# Time Alignment in Car via Impulse Response



## subterFUSE

Starting a new thread for this issue.

I'm trying to use REW's impulse response measurements for determining the time alignment settings for my car audio system.

Here are the testing parameters:

Using a Behringer ECM8000 and M-Audio M-Track sound card. REW used on a MacBook Pro.

Mic was placed in the driver seat, on a mic stand with the mic held at the center of head position.

My system is comprised of the following speakers:

2 x Horn Loaded Compression Drivers (installed under the dash)
2 x 8" Midbass drivers (installed in the front doors, stock location)
2 x 12" subwoofers (installed in a trunk baffle)

All speakers are active crossover via an Audison BitOne DSP.


For these measurements, only the horns and midbass were tested. Subwoofers are not included in these IR measurements.

These measurements are taken with no delays applied to any of my drivers. I'm hoping to determine how much delay to apply to each one.

The measurements are labelled as follows:

L HLCD = Left Horn
R HLCD = Right Horn
L MB = Left Midbass
R MB = Right Midbass

Each driver was measured independently with all others muted.

Attached is my mdat file with the measurements.


I'm hoping to get help understanding how to align these measurements to determine the correct time alignment for these drivers. As I understand it, the impulse peaks are not the correct point to align the IR. The "initial rise" is the correct point to aim for, but I don't quite understand where the "initial rise" is located.


Thanks in advance for any assistance.


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## jtalden

The IRs are way too irregular for me to do any detailed timing analysis in the manner I normally do.

> I notice that you ran 2 sweeps for each of these measurements. I don't think that should be an issue, but I would recommend single sweeps. If the relative timing is changed even slightly between the 2 sweeps it will cause measurement problems.

> I see the sample rate is reported in REW info as 12,000? I don't know if that is a problem, or not, but it doesn't seem right. I usually see 41.1k or 48k or 96k. I didn't know REW would use other rates. If there is resampling happening in the measurement (different rates between REW, the audio interface, and XO unit) maybe that is causing a measurement problem.

> When all is normal we should be able to repeat the exact results with repeated measurements of a driver. If we can't do that there is a measurement problem.

> It is easier for me if full range sweeps are used as I can better determine the direct sound phase path. The narrow range sweeps should be okay also, but that is not my normal process so I have little experience that way.

> With the mic close to a driver or at the mouth of the horn a very clean IR should result. If it is clean and repeatable that way and irregular at the LP that suggests the irregular IR is just the result of the car reflections.

I suggest you experiment with this an see if the IRs can be cleaned up. If so, I may be able to help with timing. If not, the timing you have is not too bad. There is another process we could try. I have only tried it once or twice, but it may work better under these difficult circumstances.


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## JohnM

jtalden said:


> > I see the sample rate is reported in REW info as 12,000? I don't know if that is a problem, or not, but it doesn't seem right.


If the 'Decimate IR' option is selected in Analysis preferences REW will reduce the sample rate according to the sweep end frequency to reduce the storage required.


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## subterFUSE

My soundcard is 48kHz and that's the sample rate I used in the prefs.

However, you are correct that I had the decimate IR box checked.


Thanks for the suggestions. I'm going to attempt new measurements and see if I can get better results.
Unfortunately, I am traveling right now so it will be a few days before I can do this. I will report back ASAP.


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## jtalden

There is no problem in continuing to use the Decimate IR function. The advantage is a smaller file size. The tradeoff is of no consequence to the particular job we are doing. Either way is fine for our purposes.


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## subterFUSE

Following up on this thread:

I am home from my trip and finally had enough free time to attempt new measurements.

I have changed the setup conditions since my previous attempt. A review of the conditions is below:


Computer = MacBook Pro
Sound Card = Behringer FCA610
Mic = Behringer ECM8000

Mic at listening position, driver's seat, ear level, facing forward in horizontal orientation.

Speakers tested are as follows:

L HLCD = Left Horn
R HLCD = Right Horn
L MB = Left Midbass
R MB = Right Midbass

This time I ran sweeps from 20Hz - 20kHz.
Only 1 sweep instead of 2 as before.

I took measurements with the car windows open, and another set with the windows closed. (attempting to change reflections)


Please review the attached .mdat file and help me determine if these measurements look more valid, and more importantly, the correct impulse alignment if I am trying to change my time alignment delays.

Lastly, I have included the measurement for my sound card calibration. I'm hoping someone can tell me if the sound card is inverting the inputs or not?


Thanks in advance for any assistance!


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## subterFUSE

I took a swing at this myself, but I'm just not sure if this is aligned correctly or not.
Only used the measurements taken with the car windows closed.

Here is what my impulse overlay looks like after I shifted the impulses by the amount of delay I think is required. Does this look correct now?

Delay times applied to the impulses were:

L HLCD = .5491 ms
R HLCD = 0 ms
L Midbass = 1.4072 ms
R Midbass = .6326 ms


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## jtalden

The cal file looks normal. The SC (soundcard) does not invert the phase; the SC phase follows along the 0° line of the SC phase chart. To see the efficacy of the SC cal file we could just do a loopback measurement after the SC cal file is created and active. A good cal file will measure the loopback cable at flat SPL over maybe 10-20k Hz and the phase will track at 0° over that same range. The phase may roll up or down a little at the top end as that is dependent on the exact positioning of the IR. In that we case could always manually adjust the IR position a little to confirm the phase is indeed flat. REW auto IR positioning is very good, but always perfect in this regard.

This is just background FYI. Your SC cal is good - no action needed.

*Measurements:*
Great start! :sn:
The IRs in your chart now look very normal. The IR positioning in the chart looks exactly how I would align them. The next step is to see which relative polarity of the MR to TW works best. You can swap the polarity on one set of the drivers (I would chose the TWs in this case). The relative polarity to that provides the most SPL support through the XO range is the correct one to use. A final step would then be to make small changes to the delay to assure the XO range SPL support is as smooth as possible. Changing the delay by about 20-30° relative to the XO freq is a good choice. This last step in usually not critical to the sound quality, it is more for those of us that like to fine tune the system for best measurements.

I will next take a look a the file and see what additional adjustments I would have suggested.


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## jtalden

I reviewed the data file. While I was optimistic of being able to provide optimized timing advice for your situation, I was not able to do that. There is still a lot of TW phase congestion in the XO range making is impossible for me to do any better than your chart timing above.

I am guessing that this may be because the horns may not have a direct sound path to the mic. If they are under the dash or pointed away from the mic then all sound arriving from them is reflected to some extent. There is still first arriving sound from the closest path, but the other reflections are about as strong and very closely timed, hence the chaos.

If this is the case, Then your chart above is probably the best that can be done. Minor deviations in timing from that setting is not likely to have much impact at all on the SPL support through the XO range like it would when there are fewer close reflections. Even a TW polarity change is not likely very significant although that may still be worth a try.

Possibly there is better advice from someone experienced in car audio.


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## subterFUSE

jtalden said:


> *Measurements:*
> Great start! :sn:
> The IRs in your chart now look very normal. The IR positioning in the chart looks exactly how I would align them. The next step is to see which relative polarity of the MR to TW works best. You can swap the polarity on one set of the drivers (I would chose the TWs in this case). The relative polarity to that provides the most SPL support through the XO range is the correct one to use. A final step would then be to make small changes to the delay to assure the XO range SPL support is as smooth as possible. Changing the delay by about 20-30° relative to the XO freq is a good choice. This last step in usually not critical to the sound quality, it is more for those of us that like to fine tune the system for best measurements.
> 
> I will next take a look a the file and see what additional adjustments I would have suggested.



Awesome! Thanks for the help. I definitely feel like I understand this a lot better now.

My crossover point for the midbass and horns is 800Hz @ 24dB Linkwitz. I was looking at the phase chart at 800Hz and saw that the Left Horn and Right Midbass drivers aligned very well.
The Left Midbass looks like it would align better if inverted, however. I have an invert polarity button in my DSP for each channel, should I use that and then fine tune the delay to get the phase dialed in closer?
The Right Horn is about 80 degrees off from the Left Horn. Inverting it looks like it goes more out-of-phase.
Should I use delay to bring that driver's phase more in line with the others?
That horn is the furthest driver from the LP, so obviously I would just increase delay for all the other drivers by the same amount. I attempted this shift in REW but it took about .28ms of delay, which seems like a lot.

Thoughts?


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## jtalden

Both channels drivers need to be the same. The 2 MR the same and the 2 TWs the same polarity. The relative polarity can be reversed for MRs Vs the TWs though. Just select the setting that gives the best SPL response. There may not be much advantage either way in this case however - just guessing.

I don't think you will learn much from the phase chart due to the various reflections. Moving the window range slightly make the phase tracking look very different. There is no clear way that I found to suggesting one setting is correct or even clearly better than the other. I would rely in the SPL chart to decide.


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## subterFUSE

Just spent an hour listening to the system after applying the delays from my latest measurements. I can happily report that we have achieved the best imaging my system has presented yet. Many thanks!

I did not have to invert any of the drivers' polarity. I tried inverting the left midbass, but that killed the center image.

I did invert the subwoofer by 180 degrees, and then delayed it by about 3 ms. The bass is now well anchored in the front of the soundstage.


Here are some new measurements after the delays were applied. The impulses line up almost exactly as predicted before.


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## jtalden

Good Job!


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## GCG

If that was mine I'd swap the phase of the 2 MB elements or the others (either would work) to get everyone moving the same way (Positive or Negative) out of the gate. May not be noticeable due to the difference in the impulse waveforms but I'd be inclined to try anyway.

Just my $.02

GCG


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## subterFUSE

GCG said:


> If that was mine I'd swap the phase of the 2 MB elements or the others (either would work) to get everyone moving the same way (Positive or Negative) out of the gate. May not be noticeable due to the difference in the impulse waveforms but I'd be inclined to try anyway.
> 
> Just my $.02
> 
> GCG


OK, I'll give that a try after the weekend. :T


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## subterFUSE

GCG said:


> If that was mine I'd swap the phase of the 2 MB elements or the others (either would work) to get everyone moving the same way (Positive or Negative) out of the gate. May not be noticeable due to the difference in the impulse waveforms but I'd be inclined to try anyway.
> 
> Just my $.02
> 
> GCG


Thanks for the tip. You were correct. I measured my response, and then inverted the phase on the horns and measured again. I got better SPL support through the crossover range with the inverted horns. :T


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## subterFUSE

I have another question about this topic...


When recording a measurement for the impulse response in order to determine time alignment, should we use a full range sweep? Or is it ok to limit the sweep range to an area closer to crossover point?

For example, if my midbass and horns are crossed over at 800 Hz, is it ok to sweep from 400 Hz - 1600 Hz?
Or is 20 Hz - 20,000 Hz still preferred?


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## jtalden

Yes, a narrower range should work as well.


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## subterFUSE

Should IR measurements for time alignment be done before or after EQ and X-over are set?

I was trying another go at this today but I couldn't get very good IR measurements. I had done my EQ and X-over settings first this time. Previously, the measurements for time alignment were done first before EQ and X-over.


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## jtalden

The XO changes the delay. EQ around the XO range impacts the delay a little also.

I'd suggest; set the desired XO filters, adjust the driver levels, adjust the delay, and then EQ.
It's a good Idea to confirm the delay is still good after EQ.

For additional delay adjustment after the initial setup just leave the EQ on.

That's my approach. It's the end result that counts so order is not required.


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## subterFUSE

Posting some new measurements taken today. These are only the right side horn and midbass drivers.
No delays applied. Only crossovers have been set, and a tiny bit of EQ to take down some peaks.

I'll be interested to see if we can determine anything about the phase from these?

I was attempting for an acoustic crossover @ 1200Hz. Linkwitz 24 slopes.


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## jtalden

I reviewed the data.
As before, the IRs show very strong early reflections that make it impossible for me to adjust timing using the phase tracking method. If this were in a room situation I would question if both L+R Horns and L+R MR units were both operating during their measurements. The reflections are basically as strong or stronger than the direct sound.

I quickly checked your current timing for SPL support and found it be very good. There is support across the XO range and I did not improve it when I checked a few timing adjustments. Given this setup I would say the current timing is pretty well optimized. In looking at the IR locations the MB is leading the horn a little, but the distance is well within the range that is normal. I believed this alignment is as good as possible.


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## subterFUSE

jtalden said:


> I reviewed the data.
> As before, the IRs show very strong early reflections that make it impossible for me to adjust timing using the phase tracking method. If this were in a room situation I would question if both L+R Horns and L+R MR units were both operating during their measurements. The reflections are basically as strong or stronger than the direct sound.


Not surprising. The horns are mounted underneath the dash so there is almost do direct path to the LP. The sound is going to be reflecting quite a bit.



> I quickly checked your current timing for SPL support and found it be very good. There is support across the XO range and I did not improve it when I checked a few timing adjustments. Given this setup I would say the current timing is pretty well optimized. In looking at the IR locations the MB is leading the horn a little, but the distance is well within the range that is normal. I believed this alignment is as good as possible.


This was actually my exact thought, too. When I compared the overlays of each driver independently with the combined response, I saw the SPL support was good. Keep in mind that my house curve has quite a tilt to it because in a car environment we skew heavy on the bass.

This is a higher crossover point than I have traditionally attempted to use. My horns can play down to about 800Hz if I use 24 dB slopes. This time I was attempting a 1200Hz acoustic crossover point. However, it would appear that my measured crossover point was more like 1300Hz? Do you agree?

I was initially surprised that the SPL support was so good without any delays on the drivers. But when I think back to some of the threads about Linkwitz 4th order crossovers being 360 degrees out of phase, then it makes sense that a midbass driver which is closer to the LP might align well without any delay. Especially when the distance between the horn and the midbass is about 1 wavelength @ the crossover range. I think 1300Hz is about 10 inches, correct? That's about how much closer the midbass driver is to the LP vs. the horn.

Is my understanding of this on target?


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## Niick

Have you ever considered performing the alignment using the phase trace?

Edit: oh , missed that part about early reflections making it impossible........


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## Niick

but impossible? hmmm..........windowing?????


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## subterFUSE

I believe in my situation, since I am using horn-loaded compression drivers that are mounted under the dash, there really isn't much direct sound. Everything is reflected in some way. So I don't know if even windowing could clear the phase traces up enough to be helpful?


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## Niick

No you're probably right,man that sounds very interesting. I'd love to get ahold of that project and throw my "rolly cart" on it..........I'm actually working on a really cool experiment right now, I'm at work, but is slow......


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## subterFUSE

What are people's opinions on whether or not I should be sitting in the driver seat during measurements?

These measurements thus far have been with me outside the car. I'm wondering if I should be in the car during measurements, however?


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## JohnM

Another plot which can be interesting when looking at time alignment is a wavelet spectrogram, I've been adding that to REW lately. Here is a normalised wavelet spectrogram of the combined right side data, with the peak energy arrival shown by the dashed line. Beta versions with the wavelet spectrogram are available for Windows and OS X.


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## subterFUSE

JohnM said:


> Another plot which can be interesting when looking at time alignment is a wavelet spectrogram, I've been adding that to REW lately. Here is a normalised wavelet spectrogram of the combined right side data, with the peak energy arrival shown by the dashed line. Beta versions with the wavelet spectrogram are available for Windows and OS X.
> 
> View attachment 96241


Very cool, although I have no idea what I'm looking at. LOL 

Is the goal to have the dashed line be smooth, with few of those vertical "jumps?"


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## Niick

So, as far as sitting in the car or out, I have definitely tuned both ways, i honestly think that for the most part, it doesn't matter. The parameters that we adjust when optimizing a car audio system, its been my experience, that any differences that you'd end up with at the end of the process would be so small that you couldn't rule out normal variation/tolerance. Do ya know what i mean. in fact, now that i think of it, Id say in most cases out of the car would be recommended because sometimes when were in a car and trying to tune a system, we have a greater chance of accidentally (unknowingly?) affecting our measurements, due to our physical body positioning (legs, arms blocking/unblocking sound propagation) and us moving about. Id say, try both and see what differences you come up with.


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## Niick

I've been doing experiments with measuring the step response of a system before and after alignment. Very interesting, this could be another way to verify the coherence of the different drive units thru crossover. I wish I had more time to post the pics today, but we're closing in 30 min.


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## Niick

JohnM said:


> Another plot which can be interesting when looking at time alignment is a wavelet spectrogram, I've been adding that to REW lately. Here is a normalised wavelet spectrogram of the combined right side data, with the peak energy arrival shown by the dashed line. Beta versions with the wavelet spectrogram are available for Windows and OS X.


John, this wavelet analysis look really interesting. Can't wait to experiment with that one, for sure!! On a different note, I've been experimenting with a way to be able to reform a transfer function measurement on a system that has no means of getting a signal into it, like a lot of OEM Car systems, when all they have is CD or USB. What I did was come up with a 131k periodic pink noise signal with a synchronization pulse every 3 seconds, this way I can line up the measure and reference in SysTune and once I get them aligned, turn on auto delay tracking to account for the drift (because the two clock sources aren't syncronous) and WHAMO, it worked!!........kinda, I got good data up till 4500Hz. Here is a pic of the signal I am using.

Any chance you might incorporate some type of triggered sweep, where we can play the excitation signal thru a cd or USB stick?


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## jtalden

subterFUSE said:


> This time I was attempting a 1200Hz acoustic crossover point. However, it would appear that my measured crossover point was more like 1300Hz? Do you agree?


 The crossing point changes a little depending on smoothing used. I just looked at 1/3 smoothing and that way it is near 1400. Minor differences like this between the electrical and acoustic XO frequencies is common. There is nothing to be concerned with.



> I was initially surprised that the SPL support was so good without any delays on the drivers. But when I think back to some of the threads about Linkwitz 4th order crossovers being 360 degrees out of phase, then it makes sense that a midbass driver which is closer to the LP might align well without any delay. Especially when the distance between the horn and the midbass is about 1 wavelength @ the crossover range. I think 1300Hz is about 10 inches, correct? That's about how much closer the midbass driver is to the LP vs. the horn.
> 
> Is my understanding of this on target?


 Yep, 1300Hz is about 10.4" WL. 

Your MB is leading the Horn a little and as such the direct sound phase tracking is probably not ideal. It is likely, as you suggest, that the phase is crossing near the 1300Hz XO with the MB leading by maybe 1 WL. This is not a bad situation. It is in fact a very good setup. This can provide a reasonably flat phase and GD response with the only the XO range phase being irregular. The more important SPL support is still strong. 

In a room with vertically offset drivers the frontal lobe would shift up and down a little in the XO range, but even there, that is not a major influence in most cases. In your car setup I would think this is no issue at all.


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## JohnM

subterFUSE said:


> Very cool, although I have no idea what I'm looking at. LOL
> 
> Is the goal to have the dashed line be smooth, with few of those vertical "jumps?"


Pretty much. A conventional spectrogram is produced using a fixed window width, which gives it the same time resolution at all frequencies. The frequency resolution is similarly a fixed number of Hz, 10 Hz if a 100 ms window was used, for example. At low frequencies that is a big octave fraction (1/1.4 octaves at 20 Hz), at high frequencies a very, very small octave fraction (1/1386 octaves at 20 kHz). For a time-frequency plot it would be handier if the tradeoff between time and frequency resolution varied with frequency, with higher time resolution at high frequencies and lower at low frequencies. A wavelet transform can achieve that, specifically a constant Q wavelet transform. A constant Q wavelet transform is mathematically equivalent to using a frequency-dependent window to produce the spectrogram (which is what REW does). For reference, this is how a perfect impulse looks with the same settings.


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## JohnM

Gennelle51187 said:


> What I did was come up with a 131k periodic pink noise signal with a synchronization pulse every 3 seconds, this way I can line up the measure and reference in SysTune and once I get them aligned, turn on auto delay tracking to account for the drift (because the two clock sources aren't syncronous) and WHAMO, it worked!!........kinda, I got good data up till 4500Hz. Here is a pic of the signal I am using.
> 
> Any chance you might incorporate some type of triggered sweep, where we can play the excitation signal thru a cd or USB stick?


I'm not sure why you need the sync pulse, the PN is periodic anyway so I'd have thought SysTune could align to the reference without anything else.

Generating responses with offline signals is on the REW todo list.


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## Niick

You're right John, I was over complicating it. What had me confused was the fact that in SysTune, the custom file option for excitation signals only allows just over 500,000 samples, so at 44.1k I think is right around 11 seconds. So any custom signal used, if longer than that, is chopped to 11 seconds. It never occurred to me to just record a long time file (30+min.)of the built in generator of SysTune, then burn that to disc, then use the built in generator of SysTune as my reference, and play the disc. So that's what I did today, and it worked like a champ!


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## Niick

Got another question for ya John, I can't figure out why, when I select Periodic Pink Noise in the generator in REW, and select the file button, to save the signal to wav file, then import the signal into SysTune, it looks like this: And I've tried reducing the level of the signal in REW before saving, as low as -22dB, doesn't seem to matter? The signal looks clipped, or am I missing something? The second pic is the built in generator.


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## JohnM

The PN sequences REW generates are optimised to have a crest factor (ratio of peak level to rms level) that does not exceed 6 dB.


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## Niick

JohnM said:


> The PN sequences REW generates are optimised to have a crest factor (ratio of peak level to rms level) that does not exceed 6 dB.


Ok, awesome! See that's very handy to know, I never would have known that had I not imported the signal to SysTune. Even when I try to view it on my scope, it isn't as apparent as in SysTune. So it's not clipping at all, which is what I found when I zoomed in real close, hence the confusion. Thanks John!

One more thing, can you elaborate a little on why you made the signal this way. Potential benefits and downfalls?


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## JohnM

It's done to avoid clipping, since pink noise otherwise has a crest factor of around 13 or 14 dB so it clips easily at higher levels. There's no downside I'm aware of.


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## roomeqtester

@subterFUSE: Congratulations. What head unit and what amp do you use?


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## subterFUSE

roomeqtester said:


> @subterFUSE: Congratulations. What head unit and what amp do you use?


2013 Audi S6

Head Unit = Audi MMI with Optical TOSlink output provided by mObridge DA1 preamp
DSP = Helix DSP Pro

Secondary Source = Audison Bit Play HD (FLAC media player with Optical TOSlink output)

Amps:

1 x Sinfoni Prestigio Class A
2 x Sinfoni Presto
1 x Sinfoni Grave

Speakers:

Eric Stevens Ultra Horn-Loaded Compression Drivers (Full-Size Horn Bodies)
Beyma 8G40 8" midbass
2 x Dynaudio Esotar 1200 subwoofers (Infinite Baffle)


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## 3ll3d00d

JohnM said:


> Pretty much. A conventional spectrogram is produced using a fixed window width, which gives it the same time resolution at all frequencies. The frequency resolution is similarly a fixed number of Hz, 10 Hz if a 100 ms window was used, for example. At low frequencies that is a big octave fraction (1/1.4 octaves at 20 Hz), at high frequencies a very, very small octave fraction (1/1386 octaves at 20 kHz). For a time-frequency plot it would be handier if the tradeoff between time and frequency resolution varied with frequency, with higher time resolution at high frequencies and lower at low frequencies. A wavelet transform can achieve that, specifically a constant Q wavelet transform. A constant Q wavelet transform is mathematically equivalent to using a frequency-dependent window to produce the spectrogram (which is what REW does). For reference, this is how a perfect impulse looks with the same settings.
> 
> View attachment 96265


nice work, a few Qs for you

1) how does this compare to a s(tockwell) transform? (superficially they seem like quite similar things)
2) what does the dotted line really mean? it is described as the "peak energy curve" but I'm not sure what that actually means?
3) should one normalise or not?


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## JohnM

ST and CWT with Morlet wavelet are very similar when looking at amplitude. ST retains absolute phase information, but that's not relevant for the uses these plots are put to.
The dotted line shows where the maximum level occurs at each frequency.
If the main interest is relative timing normalisation is helpful in seeing where the peak occurs vs frequency, but of course it very much distorts the amplitude since it lifts the content at each frequency up to achieve the same peak value.


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## subterFUSE

OK Guys,

I was giving this yet another try recently, this time focusing on phase aligning my subwoofer to my midbass speakers. See the attached MDAT...

Trace 1 is the Subwoofer by itself, with no time delay.

Trace 2 is the Midbass speakers playing together without the Sub.


I compared those traces in the Overlays Window using the Phase tab. I then adjusted delay on my sub and took additional measurements until I got the phase to align at the crossover range. 80Hz is where the sub and midbass are crossed over.


Trace 3 is the Sub with 1.59ms delay. This was the delay I found that aligns the phase well at the crossover. At 80 Hz, the sub and MB are showing 18 degrees phase, and the phase plot looks very stable through the crossover.

Trace 4 is the Subs w/ 1.59ms delay and the Midbass speakers playing together. I did this to observe the SPL support through the crossover, which to me looks like it was good.



For further confirmation, I applied Filtered IR @ 80Hz for the Sub w/ 1.59ms delay and the MB speakers. The filtered IR peaks line up very well in the Overlays Window for Impulse.

Let me know if this looks like I'm on the right track?


Thanks!


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## jtalden

Yes, that timing is optimized.


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## subterFUSE

:yay2:


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## Niick

subterFUSE said:


> I did invert the subwoofer by 180 degrees, and then delayed it by about 3 ms.................Here are some new measurements after the delays were applied.


 subterFUSE, I was reading over these old posts as driver to driver alignment thru crossover is an area of great research and learning for me. I perform car audio system tunings at my work, therefore I have the rather unique perspective of having worked with a very wide range of different drivers, cars, processors, etc. I see that you said you inverted the subwoofer by 180 degrees, but your IR chart clearly shows otherwise. The subwoofer polarity hasn't changed from one measurement to the next. Were you by any chance using the Helix DSP when these measurements were taken? I also experienced the same thing (when using the helix) where as most processors allow a simple polarity inversion of each channel, the helix is different.


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## subterFUSE

Sorry.... you are correct.

I did not invert polarity. I adjusted the phase control to 180 degrees. The phase control on the Helix DSP Pro is realized by a 2nd Order Allpass filter.


Edit: Also, I think the images you linked are pretty old.... when i had even less of a clue what I was doing than I do now. LOL

I'm busy tuning my car for INAC Finals this weekend, so I'll try to post some new measurements here that might be more informative.


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## Niick

subterFUSE said:


> Sorry.... you are correct. I did not invert polarity. I adjusted the phase control to 180 degrees. The phase control on the Helix DSP Pro is realized by a 2nd Order Allpass filter. Edit: Also, I think the images you linked are pretty old.... when i had even less of a clue what I was doing than I do now. LOL I'm busy tuning my car for INAC Finals this weekend, so I'll try to post some new measurements here that might be more informative.


 awesome, that should be fun. I'm currently working on a way to determine how the different crossover selection, different timing, etc. affect the transient response of a system. I have come up with a way to measure, in real time, the step response, and filter the signal coming from the mic to just the range of frequencies shared by the drivers that are sharing a crossover, so, in other words, the crossover region. So far, using this method has proved fruitful. I also have begun making recordings of live instruments, (drums so far) using the same measurement mics that I use to tune systems. This way, the mics own freq. response and IR characteristics are nullified, plus, since I was the one who made the recordings, I know what the live event sounded like. So much fun and an incredible learning experience.


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