• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). There are daily reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

Understanding How the Klippel NFS Works

AdamG247

Strive not for validation, but rather to be valued
Moderator
Forum Donor
Joined
Jan 3, 2021
Messages
2,314
Likes
5,566
I noticed today in PS Audio’s Copper magazine an article discussing spinorama where they used a graph provided by ASR! Potential for cooperation? @amirm , will you test their FR-28?
Link please?
 

thewas

Major Contributor
Forum Donor
Joined
Jan 15, 2020
Messages
4,309
Likes
9,552
Amir made also a thread about it

 
Last edited:

S34MH1

Active Member
Joined
Apr 3, 2019
Messages
167
Likes
127
The whole point of NFS is to solve your last sentence. It measures in near-field, but then computes the far field.

As to line source, it all depends on complexity of its soundfield in near field. If it is very complex then you may need a lot of measurement points, and lots of computation time.

If the line source is made out of individual drivers, then each one can be measured independently and them summed together. That is an extra cost option which I did not purchase.
How to understand far field, 3M or 10M?
 
OP
NTK

NTK

Major Contributor
Forum Donor
Joined
Aug 11, 2019
Messages
1,598
Likes
3,252
Location
US East
How to understand far field, 3M or 10M?
I think it is best to first define what far-field (and near-field) means. In internet audio fora such as this one, people often use the term "nearfield" to mean something that is completely unrelated to its meaning in acoustics. (See this article by E. Sengpiel)

Briefly, in what most internet audio forum posts, "nearfield listening" means the listening distance is close and the majority of the sound heard is direct sound from the speakers (i.e. direct field listening). The opposite case is "far-field listening", and in this case the majority of the sound heard is from reflections (i.e. reverberant field listening). Here I took from one of Genelec's monitor selection guides and use the model 8350A for illustration.
genelec1.png

The red zone is the acoustical "near-field". It is where the listening distance to the speaker is too short for its 2 drivers to integrate. In the acoustical near-field, frequency response change with listening distance because of lack of driver integration. Once beyond the red zone, we are in the acoustical far-field. The green zone is where direct sound dominates. The black vertical separator is the critical distance. This is the point the direct sound energy (which decreases with listening distance) and the reflected sound energy becomes the same. The critical distance is room dependent (mostly volume and reflectivity).

Beyond the critical distance, as in reverberant field listening, the sound heard is dominated by reflections and therefore room effects are much more prominent.

In acoustics, far-field means the listener is far enough away from the sound source that the source appears to be compact, and thus behaves like a point source. For a speaker with multiple drivers, that means the sound radiated from the drivers are well integrated at that distance. If you look at the Genelec chart, the far-field for the 8350A begins at ~2.5 m ft. For the 8351B, which has the same SPL output rating as the 8350A, because of its coaxial design, far-field begins at a much shorted distance of ~1.5 m ft. Where farfield begins is determined by the speaker design, and is not room dependent.

In the acoustical far-field, the sound radiating properties of the speaker only depends on distance. If you have measurements at one distance (in the far-field), you can easily scale it to a different distance by using the 1/r relationship (i.e. SPL decreases by 6 dB every doubling of distance). Therefore, there is little difference between the reporting numbers at 3 m and 10 m. They are just a scaling factor offset of each other. The ANSI/CTA2034 standard for anechoic chamber measurement is to measure at 2 m, and report the scaled measurements at 1 m.

[Edit] Corrected the distance unit error. Distances quoted are in ft instead of m.
 
Last edited:

S34MH1

Active Member
Joined
Apr 3, 2019
Messages
167
Likes
127
I think it is best to first define what far-field (and near-field) means. In internet audio fora such as this one, people often use the term "nearfield" to mean something that is completely unrelated to its meaning in acoustics. (See this article by E. Sengpiel)

Briefly, in what most internet audio forum posts, "nearfield listening" means the listening distance is close and the majority of the sound heard is direct sound from the speakers (i.e. direct field listening). The opposite case is "far-field listening", and in this case the majority of the sound heard is from reflections (i.e. reverberant field listening). Here I took the from one of Genelec's monitor selection guides and use the model 8350A for illustration.
View attachment 193610

The red zone is the acoustical "near-field". It is where the listening distance to the speaker is too short for its 2 drivers to integrate. In the acoustical near-field, frequency response change with listening distance because of lack of driver integration. Once beyond the red zone, we are in the acoustical far-field. The green zone is where direct sound dominates. The black vertical separator is the critical distance. This is the point the direct sound energy (which decreases with listening distance) and the reflected sound energy becomes the same. The critical distance is room dependent (mostly volume and reflectivity).

Beyond the critical distance, as in reverberant field listening, the sound heard is dominated by reflections and therefore room effects are much more prominent.

In acoustics, far-field means the listener is far enough away from the sound source that the source appears to be compact, and thus behaves like a point source. For a speaker with multiple drivers, that means the sound radiated from the drivers are well integrated at that distance. If you look at the Genelec chart, the far-field for the 8350A begins at ~2.5 m. For the 8351B, which has the same SPL output rating as the 8350A, because of its coaxial design, far-field begins at a much shorted distance of ~1.5 m. Where farfield begins is determined by the speaker design, and is not room dependent.

In the acoustical far-field, the sound radiating properties of the speaker only depends on distance. If you have measurements at one distance (in the far-field), you can easily scale it to a different distance by using the 1/r relationship (i.e. SPL decreases by 6 dB every doubling of distance). Therefore, there is little difference between the reporting numbers at 3 m and 10 m. They are just a scaling factor offset of each other. The ANSI/CTA2034 standard for anechoic chamber measurement is to measure at 2 m, and report the scaled measurements at 1 m.
Extremely grateful.
 
  • Like
Reactions: NTK

Newman

Major Contributor
Joined
Jan 6, 2017
Messages
1,553
Likes
1,777
Extremely grateful.
Except for where he said “from reflections (i.e. reverberant field listening)”. They are two different things (with partial overlap) and the reverberant field does not occur in small rooms ie home hifi. The sengpielaudio article also fails to make this clear, although I think it is talking about performance spaces where this distinction is not as important as it is in home hifi.
 

mwmkravchenko

Member
Joined
Nov 20, 2018
Messages
75
Likes
91
Location
Perth Ontario
I think it is best to first define what far-field (and near-field) means. In internet audio fora such as this one, people often use the term "nearfield" to mean something that is completely unrelated to its meaning in acoustics. (See this article by E. Sengpiel)

Briefly, in what most internet audio forum posts, "nearfield listening" means the listening distance is close and the majority of the sound heard is direct sound from the speakers (i.e. direct field listening). The opposite case is "far-field listening", and in this case the majority of the sound heard is from reflections (i.e. reverberant field listening). Here I took the from one of Genelec's monitor selection guides and use the model 8350A for illustration.
View attachment 193610

The red zone is the acoustical "near-field". It is where the listening distance to the speaker is too short for its 2 drivers to integrate. In the acoustical near-field, frequency response change with listening distance because of lack of driver integration. Once beyond the red zone, we are in the acoustical far-field. The green zone is where direct sound dominates. The black vertical separator is the critical distance. This is the point the direct sound energy (which decreases with listening distance) and the reflected sound energy becomes the same. The critical distance is room dependent (mostly volume and reflectivity).

Beyond the critical distance, as in reverberant field listening, the sound heard is dominated by reflections and therefore room effects are much more prominent.

In acoustics, far-field means the listener is far enough away from the sound source that the source appears to be compact, and thus behaves like a point source. For a speaker with multiple drivers, that means the sound radiated from the drivers are well integrated at that distance. If you look at the Genelec chart, the far-field for the 8350A begins at ~2.5 m. For the 8351B, which has the same SPL output rating as the 8350A, because of its coaxial design, far-field begins at a much shorted distance of ~1.5 m. Where farfield begins is determined by the speaker design, and is not room dependent.

In the acoustical far-field, the sound radiating properties of the speaker only depends on distance. If you have measurements at one distance (in the far-field), you can easily scale it to a different distance by using the 1/r relationship (i.e. SPL decreases by 6 dB every doubling of distance). Therefore, there is little difference between the reporting numbers at 3 m and 10 m. They are just a scaling factor offset of each other. The ANSI/CTA2034 standard for anechoic chamber measurement is to measure at 2 m, and report the scaled measurements at 1 m.
Well put together. Many people listen much to far away from their speakers. And expect sound pressure levels that are not possible when they are ten to 12 feet away. They loose the impact and immediacy that you can get when you are correctly setup. I will keep this for a few clients to look over. Saves me having to type it all. One thing I take umbrage with is the fallacy that you cannot hear below 16 hertz. Very well proven to be false. In fact given sufficient volume and clarity from harmonic and intermodulation distortions from mediocre drivers you can hear to below 5 hertz. And there are plenty of studies that have proven this. My favorite one is a compendium of the various low frequency surveys.




A little off topic but potentially useful to a few people.
 

Newman

Major Contributor
Joined
Jan 6, 2017
Messages
1,553
Likes
1,777
Those links don’t actually say we can hear below 20 Hz. Vibrating body parts is a different issue. Hearing is what the eardrum responds to with vibration and auditory nerve signal.

What you are talking about belongs in the same category as bone conduction, ie non-auditory response to sound waves.
 

dc655321

Major Contributor
Joined
Mar 4, 2018
Messages
1,432
Likes
1,839
One thing I take umbrage with is the fallacy that you cannot hear below 16 hertz. Very well proven to be false. In fact given sufficient volume and clarity from harmonic and intermodulation distortions from mediocre drivers you can hear to below 5 hertz. And there are plenty of studies that have proven this. My favorite one is a compendium of the various low frequency surveys.

Your links indicate perception, not necessarily audibility, of infrasonic stimuli. Not everyone and not at typical SPL.

Generalizations suck.
 

mwmkravchenko

Member
Joined
Nov 20, 2018
Messages
75
Likes
91
Location
Perth Ontario
Your links indicate perception, not necessarily audibility, of infrasonic stimuli. Not everyone and not at typical SPL.

Generalizations suck.

Generalizations suck on both sides of that coin.

Long held ideas are tough to let go of. But research continues. And if it is grounded research what is the problem with accepting it. There are many more studies. I simply put up a link to an easily accessible one.

I have had the joy of playing good solid undistorted 16.8 low C on well recorded pipe organs for quite a few people. With low distortion reproduction and sufficient volume you can not only hear, you can discern pitch.

The ability to discern pitch is the key. Below that I agree that there is only bodily conduction.

Again research how low we can discern pitch and it is below 16 hertz.

Not looking to kick over a hornets nest. Just open a little window into research that you may, or may not know about.
 

mwmkravchenko

Member
Joined
Nov 20, 2018
Messages
75
Likes
91
Location
Perth Ontario
Those links don’t actually say we can hear below 20 Hz. Vibrating body parts is a different issue. Hearing is what the eardrum responds to with vibration and auditory nerve signal.

What you are talking about belongs in the same category as bone conduction, ie non-auditory response to sound waves.

I am guessing that you are content with your present level of knowledge.

You have not read enough of these studies to actually have an informed opinion it appears. Pitch perception being the root of what I would consider audible.
 

dc655321

Major Contributor
Joined
Mar 4, 2018
Messages
1,432
Likes
1,839
Generalizations suck on both sides of that coin.
Touché.

Long held ideas are tough to let go of. But research continues. And if it is grounded research what is the problem with accepting it. There are many more studies. I simply put up a link to an easily accessible one.

Acceptance of research is not the issue. Your misinterpretation and evidence-free anecdotes are.

It looks like you may have the means to demonstrate your claims. Please do so.
 
OP
NTK

NTK

Major Contributor
Forum Donor
Joined
Aug 11, 2019
Messages
1,598
Likes
3,252
Location
US East
Is Klippel NFS more accurate than any anechoic chamber? Even those large fully anechoic chambers?
IMHO the Klippel NFS does seem to be better for loudspeaker measurements than a typical anechoic chamber.
  1. The Klippel NFS is more accurate than a typical anechoic chamber at frequencies below ~80-100 Hz. The depth of the wedges in an anechoic chamber needs to be more than 1/4 wavelength for them to be non-reflective. The wavelength of 80 Hz is 4.3 m, which means the wedges needs to be >1 m deep, and very few anechoic chambers have wedges deeper than 1 m.
    Low frequency response of an anechoic chamber can be calibrated with free field measurements, for example by comparing measurements to those obtained using a tall test tower (see link below). However, the results will only be accurate if the loudspeaker to be tested is similar in configuration as the one used for the calibration (e.g. a chamber calibrated with a monopole woofer will not be able to measure a dipole or cardioid woofer accurately).
    Here is a slide (from link) showing the estimated errors of the NFS measuring a conventional compact 2-way loudspeaker. For this example loudspeaker, it required an "order" of 10 for the wave expansion equation to provide less than 1% error from 30 - 10k Hz , which I guess would need about ~400 or more measurement points.
    NFS Errors.png
  2. The NFS gives much better spatial resolution than traditional measurements in an anechoic chamber. With one set of measurements, the NFS can calculate the full 3D data at any arbitrary directions. For traditional measurements, if you want to know the loudspeaker response at a certain direction (as given by the azimuthal and elevation angles), you will need to take a measurement at that specific location.
  3. The NFS can also tell you the location of the transition from nearfield to far field. (Note: the nearfield to far field transition location is different for different frequencies.) Traditional measurements depend on the assumption that the measurement mics are located in the far field, which is not necessarily true for large loudspeakers and arrays.
    The mathematics used by the NFS is most efficient for compact sound sources because their sound radiation patterns are usually relatively simple. For panels sources and arrays, the NFS will require a lot more measurement points as their sound radiation patterns are much more complicated, especially at the upper frequencies. In the Magnepan LRS review, even with more than twice the measurement points, the estimated (fitting) errors skyrocketed above ~2 kHz.
Anechoic chambers have the advantage of usually being sound insulated and are therefore less noisy environments. The lower noise floor is better for non-linear distortion type measurements (and more convenient for testing max SPL capabilities). Of course, the best of both worlds is to measure in an anechoic chamber with the NFS :)
 

amirm

Founder/Admin
Staff Member
CFO (Chief Fun Officer)
Joined
Feb 13, 2016
Messages
37,931
Likes
161,737
Location
Seattle Area
Is Klippel NFS more accurate than any anechoic chamber? Even those large fully anechoic chambers?
The key thing to understand is that the gold standard is NOT anechoic chamber, but "free field." That is a speaker suspended mid-air with large distance between it and any reflection source. At the extreme, this is impossible to do. But assuming so, an anechoic chamber is a step down from this depending on how large it is.

Klippel NFS is another approximation of free field. It has strong advantage of being accurate to very low frequencies. And by measuring in near-field, it doesn't suffer from environmental issues that impact far-field measurements in anechoic chamber or free field (unless very fancy temperature/humidity control is used in the former).
 

S34MH1

Active Member
Joined
Apr 3, 2019
Messages
167
Likes
127
The key thing to understand is that the gold standard is NOT anechoic chamber, but "free field." That is a speaker suspended mid-air with large distance between it and any reflection source. At the extreme, this is impossible to do. But assuming so, an anechoic chamber is a step down from this depending on how large it is.

Klippel NFS is another approximation of free field. It has strong advantage of being accurate to very low frequencies. And by measuring in near-field, it doesn't suffer from environmental issues that impact far-field measurements in anechoic chamber or free field (unless very fancy temperature/humidity control is used in the former).
From: https://mp.weixin.qq.com/s?__biz=Mz...r_shareid=e969fd6ae8369cd534d57dceb76c07ed#rd

limited

  • The test time is longer. This is no way, the point needs to be very dense.

  • The position of the sound source point (ie the sound center) needs to be entered manually. The determination of the location of the sound source point is another big pit. The judgment of the sound source point will inevitably introduce some errors.

  • The influence of the edge diffraction of the speaker on the sound field distribution cannot be considered. Edge diffraction still has no small effect on far-field acoustic radiation.

  • Applies only to spherical wave or products that can approximate spherical wave sound sources. Because it is fitted by spherical wave expansion.
1656170378150.png

  • The error may be larger for speakers of aspherical wave sound sources, such as line arrays/sound columns, etc.

  • The official workaround at present is to measure one, and then perform the superposition operation. For example, when three line array speakers are stacked at the same time, only the one in the middle is turned on, and the other two do not work. Because this is closer to a spherical wave than a single measurement.
1656170393285.png

  • Of course, klippel will measure two layers, calibrate each other, and comes with an error estimate, so you can know which frequency bands are more credible.
1656170402618.png

I haven't thought of a better way yet. Look forward to the progress and update of the algorithm.

Just thinking maybe it might be possible to borrow algorithms from PML (Perfect Matching Layer) or AML (Automatic Matching Layer) in Finite Elements.

Because in the finite element calculation, the acoustic radiation boundary also needs to specify the type of sound source, spherical wave, cylindrical wave, plane wave, etc. In addition to plane waves, you also need to specify the location of the sound source. Otherwise, there will be errors in the estimation from the near field to the far field.

Personally, I think these algorithms in finite element are very similar to klippel's holographic near-field test.
 

S34MH1

Active Member
Joined
Apr 3, 2019
Messages
167
Likes
127
The key thing to understand is that the gold standard is NOT anechoic chamber, but "free field." That is a speaker suspended mid-air with large distance between it and any reflection source. At the extreme, this is impossible to do. But assuming so, an anechoic chamber is a step down from this depending on how large it is.

Klippel NFS is another approximation of free field. It has strong advantage of being accurate to very low frequencies. And by measuring in near-field, it doesn't suffer from environmental issues that impact far-field measurements in anechoic chamber or free field (unless very fancy temperature/humidity control is used in the former).
I also saw some comments from other people.


  • NFS is the most inaccurate among common electroacoustic test instruments.

  • It has a protective cover for that microphone, the high frequency is easy to be wrong, take off the protective cover, if the setting is wrong, the results will be wacky.

  • NFS is a cost down thing, the anechoic chamber is too expensive.

  • I remember that the high frequency of NFS is filtered by the time window, which is different from the anechoic chamber in principle. Other problems include low frequency accuracy and SNR of the algorithm.

  • The microphone for NFS is relatively low-end GRAS.
 
Top Bottom