# Using IR Windows



## vettett15 (Jul 1, 2009)

Guys,

Had a question about using IR windows for gating and then looking at the frequency response. I have a null that goes from about 300-500Hz that I have been trying to figure out. One day I was messing with the IR window feature and noticed that in all of my sweeps the null seems to form when I hit about 12ms. The null grows larger until I hit about 16ms and then it stops. Would I be able to use that information to determine what is causing this null?

Thanks,
Pete


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## EarlK (Jan 1, 2010)

> I'll leave it to others to answer your question directly . :whistling:

> In the meantime, measure a single speaker from less than 18" out ( on axis ) to see if the null goes away ( & to get a truer picture of the speakers response ) .

> Typically you'll find there is a null area in the 300 to 500hz region caused by signal reflection ( from a hard ) floor ( or other surface ), generating some quite coherent out of phase signal over a fairly broad area .

> The null is usually a good-looking "V" shape if the surface causing the reflection is hard & smooth ( at least between the test mic & the speaker).


> Move the test mic up & down to a few different vertical positions, to see if there is any correlation to the null .
> Moving the test mic higher, should lower the frequency for the center of the null, ( if the cause is actually "floor bounce" ).


:sn:


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## vettett15 (Jul 1, 2009)

Earl, I have done some sweeps closer to the speaker and the response is "flat".

I think you are right in that I need to do some sweeps while moving the mic around. But is it better to use the ETC to determine if i'm moving closer/further away from the reflection point or the response itself..


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## SAC (Dec 3, 2009)

A few comments...

One does not propose to treat for every spot in the room. And a close mic'd response will seldom exhibit significant boundary reflection issues - although near field diffraction as well as signal alignment issues between adjacent drivers are not uncommon...

The limitation with windowing an IR is that you loose resolution. And why are you attempting to remove indications of additional indirect energy that may arrive at earlier or later times at that location if they are of sufficient gain to also cause problems - and which you would want to identify the paths and appropriately treat their incident points as well?

I guess that I am not 'getting' why we are wanting to make this process more complex or difficult than it is?

I am also guessing that you are looking only at the IR and not at the ETC response. If that is the case, i can easily understand the desire to make the process of isolating and identifying the pertinent information easier! Luckily there is a rather easy method to do exactly that.

The ETC response is ideally suited (and much improved over the IR view) for providing a high resolution view of the arriving energy with a detailed view of gain and arrival time, from which you can quickly resolve the path and point of boundary incidence of actual reflections. (You will want to properly and non-destructively 'window' the data using the "limits" function to set the x axis time range and the Y axis gain range to useful limits - in order to make interpreting the results easier and more clear.)

With the mic at the listening position, simply convolve the ETC response and address the actual high gain reflections that effect that location. By resolving each of the reflections into their vector paths (direction of travel and point of boundary incidence) you determine all of the speaker-room interaction causal factors that may be contributing to anomalies in the FR. Identifying and properly/surgically treating those indirect energy arrivals will address nulls created by the superposition of the direct and indirect energy.

There is already a best practice method for accomplishing your goal. And from the information provided in the ETC one can easily use the string method or the blocking method to resolve this information.

Just be sure that if you use the string method that you employ the hardware loopback configuration and choose the 'use loopback as timing reference' setting while leaving the 'set T=0 to IR peak' setting UNSET in the Preferences-> Analysis window !, as you will need an accurate total time of flight time figure to convert to distance.


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## vettett15 (Jul 1, 2009)

SAC,

thanks for the response, I left some info out to say the least. I have been using the ETC plots to determine the points of reflections but recently I have moved my speakers/listening position around to try and get the best response I could before heading down the room treatment road. 

The ETC plot shows some spikes that have a distance of 1.5' past the direct energy, so i'm not sure if that is caused by the baffle or perhaps the couch (listening position). 

I have read that pdf that was put together discussing the different ways to determine the location of the reflections, I plan on using the blocking method.

 I just happened to be playing around in REW and noticed that the large dip I have been trying to get rid of seems to be coming in at around 12ms in all of my graphs (left and right done separately). I was trying to make some assumptions from this (perhaps wrong) that if the dip is coming in at the same time for both speakers then the boundary causing this had to be at the same distance for both speakers. That narrows it down to the floor, ceiling, front wall and back wall so I attempted to use the string method and it seems like it is coming from the ceiling. 

Perhaps I should post my mdat files to give you a better idea of what i'm dealing with.


Thanks,
Pete


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## SAC (Dec 3, 2009)

Yeah, posting the .mdat files is ALWAYS advantageous :bigsmile: as it allows us to massage the display in perhaps a more advantageous manner... In fact, i wish the forum would adopt their posting as the default requirement for the evaluation of plots...

If you have similar results form the L & R ETCs, that usually indicates that the room and setup have good symmetry - a Very good thing!

Sometimes it can be a common surface like the ceiling, and often it can also be from similar relationships between each speaker and their proximal boundaries (e.g. the left speaker and, say.for example, the right wall, and the right speaker and the left wall...). The greater the symmetry, the greater the similarity between the L & R ETCs.

And yes, after a bit of playing with the tool, you will begin to be able to pretty easily interpret the plots with some rather uncannily accurate pattern recognition.

I am glad to hear of your growing insights and I heartily encourage your further exploration! Once you get past the 'intimidation' phase and the light goes on, you will see just how powerful and practical the tool is - and then if you are really adventurous you might consider using it to further analyze the loudspeaker itself and explore driver signal alignment, cabinet and baffle diffraction, and even the existence of internal cabinet standing waves (think modes!) that impinge on the driver cone and color its frequency response - all issues that can be treated. And that's just a beginning of what the tool can illuminate...


Oh, and one more small thought...
When one says that they observe an indirect arrival that occurs, say, 1.5 feet after the direct arrival, just be aware that while the direct signal is a simple line from source to mic, the path of the indirect energy is not - thus you are (to use terms VERY loosely) comparing a triangle base to the hypotenuse plus the rise... So the location of the boundary incidence will not be literally 1.5 feet from the mic... A small issue, but one whose casual assumption may cause some confusion.
Now that I have totally confused you where a diagram would be instantly understood


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## vettett15 (Jul 1, 2009)

SAC,

This is all good info, the "odd" (to me) thing is when I do the gating excercise and see the 300-500 null appear it doesn't correspond to a peak on the right ETC. The left ETC has a peak in the area but never the less the null seems to appear around the same time for both speakers.

I understand your comments on the indirect path, in the 1.5 feet the indirect signal has to hit the boundary and then head towards the mic. So i assume this means the distance between the speaker and the boundary or the mic and the boundary is less than 1.5 feet which makes it a little more confusing. The only thing I can think of is the couch next to the mic.

Here are the left and right mdat files, this is with no avr processing (no eq), from the listening position.


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## SAC (Dec 3, 2009)

What do you mean by "gating".

Actually gating or windowing of the source file and them convolving the data onto an ETC results in loss of resolution - you are effectively changing the data! There is No need to do this! You simply use the "limits" control in the ETC function to define the viewable area and to effectively zoom in without any change to the data!

_"I understand your comments on the indirect path, *in the 1.5 feet the indirect signal has to hit the boundary and then head towards the mic. So i assume this means the distance between the speaker and the boundary or the mic and the boundary is less than 1.5 feet which makes it a little more confusing. *The only thing I can think of is the couch next to the mic."_

No.

There is no indication of when the incident takes place! Your assumption has no legs. It simply means that the total time of flight took (1.5ft x 1.13ms =) 1.70ms longer. If you do not have an accurate total time of flight corrected by loopback this time is not going to directly help you. Thus you must use the blocking method to intercept the vector path and walk it back to the boundary in order to determine the point of incidence.

If you do have the accurate total time of flight via use of the loopback hardware propagation delay compensation, you cut a string longer than the distance corresponding to the time to flight, have one person hold the marked point at one end to the center of the speaker and the other marked end at the mic capsule location, and extend it until you find an incident point with the fully extended string.

Stop assuming the location of incident and then being confused that your assumption is incorrect. Start with the data and let IT guide you to the incident point! Your process is trying to force the data to fit your assumption, rather than allowing the data to determine the point of incidence.

_Also, you use an unsmoothed FR._


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## vettett15 (Jul 1, 2009)

Actually gating or windowing of the source file and them convolving the data onto an ETC results in loss of resolution - you are effectively changing the data! There is No need to do this! You simply use the "limits" control in the ETC function to define the viewable area and to effectively zoom in without any change to the data!

Not sure what you mean here. By gating I mean I used the IR windows and set the right window until I saw the dip in the frequency response appears (around 12ms). Is what you're talking about the same?

With regards to the extra distance, perhaps my thought was incorrect but I actually used the string method. The problem is since I am only one person I had to tape the string to the speaker and the couch (where my head would lay) so it becomes a little inaccurate. The string didn't really hit anything, but if I actually held the string where the mic was I think I would hit the couch (again where my head would lay). 

I am using the "use loopback as a timing reference" in all of my sweeps.


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## SAC (Dec 3, 2009)

Just a few comments as I am in the process of running elsewhere...


The room is already very dead.


For the right ETC:
Direct signal is at:8.75 ms (9.89ft)
Your first significant reflection is at 10.06ms (11.37 feet), the 2nd at 10.38 ms (11.73 feet), and the 3rd, 20.66 dB down(!) at 14.33 ms (16.19 feet)...with remaining sparse reflections at 18.49ms, 20.64ms and 25.45ms...

Each should be resolved, and the last three diffused.

For the left ETC:
Direct signal is at: 8.87 ms (10.02 ft)
Your first significant reflection is at 10.37ms (11.72 feet), the 2nd at 11.35 ms (12.83 feet), and the 3rd at 12.46 ms (14.08 feet), 4th at 12.70 ms, 5th 20 dB down at 13.88 ms. Additional sparse reflections occur at 14.13ms, 15.72ms, 16,21ms, 17.41ms, 20.13ms, 21.37 ms ...with remaining late arriving sparse reflections at 28.51 ms and 31.05ms. The sparse reflection at 31.05 (especially) is anomalous in its intensity. And all of the later arriving sparse reflections would benefit from diffusion.

(Don't trust my math - or my calculator as the batteries are a bit flaky as I have caught it giving strange results recently - above and beyond my goofy logic!!!)

Judging simply from the 'business' of the left ETC, there is a distinct difference in the symmetry.


Edit:

Gating or windowing a function effectively edits it and 'removes a select portion for further examination. We are NOT doing (or should not be doing) that here. All we should be doing is changing what we see without modifying the data at all. And we do NOTHING to the IR!

I do not understand at all what you mean by 'gating the IR". Maybe it is just a matter of semantics or maybe a problem with process. I hope its the former!
You do _nothing_ to the IR except generate it from an unsmoothed FR, and then convolve the ETC. And then the only 'changes' (in view only) to the ETC are in terms of the specified "limits" - the view function provided in the upper right of the program where the ETC is displayed where you can set the minimum and maximum extents of the X (time) and Y(gain) parameters that define what you see.

The string corresponds not to "where your head would lay" but from the acoustic origin of the speaker to the capsule of the measurement mic!!!

Also, if these measurements are of an already treated room, DO NOT simply start treating the results of the ETC! One would need to go back and evaluate just what the existing treatment has done - especially as it has left a significant amount of significant response artifacts in the process of rendering the roam already rather dead - where early reflections require addressing and later arriving sparse reflections would benefit from both spatial and temporal diffusion to both lower the gain and spread the energy out temporally.


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## vettett15 (Jul 1, 2009)

My future plan is on track, I plan on hunting down those reflections and attempting to do something about them with proper treatments. 

It sounds like I may have discovered nothing with regards to gating to determine which reflections were causing dips/peaks in the frequency response. Just to clear up confusion this is what I mean by "gating":

Looking at the frequency response tab I hit the "IR WINDOWS" icon at the top of the screen. The pop up window allows you to choose the section of the impulse window to use to make the frequency response (that may be a poor explanation). I leave the left window at 125ms, the window ref time to 0 and the right window is what i modify. I enter in the right window a time just after the direct signal (say 10ms), while looking at the frequency response. As I increase the time, say 11ms, I have a somewhat flat frequency response. As I get to 12ms, I can see the dip start forming in the 300-500 range. 

I know the end of the string shouldn't be taped where my head lays, but I can't tape it where the mic actually was because there is nothing there to tape it to (mic was already gone). From reading your other posts it seems the blocking method or moving the mic method may are better ways to go about finding the reflections. Perhaps another way, maybe not as accurate, would be to place some absorber at places where I believe the reflection is and see if the spike in the etc is reduced? The advantage here, I thought, is I can leave the mic where it is. One disadvantage is it may take more time to find the reflection.


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## SAC (Dec 3, 2009)

You are making this much more difficult than it need be! (and please don't mess with 'gating' the IR!)

As you have no help (and after subjecting friends to the string technique you will see a marked decrease in the number of folks who allow themselves to be considered friends!!!)

Simply set up the measurement system, run the sweep generating an UNsmoothed FR, generate the IR from the unsmoothed FR, select ETC and then adjust the view via the Limits tool to set the X and Y axis parameters for better viewing.

Then, after initial examination, rerun the sweep using the blocking technique and discover the paths and points of boundary incidence corresponding to each problem 'ETC peak'. Mark the spot with a bit of blue (non-destructive) painter's tape.

(Mark the mic capsules position with a weighted string plumb bob tacked to the ceiling - you can move it out of the way when not in use and replace it later...)

From there you can determine the appropriate treatments for each point dependent upon the nature of the response desired. And then you can rerun the tests to verify the effectiveness of the treatment, allowing modification as/if necessary.

Have fun!


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## vettett15 (Jul 1, 2009)

alright, i'll go the easy route....Here I was thinking I had discovered something cool. I will post up some post ETC graphs after I figure out where these reflections are at.


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## jtalden (Mar 12, 2009)

vettett15 said:


> ...
> 
> It sounds like I may have discovered nothing with regards to gating to determine which reflections were causing dips/peaks in the frequency response. Just to clear up confusion this is what I mean by "gating":
> 
> ...



I am setting aside issue of ETC and your path forward. I just wanted to help with your understanding of the IR gating and its impact on FR.

You have not set the IR peak at zero because you want to retain the time of flight for your ETC work - that's fine. 

In that case however If you want to apply a narrow window and look at SPL, the window needs to offset to properly capture the IR. Below, the left gate is 0.2 ms, the Reference time is at 8.8 ms and the right gate is at 2.6 ms. The reference time needs to be near the IR peak (it’s exact location is not super critical for looking at the SPL). The left gate needs to be close to the left side of the IR just where it starts to rise. The right gate can then be adjusted as needed to see as much of the FR as needed. 









With the Reference Time and left gate set properly as shown:
> The smaller the right gate value the less of the reverberant energy is captured and thus the 
HF response weights the direct signal to a greater extent. 
> The total width of the window (right gate + left gate setting) determines what the freq range will be. As the total window narrows the low frequencies are lost. In this case I set a total width of 2.8 ms and the SPL graph is automatically truncated to 357.14 Hz as shown. Also note that the second data point is at 2x that value and the third is at 3x that value. This means there is low resolution of the shape of the line until you get to 3x (~1071 Hz).

Note that:
> Even though the resolution is low at the 357 Hz first point you can see that that point is well below the higher range values (about -10 dB lower).
> If you move the left gate to a larger value the window is wider and the frequency range goes lower, but that is padding the range with zero values and the extra LF range will be meaningless. If you want more LF data the right gate needs to be increased. So move the right gate larger and you will see the 300 Hz sag is immediately apparent and is present no matter how much larger the setting.


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## vettett15 (Jul 1, 2009)

jtalden, thanks for the info. Looks like since the dip is apparent right when the resolution hits that frequency it would be hard to get any good info from it.

SAC, forgot to mention something, you said my room looked pretty dead. Can you give me what gave it away? I'm assuming its because of my reflections (or lack of). The left speaker is closer to a side wall and it has my sub on its other side, which perhaps explains why it has more going on in the ETC graphs.


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