Klippel Near Field Scanner on a Shoestring

b_force, you know how an octave works. Yes up to 800 hertz is easy 2 octaves down from there is 200 hertz. You can effectively go a little lower on a small cabinet, but not too much lower. PA is a totally different animal. You can camp out in some of those subwoofer stacks at concerts. And the subwoofer stack can be near a half wavelength of the lower frequencies they are asked to reproduce.
Oh, right, I was thinking from like 80-100Hz, so it makes sense what you're saying :)
 
Yes, for the purposes of simulating maximum SPL from specifications I agree. Simulations are very accurate. Especially from Hornresp. But, Many many woofers are very optimistically specified using the geometric distance of the voicecoil minus the top plate, or at a BL of 80% calculated and simulated! When I measure subwoofers I do this outside and I can tell you it is a rare woofer that gives you honest specifications. I recently read of a car audio driver that purported 3 inches of travel in each direction. When you design woofers, you need to pay attention to the mechanics of the basket. And I knew the basket that they attributed this massive excursion too. To get that mechanical motion was simply impossible. Secondarily to have a surround that has that much mechanical motion you loose so much SD that it becomes a game where you gain almost nothing due to the smaller and smaller Sd. Through many years of design I have come up with a rule of thumb to not try to do anything greater than 34mm one way. Most of the driver designs that I do start at 10mm one way. Examples abound of wishful engineering. Some I cannot mention. But for real world performance testing of a subwoofer you need the great outdoors. Your building will not like 130db at 14 or 16 hertz believe me!

Mark

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A very long excursion 100mm driver I recently designed. Not quite 10mm real world mechanical throw.

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I think we are mixing up things here?

Sound field is not the same as parametric behavior.
SPL can be matched with nearfield measurements and stitching methods.
As long as the type of cabinet and system are known, the sound field (aka directivity and polar plots) can be simulated.

It all depends how far you wanna go with design verification.
I personally don't care about max measured numbers, simply because I don't find them all that relevant and important.
I use those just as ballpark figures. At a certain point, it's either loud or even louder.
In my experience that is certainly true for home/studio hifi, but also for PA/sound reinforcement.

From a driver design perspective, I can totally see why you do wanna know these kind of things.
Btw, a type of field that 100% fascinates me, since it ticks so many boxes of physics! :) :) 🤓

But I totally agree with "Examples abound of wishful engineering", or worse is cherry picking.
It just doesn't serve anyone and eventually only leads to disappointment.
This happens even from a lot of known manufactures and seems to becoming worse unfortunately.

Btw, sexy woofer! :)
 
Many many woofers are very optimistically specified using the geometric distance of the voicecoil minus the top plate, or at a BL of 80% calculated and simulated!
Mark

You are mixing up small signal performance with large signal (nonlinear) performance. It's best not to do that since the two things require completely different approaches.

The large signal parameters can be obtained in many ways, with a mic, a laser, or even just through the voice coil impedance. These tests should be separate from the small signal ones which should never be done under high signal input.

I never take manufacturers specs for granted, but they are easy enough to measure directly.
 
This technique was first revealed way back in the 40's in JASA by Gabriel Weinreich at the university of Michigan. All of the Klippel, et. al. techniques are based on this paper.
Hi Earl

I can find articles by Gabriel Weinreich starting in a 1953 thesis mentioned. Then Bell Labs working with fundamental physic for semi-conductors. In 1960 he started at Michigan University. 1977 seems to be his first foray into acoustics. If you have the reference that would definitely be interesting to read.

Mark
 
Hi Earl

I can find articles by Gabriel Weinreich starting in a 1953 thesis mentioned. Then Bell Labs working with fundamental physic for semi-conductors. In 1960 he started at Michigan University. 1977 seems to be his first foray into acoustics. If you have the reference that would definitely be interesting to read.

Mark
Maybe I am wrong on the date, in thinking I have to be because he was in Europe during the war. He was definitely at UoM when he did the paper so it has to be after 1960. Keep looking, I am sure someone will find it, I did.
 
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It's not my paper to share and it can readily be downloaded from the ASA site just as I did. I have the paper somewhere but its been years since I looked at it so I don't remember the exact issue or date. (And in looking I couldn't find it.)
No problem, I will just try to use my search magic skills :)
I just thought it might be helpful if you have some keywords or title or something :)
 
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@gedlee
I think this must be:
Weinreich, G. and Arnold, E. B. (1980). Method for measuring acoustic radiation fields, J. Acoust. Soc. Am. 68 (2), 404-411.

This is also the paper Klipper is using in their references.

If you dive deeper into that specific rabbit hole, you will find heaps of other papers about the same subject.

Such as:
https://gilleschardon.fr/papers/chardon_kreuzer_noisternig_icassp_2014.pdf

Instead of a real physical array, one can also just move a single microphone to different coordinates.
 
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Probably already known and posted in this topic (can't remember, has been far too long), but eventually you'll end up at Klippel's papers;

https://pub.dega-akustik.de/DAGA_2015/data/articles/000208.pdf

https://www.klippel.de/fileadmin/kl...ld Measurement of Loudspeaker Directivity.pdf

The results are interesting to see.
They are not identical to an anechoic room.
Which leaves us with another chicken-egg discussion.

They seem to use a similar approach to;
http://www.conforg.fr/acoustics2012/cdrom/data/articles/000407.pdf

The combination with the last one incl time window measurements would make more sense to me.
 
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Yes it's the M215, the factory calibrations seems pretty decent, I compared it to a calibrated Earthworks and it all looked consistent. (thanks to @cowanaudio ! )
It rolls off a little earlier than I would prefer, the calibration takes care of that but it doesn't look as nice.
But the M215 is pretty nice for the money, construction quality is impressive and less than $350 US is definitely not crazy expensive.
Good to know, thanks for giving your impressions.
So I would take your NTi M2010 in preference, you could always put it in a slim tube if you don't want a nunchuk.
I do actually have it in a slim tube about the length of the connecting cable. It makes for a long assembly, but it does a nice job of preventing reflections off of a mic clip.
What do you use as an audio interface?
Late reply on account of being embarrassed... but I'm currently using a Behringer UMC204HD. I have an old M-Audio Delta 1010 that I plan on using when I have a dedicated measurement PC as it should have better performance.
 
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That seems similar to me as well. And I've been thinking about how sound intensity measurements are able to do direct/reflect field separation.... it is usually the p-p method. That might take less math and computing than computing spherical harmonics... but I ain't no math guy.
I cannot findcthis doc on the site, error 404, can someone attach it to a post?
 
Late reply on account of being embarrassed... but I'm currently using a Behringer UMC204HD. I have an old M-Audio Delta 1010 that I plan on using when I have a dedicated measurement PC as it should have better performance.
An ADC works with 1's and 0's for the purposes of testing this is serious overkill. Be proud you are testing! with very reasonable equipment. When I started out in this to do these types of tests as a independent consultant was not possible. It required a $100,000 in test equipment and we couldn't do much of what is a button press now.

We are well past being able to quantify what a loudspeaker does. It is sad that there are still execrable loudspeakers sold! A $100 UMIL or equivalent is more than enough to measure a great loudspeaker. And the free software tools are many times better than the paid for tools. I happily sent John Mulcahy the Pro fee. Vituix, for crossovers is very powerful. Again a modest fee. I don't do as many crossovers as I used to. Mostly drivers and enclosure/amplifier systems now. But next paying crossover work, Mr. Saunisto will get a license request too!

Speaking of embarrassed, who taught me about the REW method? And thanks for doing so! It has proven to be a most useful tool in measurement and characterization of systems.

The more you dive into a subject the less you know. It is true you know enough to have a skeletal understanding. And even a little more flesh on the bones. But total and complete, that takes a lot of time. Happily we have papers and concerted efforts to understand many of the unanswered questions. Finding the answers is the fun part no?
 
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