# 2 channel input?



## bmxbandit (Jul 25, 2015)

Does REW support 2 channel (stereo) input?

I have a pair of matched mics in an ORTF arrangement and would like to measure in stereo.

As far as I can see only L or R can be selected as input channel.

Additionally, thanks for providing this software. I have spent the last few days comparing free acoustic measurement software and REW looks the best out there to me


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## JohnM (Apr 11, 2006)

REW only analyses one channel at a time, though support for multiple input channels is on my todo list.


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## Laplace (Aug 10, 2015)

JohnM said:


> REW only analyses one channel at a time, though support for multiple input channels is on my todo list.


Regarding this issue i have a similar one:

Can REW together with the minidsp UMIK-1 microphone perform a 2-channel measurement?

For my technical understanding it is nearly impossible to get a hard timerelation between the digital usb-port as first channel and the da/ad-converted output/input of the soundcard as second channel.

Anybody knows better?

Best regards,
Michael


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## JohnM (Apr 11, 2006)

No, a fixed timing relationship cannot be established between the UMIK and another source. An acoustic reference could be created by positioning a HF driver close to (and at a fixed distance from) the mic (e.g. a tweeter) and using the peak of the IR from that driver as a timing reference for the later arriving signals of the driver being measured. Doing this more controllably with a timing ref signal at a suitable timing offset from the main measurement signal is one of the many things on my todo list.


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## Laplace (Aug 10, 2015)

I guess i understand. To check my understanding i describe now a measurement procedure to determine the difference between the points of sound wave generation of two speakers (see picture "step_s") with the goal of importing the frequency- and phase response in boxsim to calculate/design a crossover with it.

This procedure contains two steps.

Step 1: Determing the actual propagation velocity of sound wave










Measurement 1 (IR) delivers t1, then changing the position of same speaker from s1 to s2. Measurement 2 (IR) delivers t2.
Two different distances s1 and s2 result in two different times t1 and t2.

=> c = (s1 - s2) / (t1 - t2)

Step 2: With the knowledge of c i can now determine the offset of tweeter and woofer in a common baffle










IR-Measurement of tweeter delivers t1, another IR-measurement of the woofer delivers t2.

The offset can now calculated as

offset = s2 - s1 = (t2 - t1)*c

Would this procedure working?


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## JohnM (Apr 11, 2006)

Not with two separate measurements, no, because the software has no absolute reference to use to align the two measurements. The trick with another driver is to in effect measure two speakers at once, the impulse response from the nearer speaker will appear earlier in time so its peak can act as a reference. If a subsequent measurement is made with that reference speaker at the same position relative to the mic it can again provide an earlier peak against which the two measurements can then be aligned. Using a tweeter as the reference driver ensures it has a compact impulse response and doesn't contaminate the response of the main speaker being measured.


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## Laplace (Aug 10, 2015)

Hi John, i feared exactly this answer that is to say that two subsequent measurements in REW are not exaktly reproducible regarding to timing. With this in mind your suggested procedure is logical.

With your procedure it should also be possible to measure the propagation velocity of sound wave in a first step that is to say to perform two subsequent measurements with different well known distances between the "tweeter" and the DUT. The resulting time-difference of both measurements in IR between "tweeter" and DUT can be used to calculate c like mentioned in my last post.
And then the same procedure with the "tweeter" again and the different DUTs in the same baffle (woofer and real tweeter ).

Is it right?

There is only one problem i see:
The near to mic position of the "tweeter" is a problem for a fine IR of the DUT far behind, isn't it.


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## JohnM (Apr 11, 2006)

You could do that, though the speed of sound in air is pretty well understood already 

Having the tweeter IR won't contaminate the main measurement as long as there is sufficient difference in distance, which may only need to be a metre or two - to check that measure the tweeter on its own and see what the tail of the IR looks like.


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## jtalden (Mar 12, 2009)

Laplace,
There is another workaround for determination of the current XO timing when using a USB mic. It only requires 3 normal measurements; HF driver, LF driver and the both together. The accuracy is very high and it works for any XO Frequency. Trace arithmetic can then be used to determination the change in delay needed for ideal timing. 

If it will be helpful, I can either detail the method for you, or I could do the analysis on your measurements and determine the change in delay needed.


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## Laplace (Aug 10, 2015)

JohnM said:


> You could do that, though the speed of sound in air is pretty well understood already
> 
> Of coarse, but think about the temperature-dependancy. I guess to calculate c as c = (331,5 + 0,6 T/°C) in m/s makes the measuring of c obsolete .
> 
> ...


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## Laplace (Aug 10, 2015)

jtalden said:


> Laplace,
> There is another workaround for determination of the current XO timing when using a USB mic. It only requires 3 normal measurements; HF driver, LF driver and the both together. The accuracy is very high and it works for any XO Frequency. Trace arithmetic can then be used to determination the change in delay needed for ideal timing.
> 
> If it will be helpful, I can either detail the method for you, or I could do the analysis on your measurements and determine the change in delay needed.


Thanks for your offer. A more detailed description would be nice, *but only* when the result can be used for designing the filter in the software "boxsim". For exact results this software needs the offset in propagation direction between the two channels for correct phase and therefore correct amplitude response simulation.


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## jtalden (Mar 12, 2009)

I wasn't thinking of boxsim input requirements when I posted. I don't know enough about exactly what form the info is needed for boxsim having never used it or similar passive XO design simulation software. 

My previous use of this workaround involves measuring the drivers on a baffle with the XO activated. There is only one delay alignment that optimizes the phase tracking for a given XO implementation. The workaround can determine the current delay timing and the delay change needed to achieve the ideal timing. Other 'good' timings can also be identified. Those don't achieve ideal phase tracking, but they do align the phase at the XO. The SPL support in the XO range is very good for any of the good alignments.

I'm guessing that for passive design software they don't implement any XO, but instead measure the raw drivers on a baffle without any XO applied. If boxsim requires the in 'acoustic center' of the 2 drivers vs a baffle mounting position that may still possible using this workaround, but it still requires the 3 measurements. The mic would need to be equidistant from the point on the baffle plane where the axial center of each driver crosses that plane.

If you don't have two drivers already mounted on a common baffle for this test then this is more work on your part. Since I haven't tried this without an XO active. I am also not certain that it will be as effective in providing a accurate results as it is for cases with the XO active. I think it has a good chance of being successful. Wide range sweeps are needed for all 3 measurements covering the entire XO range and probably an octave beyond. Care would be needed to limit the levels so as to not damage a driver.

Possibly there are other options that are safer and more attractive. It is just offered as a way to get around the timing limitation of REW when using a USB mic.


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## Laplace (Aug 10, 2015)

jtalden said:


> ...
> 
> I'm guessing that for passive design software they don't implement any XO, but instead measure the raw drivers on a baffle without any XO applied. *If boxsim requires the in 'acoustic center' of the 2 drivers vs a baffle mounting position that may still possible using this *workaround, but it still requires the 3 measurements. The mic would need to be equidistant from the point on the baffle plane where the axial center of each driver crosses that plane.
> 
> ...


Hello jtalden,

the bold marked sentence of your last post is exactly that, what i need.

For a correct simulation boxsim needs the perpendicular distance between baffel and "acoustic center" of each driver (in Germany we say "Schallentstehungsort") for consistence simulation of phase and SPL.

I'm still interessted in your work around.

The arrangement of drivers and mic for the method of John M could also be done by the following way i guess:










The maybe lower sensitivity of the mic in this 90° case doesn't matter. It's for both sides and only the timing is of interest.

Best regards


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## jtalden (Mar 12, 2009)

Yes, you are correct, the mic sensitivity is not a factor in this process.

Below, I modified your earlier setup chart to show the needed setup for this method.

I am assuming a 2kHz or so target XO with a dome and 12-18cm mid woofer for the following. If this is not correct we may need to adjust the following setup. 

The mic is placed about 50-70 cm from the baffle the distance is not critical but we want a reasonably clean IR measurement (avoid major room influences if possible) and want to be as far enough that the angle of the mic to the center of the speakers is as small as possible. 50-70 cm from the baffle should work well based on my experience. 

The mic should be placed in the middle of the two drivers instead of on axis for all 3 measurements. Do not move the mic. Take a little care in centering the mic as it does impact the results. It is not overly sensitivity however so no need to worry about being 2-4mm off center. We want the length of the red lines shown to be equal if possible. They extend from the intersection of the speaker axis and the baffle to the capsule of the mic.

Sweep ±3 octaves from your intended XO frequency. The level can be low to avoid any possible damage to the TW.

Post the mdat and I will see if the process works as expected in this situation. The basic process is to use the step response charts to determine the relative timing alignment of the 2 drivers. The combined MW+TW step response measurement can be used to see exactly how the timing is related between the 2 drivers. From there we can make the adjustments needed to determine the acoustic centers relative to the baffle.

I will provide a brief explanation of the process and results and answer and questions.


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## jtalden (Mar 12, 2009)

Forgot to attach the setup picture. :doh:


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## Laplace (Aug 10, 2015)

jtalden said:


> Forgot to attach the setup picture. :doh:
> 
> View attachment 99033


I guess i understand your suggested procedure.
This should work *with only one measurement* i guess. The problem is the *principal error of* these method.
The real difference between the "acoustical centers" of both drivers is always bigger than the difference between the distances driver <-> micro.
The bigger the distance of both drivers on baffle or the less the distance between baffel and micro the bigger the error. I guess if the distance baffel <-> micro is *5 to 10 times bigger* than the distance woofer <-> tweeter the *error is negligible* - isn't it?


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## jtalden (Mar 12, 2009)

The process requires all 3 measurements. 

> We need the IR of each driver by itself. 
> We need the IR of both drivers operating together.
> We need the relative mic distance difference to each drivers baffle.
[I suggested we position the mic to as shown above to make the baffle distance equal so we simplify setup and minimize errors in accounting for different mic distances.]
> We use the Step Response (SR) of the combined measurement vs the separate driver SRs to determine the relative difference in arrival time of the 2 drivers. [This is the step that accounts for the loss of relative timing when using a USB mic for different measurements.]

> Then, knowing this information, we can shift the IRs of both drivers until the phase is at 0° through its bandpass frequency range. 

> Finally, the sum of the change in the shift of the 2 IRs is equal to the difference in acoustic centers relative to the baffle plane (or any other plane we chose that is perpendicular to the speaker axis).

These are the major steps needed. There are minor process steps/settings to effectively implement this within REW.

-----

Is this single baffle setup an Issue for you? if not, the easy way to see the process and result is to post the mdat and I will provide that information.

If this setup is a problem, it may well be possible to have opposing speaker positions as you show or better yet to use 2 different baffles at some angle to each other. As long as the baffle/mic distances are known and nothing moves between the 3 measurements it should work. With the single baffle setup I showed there would be less reflections present in the 3 IRs and there is less chance of confusion dealing with different baffle/mic distances. We can probably work within your setup constraints however. I just need to understand what the setup will be so I can be sure we will generate the needed information for the analysis.

If you still need better understanding of the process I will try to answer questions.


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## jtalden (Mar 12, 2009)

Laplace said:


> I guess i understand your suggested procedure.
> This should work *with only one measurement* i guess. The problem is the *principal error of* these method.


 One measurement cannot work as the relative timing of the 2 drivers is then unknown. We can't calculate the distance difference between acoustic centers if we don't know the relative timing.



> The real difference between the "acoustical centers" of both drivers is always bigger than the difference between the distances driver <-> micro.


 I don't understand this comment.



> The bigger the distance of both drivers on baffle or the less the distance between baffel and micro the bigger the error. I guess if the distance baffel <-> micro is *5 to 10 times bigger* than the distance woofer <-> tweeter the *error is negligible* - isn't it?


Yes, if I understand you correctly. I get the same result in phase tracking at my 4m LP as I do at any position closer to the speaker so long as I keep the mic on the listening axis. The axis terminates at the bisector of the line on the baffle between the axial centers of the 2 drivers. I have moved to as close as 30cm without significant error just eyeballing the mic position. My baffle speaker spacing is about 14.3cm. The closer you go though the more the error will be with a given deviation from the axis. It does not become very significant unless we are very close or get the mic significantly off the listening axis.


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## Laplace (Aug 10, 2015)

Hi jtalden,

the last days i was busy. I have resumed our discussion and i find a way to get the DIFFERENCE of the acoustical centers of both drivers with one measurement, i guess.










Use for s2 two times of s1 (e.g. 1m and 0.5m) to ensure two well seperated (two peaks) IR's with one measurement. Is s1 and s2 exactly known then you can calculate the DIFFERENCE of the acoustical centers of both drivers in the following way:
The time t1 is the time for the acoustic wave for the distance s1+ac1
and the time t2 for s2+ac2.
The DIFFERENCE of the acoustical centers shold be named delta ac for example.

Then the third equation below should solve the problem:

1. (s2 + ac2) - (s1 + ac1) = ct2 - ct1
2. delta ac = ac2 - ac1
=> 3. *delta ac = c(t2 - t1) - s2 + s1*​
S1 and s2 are known default values, t1 and t2 are measured with one IR-measurement and then you can calculate delta ac.

The absolute values of ac1 and ac2 are not necessary for xo-design.
S1 could be realized with the right speaker and s2 with the left speaker e.g..

Do you agree?


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## jtalden (Mar 12, 2009)

Yes, that is the general concept for the relative difference in Acoustic Center offset. The only issues in that stated process setup are related to the phrase; "to ensure two well seperated (two peaks) IR's with one measurement". How do we deal with that? 

Looking at many IRs it is clear that there is interaction between the responses. Separating IRs by 1-2ms is probably not enough to assure that the second peak has not been influenced due to the leading IR. With longer offsets then we need to be concerned with room effect interactions as well. Further, (depending on the type of drivers and possibly other factors) the IR peak is usually but not always indicative of the drivers bandpass linear phase. Horns and domes with phase plugs/shields and possibly other driver types/designs may cause the IR peak to be significantly shifted relative to the drivers bandpass. If we have standard stiff cone MW (with no nasty breakup modes) and a standard dome without anything in front of it then my guess is that it is likely to work acceptably well, but maybe not highly accurate. We would not have any conclusive evidence to show that it did however. If we used that method after confirming the results are always comparable to loopback results then we could have confidence in it. I haven't seen any such study although, I haven't done any related searches either. In summary: It might work well - at least in many cases.

When using a USB mic with REW, it still requires 3 measurements with the same setup to provide a demonstrably accurate result. The combined measurement allows us to see the relative timing relationship as accurately as can be done when using loopback timing. The delta offset can be based on the 2 drivers bandpass phase response and is therefore not impacted in the case of any IR peak idiosyncrasy of the drivers.


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## Laplace (Aug 10, 2015)

jtalden said:


> Yes, that is the general concept for the relative difference in Acoustic Center offset. The only issues in that stated process setup are related to the phrase; "to ensure two well seperated (two peaks) IR's with one measurement". How do we deal with that?
> idiosyncrasy of the drivers.


You are right. I didn't keep in mind that the time-difference for the two distances s1 = 0.5 m and s2 = 1 m is only round about 1.5 ms. In this short time the first IR-peak (from the nearer driver) is not faded down until the second peak arrives at the mic!
The bigger the difference between s1 and s2 the more time for seperate IR-peaks, but the room starts taking influence on the measurement, as you mentioned above.

I don't use the UMIK-1 and REW at this time but i plan to buy this mic and use it with REW. But the longer this thread becomes the more i think to buy a two channel system like the new CLIO POCKET or so. The costs are higher, certainly, but you don't need such time intensive workarounds which are hard to manage for exact results.

I also have to admit that i don't exactly understand the several 3 steps of your "3-measurements-method".
Measuring driver one, driver two and then both together: how will you get the delta of acoustic centers with it?


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## jtalden (Mar 12, 2009)

If you haven't bought the USB mic yet and intend to do more timing type work or just protect your investment for maximum utility in the future, you may want to consider an XLR mic and external audio interface. Good options are about 2-3x the price of the USB mic. It also takes a bit more effort to setup properly. It depends on whether it is a likely to be used for this type work in the future. Another advantage is in noise floor of the measurements system. It all depends on your future intentions and budget. For a 'one-off' setup a USB mic can't be beat. 

The REW program does loopback timing using the other analog channel for timing when using an XLR mic with an external audio interface so you would be able to do the acoustic center measurements with just 1 measurement on each driver you want to characterize. REW can do most everything a hobbyist is likely to encounter.


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