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Klippel Near Field Scanner on a Shoestring
Klippel Near Field Scanner on a Shoestring
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Old 3rd March 2018, 10:48 PM   #51
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by jcx View Post
MEMS pressure mics for cell phones...
OK, now I understand, my comments were about MEMS velocity sensors like the Microflown and I took it that you meant the same.
The peak makes sense then.

Quote:
arrays of pressure mics...=> velocities
The derivation of velocity information from spaced pressure sensors has been discussed in the literature.
Unfortunately, in the near field the velocity drops fast and non-linearly and the pressure and velocity phase varies so the technique doesn't work there as well as one would hope, or at least not easily.
But I don't fully understand the near field so maybe it's doable.
You have any references?

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David

Last edited by Dave Zan; 3rd March 2018 at 10:51 PM.
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Old 4th March 2018, 04:54 PM   #52
gedlee is offline gedlee  United States
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The Microflown sensors are great for fields where one worries about the transducer interrupting the field, because they are so small. But for what we do, small small isn't that critical. Spaced pressure mics, like in intensity measurements, would seem to me to be best - small mics, but not "micro". In the near field not all the velocity is not radial, that's where the problems come in.
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Old 5th March 2018, 10:38 AM   #53
Dave Zan is offline Dave Zan  Australia
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Originally Posted by gedlee View Post
...But for what we do, small small isn't that critical.
Yes, small isn't the issue here.

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Spaced pressure mics, like in intensity measurements, would seem to me to be best...
MicroFlown say that spaced pressure mics are a sub-optimal way to make measurements in the near field.
Of course, they would say that, but the claim makes sense and there are published analyses that lend credence to it - for instance >this one< looks reputable and independent AFAIK.
I noted in my previous post why I think it difficult to derive velocity from pressure in the near field.
Why do you think spaced pressure mics would be best, cost aside?
I do have some doubts, the direct measurement of velocity looks more mathematically robust but the velocity sensors are presumably less developed, which could nullify the theoretical benefits.

Best wishes
David
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Old 8th March 2018, 04:50 AM   #54
Dave Zan is offline Dave Zan  Australia
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I received a response to my Microflown inquiry.
As expected, expensive, around $11K to $15K for the versions that looked most suitable.
That is the Australian price, and Australian importers tend to have extortionate mark-ups when they are the sole source.
But clearly won't be cheap, a pity because the theory looked so nice.
Back to spaced pressure mics.

Best wishes
David
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Old 11th March 2018, 01:40 AM   #55
aslepekis is offline aslepekis  United States
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I'm back. The dust from moving to a new house has mostly settled, so I have more time to give to other projects again.

Quote:
Originally Posted by gedlee View Post
What I would like to do to further this technique is to do a circular source in an infinite baffle. I believe that this technique could reconstruct the cone motion fairly easily with reasonable accuracy. This is what I tried decades ago and failed because of the singularities, but today, I know how to do it, with one exception. That is that an infinite baffle is impossible to create. But perhaps the two radius solutions could extract the radiating field from the baffles edge diffraction, which otherwise could be an issue.
That is one of the points Klippel highlights as a benefit to their NFS.

Quote:
Originally Posted by Dave Zan View Post
I think this depends a bit on the set up, I will use my own room as an example because I want a check of my numbers anyway
Say the speaker is in the middle of a room a bit over 3.6 m wide, so about 1.8 m to a wall.
Microphone is a compromise of your previously quoted distance, say 1.2 m.
So 0.6 m clearance to the wall and 1.2 m/sound velocity echo free.
If we scan in the near field, say 0.3 m, then we have 3.0 m/v echo free, which is a nice improvement, but the speaker echoes will only be a further 0.6 m/v later, not very much further out in time.

@Aaron. This makes me rethink the scanner mechanics.
For my first proposal the limits are set by the closest wall as the scanner moves.
Perhaps better to rotate the speaker on the stand for one coordinate but keep the pivoted arm for the other. Then the microphone can stay in the plane down the centre of the room, furthest from the wall.
Stand rotation would be no harder to implement than the first proposal, maybe even a bit simpler.
I assume you are talking about for Version's 1 and 2? I do see that rotating the speaker would have its advantages. But with Version's 2 and 3, I really see ceiling height becoming the big enemy because that is going to be the smallest room dimension. To keep a ~6 millisecond IR window in a room with 2.4 meter ceilings would then require a measurement surface around the speaker to be about 0.6 meters in diameter. Perhaps Klippel uses a cylindrical scan surface to allow larger speakers to be measured in typical rooms?

Maybe if we design it light and portable enough it could be used outdoors where reflective surfaces can be further away (or nonexistent, in the case of ceilings). But then again, atmospheric conditions could prevent the acquisition of usable data.
Quote:
Originally Posted by Dave Zan View Post
I received a response to my Microflown inquiry.
As expected, expensive, around $11K to $15K for the versions that looked most suitable.
That is the Australian price, and Australian importers tend to have extortionate mark-ups when they are the sole source.
But clearly won't be cheap, a pity because the theory looked so nice.
Back to spaced pressure mics.

Best wishes
David
Yikes!

I have noticed that is a reoccurring reality with audio; that highly specialized and specific tools are often needed, and those tools are never cheap. Definitely a pity for the DIY community.
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Old 11th March 2018, 08:43 PM   #56
gedlee is offline gedlee  United States
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Quote:
Originally Posted by aslepekis View Post
That is a good summary of the problem, but I believe that Klippel is using the wrong set of basis functions for the baffle tests.

I have also been thinking that one need only measure the sound pressure and sound velocity on an enclosing hemisphere and we know all that can be know about what creates the sound within this hemisphere (Green's Theorem.) That means a sweep of just two mics where the pressure is the sum and the velocity is the difference. All room effects can thus be excluded. Kilppels discussion of symmetry is well done and an important aspect of the problem. If one is seriously going to do this project, I suggest starting with complete symmetry and working outward towards completely arbitrary.
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Old 12th March 2018, 02:21 AM   #57
Dave Zan is offline Dave Zan  Australia
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Originally Posted by aslepekis View Post
...the smallest room dimension...
Yes, I worry about that too.
There is some relief, in that the vertical polar power is likely to be fairly low for a lot of the frequency band.

Quote:
Yikes!...specialized and specific tools are often needed, and those tools are never cheap...
More or less inevitable, "specialized and specific" means low volume production so no economies of scale, little competition.
And for industrial customers like aerospace, it's only petty cash.

Quote:
Originally Posted by gedlee View Post
That is a good summary of the problem, but I believe that Klippel is...
What do believe is incorrect in their selection of basis functions?

Quote:
That means a sweep of just two mics where the pressure is the sum and the velocity is the difference...
To derive the velocity as a simple difference seems a bit problematic.
If the microphones are close then the difference in pressure is small, minor mismatches in pressure response or noise become multiplied.
As the microphones are placed further apart there is a problem with the non-linear fall off in pressure in the near field and also nulls above some frequency limit.
I assume this is precisely why Microflown can sell true velocity sensor microphones for >$10K.
The expansion in Hankel functions is much more robust I think, and not too much more complex.
This was covered in a reference I posted earlier.

Best wishes
David

Last edited by Dave Zan; 12th March 2018 at 02:26 AM.
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Old 12th March 2018, 05:59 PM   #58
gedlee is offline gedlee  United States
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Quote:
Originally Posted by Dave Zan View Post
What do believe is incorrect in their selection of basis functions?

To derive the velocity as a simple difference seems a bit problematic.
If the microphones are close then the difference in pressure is small, minor mismatches in pressure response or noise become multiplied.
As the microphones are placed further apart there is a problem with the non-linear fall off in pressure in the near field and also nulls above some frequency limit.
I assume this is precisely why Microflown can sell true velocity sensor microphones for >$10K.
The expansion in Hankel functions is much more robust I think, and not too much more complex.
This was covered in a reference I posted earlier.

Best wishes
David
I should be clear that the Hankel functions as Klippel uses for the infinite baffle are not incorrect in that they will work, but the circular aperture functions as shown in Morse VII.28 should work better for this case. They are simpler and should converge more rapidly.

Deriving velocity with two mics is done all the time in intensity probes. It is a very common technique. Some things to watch out for, like singularities for lambda / 2 spacing, but these are well known and easily handled.
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Old 13th March 2018, 03:08 PM   #59
Dave Zan is offline Dave Zan  Australia
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Originally Posted by gedlee View Post
I should be clear that the Hankel functions as Klippel uses for the infinite baffle are not incorrect in that they will work, but the circular aperture functions as shown in Morse VII.28 should work better for this case. They are simpler...
Is that reference to "Sound and Vibration' and the functions illustrated in Fig 71?
I believe Zernike polynomials are canonical for circular aperture functions so that's what I would use.
But I haven't studied Hilbert space theory sufficiently to know if Morse's technique would work better.
Any basis set will be an infinite set of orthonormal functions so how does one choose a particular set to optimize the calculation?
Is there a metric to optimize?

Best wishes
David

Last edited by Dave Zan; 13th March 2018 at 03:22 PM.
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Old 13th March 2018, 03:34 PM   #60
gedlee is offline gedlee  United States
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Yes, those are the functions. I don't think that there is a metric that compares different functional representations of a solution. Of course there will be a lot of different possible solution sets, but since Morse has worked this all out for the circular piston, it just seems to me to be the best candidate. Being simple Bessel functions doesn't hurt either. Bessel functions are also what are used for circular apertures in optics, so they must be a natural choice.

For that matter, one could use sine and cosine expansions, but convergence would be slow. As I am sure you know, speed of convergence is a big issue in the solution of PDEs.

In FORTRAN, Bessel functions are intrinsic (like sine and cosine) and very fast, but I don't think the Zernike functions are readily available. I don't have code for them. I had not even heard of them until I looked them up. Even in the classic text "Fourier Optics" by Goodman, he does Zernike's problem using the Bessel set and there is no mention of the Zernike functions.
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Last edited by gedlee; 13th March 2018 at 03:46 PM.
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