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Understanding How the Klippel NFS Works

MZKM

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In a nutshell, the NFS is just a tool to obtain free-field (anechoic) measurements without having to have an anechoic chamber. The results from the NFS allow us to predict what the sound field radiated by a loudspeaker at any distances* and directions in 3D space (in the free field/anechoic condition).

The NFS can tell us at what distance far field will begin for the speaker. When I say "far field", I am referring to the term's meaning in acoustics, which is at what distance the loudspeaker begins to behave like a point source (i.e. the listener is far enough away from the loudspeaker that it looks small and integrated). Very often people confuse the terms "near field" with "direct field" and "far field" with "reverberant field". (See http://www.sengpielaudio.com/DirectFieldAndReverberantField.pdf)

The following posts explains how the NFS determines at which distance far field begins.
https://www.audiosciencereview.com/...tudio-monitor-review.15963/page-4#post-513378
https://www.audiosciencereview.com/...tudio-monitor-review.15963/page-4#post-513422



Note: * At any distances further than the measurement locations.
Others should take note that Amir has posted the Apparent Power graphs, but interpreted them incorrectly. He pointed to where the highest expansive flattened out, but instead it is where the Total Apparent Power flattens, and also didn’t choose an appropriate frequency.

Not sure if you watched Erin’s (@hardisj, just to tag him) talk with Klippel (1hr 23min mark):

You don’t look at the highest order to determine far-field, you look at Total Power. Look at how huge your y-axis is, that is throwing off the intuitive nature, we don’t need the 13th spherical harmonic to be -200dB down from the monopole. He states (as well as one of Klippel’s PDFs) that the far-field limit is when Total Power reaches 0.5dB from being monopole.

Also, showing it at 400Hz just shows it at 400Hz, not it and above. So at 400Hz it’s not 1.54m but around 30cm (your scale is too huge to see 0.5dB increments). This makes sense as it’s mostly the woofer playing that frequency. You stated as you went lower in Hz that the distance increased, which makes sense as this is a rear-ported speaker so the sound field is more complex in the bass and you need to be further away for the port and woofer to sum.

The guy at Klippel said that they are working on generating far-field transition distance for all frequencies, but that it needs work.

However, due to my understanding, there is a manual way. You first look at the Radiated Sound Power graph:
View attachment 117966

And look at where the monopole nature is most reduced, in this case around 1500Hz.
I am willing to bet if you look at the Apparent Sound Power for any frequency for this speaker above that ~1500Hz will have the furthest distance to be considered far-field.

_______
Assuming the Apparent Sound Power data can be exported, can it only be exported as 1 frequency (what the graph is limited to), or can you export it for all frequencies? Because if so and you are willing, I can find the far-field distance.
 
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amirm

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Others should take note that Amir has posted the Apparent Power graphs, but interpreted them incorrectly. He pointed to where the highest expansive flattened out, but instead it is where the Total Apparent Power flattens, and also didn’t choose an appropriate frequency.
You mean Erin, not me, correct?
 

MZKM

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You mean Erin, not me, correct?
You did it for the Dynaudio LYD & Magnapan LRS.

index.php


~0.16m not 1.54m, but not done at the right frequency.
index.php


~1.1m, not 3m, but not done at the right frequency.
 

BenB

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Does the NFS calculate the individual driver contributions, or are those measurements taken in another way? I ask because those plots are very misleading. Using the recent Polk Monitor 40 Series II review as an example, you can see that at 100 Hz, the tweeter output is depicted as being only 10 dB below that of the woofers.

https://www.audiosciencereview.com/...se-measurements-bookshelf-speaker-png.135107/

Since most speakers utilize parallel crossovers, I'm used to seeing these types of plots made by simply connecting each "way" in isolation. Of course, that doesn't work for calculating the port contribution, which is typically performed by close micing the woofer and port separately, and comparing to the summed response.

I'm curious if we're stuck with these misleading plots, and why.
 

alex-z

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Does the NFS calculate the individual driver contributions, or are those measurements taken in another way? I ask because those plots are very misleading. Using the recent Polk Monitor 40 Series II review as an example, you can see that at 100 Hz, the tweeter output is depicted as being only 10 dB below that of the woofers.

Pretty sure those measurements are simply raw measurements with the crossover in place , so there is natural overlap. The individual driver graph is used primarily to show resonances + crossover point.
 

restorer-john

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I'm curious if we're stuck with these misleading plots, and why.

The response of the tweeter as shown is not just the response of the tweeter. And neither is the woofer's response just the woofer. Once past the crossover point all bets are off, as the microphone is picking up the woofer's response in the case of the tweeter and vice versa. It's plainly obvious.

No tweeter on earth will be outputting exactly the same response as the woofer, just down 10dB, all the way down to 20Hz. That's patently obvious and clearly a very misleading and wrong plot, but I'm sure there's the ability to cut off the driver's responses at the point they no longer are representative of the actual output from those drivers alone. In the case of the tweeter about 1.5Khz in this case.

We know the tweeters output is not increasing below 1.5KHz. It is rolling off hard due to the series capacitors (8.22uF) in the crossover as designed.

We also know the woofer cannot and is not responding at the HF frequencies as depicted. The 7-10kHz jagged mess is the Klippel getting horribly confused.
1624330020014.png
 
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Jukka

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If a speaker has multiple binding posts (a pair for each driver), is it possible to make these individual driver measurements so that only the corresponding bindings are used?
 

restorer-john

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If a speaker has multiple binding posts (a pair for each driver), is it possible to make these individual driver measurements so that only the corresponding bindings are used?

Absolutely, yes. But the issues of acoustic interference/cancellation/reinforcement between the drivers won't show when done individually.
 

dasdoing

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I have questioned before the conclusions that are made with the peaks that appear high in the port plot.
people have sugested/answered that those resonances probably represent comb filtering inside the case. how is the port outputting at 1800HZ? impossible right?
 

hardisj

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I have questioned before the conclusions that are made with the peaks that appear high in the port plot.
people have sugested/answered that those resonances probably represent comb filtering inside the case. how is the port outputting at 1800HZ? impossible right?

You have to plug the ports to know for sure. I've had to do this a few times to help me figure out if the port is the source of the issue or if it is simply a standing wave leaking from the port. For example, my review of the Pioneer DJ VM-50:
Stuffing the Ports
When looking at the SPIN data and the horizontal data, you can see some resonances around 600Hz, 800Hz and 1200Hz. I thought it was possible the port was resonating and sending energy around the speaker through a mechanical means. So, I tested this theory by measuring the speaker with the port open and then stuffing the port. Per the graphic below, there is no difference in the region I am concerned with. This means the port itself is not the cause.
Pioneer%20DJ%20VM-50%20--%20Port%20Stuffed%20vs%20Open.png


The width of the speaker is about 7.8-inches, but assuming 0.50-inch thick walls, this puts the internal width closer to 6.8-inches. This results in a half-wave of about 1kHz. Figuring the internal dimensions may be a bit off, this leads me to believe what I am seeing is a standing wave inside the enclosure that is simply leaking out of the enclosure in the back. The same assumption applies to all three resonances as the internal dimensions work out to be approximately at these frequencies.

All of that said, these resonances aren’t anything I was actually hearing, at least not to the detriment of my enjoyment of the speaker. Certainly not like other speakers I have experienced resonances with, where in those cases, completely ruined my enjoyment of the speaker. More than anything, it is the sequence of resonances that cause the midrange from 500Hz to 1kHz to be boosted and alter the tonality. In my case, I simply used the built-in EQ of my computer to pad this down about 2dB and it made my musical selection more pleasant to listen to.
 

dasdoing

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You have to plug the ports to know for sure. I've had to do this a few times to help me figure out if the port is the source of the issue or if it is simply a standing wave leaking from the port. For example, my review of the Pioneer DJ VM-50:

I did the test, too....and there was also no diference. my ported mains are crossed way above port range
 

Thomas_A

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Don't know if I've seen that, but can the Klippel calculate the "direct" (<5ms) response including nearby reflecting surfaces given a certain room size and speaker position relative to the nearby surfaces?
 

amirm

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Does the NFS calculate the individual driver contributions, or are those measurements taken in another way? I ask because those plots are very misleading. Using the recent Polk Monitor 40 Series II review as an example, you can see that at 100 Hz, the tweeter output is depicted as being only 10 dB below that of the woofers.
The right answer has been given. But just to add, this thread is about Near-field Scanner. This is what is used to generate the "spin" graphs. All other measurements are using Klippel's base measurement system which has nothing to do with NFS. They are static measurements and don't have any "magic" in them. You could very well produce them with REW or any other similar program.

Also, I have started to cut off the low and upper bounds of the near-field measurements to avoid the conclusion that has led to this question. It requires more work :), but it makes things more clear. You can see it in my latest measurements:

index.php


I am making a judgement call on where to start and stop the sweep as there is no perfect method to compute it.
 

amirm

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I have questioned before the conclusions that are made with the peaks that appear high in the port plot.
people have sugested/answered that those resonances probably represent comb filtering inside the case. how is the port outputting at 1800HZ? impossible right?
The port basically lets out whatever is in the box. So it could be anything.
 

Ashoka

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Can't believe I didn't think to post this here. I had Christian Bellmann (one of the inventors of the NFS) on my channel to discuss the NFS. You guys may find this useful.


1. At 1.00.00hr speakers come in different sizes, some inches to some feet so graph also should show real speaker in size and measurements around it.
2. Edited video is better, Try to reduce duration by half.
 

mwmkravchenko

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Anything happening on the front of the cone on a woofer type driver, is also happening on the rear of the cone. This includes the entire passband of the driver. And yes this leaks out through the port. There are also internal resonances that are usually fairly easy to find in the impedance phase plot once you understand some basic math in regard to frequency wavelengths. If you have parallel walls, or parallel top and bottom you will have internal reflections. Having gone to great lengths to investigate this I can tell you that it is more noticeable when there is an absence of these colourations. You notice them much easier once you have become accustomed to a sound source that is devoid of these little gremlins.
 

pozz

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kyle_neuron

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I gotta git me one of these Klippel thingys, they sound fun
How many kidneys do you possess, and can you manage to live without any of them? :)


Handmade acoustic holography. @NTK I figured you'd like this.
If you have the time and patience to match (or make correction curves for) a whole bunch of MEMS mics, you can do your own too with some aluminium extrusion. The software is the harder part! MiniDSP actually sell a ready to roll unit, albeit at a fairly small size:

Moving the thing in a smooth, predictable manner is also a tricky part for doing things like the Klippel NFS approach. You can make fairly reliable stepper motor driven turntables and the like, but having one that scales up or down appropriately to fit various items inside the field is tricky. It's a problem even with robot arms such as this one, which I've used a few times in the past:

Realising the thing has run afoul of a large speaker is a heart-wrenching moment....
 
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