# Time Alignment between mid + tweeter -- measuring step response using REW



## rlee777 (Apr 28, 2009)

Found a good method of time-aligning the mid/tweeter transition using the Step Response measurement function within REW.

System: Two-way active system + sub with MiniDSP for full range EQ. Using REW for measurements.

Background: Started with tape measure T/A for ballpark settings. Used HolmImpulse to align the Left Mid and the Right Mid. However, when I adjusted the tweeter vs mid T/A settings, HolmImpulse was not consistently relaying changes. The REW acoustic timing ref measurement was not consistent either.

Found an interesting article on step response measurement with speakers -- https://www.stereophile.com/content/measuring-loudspeakers-part-two-page-3

REW has an effective step response measurement built-in under the IMPULSE tab. The BEFORE attachment shows what the step response looks like before tweaking.

Here is the basic process:

1. T/A the left + right mids using tool of choice (I used HolmImpulse)
2. Turn off the sub (step response measurement works best with HF range)
3. Run the MEASURE sweep with REW (one channel, both mid LF and tweeter HF active)
4. Select the IMPULSE tab and confirm that STEP RESPONSE is selected on the bottom
5. You may see two peaks -- the sharp peak would be the tweeter/HF driver, the peak with the slow rise+decay would be the LF driver
6. Adjust the tweeter/HF T/A delay to align the upper peaks as much as possible. Note that you may see multi spike with the LF driver due to reflections (the Stereophile article has speakers measured in an anechoic environment, so the graphs in our cars will not look as clean)

I found the imaging to improve significantly after aligning the HF and LF drivers -- sounds like I have a center channel in the dash. Integration between the drivers is now excellent. The nice thing is how accurately T/A adjustments are shown in REW -- even changes as small as 0.02ms.


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## dcfis (Sep 9, 2016)

What are the two charts?


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## Ifixtheinternet (Jan 29, 2016)

dcfis said:


> What are the two charts?


Looks like the impulse response for the mid and tweeter measured together.

First one is with time alignment roughly set with measuring tape, showing 2 different peaks.
Second one is after tweaking the time alignment to get the initial impulse from both drivers to line up.

I've tried several methods for time alignment with my 80prs, (auto-TA with mic, using pink noise with reversed phase, sine waves) and the ear methods are tedious and difficult to do.

This gives a good visual representation that makes it easy to see what is going on. 

I have a Umik-1 and REW, and I want to try this to see what I get.

It will be interesting to see how close it is.









Sent from my Pixel XL using Tapatalk


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## rlee777 (Apr 28, 2009)

Looks like the attachment names did not transfer -- the top chart is the BEFORE and the second chart is the AFTER (step response charts out of REW for a single channel). The linked Stereophile article can explain what is being shown here.

Using the powerful MINIDSP with this REW step response adjustment, with the minimal EQ filters (https://www.diymobileaudio.com/forum/5607087-post90.html) my system has never sounded better. I have had a good house curve (B&K with bump in sub bass) with smooth EQed response, but dynamics and imaging were just not there. Listening to a time-aligned (coherent) system does wonders for imaging/staging with upfront midbass that is very musical.

Good tuning with a solid DSP can transform a system dramatically.


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## Holmz (Jul 12, 2017)

Are you happy with that impulse response?


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## rlee777 (Apr 28, 2009)

After a week with the tweeters + mids aligned this way, I am convinced that this is much more precise T/A than using a tape measure method. I went ahead and used this method on our second car that also has a MiniDSP running full range active also with excellent results. Interesting enough, in both cars, only the tweeter delays needed to be increased to match up the step (combined impulse) response over using tape measure T/A conversion. 

Precise imaging and a transparent quality that makes the drivers hard to localize. After working hard to EQ toward the target "house" curve, confirming proper polarity for all drivers, and finding the best Xover+slope for this 2-way + dual sub setup, this step response measurement for T/A was the final tweak that really has me enjoying the music once again.


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## Truthunter (Jun 15, 2015)

Thanks for posting this. It furthered my understanding of the effects of crossovers have on coherence between the drivers. And confirms what I had discussed with others about having to add some more delay to the high passed drivers in order to get everything playing at the same exact time.


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## subterFUSE (Sep 21, 2009)

Using impulse response to time align drivers is essentially a much faster way of doing measuring tape, without the measuring tape.

The ETC view can help you see the impulse response start more clearly because it accentuates the data by squaring all the values. Gives you a faster initial rise, in other words.

The Phase Plot is the best method for aligning drivers at the crossover point, although for midranges and tweeters it is very difficult to measure phase without good windowing. Phase Plot is much easier with subwoofers and midbass transitions. Smaart is the only software available that does a good enough job with phase at higher frequencies to be able to reliably measure it for mids and tweeters.


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## CDT FAN (Jul 25, 2012)

rlee777, How far away from the midrange is the tweeter?


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## Holmz (Jul 12, 2017)

I may be easier to look at the impulse response from each speaker separately first.

I am not sure if that 1mS secondary peak is the impulse response of the system, or a reflection/echo. Generally the tighter the impulse response the better, and that seems a bit smeared out. But the impulse peak is a function of the bandwidth so the midrange will be more difficult to resolve as it is lower frequency than the tweeter.


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## rlee777 (Apr 28, 2009)

Holmz said:


> I may be easier to look at the impulse response from each speaker separately first.
> 
> I am not sure if that 1mS secondary peak is the impulse response of the system, or a reflection/echo. Generally the tighter the impulse response the better, and that seems a bit smeared out. But the impulse peak is a function of the bandwidth so the midrange will be more difficult to resolve as it is lower frequency than the tweeter.


Definitely not as clean as the classic step response graphs. But do note that we are not measuring in an anechoic environment as in professional loudspeaker reviews. Reflections smear the impulse response graph for the midbass, but the HF tweeter is pretty clean. 

What is fascinating is to vary the T/A (adjust the tweeter delay since the midbass is pretty accurate with the tape measured delay) and watch the step response graph shift -- you can easily see the slower LF driver response shift along the horizontal axis. Then you can see how the first peaks from each driver should line up together. Easy and fast to measure with REW sweeps. The tighter the initial peaks of both drivers coincide, the better -- as you mentioned.

The difference with a time-aligned (time-coherent) full range system is clearly audible, even in the reflective car environment.


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## rlee777 (Apr 28, 2009)

CDT FAN said:


> rlee777, How far away from the midrange is the tweeter?


Here are some of the numbers:

C-C distance between A-pillar mounted tweeter and lower door mounted midbass = 20 inches

Tape-measured distance from each driver to driver's head with calculated delay (www.tracerite.com/calc.html)--
LT 27 in 2.49ms
LM 33 in 2.05ms
RT 46.5 in 1.06ms
RM 52 in 0.66ms
Sub 61 in

After T/A alignment with the step response method (MiniDSP .02ms intervals):

LT 3.3ms
LM 2.06ms
RT 1.78ms
RM 0.68ms

The midbass T/A remained essentially the same as the distance (tape) measurement method. However, do note that the tweeter delay was substantially increased to create an aligned step response graph. Not sure why this is the case (maybe dome tweeters vs cone midbass?), but the REW step response graph ability to track delay changes is very consistent and repeatable. And the audible improvement in coherency and soundstage imaging is not trivial.


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## subterFUSE (Sep 21, 2009)

rlee777 said:


> Here are some of the numbers:
> 
> C-C distance between A-pillar mounted tweeter and lower door mounted midbass = 20 inches
> 
> ...




Do you have crossovers implemented when doing the step response adjustment method? 

If yes, what is the acoustic crossing point and approximate slope and filter type?


I am wondering if the difference you are seeing is the result of the phase rotation induced by the crossovers. In a typical Linkwitz-Riley 4th order filter alignment, the high frequency driver's phase gets rotated 180 degrees forward, while the low frequency driver's phase gets rotated 180 degrees backwards. The result is 360 degrees of phase shift, which is "in-phase" but technically it is a full wave period change with the high freq driver leading the low. A delay on the HF driver of 1 period at the crossover frequency would bring the impulse responses back into alignment.

If we know the crossover point you are using and the filter order, we could determine how much phase shift is expected and whether that difference coincides with your delay settings.


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## Truthunter (Jun 15, 2015)

subterFUSE said:


> Do you have crossovers implemented when doing the step response adjustment method?
> 
> If yes, what is the acoustic crossing point and approximate slope and filter type?
> 
> ...


This is the most straight forward explanation, that I've yet to read, of what I've been trying to wrap my head around. Thank You Sir!


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## rlee777 (Apr 28, 2009)

subterFUSE said:


> Do you have crossovers implemented when doing the step response adjustment method?
> 
> If yes, what is the acoustic crossing point and approximate slope and filter type?
> 
> ...


You might be onto something here: running active with a Xover at 2200Hz with 48db/oct LR slopes, so would that theory apply since each driver would have 360 degree shift?

Perhaps that is why many purists prefer the sound of lower order crossovers due to this time alignment shift. If so, then compensating the HF delay would bring the best of both worlds (steep slopes to protect drivers + time coherency)


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## subterFUSE (Sep 21, 2009)

rlee777 said:


> You might be onto something here: running active with a Xover at 2200Hz with 48db/oct LR slopes, so would that theory apply since each driver would have 360 degree shift?
> 
> 
> 
> Perhaps that is why many purists prefer the sound of lower order crossovers due to this time alignment shift. If so, then compensating the HF delay would bring the best of both worlds (steep slopes to protect drivers + time coherency)




Steeper slopes yield more phase shift. 48 dB Linkwitz filter is actually 720 degrees of phase shift at the crossover point.

A butterworth 12 dB alignment is 180 degrees out which is why we invert polarity on 1 driver to compensate.


At 2200 Hz the period is .45 ms

2 periods would be .9 ms

How much different were your delays? I can’t see that older post in Tapatalk while I’m typing this. 


Sent from my iPhone using Tapatalk


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## subterFUSE (Sep 21, 2009)

OK your delays went from 2.49 ms to 3.3ms on the tweeter.

That is a .81 ms difference, which is very near the .9 ms expected at 2200 Hz for 720 degrees of phase shift induced by a Linkwitz 8th order filter.

The .09 ms difference from expected could be due to a minor difference in accuracy of the tape measure method at the beginning. It's not always possible to get the tape measure all the way to where the voice coils are located, and there is also a minor difference in measuring the point in space for the listening position.

Other than that, I think the crossover phase rotation explains the .81 ms.


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## CDT FAN (Jul 25, 2012)

I have been wondering if adding a bass blocker cap to my active tweeters would affect the time alignment. I guess this answers that question.

In my particular case, both the mids and tweeters are next to each other in the kick panel, so the PLD between them is negligible, but I was thinking about trying the tweeters in the sail panel to raise the soundstage. According to some experts, the time alignment of the higher frequency drivers isn't as important as the height (that we don't need to worry about PLD), but that seems to contradict this discussion.

So, I guess you're saying that you can hear a difference.




subterFUSE said:


> Do you have crossovers implemented when doing the step response adjustment method?
> 
> If yes, what is the acoustic crossing point and approximate slope and filter type?
> 
> ...


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## rlee777 (Apr 28, 2009)

CDT FAN said:


> So, I guess you're saying that you can hear a difference.


If you are using a DSP in a full range active config, this is easy to adjust and see if you can hear differences. What is nice about using REW to measure step (impulse) response is how consistent and precise this process is -- simple to "zero" in the T/A between LF mid and HF tweeter.

(My home theater has constant directivity HF horns which are time-aligned well with the LF driver, so I am used to a broad and precise soundstage. That may be why I am more sensitive with time coherency between LF+HF drivers.)


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## gijoe (Mar 25, 2008)

CDT FAN said:


> I have been wondering if adding a bass blocker cap to my active tweeters would affect the time alignment. I guess this answers that question.
> 
> In my particular case, both the mids and tweeters are next to each other in the kick panel, so the PLD between them is negligible, but I was thinking about trying the tweeters in the sail panel to raise the soundstage. According to some experts, the time alignment of the higher frequency drivers isn't as important as the height (that we don't need to worry about PLD), but that seems to contradict this discussion.
> 
> So, I guess you're saying that you can hear a difference.


TA is still valuable for the lower high frequencies, but as you go higher TA becomes less and less important. 

Think of it this way, the wavelength of a 12k sound is about 1". So, half an inch of movement puts it 180 degrees out of phase, and just 1/4 of an inch of movement puts it 90 degrees out of phase. Our heads move that much just sitting still and breathing. If we relied on phase for high frequencies, then all of the high frequency noise we hear would be constantly changing with even the slightest head movements, it would drive us crazy unless we sat perfectly still all the time. 

Don't ignore the TA on your tweeters, especially if they are playing pretty low, but a tweeter playing down to 2khz has a wavelength of almost 7", 180 degrees out of phase is about 3.5", so at 2khz TA becomes much more important.


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## Holmz (Jul 12, 2017)

gijoe said:


> TA is still valuable for the lower high frequencies, but as you go higher TA becomes less and less important.
> ...


Batman would be dizzy reading this....


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## CDT FAN (Jul 25, 2012)

gijoe said:


> Don't ignore the TA on your tweeters, especially if they are playing pretty low, but a tweeter playing down to 2khz has a wavelength of almost 7", 180 degrees out of phase is about 3.5", so at 2khz TA becomes much more important.


So, if I understand correctly, at 4khz the distance would be 1.75" with a 12db crossover (180 degrees). With a 6db crossover (90 degrees), it would be .88" out of alignment. So, if my tweeter's voicecoil was .88" closer than the mid's, it would actually be back in phase. At least at 4khz.


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