# Does REW have the ability to ETC?



## nottaway (Jan 19, 2009)

I'm looking into how to isolate particular reflections. My understanding is that I need to measure impulse and later occuring signal(s) and then extrapolate the offending reflection from the distance calculated from the delay from the primary sound.

Am I on the right track?

If not whats best practice for isolating detrimental reflections?

I'm already aware of the mirror trick, I just want a more surgical approach now that thats done.


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## EarlK (Jan 1, 2010)

> See fottos' thread called ;  *  "Help with ETC interpretion" * 

:sn:


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## nottaway (Jan 19, 2009)

EarlK said:


> > See fottos' thread called ; *"Help with ETC interpretion" *
> 
> :sn:


Hi Earl,

Thanks for the link.

That thread has fotto already taking the measurements. I read it and understood the converstion. It was good info.

However, I really need the basics. I can't figure out how to setup and measure the ETC. I understand kinda how to interpret the data but need to know how to get there first.

I looked and could not find the how to thread........


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## EarlK (Jan 1, 2010)

> However, I really need the basics. I can't figure out how to setup and measure the ETC. I understand kinda how to interpret the data but need to know how to get there first.
> 
> I looked and could not find the how to thread........


> The ETC is simply another form of displaying the captured impulse / & that impulse already exists for each & every measurement that has been made . 

> Therefore, if you have already made Frequency measurements, you already have the captured Impulse / meaning , there is nothing new to measure . 

> ETC is a display choice (check-box), found within the Filtered IR window . 

> If you haven't yet learned how to make ( any type ) of measurement with REW , then study the stickies at the top of this forum .

:sn:


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## nottaway (Jan 19, 2009)

Thank You Earl. I see it now.

I've done some measuring mostly EQing subs and trying to evaluate treatment needs and performance. Ive now treated most the primary reflections with the mirror trick. I want to finish my room treatments but don't want to make the room too dead.

All my measurements are with all channels driven so the ETC data is not very useful on them. I need now to do some measurements on single channels to evaluate some additional surgically placed treatments to try to get an inch closer..... 

I'll now re-read fottos documentation to see how to do it correctly and post back after.

Thank you,
Nicholas


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## EarlK (Jan 1, 2010)

> Here's a really great video from  *RealTraps, allowing one to listen to a rooms' acoustics*  "before & after" acoustic treatments have been made to the ( mixing ) room .

> This video is highly educational, since RealTraps uses REW to display relevant data that allows the viewer to make a reality based connection between what is heard and the numbers on the screen .

> Make sure to listen with headphones on .

:sn:


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## EarlK (Jan 1, 2010)

> I can't think of a better resource ( for helping make a visual connection between applied room correction & what one sees on an ETC display & the displayed Frequency Response .
*this link to Doug Jones study using Sonex panels to treat a room .* 

> You'll want to study the pics in the link to make your own connection between the elimination of ETC spikes & a corresponding smoothing of the frequency response .

> The linked to ( preview ) pages exist within the second edition of this book ( the following link is to the newer third edition, which also has the above content );
 

:sn:


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## nottaway (Jan 19, 2009)

OK so I have a couple questions on setup for taking measurements to analyze the ETC.

For example If I were measuring the Center:

Here is what I understand:
:dumbcrazy: 

Unplug or disable other speakers except Center Channel.
Use direct/analog mode to keep signal in the analog domain
Disable Auydyssey Volume, EQ, and other processing
I need some analog signal post processor to "loop back" to the measurement rig to quantify T0.
Questions:


How do I setup the Loopback? Where do I get the signal? Use a Y connector off pre-out for center? Run in all channel stereo and use another pre-out?
What level is appropriate for this type measurement?
:huh:lease::dontknow:


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## EarlK (Jan 1, 2010)

> Questions:
> How do I setup the Loopback?
> Where do I get the signal?
> Use a Y connector off pre-out for center?
> ...


> Using a two chnl soundcard , you connect your soundcards' output to its' line input ( for just one of the 2 channels / usually the left chnl ). 
> The other soundcard channel drives your AVR .
> This is mentioned in REWs help file. 

> Only use the "Y" connector on the ( now ) single channel feeding your AVR ( so that you can drive signal into both of its' input channels ) .

> 75 to 85db is the recommended measurement level . 


:sn:


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## fotto (Jan 17, 2010)

nottaway, I just updated me previous thread that Earl noted with some pics, explanations etc. which may be of value to you.


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## nottaway (Jan 19, 2009)

fotto said:


> nottaway, I just updated me previous thread that Earl noted with some pics, explanations etc. which may be of value to you.


Thanks yes its more clear now.

Which frequency were you seeing the reflecton? Mine has a dropdown on the left to select what appears to be frequency range......

I cant wait to get home and give this a try.....

Thank You Earl for the clarification on hiw to setup the loopback.

So what does the signal from the loopback look like on the plot? i would assume a sharp peak without the characteristic tails and secondary peaks of actual sound arrival, and then a "quiet" period until T1 arrives?


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## fotto (Jan 17, 2010)

I'm not sure what you are referring to in the "drop down" as I don't believe you can view actual frequency involved in the impulse response/ETC window. I am under the impression that you would need to do band limited sweeps in order to find frequency content. What the ETC deals with is the time domain and it's suggested that you really have no reason to stray out of that. You are measuring a energy peak at a particular time, which needs to be attenuated via absorption, diffusion, or whatever suits your goal/model.

Here's a pic of the loopback engaged. There is some disagreement as to whether the spike at time = 0 is actually the loopback signal or not, but with the system delay out of the picture, you can measure the total time of flight for a particular reflection starting at 0 point to the actual reflection peak which in this case is measured at 14' 6" again. In the previous example I showed where you set T=0 for IR peak, I needed to know the speaker distance to the mic (12') and then add in the "extra" distance of the reflection (2') in order to find the total distance of the reflection. Using loopback as timing method is more accurate for a variety of reasons, with one being how do you know where your speaker's acoustic center really is to locate that endpoint? 










SAC advises that the string method is to really just demonstrate the concept and that it's more efficient to use the "blocking method", where you take a 6-12" square piece of insulation, OC 703 or whatever you have available and start at the mic taking sweeps while positioning the insulation around different quadrants in space to see where the reflection is blocked at. Continue taking sweeps while "walking" the reflection back to it's surface/source. YMMV as to whether you consider that an easier task than the string method or not, but using blocking you really don't have to worry about loopback vs. non as you're reading the effect on the spike's attenuation.

Hope that makes sense.:sweat:


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## nottaway (Jan 19, 2009)

OK I measured the L and R channels and looks like I have a very early reflection. Its my subs which are just in front and outside my screen wall.

Any chance someone will help me interpret these? I have treatment on the side walls partially not including the reflection which occurs off of the subs(needs treatment). Can someone tell me the path length? The main signal calculates to roughly the distance to my L and R channels.

I am using loop back.


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## fotto (Jan 17, 2010)

Here's a pic of your left channel in the Filtered IR window with ETC checked. Manipulate your limits window to look similar to this. Put your left cross hair on the y axis at 0, then press your control key(hold) then right mouse button, hold and drag the cursor over from 0 to the reflection you want to measure. You will have an actual distance in feet and meters displayed. That's the distance the sound traveled from your speaker to the mic, which in this case is about 14.8 ft.










One set of guidelines noted here (http://www.acousticfrontiers.com/Strong-Early-Reflections.html) recommend the following targets:

Be visually identical (with only minor deviations) from 0-40ms
Be down to -10dB by 40ms to prevent breakdown of the precedence effect
Clearly show a decrease in the amplitude of energy over 0-40ms. The decay pattern may or may not be continuous.
Show the consecutive peaks of the highest amplitude reflections viewed across the time axis to be relatively smooth in pattern and density.
The criteria above should be considered in conjunction with the other stated targets for reverberation time and L/R frequency response.


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## EarlK (Jan 1, 2010)

> Fotto, thanks for your post mentioning Acoustic Frontiers and their recommended guidlines for good 2-chnl listening .

> Here's their  *pdf file with those guidelines mentioned ( & more ) . * It's a very good read .

:sn:

PS : Here's your ( Acoustic Frontiers generated ) list posted again ( for emphasis ) ;


Be visually identical (with only minor deviations) from 0-40ms
Be down to -10dB by 40ms to prevent breakdown of the precedence effect
Clearly show a decrease in the amplitude of energy over 0-40ms. 
The decay pattern may or may not be continuous.
Show the consecutive peaks of the highest amplitude reflections viewed across the time axis to be relatively smooth in pattern and density.
The criteria above should be considered in conjunction with the other stated targets for reverberation time and L/R frequency response.


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## nottaway (Jan 19, 2009)

Thanks ill work with it tonight!

What does the dropdown in lower left corner equate to? Looks like frequencies to select?


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## EarlK (Jan 1, 2010)

nottaway said:


> What does the dropdown in lower left corner equate to? Looks like frequencies to select?


> It's a frequency filter ( 1 octave wide I believe ) for the ETC display .

> The ETC gets filtered through it / so that one gets to see how that octave wide band of frequency content decays in millisecinds of time . 
> ( This may be mostly useful to the fully accredited acoustician )


:sn:

BTW : Read the pdf I just linked you to, it displays this feature in action ( using REW ) on page 9 .


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## SAC (Dec 3, 2009)

nottaway said:


> OK I measured the L and R channels and looks like I have a very early reflection. Its my subs which are just in front and outside my screen wall.
> 
> Any chance someone will help me interpret these? I have treatment on the side walls partially not including the reflection which occurs off of the subs(needs treatment). Can someone tell me the path length? The main signal calculates to roughly the distance to my L and R channels.
> 
> I am using loop back.



For the ETC you want to be driving only one speaker at a time.
If this is the case...Why do you say the reflection is due to a sub which not only should not be driven during the test, but which has negligible specular energy output?

In the attached, you will want to resolve each of the indirect signals in the rectangular box and treat them.

The reason you want to diminish their gain is that they occur within the Haas interval and their perception is smeared into a single impulse resulting in significantly diminished intelligibility - what Heyser termed 'time smear distortion'. Damping these indirect arrivals with sufficiently broadband treatment will result in a much more distinct and intelligible speech - which is a _critical _component in a home theater application. (unless you simply prefer the unintelligible quality of the high school gymnasium PA system...)


As far as bandwidth limited ETC - a feature that has existed from the incept of the ETC and which is a very old idea (and of which Toole became aware of in the later 1980s...), unless you are retrofitting a very poorly designed (bounded) or treated room, you need not worry about it.

If you have either massive exterior room walls (which will reflect all energy, low or high) or 2x4 stud drywall walls (which will effectively reflect 'all' specular energy - as I doubt most folks want to get into the treatment of low mid resonant nulls that involves beefing up the wall surface), they will be (for all intents and purposes) reflective in the specular passband. 

Where they may be selectively reflective is in the low modal frequencies which are insignificant with regard to the specular analysis - you will properly examine that behavior in the modal range and address that appropriately using treatment that absorbs only the modal frequencies and reflects the specular range (e.g. tuned resonant traps or properly faced porous traps).

Thus unless one has made a mistake and are measuring a room suffering with insufficient previously applied broadband(specular) treatment that exhibits a highly reactive acoustical impedance that will effectively EQ the indirect signals that will, if present, subsequently cause coloration upon their combining with the direct signal, you will NOT need to worry about band limited ETC measurements! Instead, appropriately broadband treatment applied 'surgically/strategically' will address the indirect signal that will be a close approximation of the the source signal component - whatever it might be. (And if you don't like that, address it properly by speaker choice).

The fact is, control of the boundaries has been a critical and fundamental aspect of room design for longer than any of us have been doing this. It has also been a best practice employed by many (and fundamental to the LEDE approach dating back to 1977) to utilize a massive externally bounded room with an interior low pass, specularly reflective room that is placed asymmetrically. This arrangement allows for modal energy to pass through the inner boundaries to be treated in the space between the inner and external room, while the inner room is positioned asymmetrically moving the typical primary listening position out of the predominant centrally located modal null. Additionally, the internal walls are designed to be uniformly reflective above the modal frequencies. In such a case, the indirect signals will very closely mirror the source of the indirect signal.

In other words, you need not make this issue unnecessarily complicated unless you have a room that exhibits severe issues related to inadequate construction techniques or that already suffers from lousy treatment.


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## nottaway (Jan 19, 2009)

SAC said:


> For the ETC you want to be driving only one speaker at a time.
> If this is the case...Why do you say the reflection is due to a sub which not only should not be driven during the test, but which has negligible specular energy output?
> 
> In the attached, you will want to resolve each of the indirect signals in the rectangular box and treat them.
> ...


Hi SAC,

i appreciate all you guys helping me along Im learning gobs.....

I measured each channel infependently.

i did drive the subs during measurement so next time ill nix that

The sub reflection im referring to is not the output o the sub but a specular reflection of the main off the sub enclosure which is just in front and to the side of my R and L. I need a treatment on the side of the box.

Wouldnt the octave filter also help assess treatment effectiveness over the frequency range after application?


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## SAC (Dec 3, 2009)

I would research the reflection off the sub notion a bit further. The reason for this is that energy will only be reflected off it that is of a frequency whose wavelength is smaller than the dimensions of the sub boundary surface. Wavelengths that are longer will diffract around it. So you might want to verify that its the sub...how did you validate that assumption? Typically the reflective surface will be a larger proximal boundary. Note, you do not simply note an early arrival and then subjectively 'guess' as to what might be reflecting it! That defeats the capabilities of the ETC!

I have already mentioned the factors that contribute to a modification of the spectral content of an indirect signal. 

We don't nirmally care what the spectral content is in an energy arrival UNLESS it is different (in other words, it has been modified by the acoustical impedance of an incident surface) from the originating source. 

Are you proposing treating each incident spot with a different 'tuned' treatment? And if so, how would that effect the other incident energy that is not excessively high gain as you would then be effectively and detrimentally EQing its response??


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## nottaway (Jan 19, 2009)

SAC said:


> I would research the reflection off the sub notion a bit further. The reason for this is that energy will only be reflected off it that is of a frequency whose wavelength is smaller than the dimensions of the sub boundary surface. Wavelengths that are longer will diffract around it. So you might want to verify that its the sub...how did you validate that assumption? Typically the reflective surface will be a larger proximal boundary. Note, you do not simply note an early arrival and then subjectively 'guess' as to what might be reflecting it! That defeats the capabilities of the ETC!
> 
> I have already mentioned the factors that contribute to a modification of the spectral content of an indirect signal.
> 
> ...


SEC, this is boil your blood, but yes I guessed. I intend to block and isolate, i was just admitting im aware there are issues knowingly present.  The direct flight path is about 13'(estimated acoustic center) and the reflected signal has 14' something flight path so i was(grit teeth) assuming it.........

There are a number other reflections which appear needing treatment and ill string trick them block and confirm......

The subs are quite large but may not be large enough to be problematic ill confirm and see. They are 16x16x36 standing upright in front and to the side of the mains.......



> "We don't nirmally care what the spectral content is in an energy arrival UNLESS it is different (in other words, it has been modified by the acoustical impedance of an incident surface) from the originating source."


This is exactly what im saying. Since there is so much discussion on broadband absorption treatment effectiveness at lower frequencies it might be informative to see the spectral content after incidence on the treatment and subsequent arrival at mic. This could possibly indicate a need for bigger/thicker treatment at that position if the lower spectrum is not effectively attenuated right?


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## SAC (Dec 3, 2009)

If the difference in signal path is indeed only ~1foot in a 13 foot tonal path length, it is going to most likely be driver signal (mis) alignment or diffractive issues.

The reason I say this and why I doubt it is the sub is 1, the path length, and 2, the fact that the angle of incidence equals the angle of reflection. If your sub is mounted on the same plane as your front speakers, I doubt you can find a vector that will satisfy condition two and still intercept the listening position while satisfying only a 1 foot difference in the total flight distance.

Many things are possible, but I would question why one would utilize inadequate treatment and then go and see if you can isolate a particular reflection and look at its composition. Remember, the treatment affects ALL incident energy , not only the high gain reflections we seek to go out of our way to treat. If the treatment is not sufficiently broadband, all of the ambient energy that is incident where the treatment is located will effectively be EQ'd! And that modification in the total environment is as important as the spectral content of an isolated high gain reflection! And simply analyzing one small isolated part of the total response will not be very beneficial, even if you do learn that you must now go back and properly construct a and install the proper treatment! I think that far too many are making the assumption that only the high gain reflections that we seek to mitigate contribute to the perceived response! The lower gain reflections are critical as well - we just do not have to treat them - yet an 'insufficient' treatment detrimentally affects them as well!

Me? I would do it properly the first time, knowing that there are other factors that you will not be measuring that are adversely affected by the treatment and which still contribute to the total response of the room.

And I am curious... I would_ love_ to hear the rationale that supports your assumption that one should reasonably assume that low-mids would not be involved in the spectral content of the indirect specular energy.


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## nottaway (Jan 19, 2009)

SAC said:


> If the difference in signal path is indeed only ~1foot in a 13 foot tonal path length, it is going to most likely be driver signal (mis) alignment or diffractive issues.
> 
> The reason I say this and why I doubt it is the sub is 1, the path length, and 2, the fact that the angle of incidence equals the angle of reflection. If your sub is mounted on the same plane as your front speakers, I doubt you can find a vector that will satisfy condition two and still intercept the listening position while satisfying only a 1 foot difference in the total flight distance.


Agreed. Its way too soon to be anything but something close to speaker, possibly speaker itself. Here's a photo of the sub and where it sits, speakers are behind screen and to the right in this picture:












> Many things are possible, but I would question why one would utilize inadequate treatment and then go and see if you can isolate a particular reflection and look at its composition. Remember, the treatment affects ALL incident energy , not only the high gain reflections we seek to go out of our way to treat. If the treatment is not sufficiently broadband, all of the ambient energy that is incident where the treatment is located will effectively be EQ'd! And that modification in the total environment is as important as the spectral content of an isolated high gain reflection! And simply analyzing one small isolated part of the total response will not be very beneficial, even if you do learn that you must now go back and properly construct a and install the proper treatment! I think that far too many are making the assumption that only the high gain reflections that we seek to mitigate contribute to the perceived response! The lower gain reflections are critical as well - we just do not have to treat them - yet an 'insufficient' treatment detrimentally affects them as well!
> 
> Me? I would do it properly the first time, knowing that there are other factors that you will not be measuring that are adversely affected by the treatment and which still contribute to the total response of the room.


Agreed. I'm really more concerned for my own knowledge how broadband my intended treatments actually are. I intend on using 4" OC703 with 4" air gap for broadband absorption. However, each application of the same treatment might perform slightly different due to angle of incidence right? This is why I want to see spectral performance. For my education and curiosity......



> And I am curious... I would_ love_ to hear the rationale that supports your assumption that one should reasonably assume that low-mids would not be involved in the spectral content of the indirect specular energy.


Maybe I wasn't clear enough as I'm still learning.....:bigsmile: I'm assuming that they *would* be in the spectral content due to the added level of difficulty in proper treatment of the "low-mids" Thats why I was wanting to breakdown by octave to see how the treatment performs on the lower stuff.


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## SAC (Dec 3, 2009)

Glad to hear the rationale behind your reasoning!

The fact is, in terms of the spectral content of the indirect energy, it should generally reflect (hmmm, an unintended pun!:rolleyesno the source, whatever it might be. The fact is, except for issues with uneven power response, where say there may be more low-mids reflected off the boundaries by virtue of the low-mid polar response exhibiting a low Q (wider) dispersion and the mids and highs featuring a higher Q (narrower) dispersion, the energy should track whatever the source is doing at that moment in time - but which over the course of events will assumedly encompass the full spectrum.

Thus, the use of absorption such be surgical and full range in order to avoid EQing any of the incident energy with which that it interacts.

The 4" panel with a 4 inch gap consisting of either ~3lb/ft^3 Fiberglass or ~4 lb/ft^3 mineral wool has been generally verified via both tests and complex simulations based upon its gas flow resistance per a variety of models in 'proper' test environments. (And for anyone familiar with the 'recent' scandals involving acoustical testing, they will understand just how significant a claim this is!)

All incident angles have been examined.

And as you are questioning assumptions, one additional factor to take into account is that the porous Fiberglass or Rockwool should be oriented vertically, thus placing the 'grain' of the material perpendicular (and not perpendicular) to the preponderance of the incident energy. 

Fiberglass and mineral wool are often manufactured in oriented layers exhibiting a distinct grain. And in this manner the material anisotropic, where the acoustic properties vary depending on whether sound is incident parallel or perpendicular to the fiber orientation, with the resistance being greatest with the grain oriented perpendicularly to the incident energy.

So, rather than the need to make many additional measurements, these characteristics have been studied and one may at this point simply utilize best practices.

If you do wish to validate such research, beware that the testing is a bit more complex than what may at first seem necessary. And this is an example of a case where a 2 channel FFT analyzer such as ARTA, etc., becomes very useful.


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