# Monster null out of no where!



## tboooe (Mar 4, 2016)

Folks, check this out. Attached is the final filtered response for my room and speakers. The top blue line is the combined response with both speakers, the red line is the left speaker only, and the green line is the right speaker only. Overall I am happy with the graph aside from the various nulls I have. For the most part, the combined graph shows where I have dips in either the Left or Right speakers. What really got me perplexed is the monster null at around 80hz which seems to show up out of no where. As you can see in either of my individual speaker graphs, there is no dip at 80hz. 

Curious to know what the cause may be?


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## Tonto (Jun 30, 2007)

I'm no expert & I'm sure someone with more REW knowledge will chime in shortly, but if I had to guess...I'd say it is a phase issue between the two speakers. Just a guess.


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## willis7469 (Jan 31, 2014)

Try adding and subtracting a couple feet incrementally to the subwoofer distance and see if it changes. It looks like a phase issue between the subs and mains. Or try adjusting the phase control on the subwoofer. Re-sweep and compare results. Adjust accordingly. Disclaimer: also not an expert. 


Sent from my iPhone using Tapatalk


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## tboooe (Mar 4, 2016)

Thanks guys...I think I may have found something. I was looking at an older graph of my right speaker taken about a week ago before I repositioned it. It shows the same massive, and steep null at 80hz. But I thought I remedied this by moving the speaker and by also closing the door to my listening room. As you can see from the graphs I included above taken just yesterday, the right speaker no longer has this null. I have taken many many measurements of my right speaker since I moved it and starting closing the door to my listening room and REW does not show this 80hz dip any more.

In this graph, the blue line is my current room response with both speakers, and the green line is just my right speaker from a few days ago.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Tonto said:


> I'm no expert & I'm sure someone with more REW knowledge will chime in shortly, but if I had to guess...I'd say it is a phase issue between the two speakers. Just a guess.


I’d tend to agree. Low-frequency nulls are typically a function of the distance between the speaker and a boundary. However the signal also “sees” another low frequency source as a boundary. That would explain the appearance of a null when both speakers are played together.

Regards, 
Wayne


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## tboooe (Mar 4, 2016)

Wayne A. Pflughaupt said:


> I’d tend to agree. Low-frequency nulls are typically a function of the distance between the speaker and a boundary. However the signal also “sees” another low frequency source as a boundary. That would explain the appearance of a null when both speakers are played together.
> 
> Regards,
> Wayne


Did more testing....using REW signal generator and the SPL meter, I tested 70, 80, 90 hz signals for each speaker and combined. At 70 and 90hz, each speaker and the combined response of both speakers measures above 74db. As suspected by others in this thread, at 80hz, even though each speaker measures around 73db individually but when I measure both there is huge null. So, the question is...is there anything I do about this? Unfortunately, in my little room I do not have a lot of freedom to move things around.


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## Tonto (Jun 30, 2007)

Adjust the phase of one of the speakers in small increments & measure the effect. Or both in different directions.


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## tboooe (Mar 4, 2016)

deleted due to my stupidity

EDIT..ok maybe not so stupid. I think there is something wrong with either my mic or REW. I just did 2 measurements and the 80hz dip was gone. Took another one and it came back.


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## tboooe (Mar 4, 2016)

Right now at 5:30pm I can measure both speakers outputting 80hz. REW SPL meter reads 74.3.


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## Gdaddy (Oct 29, 2016)

Check it everyday at 5:30. Maybe it's 'time of day' related. onder:


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## tboooe (Mar 4, 2016)

No change to system, now at 7:40pm both speakers measuring null at 80hz again, verified with my own ears at listening position.

How is it possible that there is sometimes a null at 80hz and sometimes there isn't? For the past hours I've no problems getting 74db measured at 80hz. Now all of sudden there is a null??? I am losing my mind.


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## tboooe (Mar 4, 2016)

UREKA!!! I finally started to figure out whats going on! Sort of.

Let me explain my setup, I use Jriver as the output device. I use Jriver's convolution so that I can test out my filters to see how they actually perform in my room instead of relying on the predicted curve in REW. In my room, the left speaker as a massive +10db peak at 77hz and the right speaker has a similar peak at 62hz. When I create a filter to flatten out these peaks so they match my target curve, I have to use a -10 gain with a BW60 of 8-10. The result is a very nice flat curve for the left and right speaker. However, when I measure with both speakers I get inconsistent results. Sometimes I get the massive nulls precisely at where I had created a filter to deal with the large peaks for the left and right speaker. Other times I get a smooth curve with no nulls but with a large trough between 60-80hz. When I do a measurement with convolution turned off in JRiver the nulls at 62hz and 77hz goes away.

So clearly Jriver's convolution engine is doing something strange with the filter's I created. Why would I get a flat curve when only measuring 1 speaker but get inconsistent results with both speakers? Also, when I do get measurements that result in a trough versus 2 nulls, the trough actually dips below the db level of each individual speaker. How is that possible?

Here are some pics that may help.

This are two different measurements of both speakers, with Jriver's convolution engine using the same filters. Notice in one measurement there are two nulls at 62 and 77hz but with a good response in between. In the other measurement, there is a wide trough. 








Here is a measurement of both speakers with JRivers convolution engine turned off (no filters applied).








Here are measurements of each speaker and combined. Notice how the trough in measurement for both speakers actually extends below the db level of each individual speaker in the 60-80hz range.


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## tboooe (Mar 4, 2016)

More testing....when I use the Trace Arithmetic feature in REW and sum the filtered Left and Right response, I get the combined response with the two dips at 62 and 77hz (BTW I am amazed at how closely the predicted sum matches one of the actual measured response). So my question is why do the big dips occur precisely at the center of the large negative gain I applied to the Left and Right filters but these dips do not show up in the measurement of each individual speaker? Since my Left and Right speakers do not show the big dips at where I applied the filters, are these dips showing up in the combined response real or just some glitch in REW?


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## jtalden (Mar 12, 2009)

Tonto was on point (see Post 2). 
Room modes and reflections impact the phase of the sound at various frequencies of each speaker. Nulls will occur at frequencies related to major room modes at certain locations in the room. The 2 speakers are impacted differently in all but the most symmetrical room setups. When both speakers are active with a different phase response, the phase interaction of the 2 results in a different net response. This is very common and can impacted by significant changes to the location of the speakers, the LP, or significant changes to the room.


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## tboooe (Mar 4, 2016)

jtalden said:


> Tonto was on point (see Post 2).
> Room modes and reflections impact the phase of the sound at various frequencies of each speaker. Nulls will occur at frequencies related to major room modes at certain locations in the room. The 2 speakers are impacted differently in all but the most symmetrical room setups. When both speakers are active with a different phase response, the phase interaction of the 2 results in a different net response. This is very common and can impacted by significant changes to the location of the speakers, the LP, or significant changes to the room.


Thank you and I totally understand the phase explanation. What I am not understanding is why I am getting different measurements even though nothing has changed?


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## tboooe (Mar 4, 2016)

More testing...I manually entered the REW filter settings into JRMC parametric eq and reran the measurements. The combined graphs show the double dip at 62 and 77hz (precisely where I have my negative gain filter on the R and L speaker respectively). The measurements for each speaker is also very similar to when I use JRMC convolution engine. Therefore, I have to believe the inconsistent results of the combined measurement has to do with the JRMC convolution engine. 

Regarding the phase...I noticed that when I have all filters off and just measure individual frequencies in the range of 60-80hz, the measured combined output is about 4-5db less than the individual levels. For example, at 70hz the left and right speakers measure 77db but the combined output is 73db. So does this mean I have a slight phase issue in this range? If so, is there a way I can fix it?


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## jtalden (Mar 12, 2009)

In general using IIR EQ filters; the smoother the SPL, the smoother the phase will be. Room modes and strong reflections are special cases and nulls due to those cannot be effectively addressed with EQ. 

I understood the original question to be; 
Q; why is there is a SPL difference when both speakers are active? 
A; Added speaker interaction; Phase differences. You are now good on that one.

Q; Why different results when the EQ is applied? 
A; EQ changes the SPL / Phase so an SPL / phase /interaction change is expected. 

Q; Why different results when nothing is changed.
A; Something has changed, otherwise the results would be the same. EQ is a change, but also the change may be in the room setup, the measuring system or possible the REW settings used for the analysis.

Q; Regarding the phase...I noticed that when I have all filters off and just measure individual frequencies in the range of 60-80hz, the measured combined output is about 4-5db less than the individual levels. For example, at 70hz the left and right speakers measure 77db but the combined output is 73db. So does this mean I have a slight phase issue in this range? 
A; Yes, a phase issue. 

Q; If so, is there a way I can fix it? 
A; Possibly, but not likely with EQ, as this is usually the result of room modes or strong reflections. 

Sharp nulls normally do not sound as bad as the charts look. Most of us live with some of them as we are not willing to take the more difficult steps needed to avoid them.


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## tboooe (Mar 4, 2016)

jtalden said:


> In general using IIR EQ filters; the smoother the SPL, the smoother the phase will be. Room modes and strong reflections are special cases and nulls due to those cannot be effectively addressed with EQ.


What do you mean "smoother" SPL? You mean smooth response curve? That is what I've been trying to do! 




jtalden said:


> Q; Why different results when nothing is changed.
> A; Something has changed, otherwise the results would be the same. EQ is a change, but also the change may be in the room setup, the measuring system or possible the REW settings used for the analysis.


I am telling you, nothing changed. I made a measurement, left for about 30 mins, came back, took another measurement and the response was dramatically different. I have to believe something is wonky with JRMC convolution.



jtalden said:


> Q; If so, is there a way I can fix it?
> A; Possibly, but not likely with EQ, as this is usually the result of room modes or strong reflections.


This is actually good news. I was hoping I could attempt to remedy the problem with some more room treatments.



jtalden said:


> Sharp nulls normally do not sound as bad as the charts look. Most of us live with some of them as we are not willing to take the more difficult steps needed to avoid them.


100% agree here. Deep, narrow nulls do not bother me as much, so long as I do not have a lot of them. In my case, with the nulls at 62 and 77hz, these are caused by the large negative gains I had to apply at peaks. When i remove those particular filters, the two deep nulls go away but of course that leaves two massive peaks. Hopefully I can reduce the peaks a bit with some room treatments.


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## willis7469 (Jan 31, 2014)

jtalden;
Sharp nulls normally do not sound as bad as the charts look. Most of us live with some of them as we are not willing to take the more difficult steps needed to avoid them.[/QUOTE said:


> Agreed. They usually don't cover a wide enough frequency range to make a huge difference. Everyone's mileage may vary of course.
> 
> 
> 
> ...


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## Tonto (Jun 30, 2007)

Diffusion will help as long as it is not the speaker to speaker distance that is causing the null. You might also consider a resonant panel for the ceiling. How much toe in are you using?


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## tboooe (Mar 4, 2016)

Tonto said:


> Diffusion will help as long as it is not the speaker to speaker distance that is causing the null. You might also consider a resonant panel for the ceiling. How much toe in are you using?


I have tried all sorts of toe in but my speakers give the best response when its pointed straight ahead.

As for diffusion, I am not sure if that will for me because my room is very small. As I understand it, diffusion works best in larger rooms right?


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## jtalden (Mar 12, 2009)

tboooe said:


> What do you mean "smoother" SPL? You mean smooth response curve? That is what I've been trying to do!


Yes, smoother SPL response. - Just a simple statement of the relationship between SPL and Phase Response.




> I am telling you, nothing changed. I made a measurement, left for about 30 mins, came back, took another measurement and the response was dramatically different. I have to believe something is wonky with JRMC convolution.


Again this was a simple statement of truth. Something has change by definition if the measurement results are significantly different. You are attributing it to a flaw in JRMC and that may be, but I would assume that is not the case. What does the JMRC forum say about other users experiences with their convolution engine? I have only heard good things.



> In my case, with the nulls at 62 and 77hz, these are caused by the large negative gains I had to apply at peaks. When i remove those particular filters, the two deep nulls go away but of course that leaves two massive peaks. Hopefully I can reduce the peaks a bit with some room treatments.


Yes, large sharp filters may cause problems and are not normally recommended. The typical advice is to find another spot for the speakers or LP to minimize the need for large EQ and to consider room treatments. I would try to keep the total EQ range no more than 15dB - less is better. With you room and setup possibly that is not possible? Some rooms are very hard to tame.

We can only keep repeating general guidelines that you are probably already very familiar with, as we have no understanding and experience with your room. If you want to post mdat files, possibly we can understand the situation a little better. Below are possible mdat options depending on what you want us to take a closer look at. Your EQ settings would also be helpful to see what you mean by; 'large negative gains that cause problems'.

> L, R, L+R (with no EQ)
> L, R, L+R (with EQ)
> L, R, L+R (with EQ but is a second measurement that shows a change in response with no change to the setup)


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## tboooe (Mar 4, 2016)

jtalden said:


> We can only keep repeating general guidelines that you are probably already very familiar with, as we have no understanding and experience with your room. If you want to post mdat files, possibly we can understand the situation a little better. Below are possible mdat options depending on what you want us to take a closer look at. Your EQ settings would also be helpful to see what you mean by; 'large negative gains that cause problems'.
> 
> > L, R, L+R (with no EQ)
> > L, R, L+R (with EQ)
> > L, R, L+R (with EQ but is a second measurement that shows a change in response with no change to the setup)


Thank you for the offer for assistance. Attached are my mdat files and the EQ. Baseline files are all without any EQ. The EQ is essentially what REW came up with just some minor tweaks to get my response to meet the target curve. 

My room is very small at only 10' L x 9' W x 9' H. The back wall behind my head is only 36" and above that the space opens up to my family room and kitchen.


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## Tonto (Jun 30, 2007)

You need at least 4 feet from the PLP for diffusion to be affective.


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## jtalden (Mar 12, 2009)

The data looks very good. Nothing too unusual. I will take a closer look tomorrow.

What speakers are these?
What is the SW XO freq?


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## jtalden (Mar 12, 2009)

I took a closer look and the following are my thoughts:

> The nulls that result in the L, R SPL response are due to room modes and reflections. Filters shift the phase response to some extent and can thus shift the location of the nulls. Interactions of the speakers when both are playing change the number and locations of the of the nulls as expected.

I saw nothing unusual in this data. 

> The EQ'ed SPL response is very smooth following a reasonable house curve and was achieved with 9 (left), 11 (right) filters from about 45-500 Hz. most of those filters are very sharp being; Q > 10. It appears that a 1/12 octave setting was used for the REW filter calculation. The target level chosen was appropriate for the filter calculation. The range of the filters is 16dB; peak / cut filters are +6 dB / -10 dB. Filters are independent between the 2 channels. 

Given this very aggressive EQ approach there is nothing to improve on. Many use an aggressive filter set as is implemented here and report satisfying sound quality results. Most all the recommendations here and by experts that I have read suggest a less aggressive filter approach. You may want to make and alternate less aggressive filter set for comparison and then do your own evaluation of the two.

I found it difficult to do this using the REW automated EQ function even when limiting the Q to 10 by selecting the DCX equalizer rather than the generic. Smoothing did not seem to help, nor did limiting the number of filters. Manually, I had no problem finding a good filter set with only 2-4 filters to cover the same 45-500Hz range. Below is an example for the left channel. I am unclear however on what the SW XO is and how the filters are thus split between the Sub channel and mains. My estimate of the house curve may also be in error, thus this example may need to be modified to fit the actual XO/Channel/house curve situation. It is just an example of a less aggressive filter set that may achieve good results. There is also middle ground available between these 2 extremes. Filters 3 and 4 are probably not really needed, but are included here to soften those peaks a little. I do not see the need to do much more than this. The fact that the filters are all negative in this case is only a result of the particular SPL response and the target curve. It is the best method to achieve the target with a minimum number of filters. It has the lowest total EQ boost/cut range (6dB) and it does not suppress the overall level of the response. In most cased a mix of boost and cut are more appropriate.

No Smoothing:








1/6 Order Smoothing:








Psychoacoustic Smoothing:


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## tboooe (Mar 4, 2016)

jtalden said:


> The data looks very good. Nothing too unusual. I will take a closer look tomorrow.
> 
> What speakers are these?
> What is the SW XO freq?


Thank you very much for reviewing my data. My speakers are Sonus Faber Guarneri Evolutions. I also have 2 Rel T 5i subs. I don't don't cross over the Evolutions. These speakers are good down to around 40hz so that is about where I set the subwoofer cross over. When I measure the subs alone and the speakers alone, I see a nice transition between the two. 

For my measurements I used the Variable smoothing as I felt it was a good compromise and allowed to see what was really happening below 500hz which was the range I was most interested in. I realized I should have Included my target curve settings too. I try to be flat from 20-100hz, sloping down -5db from 100-1000hz, then sloping down another -5db from 1000-15000z. 

Overall, I am quite happy with the curve I was able to achieve but I am not happy with the large negative gains I had to apply at 62 and 77hz. I am hopeful I can use room treatments to tame these peaks a bit so I can use less native gain and reduce the massive dips at those two frequencies.

What do you mean by "less aggressive" filters?


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## jtalden (Mar 12, 2009)

Great speakers and SW's!

The charts I posted are with the filter set I posted. Only 4 filters are used and the total filter range is 6dB instead of the 16dB range originally used. Two of those 4 filters may not be really be needed as stated above.

The 4-filter compliment is likely to do at least as well as the original 9-filter array for sound quality. Only your evaluation will determine what you personally prefer however. As I said above, the original filter set is well done for SPL response. It just may have utilized too many sharp filters. If you average several measurement point around the head and shoulders at the LP some of those sharp peaks and dips will partially even out in the range >200 Hz. That is reportedly a little more representative of what we hear. 

The SW to mains overlap chosen is not normally recommended, but appears to be very well implemented. That being the case, the results are likely as good or possibly better than a more normal XO.


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## jtalden (Mar 12, 2009)

By 'aggressive' I am referring to many closely spaced sharp filters for the range being covered.


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## tboooe (Mar 4, 2016)

jtalden said:


> Great speakers and SW's!
> 
> 
> The SW to mains overlap chosen is not normally recommended, but appears to be very well implemented. That being the case, the results are likely as good or possibly better than a more normal XO.


Thank you. I really love those speakers and the REL subs are great at any price. I actually got a flatter response from 20-40 but where I positioned my speakers, I can not put the subs where they respond best. As for the cross over, I would ideally like to cross over at a higher freq (80-90hz) but that would require me to get a separate cross over. I havent researched it much so I do not know which cross overs are really great and how much they cost.



jtalden said:


> By 'aggressive' I am referring to many closely spaced sharp filters for the range being covered.


Thank you for the clarification. I too would like to use less filters. i am going to redo my filters once I get more room treatments.


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## burnhamjs (Oct 11, 2018)

jtalden said:


> No Smoothing:
> View attachment 131065
> 
> 
> ...


While trying to figure out how to route the REW output through JRMC on the way to my speakers so I can measure the response after EQ filters applied in JRMC I came across this old post. 

I am trying to figure out the manual filters that were applied to get the final graph (Psycoacoustic Smoothing). Particularly the -6.0 gain at 66.1. Looking at the 1/6 Order Smoothing Graph it would appear a +3dB boost would be needed, though your filters seem to work well. I am new to this and just trying to understand the methodology to the filters that were applied. 

(and if anybody knows how I can route REW though JRMC so I can measure the applied filters I would appreciate that info as well)


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## jtalden (Mar 12, 2009)

burnhamjs said:


> I am trying to figure out the manual filters that were applied to get the final graph (Psycoacoustic Smoothing). Particularly the -6.0 gain at 66.1. Looking at the 1/6 Order Smoothing Graph it would appear a +3dB boost would be needed, though your filters seem to work well. I am new to this and just trying to understand the methodology to the filters that were applied.


I see I used 'L+S Baseline.mdat' for the example. I am not sure why I didn't use 'L+R+S Baseline.mdat'. That probably would have been the more appropriate file to use for the OP's case. The filters would have been different. Maybe that is why it didn't work for you. Either way the message that was intended was; it may not be necessary/helpful to use aggressive filters. 

I repeated the filters on the original file (L+S) and got the same result. Possibly you did not select 'Psy' for the smoothing or selected a different reference level? See the screen shot below. It better shows all the settings used. I left both the original trace and filtered trace visible so its easier to see the filter impact.

Most users have no issue letting REW calculate the filters. It just requires setting suitable parameters. The only reason I used a manual filter setting is that it is the method I have used for years and have gotten reasonably proficient at it. I suspect that it rare, if ever, necessary to use manual filters. The auto filter feature works very well and is easier for the casual user. I just went to the my comfort method when I didn't quickly find good parameters for the auto filter feature.



> (and if anybody knows how I can route REW though JRMC so I can measure the applied filters I would appreciate that info as well)


Users of JRMC likely have other better suggestions, but one option is to save a measurement sweep using the REW signal generator. Then apply the REW acoustic timing feature and 'Wait for timing reference' and select 'measure'. REW will start to measure and then pause to wait to hear the reference sound. Then play the sweep file in JRMP. I think this should work, but I don't use JRMP. I'm pretty sure there is any easier way using recent versions of JRMP based on users comments I've seen here. The JRMP site may be a better place to ask.


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## burnhamjs (Oct 11, 2018)

thanks jtalden - I was looking at the 1/6 order smoothing graph and trying to understand the filters. With the L&S basline you just posted the filters make for sense now. I wasn't working on an actual calibration - I was just trying to understand the manual filters you selected. 

WRT JRMC -yes, that is the only thing I can come up with that would work.


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## Jens Toft (Oct 24, 2018)

Hello

I once spend weeks trying to figure out why some of my low-end response went missing.

So here's one you could try:
Are you sure, that all doors are either open or closed during these measurements?

Try closing them, measure your response - Then open them, and measure again.

Best regards,
Jens


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