# Allpass filter help



## jtrosky (Jul 19, 2019)

I was hoping that someone can help me understand allpass filters a littler better - basically, how to identify when/where an allpass filter may be beneficial and how to determine the allpass parameters to use in when/where needed (1st order, 2nd order, Q, etc).

Most of the info I find when I search for allpass filters is in regards to audio mixing, when is a different use-case, I think.

I use a Helix DSP.3, which does aupport allpass filters. I just don't know when/where to use them or even if I should use them.

Thank you!


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## Mauian (Jul 25, 2019)

jtrosky said:


> I was hoping that someone can help me understand allpass filters a littler better - basically, how to identify when/where an allpass filter may be beneficial and how to determine the allpass parameters to use in when/where needed (1st order, 2nd order, Q, etc).
> 
> Most of the info I find when I search for allpass filters is in regards to audio mixing, when is a different use-case, I think.
> 
> ...


Yes! I am also wondering about this. This would be a great topic for a video or really anything else 


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## opekone (Mar 24, 2020)

Starting at post 41 I show the phase response in the DSP. I then offer some resources to help understand how to apply filters to correct it. I'm sure someone can correct my mistakes.









2018 Subaru WRX budget build


Nice work. Tighten up those front doors... If you insist! Ordered some Acoustimac 1" sheets, it's a recycled paper insulation that just edges out Owens Corning below 250hz. Perfect for doors! ECO-CORE Eco Friendly Insulation (4 lbs/ft) 48"x24"x1" -Case of 6 It seems like it's only going to...




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## jtrosky (Jul 19, 2019)

Just to clarify, the phase lines shown in the Helix are not the actual "measured" phase in the car. That is just showing you how the phase changes with the EQ changes that you've applied. So just because the phase lines in the Helix PC-Tool app "line up", that has nothing to do with the actual phase you're getting in your car. 

I don't think there is an easy way to measure phase with inexpensive gear. Maybe via sweeps? I've never really tried. I just know you can't really measure phase with pink noise. I do believe that you can see phase issues when measuring multiple speakers together (if you see peaks or dips in a multi-speaker measurement that don't exist with the single-speaker measurements), but that isn't actually measuring phase itself.


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## opekone (Mar 24, 2020)

jtrosky said:


> Just to clarify, the phase lines shown in the Helix are not the actual "measured" phase in the car. That is just showing you how the phase changes with the EQ changes that you've applied. So just because the phase lines in the Helix PC-Tool app "line up", that has nothing to do with the actual phase you're getting in your car.
> 
> I don't think there is an easy way to measure phase with inexpensive gear. Maybe via sweeps? I've never really tried. I just know you can't really measure phase with pink noise. I do believe that you can see phase issues when measuring multiple speakers together (if you see peaks or dips in a multi-speaker measurement that don't exist with the single-speaker measurements), but that isn't actually measuring phase itself.


I expand on that quite a bit in the link.

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## opekone (Mar 24, 2020)

Bueller.

I'll stick this here:


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## dumdum (Feb 27, 2007)

jtrosky said:


> Just to clarify, the phase lines shown in the Helix are not the actual "measured" phase in the car. That is just showing you how the phase changes with the EQ changes that you've applied. So just because the phase lines in the Helix PC-Tool app "line up", that has nothing to do with the actual phase you're getting in your car.
> 
> I don't think there is an easy way to measure phase with inexpensive gear. Maybe via sweeps? I've never really tried. I just know you can't really measure phase with pink noise. I do believe that you can see phase issues when measuring multiple speakers together (if you see peaks or dips in a multi-speaker measurement that don't exist with the single-speaker measurements), but that isn't actually measuring phase itself.


You can indeed use pink noise to measure phase but only with certain software and hardware... I use a Dayton xlr mic and a second hand interface I bought 👍🏼 smaart and the like use pink noise to measure live phase, basically it can see the phase of the noise looped back into the sound card and also the phase that’s sent back by the mic, so it can do big amounts of maths on the fly and compare the two to give phase information

as for all pass filters, you can’t use an all pass to correct a single speaker where a reflection is causing a phase issue (anything you do the direct sound will also be applied to the reflected sound which is causing the cancellation)

what you can apply them to is when one sides phase issue causes a big dip at the listening position when playing with another speaker (or as in nicks example when playing through the crossover region with another driver... however id rather avoid phase anomalies in the crossover region if I can by adjusting crossovers up or down), in that situation you can sometimes apply an all pass to the troubled side/driver and adjust the phase so it’s back in phase with the opposite side or neighbouring driver when played together

it’s a little bit hit or miss if you don’t have phase information to see that you know is also good and accurate

I have several pictures of phase traces which show issues of varying sizes... just not on my phone right now...

All pass filters are very powerful, however I will also add that they are very easy to mess up with 👍🏼


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## Mauian (Jul 25, 2019)

This thread was fairly useful on APFs.









lets talk about allpass filters?


I know how they work..basicly..and I know how they can help with the crossover points and improve dispursion in pro audio, but where do you guys see there effectiveness in a car? now that we have micro adjustable delays, are they redundant/just a different approach? I want to learn how to...




www.diymobileaudio.com





After some reading, as I understand them they are a way to let frequencies “pass” through with their gain untouched but change the phase for those frequencies. Pretty cool!

It seems they are very useful when correcting a specific range of frequencies in a car that are out of phase with another speaker. (As dumdum said)

I found it helpful when checking crossover region when the phase issue can’t be corrected with a simple flip of speaker polarity for example.

I actually just ran into a use case last night.

I was tuning my car and checking phase alignment as in Nicks video above. It wasn’t summing properly (see green line below):










You can see that part of the crossover region summed well but the lower frequencies did not.

I tried adjusting the gains on the individual drivers which did not help. A simple flip of polarity did not help (it made it worse).

I decided to try an all pass filter (APF) and found that a second order APF at 160 Hz and Q of 2 worked nicely. Second order means it changes the phase by 180 degrees which seems most useful when it appears you have a focused area of cancellation going on as above. First order means 90 degrees and I think would be more helpful if you’re noticing a regular repeating pattern of cancellation or constructive interference (regular peaks and valleys) such as in a comb filter (but please correct me if this is not correct).

I assume Q sets the “width” of frequencies for which the APF is applied as in other filters. 

As dumdum said, this was done with trial and error as I’m only using REW with UMIK-1. 

Here is the plot with the filter in place:









You can see it is summing nicely now (the orange line).

Pretty powerful stuff.

Interestingly, I didn’t need an all pass filter with the GS25 installed but needed one with the HAT Unity U2. Not sure why it’s happening in a speaker specific way. Maybe an issue with the individual driver tunes is causing it? Maybe it’s due to the size difference in the drivers? (Between the GS25 and U2)?


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## jtrosky (Jul 19, 2019)

Can't you measure phase with REW using sweeps (instead of pink noise)? I could have sworn that I read that somewhere. Haven't spent the time to look into it any further yet though.

I would just love to be able to actually _measure_ phase without needing $1k software (Smaart), if possible. Would be nice to _know_ for sure what are phase issues (vs reflection issues or some other issue). Maybe it's just not possible though.

Every time I''ve tried to use an allpass filter to correct a dip that exists when playing multiple speakers together (that doesn't exist when playing each of the speakers individually), the allpass filter didn't change anything at all, so based on that, I'm _assuming_ that the issue(s) I'm talking about aren't actually phase issues, but probably reflection-related issues?


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## Truthunter (Jun 15, 2015)

The "Measure" function (sweeps) in REW will produce phase plots. I have not tried it for myself. I've been told by people with way more experience than me that it doesn't produce consistent results.


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## preston (Dec 10, 2007)

> I decided to try an all pass filter (APF) and found that a second order APF at 160 Hz and Q of 2 worked nicely.


I'm curious - you made a nice improvement in a line on the graph. Do you think you can detect this with your ear ? If you defeat the all pass filter does it even sound different ? does it sound better or worse ? 

I too am interested in how all phase filters are used - personally when i see all the lines on REW I'm happy to just get a decent response, I don't think my results are consistent enough or my understanding coherent enough to properly apply such. And then I start wondering if it makes any difference in the music ? Just curious what listening revealed vs REW plots.


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## Mauian (Jul 25, 2019)

preston said:


> I'm curious - you made a nice improvement in a line on the graph. Do you think you can detect this with your ear ? If you defeat the all pass filter does it even sound different ? does it sound better or worse ?
> 
> I too am interested in how all phase filters are used - personally when i see all the lines on REW I'm happy to just get a decent response, I don't think my results are consistent enough or my understanding coherent enough to properly apply such. And then I start wondering if it makes any difference in the music ? Just curious what listening revealed vs REW plots.





preston said:


> I'm curious - you made a nice improvement in a line on the graph. Do you think you can detect this with your ear ? If you defeat the all pass filter does it even sound different ? does it sound better or worse ?
> 
> I too am interested in how all phase filters are used - personally when i see all the lines on REW I'm happy to just get a decent response, I don't think my results are consistent enough or my understanding coherent enough to properly apply such. And then I start wondering if it makes any difference in the music ? Just curious what listening revealed vs REW plots.


Yes, but it was subtle. It’s a difference of about 2-3 dB at those frequencies so just barely perceptible. Making sure the crossover region sums nicely is definitely noticeable with getting the bass upfront, however (for between sub and mid bass). It’s something that you may not notice until you get it right and then you’re like “oh, THAT’S what upfront bass can sound like”


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## ckirocz28 (Nov 29, 2017)

jtrosky said:


> I was hoping that someone can help me understand allpass filters a littler better - basically, how to identify when/where an allpass filter may be beneficial and how to determine the allpass parameters to use in when/where needed (1st order, 2nd order, Q, etc).
> 
> Most of the info I find when I search for allpass filters is in regards to audio mixing, when is a different use-case, I think.
> 
> ...


View "predicted phase" tab in REW's "overlays" window, select only the speakers you want to match, unwrap the phase (gear icon, top right), open any speakers eq filters window, add an all pass filter, adjust it and watch what happens. The all pass filters will increase the slope of the phase (as the freq increases), so you will want to leave the steepest slope as-is and try to match the rest.


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## ckirocz28 (Nov 29, 2017)

jtrosky said:


> Can't you measure phase with REW using sweeps ?


Yes!
I should have mentioned this in my previous post. Hanatsu's REW guide shows how to get useful measurements from multiple sweep measurements.


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## jtrosky (Jul 19, 2019)

ckirocz28 said:


> Yes!
> I should have mentioned this in my previous post. Hanatsu's REW guide shows how to get useful measurements from multiple sweep measurements.


Thank you! I had forgotten all about that guide (it's probably where I see that it could be done!).  I did find a .pdf version of it on my Google Drive, so here is a link to it in case anyone is interested in reviewing it. I'll have to revisit it myself and try the phase-related stuff. I haven't done any sweep-based measurements yet.

Anyway, here is a link to the Hanatsu REW Guide in PDF format from my Google Drive:
Hanatsu's REW Guide.pdf


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## opekone (Mar 24, 2020)

Oh my lord that's just beautiful 

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## jtrosky (Jul 19, 2019)

Ok, I _think_ I'm finally making some progress with Allpass filters!  Hopefully, I'm using them like I'm supposed to. Here are some pictures...

This first picture is of my door and dash speakers - individual left and right measurements (Red and Blue) and the summed L+R response for each speaker pair. Notice the significant dips in the L+R combined measurements at approx 200hz, 400hz, 600hz (door speakers) and 700hz (dash speakers). There are a few other areas that could be fixed with the dash speakers, but let's stick with the 4 areas for now.












So then I tried using allpass filters at those 4 locations - and guess what? A _major_ improvement! Completely removed those significant dips without using any EQ whatsoever!









Is this a proper use of allpass filters? Does the fact that I have the phase-induced dips to begin with tell me that something is wrong with time alignment? In this case, I put the allpass filters on the LEFT speaker in both cases.

When I originally tried using allpass filters for this, I was putting the allpass filters on both the left AND right speakers, so I think they were basically cancelling each other out - which is probably why they did absolutely nothing. 

However, I still have an issue that I don't quite understand.... Even though the L+R door and dash speakers are summing good now without any big dips, look what happens at the crossover (500hz) when playing door, dash and sub all at the same time:









The crossover itself seems to be summing well, but not sure why I have those deep dips on each side of the 500hz crossover?! Any ideas on that?

All images at 1/12 smoothing in order to show the dips well. Smoothing too much tends to "hide" the dips.

Thank you!


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## Mauian (Jul 25, 2019)

At 400 and 600 it looks like your left and right are a bit off. I’m wondering if you were able to match those up better would those dips improve?

I noticed that in some cases if I could match the L and R a bit better sometimes these phase issues would go away. As the gain is adjusted with the EQ filters the phase adjusts as well so sometimes these small adjustments can make a difference. But sometimes they don’t! Just worth a try...

Also what all pass filter settings are you using?


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## Mauian (Jul 25, 2019)

Just to add to this, I found that around the crossover region, the closer I’m able to get the response to match the smooth target curve the better. If I can get it to be less jagged in this region the signals seem to sum much better in my experience.

This doesn’t always do it, but I’ve found it helps in some cases.


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## Mauian (Jul 25, 2019)

Is this with the GS690?  just wondering if you were able to install them yet


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## Mauian (Jul 25, 2019)

Also, I looks like the combined signals are crossing over at around ~540 Hz. Instead of your desired 500 Hz. Moving the crossover is not a huge deal but it goes along with trying to match the target curves as much as possible and as smooth as possible around the crossovers so that the crossover point is where you wanted it and the relative slopes are where they need to be to make things sum up right.

When I missed the crossover point by a bit or passed through it at an angle that was sharper or flatter than my target I often found that I would get these summing issues that sometimes would go away when I addressed these items.

Either way though, it’s looking good and I’m sure it’s sounding very nice!


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## jtrosky (Jul 19, 2019)

Mauian said:


> Is this with the GS690?  just wondering if you were able to install them yet
> 
> 
> Sent from my iPhone using Tapatalk


No, this is still with the CDT CL-69S. I'm hoping to put the GS690's in today (not sure why I keep putting it off - the door speakers are quick and easy to replace!). Honestly, the system sounds fantastic now, especially since I widened the rear speaker bandpass range significantly and increased their volume a little - so I'll just leave the dips there before i worry about it too much (it's very easy to worry too much about how the measurements look instead of how the system sounds - been there, done that!!) - but was just kind of curious what may be causing them. Very strange how the dips are almost exactly the same on both sides of the crossover summation area.

The left and right 400hz and 600hz areas ore only off between 1dB and 2dB - it just looks funky since it's 1/12th smoothing. If I smooth to 1/3, they match up extremely close. However, the dips on each side of the crossover are over 5dB. I mean I could probably make the left and right match a little closer, but at 1dB-2dB difference, we're talking such small differences that they could even just be measurement variances (it's easy to see 1dB or 2dB differences between measurements with moving mic measurements. There is something more to it than those small differences - I mean at 1/12th smoothing, I have 1dB-2dB differences throughout the entire frequency range.  There is no way those 1dB or 2dB differences should cause those 5dB+ dips like that

If you measure individual speakers and L+R speaker pairs, do you see similar phase-related dips in the L+R combined measurements at all? I see those same phase issues at the exact same freqs no matter the crossover freq I use. Also, the dips on the sides of the crossover are there regardless of the allpass filters, so the two things are not related in any way (just wanted to mention that). 

I'm just glad that the allpass filters fixed the L+R combined measurements - I was amazed how well they fixed the big dips! I'm using 2nd order allpass filters - Q varies depending on how wide the dip is. the 200hz dip I have at a Q of 6 and the other three allpass filters are using a Q of 8 right now. I haven't really fine-tuned them yet - was just happy to see that the allpass filters worked at all - especially with how well they worked! Who knows, I mean even be able to use allpass fitlers to fix the dips on the sides of the xover as well.  Although, I don't want to spend much time on it since I'll be replacing the 6x9s anyway - not sure if the GS690's will have the same issue or not - probably will based on my experience though. Just wondering if these issues are time alignment related. I will take new measurements to the speaker dustcap when I remove the door panels - just in case my measurements are off by an inch or two (never measured with the door panels off).


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## bnae38 (Oct 3, 2015)

Yabut, how does it sound? If you do a frequency sweep is the stage stable in the center?

I was excited about apf years ago when they first released it because of the 180-200hz deep null I have with midbass's playing together. Sure enough, it fixed it!

I still have that apf in place, but i'm not totally sure its a overall win. It seems to smear the stage a bit. Tbh, i haven't played with tuning in the car for at least 2 years.


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## jtrosky (Jul 19, 2019)

bnae38 said:


> Yabut, how does it sound? If you do a frequency sweep is the stage stable in the center?
> 
> I was excited about apf years ago when they first released it because of the 180-200hz deep null I have with midbass's playing together. Sure enough, it fixed it!
> 
> I still have that apf in place, but i'm not totally sure its a overall win. It seems to smear the stage a bit. Tbh, i haven't played with tuning in the car for at least 2 years.





jtrosky said:


> *Honestly, the system sounds fantastic now*, especially since I widened the rear speaker bandpass range significantly and increased their volume a little - so I'll just leave the dips there before i worry about it too much (it's very easy to worry too much about how the measurements look instead of how the system sounds - been there, done that!!) - but was just kind of curious what may be causing them. Very strange how the dips are almost exactly the same on both sides of the crossover summation area.


It sounds great right now - but really, at this point, I'm not ever sure if I can hear the differences the individual allpass filters make. Obviously, it improved the frequency response at the listening position based on the graphs, which is a good thing, but like I said, I'm also realizing that it's easy to get carried away with graphs and lose sight of the ultimate goal - and that is that is *sounds good* and makes you happy.  

A lot of these small things we change in the tune are probably not even audible individually, but with enough small improvements, I'm sure the minor stuff has a positive impact as a whole. I mean that is the whole purpose of a tuning mic - to be able to measure and "see" things that are hard to quantify with our ears alone. The differences may only be apparent in some songs or certain types of music, etc.

But year, the system sounds great right now. Honestly, since installing the GS25's and having the rear speakers do a little more, it's the best it's ever sounded. Just in time to tear it apart and replace the door speakers - and start all over again.


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## Mauian (Jul 25, 2019)

Totally hear ya on the small things. Just sharing my experience with those small variances (which I agree can be the same as measurement variances) that at times (not always) add up when you start combining mid bass and wideband, L and R, and adding rear speakers on top of it. 

I think the individual graphs of an RTA don’t tell the whole story with phase so just because you only see a small 1-2 dB dip from one of them, doesn’t mean that there isn’t a larger phase issue caused by reflections, nulls/cancelations, when multiple speakers are playing. Sometimes these can be fixed, in my experience, with small adjustments of gain (and consequently phase) at those jagged frequencies near the crossover. Sometimes you can’t and it’s amenable to an APF.

Either way, awesome that it’s sounding great - the most important thing for sure.


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## garysummers (Oct 25, 2010)

I have been utilizing the all pass filters in my two MiniDSP CDSP 8X12's I have in my car. They do not always work to fix an issue but when they do whoa. I have had very good results both measured and to my ear with correcting the summed signals in my 5.1 system in my car. Also have had some good results correcting in the crossover regions. As has been mentioned here, I look at the combined response on the RTA, where there is a null in the target response, apply an APF, null improves graphically, get in the car and A\/B each place that I have placed an APF. I can hear the coherencey of the sound image improve when the filter is justified.


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## tjk_bail (Feb 2, 2012)

garysummers said:


> get in the car and A\/B each place that I have placed an APF. I can hear the coherencey of the sound image improve when the filter is justified.


Can you explain this in a little more detail, I'm not sure I follow what you're saying .....


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## garysummers (Oct 25, 2010)

Very simple! I verify on the RTA that the APF produces a improvement in the combined response relative to the target curve. Then I verify that APF by "Ear" in the car while listening to music. My reference to A/B means a comparison of the filter in, then the filter out. Which sounds better to my ear.


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## jtrosky (Jul 19, 2019)

Ok - stupid question.... So how do you know whether to place an allpass filter on the L or R speaker when you see an obvious phase-related issue when playing both L+R speaker pairs? Or doesn't it matter which one you place the allpass filter on?


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## garysummers (Oct 25, 2010)

Doesn’t mater! Phase is a relationship. You are modifying one within that relationship.


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## dumdum (Feb 27, 2007)

jtrosky said:


> Can't you measure phase with REW using sweeps (instead of pink noise)? I could have sworn that I read that somewhere. Haven't spent the time to look into it any further yet though.
> 
> I would just love to be able to actually _measure_ phase without needing $1k software (Smaart), if possible. Would be nice to _know_ for sure what are phase issues (vs reflection issues or some other issue). Maybe it's just not possible though.
> 
> Every time I''ve tried to use an allpass filter to correct a dip that exists when playing multiple speakers together (that doesn't exist when playing each of the speakers individually), the allpass filter didn't change anything at all, so based on that, I'm _assuming_ that the issue(s) I'm talking about aren't actually phase issues, but probably reflection-related issues?


Reflection and phase issues are one and the same... room modes are reflections from the rear/sides/top/bottom of the car which reinforce and cancel in different frequencys, however due to wavelengths they don’t tend to change a lot below a certain frequency, the shorter the wavelength the shorter the distance you have to move to see a difference on the rta and hear the difference in a car, try it... move your head while listening to ten kHz, as you rock left to right it will come, go, come again... now do the same for 100hz... it won’t change...

it may be that your all pass wasn’t the correct amount of phase shift, or it moved it 180 degrees so it shifted it the other way if it only needed 90 degrees, the net change would be 90 degrees out either way, so the response would remain the same

I’d much rather move drivers to get the best phase response with as little phase issues at the listening position then deal with smaller issues after the fact if you see what I mean, at least then you stand a chance of listening to the drivers own response instead of boosting and cutting way more which then adds boosts and cuts to reflected and direct sound... a good installation makes tuning way easier 👍🏼


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## dumdum (Feb 27, 2007)

garysummers said:


> Doesn’t mater! Phase is a relationship. You are modifying one within that relationship.


However if the phase issue is with one side you will hear it and it’s relationship to frequencys next up and down... so ideally I’d try both sides to see if one way or another sounds better and more phase coherent, after all you want phase to be correct for each side so the sum gives you a good centre

a perfect speaker is a straight line for phase... if you play them together and it doesn’t sum well you have one side or the other that’s not a straight line... So it is kind of important to find the correct side


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## opekone (Mar 24, 2020)

Don't forget allpass filters have an effect far above the frequency they are set at.


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## Holmz (Jul 12, 2017)

dumdum said:


> However if the phase issue is with one side you will hear it and it’s relationship to frequencys next up and down... so ideally I’d try both sides to see if one way or another sounds better and more phase coherent, after all you want phase to be correct for each side so the sum gives you a good centre
> 
> a perfect speaker is a straight line for phase... if you play them together and it doesn’t sum well you have one side or the other that’s not a straight line... So it is kind of important to find the correct side


One can always compare them to the input signal and figure out which side(s) needs fixing.


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## jtrosky (Jul 19, 2019)

Mauian said:


> I actually just ran into a use case last night.
> 
> I was tuning my car and checking phase alignment as in Nicks video above. It wasn’t summing properly (see green line below):
> 
> ...


I'm running into the EXACT same type of issue right now and I'm wondering what made you try an allpass filter at 160hz, with a Q of 2 to correct that dip, just trial and error?

In my case, I fixed 4 or 5 other phase-induced dips throughout the frequency response using allpass filters. But in every case, the allpass filter was for a frequency in the middle of the dip, with a Q value related to how wide the dip was (just like you'd do to correct a dip with parametric EQ). However, this last dip in my crossover region did not respond well to an allpass filter (it did change it, but still not near correct). 

Then I noticed your post where you used an allpass "center" frequency that was actually _before_ the freqs related to the dip - and with a really wide Q (2) - I would think around 4 or 5 for that dip as it relates to parametric EQ.

I'll have to try doing something similar for my dip, but was curious what made you try that. It kind of feels like I'd just be trying random freqs/Qs - instead of a center freq and Q that makes "sense". For your issue, I would have assumed values like 250hz with a Q of 4 (or maybe 5) to correct that dip. Was really surprised when I saw that it was an allpass filter of 160hz/Q of 2 that fixed it... 

I'll post some graphs of what I've been able to correct with allpass filters, where I'm at and the last little remaining issue I have (that dip at the crossover). 

Thanks man!


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## jtrosky (Jul 19, 2019)

OK, so here are some pictures. This first set of pictures just shows my Door and Dash speakers - individually and the combined L+R response of each set (L+R Door and L+R Dash).

Here is what the combined responses looked like before applying any allpass filters, with little red arrows showing the phase-related dips (3 in combined midbass response and 2 in combined dash speaker response) - I could probably use another one for the dash speaekers, but...:










Here is what the combined response looks like after I applied allpass filters to those dips - they magically just disappear that easily! Awesome! This is with 1/6th smoothing (with 1/3rd smoothing, things look perfect):









I applied the following allpass filters to fix those dips shows with the arrows (no EQ, just an allpass filter per dip):

*L+R Door (all 2nd order)*:

197hz, Q of 4
414hz, Q of 9
630hz, Q of 9

*L+R Dash (all 2nd order)*:

672hz, Q of 6
985hz, Q of 5

No idea if this is "normal" (to need to apply so many allpass filters), but they definitely addressed the big dips, so I'm happy.

So I figured - cool! I'm done - everything is lining up just right phase-wise. NOT.  I forgot that I haven't deal with the DashDoor crossovers yet! So far I just fixed the phase-related issues when playing speakers in L+R pairs....

I'll be back to show the response of each side (Left Door+Left Dash and Right Door+RIght Dash). I can't seem to get those right - at all, which obviously affects the overall response...


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## jtrosky (Jul 19, 2019)

So then when I checked the left and right sides separately, I found that I had these huge dips around the 400hz crossover!









No matter what I tried, I couldn't get rid of the dips - this is about the best that I could get them:









Anyone have any ideas? Time alignment is all correct and physical polarity is correct everywhere. Not quite sure how to fix this...

Thank you!


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## Mauian (Jul 25, 2019)

jtrosky said:


> I'm running into the EXACT same type of issue right now and I'm wondering what made you try an allpass filter at 160hz, with a Q of 2 to correct that dip, just trial and error?
> 
> In my case, I fixed 4 or 5 other phase-induced dips throughout the frequency response using allpass filters. But in every case, the allpass filter was for a frequency in the middle of the dip, with a Q value related to how wide the dip was (just like you'd do to correct a dip with parametric EQ). However, this last dip in my crossover region did not respond well to an allpass filter (it did change it, but still not near correct).
> 
> ...


It was trial an error. I agree that something in the middle with a smaller Q would make more sense. I tried that and it didn’t quite address it fully so with some trial and error I landed on the above and it worked.


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## Mauian (Jul 25, 2019)

jtrosky said:


> So then when I checked the left and right sides separately, I found that I had these huge dips around the 400hz crossover!
> 
> 
> 
> ...


With those huge dips at the crossover, did you try flipping the polarity of your widebands? I’ve thought polarity was all in sync and then I ran across that exact same problem and flipped polarity and boom! Fixed. May not do it, but work checking.


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## jtrosky (Jul 19, 2019)

Here is a better picture showing the crappy xover summing - so far, it's the best i can get with allpass filters...


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## opekone (Mar 24, 2020)

jtrosky said:


> OK, so here are some pictures. This first set of pictures just shows my Door and Dash speakers - individually and the combined L+R response of each set (L+R Door and L+R Dash).
> 
> Here is what the combined responses looked like before applying any allpass filters, with little red arrows showing the phase-related dips (3 in combined midbass response and 2 in combined dash speaker response) - I could probably use another one for the dash speaekers, but...:
> 
> ...



It's becausse first order allpass filters don't affect the frequency you are setting them at. They roll the phase of things 1+ octaves above it. The second order has a much more pronounced impact at the frequency you set it at. You can see clearly where the filters affect phase if you watch this video. Then you can use first order filters as well, the next time!


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## opekone (Mar 24, 2020)

Damn dude, 1/3 octave smoothing. You're really going for it, huh.


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## tjk_bail (Feb 2, 2012)

jtrosky said:


> I applied the following allpass filters to fix those dips shows with the arrows (no EQ, just an allpass filter per dip):
> 
> *L+R Door (all 2nd order)*:
> 
> ...


THANK YOU for this write up !! This type of explanation is so super helpful to a DIY guy like me. Thanks again for taking the time to put your post together! You Rock the House Jtrosky!


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## jtrosky (Jul 19, 2019)

Mauian said:


> With those huge dips at the crossover, did you try flipping the polarity of your widebands? I’ve thought polarity was all in sync and then I ran across that exact same problem and flipped polarity and boom! Fixed. May not do it, but work checking.


Yeah, unfortunately, it's not that simple - I did try that. Flipping the polarity on the dash speakers just moves the dip a tiny bit - but very similar results. I just did some more testing and found that I can either get the L+R speaker pairs to sum correctly (no big dips/nulls) _or_ the dash/door crossovers to sum correctly - but not both - fixing one "breaks" the other.  Not sure which one is "better" at this point. But I can't get all combos of all speakers to sum correctly with the same settings. I'll change things (add or remove allpass filter(s)) to get the door/dash crossovers to sum correctly, but then that change causes the L+R pairs to not sum correctly anymore. Or I can get door/dash crossovers to sum correctly, but then it creates issues with the L+R speaker pairs. Just can't get them all correct at the same time, with the same settings.  Honestly, not so sure I can actually hear the difference anyway though, so I'm not going to stress over it anymore. I'll probably fix the door/dash summing and just deal with a null in the L+R speaker pair summing...




opekone said:


> It's becausse first order allpass filters don't affect the frequency you are setting them at. They roll the phase of things 1+ octaves above it. The second order has a much more pronounced impact at the frequency you set it at. You can see clearly where the filters affect phase if you watch this video. Then you can use first order filters as well, the next time!


Right, but it this case, we're using all 2nd-order allpass filters - and it's still affecting an area outside of the area we're trying to impact (that one issue that @Mauian fixed at the crossover). Strange - especially since the Q is so wide for such a relatively narrow dip. So hard to know what's _really_ going on with phase without being able to actually see the phase as you adjust things. Unfortunately, it's too expensive to buy Smaart just for stuff this.




opekone said:


> Damn dude, 1/3 octave smoothing. You're really going for it, huh.


Not following? If you're saying 1/3rd isn't detailed enough - it's kind of funny - originally, I used to use mainly 1/6th and 1/12th for everything - then everyone got on me saying that I didn't need to use that level of detail. Now I mainly use 1/3rd for everything (with brief looks at 1/6th and 1/12th) - and now I'm getting slack for using 1/3rd.  Can't make everyone happy! Everything I posted above was actually 1/6th though - except for the on pic showing how the crossovers sum.




tjk_bail said:


> THANK YOU for this write up !! This type of explanation is so super helpful to a DIY guy like me. Thanks again for taking the time to put your post together! You Rock the House Jtrosky!


Awesome - glad you found it useful. I was actually looking for help myself though, so I'm certainly not the expert on the subject! I'm in the process of trying to understand it all myself. But again, ,glad you found it useful. I wish more people posted graphs and how they deal with things - you can learn a lot just seeing how other people do things - and the graphs really help clear things up sometimes. You rarely see people actually post graphs though. Just a few people, for the most part.


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## opekone (Mar 24, 2020)

REW nerds tend to use less smoothing on lower frequencies (no smoothing or 1/24 on sub) and will limit their smoothing to 1/6 on mids/tweets. The argument this author makes for 1/3 I think really suggest 1/6 is the way to go. If we can discern 1/3 octave changes then we can be sure 1/3 octave smoothing will drop some of that data, while smoothing at 1/6 would preserve far more. Other people argue that you can hear 1/6 octave changes. So then... Anyway, you can't really go wrong.









Why Do We Equalize In 1/3-Octave Bands?Why Do We Equalize In 1/3-Octave Bands? | Prosoundtraining


Richard Honeycutt explains why most equalize in 1/3 Octave Bands. He included 30 sound clips so readers can evaluate the perceptibility of frequency-response deviations.



www.prosoundtraining.com


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## opekone (Mar 24, 2020)

jtrosky said:


> Right, but it this case, we're using all 2nd-order allpass filters - and it's still affecting an area outside of the area we're trying to impact (that one issue that @Mauian fixed at the crossover). Strange - especially since the Q is so wide for such a relatively narrow dip. So hard to know what's _really_ going on with phase without being able to actually see the phase as you adjust things. Unfortunately, it's too expensive to buy Smaart just for stuff this.


Allpass filters have a very significant impact over a wide range of frequencies, it's very much unlike a parametric filter. A lower Q would mean the filter applies to a wider frequency band, isn't that what you are describing?


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## Mauian (Jul 25, 2019)

opekone said:


> REW nerds tend to use less smoothing on lower frequencies (no smoothing or 1/24 on sub) and will limit their smoothing to 1/6 on mids/tweets. The argument this author makes for 1/3 I think really suggest 1/6 is the way to go. If we can discern 1/3 octave changes then we can be sure 1/3 octave smoothing will drop some of that data, while smoothing at 1/6 would preserve far more. Other people argue that you can hear 1/6 octave changes. So then... Anyway, you can't really go wrong.
> 
> 
> 
> ...


Yeah, I read recently Andy (Audiofrog) recommends 1/3 for frequencies above 1kHz and 1/6 for below.


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## dumdum (Feb 27, 2007)

In the above case where you can’t get the phase Matching at the crossover, it’s very likely you have a phase issue at or around the crossover point, which will never help the summation and get decent summation, I have a phase issue at 300hz on one midbass, but the pair play nicely upto 200hz so I cross there, it keeps any phase issues well out of the crossover regions and gets me excellent summation and a very good blend so the two speakers melt into the tweeters and my stage is all 12” off the dash... I have midbass under my seats, and the sub is on the dash 90% of the time, I think I need to work on its eq and rattles as the phase trace overlaps perfectly from 30hz to 300+ 👍🏼

I measure with smaart by the way

I should also add i will be testing with 12db per octave crossovers to see if that further improves the blend as the midbass will play a little lower and the sub will play a little higher... although I’ll do this for Emma sq only to help the really awkward subwoofer test track at lower levels 👍🏼


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## Mauian (Jul 25, 2019)

Jtrosky, I wonder if you could have success if you didn’t do any all pass filters until playing L+R together AND dash/doors together - as in see where the phase issues are with all drivers playing and address them then. Could be worse, but maybe better? Worth an experiment? (I mean, you’re clearly having success already, but we’re talking about the little things)

I agree with dumdum. Moving the crossover could get you out of a trouble spot.


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## jtrosky (Jul 19, 2019)

Actually, I went back out shortly after my last post (can't give up that easily!) and was able to get everything "in sync". Just had to find that "just right" combination of allpass filters. Basically, I just had to add a single allpass filter to each of the dash speakers. Now, the left side and right side have no more dips at the door/dash crossover and the L+R speaker pairs are also "dip free". 

I'll post the updated graphs shortly.

Previously, I was concentrating too much on the midbass speakers to address the dips (since the dips were mainly on the "midbass side" of the door/dash crossovers). However, in the end, all that I needed was a single allpass on each dash speaker and all is good now everywhere. Now I can concentrate on some tonality and other minor details.

I will say - I think my system is sounding the best it's ever had with those CDT Unity 8.0 speakers. It's hard to put in words, but they are just so "smooth" sounding. Not a hint of "harshness" - and I hear every little detail. VERY happy with those little speakers so far!

Thank you all for the help - I appreciate it!

EDIT: Oh - by the way, these allpass filtes are awesome! Before I started using allpass filters, I would fix all of these dips with straight-up EQ (boosts), which was 100% the incorrect way to handle these issues. Now, I was able to fix every "unexplained" dip without using a single bit of EQ. Pretty cool how that all works and everything comes together.


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## Mauian (Jul 25, 2019)

jtrosky said:


> Actually, I went back out shortly after my last post (can't give up that easily!) and was able to get everything "in sync". Just had to find that "just right" combination of allpass filters. Basically, I just had to add a single allpass filter to each of the dash speakers. Now, the left side and right side have no more dips at the door/dash crossover and the L+R speaker pairs are also "dip free".
> 
> I'll post the updated graphs shortly.
> 
> ...


Sweet! 


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## mattkim1337 (Jan 31, 2018)

Just a heads up based on my experience with all-pass filters. When you see a phase cancellation between two speakers, you may be tempted to automatically use an all-pass filter to get summation on the RTA. However, you need to consider what caused the phase between the two speakers to diverge in the first place. The original sound wave left each speaker at the same time (assuming correct delay settings), and the direct sound definitely reaches your ears in phase at all frequencies. The reason why there is a phase cancellation is due to a reflection that arrives AFTER the direct sound that is out of phase with the opposite speaker. When you use an all-pass filter to correct this issue, you're compensating for the phase change due to the reflection, but also ruining the phase response of your direct sound. Which is more important to correct, your direct sound, or the reflection? Remember that the RTA is a representation of frequency response OVER TIME. Direct sound and reflections get mixed together and you can't tell them apart. That's a tradeoff you need to experiment with. 

For example, my midbass to midrange crossover point was 300hz. The midrange behaved as it should (minimum phase) and the phase trace was ideal. The midbass, however, had a nasty reflection some milliseconds later which I could clearly see ruined the phase trace. Naturally, I applied a 2nd order all-pass filter to the midrange channel and got the phase to match perfectly. I had a perfect 6db summation on the RTA afterward. When it came time to listen, frequencies around 300hz did not produce a tight center image and were unintelligble. I was baffled because the phase traces matched so perfectly. I undid the all-pass filter and went back to normal settings. Even though the dip showed up on the RTA, I had my tight center image back around 300hz. My best hypothesis is that the direct sound of the midbass driver had a greater influence on phase/imaging than the later reflection did. The measurement software unfortunately gives the reflection too much weight in terms of perceived importance.


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## dumdum (Feb 27, 2007)

mattkim1337 said:


> Just a heads up based on my experience with all-pass filters. When you see a phase cancellation between two speakers, you may be tempted to automatically use an all-pass filter to get summation on the RTA. However, you need to consider what caused the phase between the two speakers to diverge in the first place. The original sound wave left each speaker at the same time (assuming correct delay settings), and the direct sound definitely reaches your ears in phase at all frequencies. The reason why there is a phase cancellation is due to a reflection that arrives AFTER the direct sound that is out of phase with the opposite speaker. When you use an all-pass filter to correct this issue, you're compensating for the phase change due to the reflection, but also ruining the phase response of your direct sound. Which is more important to correct, your direct sound, or the reflection? Remember that the RTA is a representation of frequency response OVER TIME. Direct sound and reflections get mixed together and you can't tell them apart. That's a tradeoff you need to experiment with.
> 
> For example, my midbass to midrange crossover point was 300hz. The midrange behaved as it should (minimum phase) and the phase trace was ideal. The midbass, however, had a nasty reflection some milliseconds later which I could clearly see ruined the phase trace. Naturally, I applied a 2nd order all-pass filter to the midrange channel and got the phase to match perfectly. I had a perfect 6db summation on the RTA afterward. When it came time to listen, frequencies around 300hz did not produce a tight center image and were unintelligble. I was baffled because the phase traces matched so perfectly. I undid the all-pass filter and went back to normal settings. Even though the dip showed up on the RTA, I had my tight center image back around 300hz. My best hypothesis is that the direct sound of the midbass driver had a greater influence on phase/imaging than the later reflection did. The measurement software unfortunately gives the reflection too much weight in terms of perceived importance.


It’s not that it gives anything any weight... it just measures everything, it’s impossible to get it to measure only direct sound as gating won’t be possible due to how enclosed a car environment is... I do agree though that a dip can sometimes be better left alone 👍🏼


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## jtrosky (Jul 19, 2019)

mattkim1337 said:


> Just a heads up based on my experience with all-pass filters. When you see a phase cancellation between two speakers, you may be tempted to automatically use an all-pass filter to get summation on the RTA. However, you need to consider what caused the phase between the two speakers to diverge in the first place. The original sound wave left each speaker at the same time (assuming correct delay settings), and the direct sound definitely reaches your ears in phase at all frequencies. The reason why there is a phase cancellation is due to a reflection that arrives AFTER the direct sound that is out of phase with the opposite speaker. When you use an all-pass filter to correct this issue, you're compensating for the phase change due to the reflection, but also ruining the phase response of your direct sound. Which is more important to correct, your direct sound, or the reflection? Remember that the RTA is a representation of frequency response OVER TIME. Direct sound and reflections get mixed together and you can't tell them apart. That's a tradeoff you need to experiment with.
> 
> For example, my midbass to midrange crossover point was 300hz. The midrange behaved as it should (minimum phase) and the phase trace was ideal. The midbass, however, had a nasty reflection some milliseconds later which I could clearly see ruined the phase trace. Naturally, I applied a 2nd order all-pass filter to the midrange channel and got the phase to match perfectly. I had a perfect 6db summation on the RTA afterward. When it came time to listen, frequencies around 300hz did not produce a tight center image and were unintelligble. I was baffled because the phase traces matched so perfectly. I undid the all-pass filter and went back to normal settings. Even though the dip showed up on the RTA, I had my tight center image back around 300hz. My best hypothesis is that the direct sound of the midbass driver had a greater influence on phase/imaging than the later reflection did. The measurement software unfortunately gives the reflection too much weight in terms of perceived importance.


Great post - thank you for that! As I was applying all of these allpass filters, I was kind of wondering if there were any unintended consequences - and it sounds like there could be.... 

To be 100% honest, I'm not even sure I could tell the audible difference between the original RTA response with the dips throughout and the "corrected" response after I fixed all of the dips with the allpass filters.  The "after" RTA response definitely _looks_ a lot better, but not so sure it really translates to actually sounding any better.

RTA measurements are really cool - but I think they also have you chasing your tail a lot of the times for things that you can't actually hear anyway. I guess that is what makes an experienced tuner so much better than a novice like myself - being able to identify the stuff that really needs addressed vs what doesn't really matter (or items that can actually make things worse!).


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## jtrosky (Jul 19, 2019)

Is it considered somewhat "normal" to have 5 or 6 allpass filters in a tune? Or would that be highly "irregular"? I'm just curious if what I'm doing adding these allpass filters is a "common" practice or not....


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## Mauian (Jul 25, 2019)

mattkim1337 said:


> Just a heads up based on my experience with all-pass filters. When you see a phase cancellation between two speakers, you may be tempted to automatically use an all-pass filter to get summation on the RTA. However, you need to consider what caused the phase between the two speakers to diverge in the first place. The original sound wave left each speaker at the same time (assuming correct delay settings), and the direct sound definitely reaches your ears in phase at all frequencies. The reason why there is a phase cancellation is due to a reflection that arrives AFTER the direct sound that is out of phase with the opposite speaker. When you use an all-pass filter to correct this issue, you're compensating for the phase change due to the reflection, but also ruining the phase response of your direct sound. Which is more important to correct, your direct sound, or the reflection? Remember that the RTA is a representation of frequency response OVER TIME. Direct sound and reflections get mixed together and you can't tell them apart. That's a tradeoff you need to experiment with.
> 
> For example, my midbass to midrange crossover point was 300hz. The midrange behaved as it should (minimum phase) and the phase trace was ideal. The midbass, however, had a nasty reflection some milliseconds later which I could clearly see ruined the phase trace. Naturally, I applied a 2nd order all-pass filter to the midrange channel and got the phase to match perfectly. I had a perfect 6db summation on the RTA afterward. When it came time to listen, frequencies around 300hz did not produce a tight center image and were unintelligble. I was baffled because the phase traces matched so perfectly. I undid the all-pass filter and went back to normal settings. Even though the dip showed up on the RTA, I had my tight center image back around 300hz. My best hypothesis is that the direct sound of the midbass driver had a greater influence on phase/imaging than the later reflection did. The measurement software unfortunately gives the reflection too much weight in terms of perceived importance.


I love this. Great idea to use a little band passed pink noise on the area with the phase issue / APF and see if it centers up better before or after. Thanks!


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## jtrosky (Jul 19, 2019)

So here is where my tune ended up for now. Keep in mind that I've never had a tune as "complete" as this. Previously, before I understood allpass filters, I would just use EQ to boost areas where I had phase-induced dips, which is obviously not the correct way to handle those issues.  That caused my ending "overall" response to look "correct", but it really wasn't since I was just using EQ to account for the phase-induced dips, instead of actually fixing the phase-related issues). I no longer use ANY EQ to "correct" any phase-related dips (as you can see from the picture below since the combined L+R measurements match the "shape" of the individual speaker measurements almost exactly).

This is what my tune looks like right now with CDT ES-690CFS 6x9 midbass speakers, CDT Unity 8.0 wideband dash speakers and a JBL BassPro SL under-seat sub. I do also have some Kenwood Excelon KFC-X174 6.5 coaxials speakers in the rear-deck, but they are not included in any of these measurements. I've also decided to let my 6x9 door speakers give more deep bass (set their HP xover to 50hz instead of 65hz, which boosted the ~40hz output from them significantly). As long as it doesn't cause any significant vibration-related issues (which it hasn't so far), I figured "why not?". 

I _definitely_ don't have any issues with my bass not being "up front" - since all of my deep bass actually comes from the front (front 6x9 door speakers and under-seat sub under passenger seat). "Up front" bass is a complete non-issue in this build.


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## cathul (Jul 2, 2018)

Damn... i wish Mosconi would implement more than 5 filters in their V1 GUI for the D2 series of DSP amps and include allpass filters for them.


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## Selkec (Nov 18, 2018)

jtrosky said:


> OK, so here are some pictures. This first set of pictures just shows my Door and Dash speakers - individually and the combined L+R response of each set (L+R Door and L+R Dash).
> 
> Here is what the combined responses looked like before applying any allpass filters, with little red arrows showing the phase-related dips (3 in combined midbass response and 2 in combined dash speaker response) - I could probably use another one for the dash speaekers, but...:
> 
> ...


What does The areas where these phase dips are at look like with no smoothing? 
I started checking my response yesterday and going to today some more. And where I thought I had a phase issue was actually a -10db sharp dip that when smoothed wasn’t noticeable.


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## jtrosky (Jul 19, 2019)

Selkec said:


> What does The areas where these phase dips are at look like with no smoothing?
> I started checking my response yesterday and going to today some more. And where I thought I had a phase issue was actually a -10db sharp dip that when smoothed wasn’t noticeable.


They basically look the same, just deeper dips. Usually, the combined L+R response will actually dip down to the leve of the individual speaker measurements and maybe even slightly lower sometimes. I'll see if I can find those measurements and respost them with less smoothing (or no smoothing).

EDIT: Also if it's a phase-related issue, the dip will be present in the L+R combined response, but not in the individual response. So if there is a dip in the combined L+R response that does not exist in the either of the individual responses, then it's a phase-related issue (or a bad measurement!). So it's pretty easy to tell whether or not it's phase-related or not - but yes, you do need to review the measurements with little smoothing (usually, 1/12th will show any dips easily). Even though I use 1/3rd smoothing a lot of the times, I still look at things with 1/6th and/or 1/12th smoothing as well - because as you noted, sometimes 1/3rd smoothing can hide things.


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## Frequentflyer (Mar 3, 2020)

Selkec said:


> What does The areas where these phase dips are at look like with no smoothing?
> I started checking my response yesterday and going to today some more. And where I thought I had a phase issue was actually a -10db sharp dip that when smoothed wasn’t noticeable.


I've got a lot of similar dips with my dash speakers. I will follow your lead on this the next time I can get out and do some tuning.


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## jtrosky (Jul 19, 2019)

Oh - here are those same measurements with 1/24th smoothing, just FYI.... As you can see, the dips are just more "severe". Also, look how far it moved my "L+R combined" crossover point compared to where the individual speakers crossed over (because of the phase-related dips)... Looking at these measurements again, there were quite a few phase-related issues between 600hz and 2500hz...

Allpass filters are very useful for getting proper summing at crossovers. I never realized how bad the summing was between my door and dash speakers. After using an allpass filter or two, summing is near perfect in the crossover region in my current tune - really helps the door and dash speakers "mesh" better. Looking at these measurements, it's amazing how much better my tunes measure (and sound!) now, since I started using allpass filters. When taking measurements, make sure to also check the crossover summing! 

When taking measurements, I now like to measure all individual speakers, combined L+R for each speaker "pair" (to check L+R summing), whole side measurements (to check crossover summing) and whole front (just to see it all together).










I'm curious to hear how you make out after you experiment with allpass filters.


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## Selkec (Nov 18, 2018)

jtrosky said:


> Oh - here are those same measurements with 1/24th smoothing, just FYI.... As you can see, the dips are just more "severe". Also, look how far it moved my "L+R combined" crossover point compared to where the individual speakers crossed over (because of the phase-related dips)... Looking at these measurements again, there were quite a few phase-related issues between 600hz and 2500hz...
> 
> Allpass filters are very useful for getting proper summing at crossovers. I never realized how bad the summing was between my door and dash speakers. After using an allpass filter or two, summing is near perfect in the crossover region in my current tune - really helps the door and dash speakers "mesh" better. Looking at these measurements, it's amazing how much better my tunes measure (and sound!) now, since I started using allpass filters. When taking measurements, make sure to also check the crossover summing!
> 
> ...


Damn it wasn’t Summing at all at the crossover 😳


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