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Bass Response Correction for Klippel NFS Measurements

hardisj

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A win win if also Klippel take this feedback and improve their product . That's peer review is so powerful.

Well, to be fair, there are plenty of other outlets measuring large speakers without these issues. I think Amir and I were both in the same boat of not having the information needed to arrive at the proper result. I had planned to email Warkwyn and ask them about it as I know they test other large speakers and we have seen Magico testing large floorstanding speakers as well. I don't think there is anything for Klippel to improve other than just better documenting this information in their instructions.
 

NTK

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Now, the only question I still have is... how do you *know* your result is A-OK? Now, please, please, please don't take this as criticism. I'm asking a bit of a rhetorical question as a fellow NFS user. I think we may be at the 98% solution here. However, in my mind, there's no way to just *know* the result is correct without some other sanity check such as a ground plane measurement or comparison against other available data (like the Harman data).

Remember, I went through this with the HDI-3800; the fitting error was well below 1% for that speaker. But fitting error isn't the tell-all. Everyone pointed out the result didn't seem right and it didn't match James' ground plane testing of the 3800 on Audioholics. I agreed that something wasn't right and would provide ground plane measurements for large speakers in the future to supplement the NFS data. I didn't have a chance to do that with the 3800 before I sent it back.

However, I did do that with the HDI-4500 and that data nearly perfectly matched the ground plane data (shown below).

So, I have two cases of rather large speakers with multiple bass drivers. And in the case of the 4500, I honestly expected that one to give me the most unlikely results but it was practically spot on to the GP measurement.

I think that when in doubt it will still be necessary - or at least useful - to provide a ground plane measurement (assuming there is none other available) to remove all doubt. At least initially to build confidence in this method. Still, this is certainly a giant leap forward in getting 100% accurate results.
Please see my earlier post:
https://www.audiosciencereview.com/...-gets-a-klippel-nfs.19445/page-23#post-681517

If the expansion center is skewed to one end of the speaker, the radius of the measurement cylinders needs to be significantly larger than if the expansion center aligns with the center of the speaker.
 

richard12511

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Now, the only question I still have is... how do you *know* your result is A-OK? Now, please, please, please don't take this as criticism. I'm asking a bit of a rhetorical question as a fellow NFS user. I think we may be at the 98% solution here. However, in my mind, there's no way to just *know* the result is correct without some other sanity check such as a ground plane measurement or comparison against other available data (like the Harman data).

Remember, I went through this with the HDI-3800; the fitting error was well below 1% for that speaker. But fitting error isn't the tell-all. Everyone pointed out the result didn't seem right and it didn't match James' ground plane testing of the 3800 on Audioholics. I agreed that something wasn't right and would provide ground plane measurements for large speakers in the future to supplement the NFS data. I didn't have a chance to do that with the 3800 before I sent it back.

However, I did do that with the HDI-4500 and that data nearly perfectly matched the ground plane data (shown below).

So, I have two cases of rather large speakers with multiple bass drivers. And in the case of the 4500, I honestly expected that one to give me the most unlikely results but it was practically spot on to the GP measurement.

I think that when in doubt it will still be necessary - or at least useful - to provide a ground plane measurement (assuming there is none other available) to remove all doubt. At least initially to build confidence in this method. Still, this is certainly a giant leap forward in getting 100% accurate results.

That's a tough one. Only way I can think of to know for sure if it's correct is to do a ground plane measurement. That kinda defeats some of the purpose of having the NFS, though, so I'm not a huge fan of that solution. Also, it's not doable in all weather.

Another way is to check the results against other NFS measurements of the speakers in the same line. For the HDI 3800 for example, you could compare the NFS measurement of that speaker against the NFS measurement of the HDI 1600 to see that something was likely wrong with the 3800, as the NFS reported a lower F6 for the 1600 than it did the 3800. Problem with this is that it's somewhat rare that we have such comparable NFS measurements to highlight these flaws. Also, it doesn't tell you how wrong the 3800 measurement is, but rather that it's likely wrong by some unknown amount.

Another way is to check against the manufacturer specs, but I'm of the opinion that not all manufacturers can be trusted in this way. Even the ones I do trust (JBL for example) still show a slightly different(higher) F6 than the ground plane or corrected NFS measurements. Not sure if this is do due to inaccuracy on their end, or the fact that they seem to slightly exaggerate sensitivity(thus raising the F6).


On the HDI 4500 vs 3800, I have to say your ground plane measurement of the 4500 almost convinced me that maybe the 3800 measurement was correct. I mean, those speakers are of a comparable size, and it didn't make sense to me at the time why it would be right for the 4500, but wrong for the 3800. Now though, if I understand Amir's explanation correctly, I think I get it. I think it has to do with where the tweeter is located on the speaker. For the 4500, the tweeter is in the exact center, which probably means that the acoustic center for the tweeter is in the exact same spot as the acoustic center for the woofers. The 3800 tweeter is at the top of the baffle, so it makes sense that the acoustic center for the tweeter and woofers are in different locations.
 

hardisj

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I think it has to do with where the tweeter is located on the speaker. For the 4500, the tweeter is in the exact center, which probably means that the acoustic center for the tweeter is in the exact same spot as the acoustic center for the woofers. The 3800 tweeter is at the top of the baffle, so it makes sense that the acoustic center for the tweeter and woofers are in different locations.

That was my thinking as well.
 
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An outdoor ground-plane measurement as a sanity check for the NFS measurement? Now there's a concept! :)

Dave.
 

eddantes

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NVM
 
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Matias

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Question: will Amir and Erin recalculate and update all the 100+ speaker reviews they have combined? That is quite a burden...
 

hardisj

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Question: will Amir and Erin recalculate and update all the 100+ speaker reviews they have combined? That is quite a burden...

I will update my JBL HDI-3800 review because that's the only one that is affected by the bass issues. All the others I have tested thus far (with the NFS) haven't been as complex.

I'm pretty sure I know the answer for Amir but I won't speak for him.
 

Juhazi

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Well, to be fair, there are plenty of other outlets measuring large speakers without these issues. I think Amir and I were both in the same boat of not having the information needed to arrive at the proper result. I had planned to email Warkwyn and ask them about it as I know they test other large speakers and we have seen Magico testing large floorstanding speakers as well. I don't think there is anything for Klippel to improve other than just better documenting this information in their instructions.

Well, perhaps it is business logic... These addon modules are pretty expensive, I believe (Amir said?) ...
http://www.klippel.de/products/rd-system/modules/nfs-near-field-scanner.html

klippel nfs multiplexer.jpg
 

Thomas savage

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Almost since I started measuring speakers using Klippel NFS, we have been having trouble with tower speakers and ports. The more drivers, the more ports, the worse the results there. A few months ago I experimented and realized that increasing the microphone distance from the speaker helps fair bit. The low frequency sound field is quite complex due to multiple radiating surfaces. The closer you sample to the speaker, the harder the prediction for the far-field is. Increasing distance helped with that but seemingly it was not enough.

So I reached out to Klippel with carefully scenario showing the improvements one gets from increasing distance asking if there is a better solution. I am at the limit of how wide I can go and so is the fixture. To my pleasant surprise, Christian who is the architect of NFS responded that there was an alternative undocumented way to make even better progress. The issue currently is that I set one reference point for measurements and that is set at tweeter axis. High-frequency is where complexity is very high and we need that center to be more optimized. With these tower speakers, the acoustic center for the bass is way lower causing that to be an added problem. Numerical precision is lost quickly and as such, NFS software limits the expansion order in bass. This in turn causes too little energy to be summed to represent the total bass output. With me so far? It is fine if you are not. :) Let me show the results.

Turns out it is possible to set the reference point for different frequency ranges. That allowed me to set a much more optimal point for bass frequencies, fixing the issue we had.

Revel F328Be with Numerical Optimizations for Bass
I pulled the reference point below 300 Hz to 1/3 of a meter instead of 1.3 meters that it used to be. I guess at the former as my speaker is now packed ready to go to its new owner. That guess was good enough to generate far better results in bass response. Here is a before and after:

View attachment 124083

We now have both more extension and higher levels in bass. Resulting improvement in preference score is quite high -- one full point! This is due to computation being based on F6 which is now a lot lower in frequency.

Here is the full, new, spin graph:

View attachment 124084

And comparison to Harman data:

Spin%2B-%2BRevel%2BPerforma3Be%2BF328Be%2Braw.png


Harman graph shows some waviness between 20 and 40 Hz which is due to their room no longer being anechoic (and hence have "room modes"). We don't have that with Klippel NFS so our accuracy is higher. To my eyes, looks like we have a match in bass but I let you all analyze it further.

Revel F35 with Numerical Optimizations for Bass
F35 measurements which was a year ago was the first time I realized bass response was not correct by good bit. I ran the results by Klippel then but we didn't arrive at a solution until now:

View attachment 124087

Much more sane results now. :) Here is the full spin:
View attachment 124088

Summary
Looks like the problem in bass response is now resolved despite my guessing as to the optimal values. I will have to work a bit to fully establish the correct parameters. Once there, I will go back and re-run the analysis for speakers most impacted.
This was always a issue and I think the times we live in and this platform have enabled a hugely valuable ' peer review ' and R&D facility for companies that just wasn't possible for them before , unless they were huge corporations .

The better that NFS is the better the industry is as its a way to track performance without the privileged access to a chamber .

Its revolutionary and im so pleased the audio community are coming together to make a true standard.
 

HooStat

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Question: will Amir and Erin recalculate and update all the 100+ speaker reviews they have combined? That is quite a burden...
I don't think that many speakers are affected. It shouldn't be too bad to do, assuming it applies to 4-5 speakers (2 of which are already done).
 

hardisj

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I just updated my SPIN for the JBL HDI-3800. I discussed things in my review thread but am also sharing here.


Thanks to @amirm starting this thread, I resolved the bass issue in my data and have updated it on my site already.

For those curious about the difference, I have plotted the old vs the new (black vs red).

1618513100455.png






Then, I looked at the HARMAN spin data; I used the Trace SPL tool in VituixCAD to trace Harman's anechoic on-axis response. I compared that to my update on-axis response and also I overlaid Audioholics' ground-plane measurement (which is only valid up to 400Hz). I smoothed all to 1/12 because the higher resolution of my data made it harder to see "through" the data. I tried to align the responses between 100-400Hz, within reason.

Mine = Red
Harman = Blue
AH = Green

1618513110932.png


You can see a generally good match. Especially between mine and the Harman anechoic response. Interestingly, the ground-plane measurement by AH is even further off from the Harman response.

So, yep. Looks like we as a community got this resolved. Thanks again to Amir for reaching out to Klippel and finding out what was going on.
 

hardisj

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What make you say that? From what I can tell, as long there is no “checkmark” response in the bass, it is accurate.

FWIW, there are plenty of cases where a checkmark may not mean the bass profile is entirely incorrect. That is more a function of output vs room modes (noise). In my case there is a room mode at around 25Hz and I've noticed that active bookshelf speakers with a steep roll off just don't have enough output that low to overcome the mode and thus you get some cancellation. So, while the checkmark may be an indication of inaccurate LF data, it doesn't mean that the bass is wrong for sure.
 

hardisj

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I think the Magnepan might benefit from a re-measure. :)

That's actually a good point. I don't know how the magnepan acts as a bass source... maybe its reference point is somewhere other than the axis Amir set for expansion? You guys would have to answer that. I'm just thinking you might be on to something here.
 

napilopez

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Its revolutionary and im so pleased the audio community are coming together to make a true standard.

It is pretty remarkable to think back on the huge difference in the availability of available measurements over the last year or two. We had soundstage Network, Audioholics, Stereophile, Audioxpress for the English sites. But now we have such a wealth of data with Amir and Erin (and a little bit from me) and the spinorama is becoming increasingly common. When Dr Toole wrote the latest edition of his book, it was pretty much only Harman doing spins... now it's showing up more and more places :D
 

MZKM

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FWIW, there are plenty of cases where a checkmark may not mean the bass profile is entirely incorrect. That is more a function of output vs room modes (noise). In my case there is a room mode at around 25Hz and I've noticed that active bookshelf speakers with a steep roll off just don't have enough output that low to overcome the mode and thus you get some cancellation. So, while the checkmark may be an indication of inaccurate LF data, it doesn't mean that the bass is wrong for sure.
Now let‘s reverse it; can the bass be incorrect without the ”checkmark”?

That would be interesting to know.
 

richard12511

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This was always a issue and I think the times we live in and this platform have enabled a hugely valuable ' peer review ' and R&D facility for companies that just wasn't possible for them before , unless they were huge corporations .

The better that NFS is the better the industry is as its a way to track performance without the privileged access to a chamber .

Its revolutionary and im so pleased the audio community are coming together to make a true standard.

I was thinking earlier that surely some of these companies like Magico who've been using the NFS for awhile have encountered these issues, especially after seeing that giant M9 being measured. I wonder if they ran into the same issues awhile back?

*Edit: M9 may be a bad example. While it's a humongous speaker, the tweeter is in the center, and it's a sealed speaker. Other big Magico towers however could show issues.
 

MrPeabody

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Now, the only question I still have is... how do you *know* your result is A-OK? Now, please, please, please don't take this as criticism. I'm asking a bit of a rhetorical question as a fellow NFS user. I think we may be at the 98% solution here. However, in my mind, there's no way to just *know* the result is correct without some other sanity check such as a ground plane measurement or comparison against other available data (like the Harman data).

Remember, I went through this with the HDI-3800; the fitting error was well below 1% for that speaker. But fitting error isn't the tell-all. Everyone pointed out the result didn't seem right and it didn't match James' ground plane testing of the 3800 on Audioholics. I agreed that something wasn't right and would provide ground plane measurements for large speakers in the future to supplement the NFS data. I didn't have a chance to do that with the 3800 before I sent it back.

However, I did do that with the HDI-4500 and that data nearly perfectly matched the ground plane data (shown below).

So, I have two cases of rather large speakers with multiple bass drivers. And in the case of the 4500, I honestly expected that one to give me the most unlikely results but it was practically spot on to the GP measurement.

I think that when in doubt it will still be necessary - or at least useful - to provide a ground plane measurement (assuming there is none other available) to remove all doubt. At least initially to build confidence in this method. Still, this is certainly a giant leap forward in getting 100% accurate results.

I was glad to see this post. You understand the issue perfectly and stated it well. To have confidence that the data obtained with the newly identified procedural adjustment is accurate and correct, you have to check it against data obtained using some other, tried and true method. After many tower speakers with ports and passive radiators in various locations have been measured, it should become apparent whether the issue has been fully resolved or whether it remains necessary to take separate bass measurements using another method and then splice the curves.
 
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