP, and we get a phase chart that looks like this:
> 
> Is it OK to use the Unwrap Phase button to view it that way?


 No, that will not normally help. When the IR is shifted very far away from 0ms there is just too much phase rotation to read the chart. Any reflections in the chart causing phase rotations also make the phase unreadable when unwrapped. The best action is follow the advice in Post 6 when the chart looks like this.


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## jtalden (Mar 12, 2009)

subterFUSE said:


> So I definitely understand that the crossover point must be set first because that will affect the timing.
> 
> What about EQ? If there will be a lot of channel-independent EQ applied, should the EQ be done before or after using this approach to adjusting time alignment/phase?


 For a new setup I would prefer to:
> Set the XO freq and filters
> Set the delay via; phase tracking, or maximum SPL in the XO range, or this new filtered IR method.
> EQ to the house curve.

That said, having at least rough EQ in place to the house curve makes it easier to set the delays as the IR and phase charts are a little cleaner. I just leave the any old EQ in place if I decide to change XO settings and delays. Then revisit the EQ settings as needed. There is nothing wrong with adjusting back and forth as needed to assure that the best combination has been found.


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## subterFUSE (May 10, 2014)

First of all, thanks so much for all of the guidance. I am having a lot of fun learning about this subject, and have spent a fair amount of time testing things. Although I'm still not achieving good results using this method, I think my environment being a car is largely to blame. Unequal path lengths to the different speakers from the LP in the driver's seat definitely make time alignment and phasing tricky.

I do have more questions:

My subwoofer to Midbass Xover point is currently 90 Hz. I have tried using Filtered IR at both 80Hz and 100Hz. Is there a preference for which filter to use when the xover point is between 2 filters?

Same is true for my horns to midbass transition, which currently is at 900 Hz.
Do I use 800Hz, or 1000Hz filter?

I have noticed that using the filtered IR method for my time alignment has yielded some wildly different results than expected when I take rough measurements of the distance to each speaker with my measuring tape. I have also not yet achieved good results with getting the proper phasing using this method. While the phase charts look correct, my ears are certainly telling me otherwise. My results have been better setting things by ear with pink noise.


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## jtalden (Mar 12, 2009)

I can imagine a car environment is very different and challenging for measuring phase. I am a little concerned however regarding your comment on your ear indicating a phase problem. Most if not all reputable references to driver phase differences for a room setup indicates phase itself is a non issue. There apparently may be an extremely small detectable impact in a midrange XO with some signals. This presumes that there isn't a gross difference in time (several wavelengths) and the SPL has been controlled. I'm thinking that what you are hearing is not because of phase differences between drivers but something else.

We do know that we are more sensitive to SPL variations and those can be caused by modes, reflections, diffraction as well as phase differences between drivers. 

We also know it is difficult to find a good EQ scheme that corrects issues that are correctable and avoids those that are not.

I'm just sharing my thought that you may be focusing on the wrong issue. It may be more related to car acoustics, speaker/LP placements, EQ related, or ??

To your question of which filter to choose: I would think either choice would work fine in room environment. If this method is not working in your case it should be good to just enter the delays from distance measurements and make any minor corrections based on achieving the smoothest SPL in the XO range.


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## geopango (Jun 24, 2012)

Good luck with you quest. Personally I wouldn't go anywhere near attempting this kind of measurement in such a chaotic environment where probably 60% of what you hear comes from reflections.


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## subterFUSE (May 10, 2014)

Here are some IR measurements I just took. I have not messed with the Filtered IR on these yet.

Here are the physical measurements to the LP micropone using a tape measure:


R HLCD = 73"
L HLCD = 62"
R Midbass = 61"
L Midbass = 46"
Subwoofers = 38"

My Right Horn-Loaded Compression Driver (HLCD) is the furthest speakers from the LP @ 73"


I've attached an MDAT file with the 0ms delay measurments for each driver, and then I used the unfiltered IR to delay everything so that the initial rise of the IR closely matches the Right HLCD.

Obviously the subwoofer is almost impossible to align this way due to the size of the IR vs. the other drivers.


Does this look like I am at least in the ballpark?


After this I can apply some crossovers and then try using the filtered IR technique again.


Thanks in advance.


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## jtalden (Mar 12, 2009)

I wouldn't have confidence with any analysis based on this data. The problem is there is nothing to align until the XOs are chosen and active. The XO filters change the needed delays so any delay settings done without XOs in place doesn't help. If XOs had be applied your delays were correct for a reasonable RTA alignment starting point. You could just make that change and then set the delays in your preferred manner.

I would also recommend full range sweeps for all drivers. The XOs protect them and with full range sweeps the timing accuracy will be assured. [I am unsure if REW will always maintain relative timing accuracy when the sweep ranges are so different. I haven't really found a problem, but worry that SW sweeps may not always align at the same trigger point as a HF sweep. This concern may not be justified.]

Other changes that may help when setting the timing: 
> The SPL response is extremely rough it would be better to apply some basic EQ to the drivers to remove the largest peaks to help smooth the response a little. The ragged response can make it more difficult to determine phase timing.

> I also would bring down the SW level to nearer the MW level. The large SPL discrepancy is a minor irritant when using trace arithmetic. If you don't use trace arithmetic method this probably makes no difference.

After the timing is set then the Levels and EQ can be reset as needed to fit your house curve. Those changes will not disrupt the XO phase timing.

If you want me to provide recommended delays for your setup I will do that. To do that I would need new measurements.
> Apply the XOs
> Smooth the drivers SPL a little by removing the larger peaks.
> Use the REW loopback timing feature as before.
> Set all delays to zero
> It would be helpful to reduce the SW level to nearer the MW. [Scaling it in REW does not help.]
> Sweep each driver full range


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## subterFUSE (May 10, 2014)

jtalden said:


> I wouldn't have confidence with any analysis based on this data. The problem is there is nothing to align until the XOs are chosen and active. The XO filters change the needed delays so any delay settings done without XOs in place doesn't help. If XOs had be applied your delays were correct for a reasonable RTA alignment starting point. You could just make that change and then set the delays in your preferred manner.
> 
> I would also recommend full range sweeps for all drivers. The XOs protect them and with full range sweeps the timing accuracy will be assured. [I am unsure if REW will always maintain relative timing accuracy when the sweep ranges are so different. I haven't really found a problem, but worry that SW sweeps may not always align at the same trigger point as a HF sweep. This concern may not be justified.]
> 
> ...


Here are some new measurements with XOver and EQ applied.
Mic at LP, Driver seat.
All drivers @ 0ms delay.
No phase adjustments.
Everything confirmed wired in correct polarity.


Thanks for the help!


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## jtalden (Mar 12, 2009)

Okay, here is the timing I suggest:

SW (Inverted) = 0ms
L MB = 6.643ms
R MB = 5.465ms
L HLCD = 5.715ms
R HLCD = 4.966ms

The SW polarity needs to be inverted and set at 0ms delay. The other drivers to be delayed as indicated.

> This will achieve close phase tracking through the 50HP/55LP lower XO and will result in good SPL support.

> I aligned the direct sound phase of the left channel 892LP/779HP XO and right channel 892LP/689HP XO without regard to the reflections. The right channel results in good SPL support through the XO. The left channel results in a deep null right at the XO due to a very strong reflection in the L HLCD. I do not know if the very unusual XO settings contributed to the issue.I would not be concerned that there is a null in that XO as there are several others scattered around the area in both channels anyway.

What was the reasoning for the overlapped and asymmetry of the XO settings? Is this something that is done in car setups?

Is locating the listening position at the driver also typical of car setups? I would have guessed better results might be achieved with a mic placement centered between the front seats and then EQ averaged in that area. I have no experience nor done any reading on the subject though.


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## subterFUSE (May 10, 2014)

jtalden said:


> Okay, here is the timing I suggest:
> 
> SW (Inverted) = 0ms
> L MB = 6.643ms
> ...


This is great. Thanks for the work you put into this. I can't wait to give these delays a try.

I actually have the ability in my DSP to adjust the phase angle in 11.5° steps. So I can invert the sub 180 degrees, or I could rotate the phase to almost any angle desired.



> What was the reasoning for the overlapped and asymmetry of the XO settings? Is this something that is done in car setups?


I was trying for an acoustic crossover of 900 Hz for the horns. Using REW's EQ section and a house curve, I set the crossover for 900 Hz @ 24 dB slope and then measured sweeps at different crossover settings in the DSP until I found the setting that made the response follow most closely to the target line. In this case, that meant a different crossover frequency point for the right horn vs. the left.

The horns can play lower than 900Hz, but when I was tuning by ear I noticed that below 900Hz the horns began to pull the stereo image lower. With an 800Hz crossover, pink noise samples were coming from the dash below the air vents. When I moved the crossover to 900Hz they moved upward to the center of the windshield where I want them.

I might need to play with the crossover points some more to get them right. Do you think I have too much overlap with the midbass?




> Is locating the listening position at the driver also typical of car setups? I would have guessed better results might be achieved with a mic placement centered between the front seats and then EQ averaged in that area. I have no experience nor done any reading on the subject though.


Yes, driver seat as the LP is common for car audio. The asymmetrical distances to the speakers make it difficult to achieve good stereo imaging for both passengers simultaneously. It can be done, but installation locations of speakers becomes a huge deal for 2 seat listening.

With 1 seat listening, DSP with time alignment can be used to overcome the less-than-ideal speaker placements in a car. But when you use time alignment to fix the stereo imaging for 1 person, it usually has a detrimental effect for the other seat.


When I am measuring for frequency response, I usually take 6-8 measurements around the head area and then average them together. But for time alignment, I just used a stationary microphone at the center of when my head would be. I figured taking an average for impulse response TA would probably make things more difficult.


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## jtalden (Mar 12, 2009)

Setting a variable phase control to 180° is not the same effect of reversing the polarity. If the DSP does not provide an option to invert the polarity then just reverse the wire connection at the SW driver or at the SW P-Amp. The variable phase control should be left set to 0°.

There is nothing wrong with setting XO filters to under-lap or even overlap if careful experimentation shows that is advantageous. I was just wondering how you tested it. Unless phase is accounted for in the experiment it may be misleading. I cannot really help with that question as there is significant work to setup each timing correctly to properly evaluate different XO settings. 

Setting the XO filters differently on the 2 channels is something I have never done. It is considered good practice in having a symmetrical room, identical XOs, and even similar EQ for both channels in the MF and HF. With your car situation it's possible there is something to gain in having XO differences. I wouldn't know.

I agree that an average near the LP for EQ purposes is a good idea, particularly so when there are so many strong reflections.


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