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  1. #1
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    Default The Real Farfield Measurement Distance

    As a general rule, software and people in general speak of the accurate farfield measurement distance being a function of the distance between the measurement microphone and source and the nearest reflectie surface. Others indicate that though this provides a reflection free window, true farfield cannot me measured unless the measurement microphone is at least 2 wavelengths away from the sound source of the lowest frequency of interest. This would put a damper in the accuracy of full range (that is, for the low frequency) ground plane measurements. I was curious to hear the thoughts of any informed engineers.

    I did some tests and seem to get some consistent results ignoring the wavelength limited distance requirements, but still, my scientific curiosity got the best of me.

    Thanks, Jay
    Jay
    Our greatest glory lies not in never falling, but in rising each time we fall.

  2. #2
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    Default Re: The Real Farfiedl Measurement Distance

    2 wavelengths throughout the pass band isn't practical for a subwoofer, but that's OK as you don't have to be that far away to be in the farfield, because the farfield distance is determined by the size of the radiating plane. The current de facto standard for measuring pro-sound subs as practiced by Danley Sound Labs is 100 watts at 10 meters ground plane, which not only ensures a farfield measurement but also gives a result far more real world than 1 watt.

  3. #3
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    Default Re: The Real Farfiedl Measurement Distance

    Jay - you may find some of this of interest:
    http://www.etcinc.us/tech/nl043_far_field_criteria.pdf
    The Klippel web site had at one time details for "stitching" near and far field measurements
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

  4. #4
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    Default Re: The Real Farfiedl Measurement Distance

    Thanks, Sydney. I had seen that and Klippel's write up, as well. It is just that I am attempting to grasp the mathematical understanding of the why. This paper helps to some degree, but I can't seem to get my hands around how you can have an accurate read being within a wavelength of the source.

    Bill, that ground plane distance is impressive and would certainly require a fairly quiet environment though a 100 dB source would mitigate some if this issue. It would be even more difficult to get a reflection free area to really get down to the real low frequencies if you were that far away. I think that for most of us DIY'ers, having access to a facility like this might be a challenge.

    Jay
    Jay
    Our greatest glory lies not in never falling, but in rising each time we fall.

  5. #5
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    Default Re: The Real Farfiedl Measurement Distance

    You may have seen a distinction draw between the geometric near field and the acoustic near field?
    but I can't seem to get my hands around how you can have an accurate read being within a wavelength of the source.
    Multiple measurements on-axis or Multiple positional measurements in a defined matrix that are summed and averaged are approaches.
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

  6. #6
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    Default Re: The Real Farfiedl Measurement Distance

    Interesting discussion.

    If measuring at 4 watts and 2m in the ground plane, I wonder how accurate that would be. I could find an area that good. 10m is tough though.

  7. #7
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    Default Re: The Real Farfiedl Measurement Distance

    Ryan: Do you tilt the cab forward?
    Jay: Just to be clear -
    Is the goal to measure on-axis FR of a single driver in a "theoretical" infinite baffle?
    A driver or driver(s) in a cab?
    Or something else?
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

  8. #8
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    Default Re: The Real Farfiedl Measurement Distance

    Nope. Do you think that's necessary? I'm only interested in <200hz if doing ground plane.

  9. #9
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    Default Re: The Real Farfiedl Measurement Distance

    IMO: Probably not if you are after woofer output.
    I could find an area that good. 10m is tough though.
    A speaker pit for flush half space measurement isn't an option for many either.
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

  10. #10
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    Default Re: The Real Farfiedl Measurement Distance

    Quote Originally Posted by jmb View Post

    Bill, that ground plane distance is impressive and would certainly require a fairly quiet environment though a 100 dB source would mitigate some if this issue. It would be even more difficult to get a reflection free area to really get down to the real low frequencies if you were that far away. I think that for most of us DIY'ers, having access to a facility like this might be a challenge.
    Danley uses their parking lot. I use my back yard, but since I only have a standard quarter-acre plot, and neighbors, I measure at 4 meters on average.

  11. #11

    Default Re: The Real Farfiedl Measurement Distance

    This is Don Keele's near field measurement AES paper and I think it provides the analysis that you're looking for. He uses terminology that looks to be in agreement with Beranek's classic text on Acoustics IIRC - it has been a while since I read that chapter:
    http://www.xlrtechs.com/dbkeele.com/...ld%20Paper.pdf

    More papers:
    http://techtalk.parts-express.com/sh...d.php?t=230830

  12. #12
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    Default Re: The Real Farfiedl Measurement Distance

    Hi Pete,
    Keele's discussion really focusses on Nearfield and does not really explain the farfield distance rationale from what I can tell.

    Sydney, the utility to me and I suspect to most would be to be able to measure a boxed speaker (i.e. measurement of both individual drivers and later the combined unit) full range (outdoors, ground plane) or semi anechoic (indoors or outdoors).

    I did several measurements with a gated measurement (so it cut out the bottom 250 Hz or so and I see the high frequency issues as one gets too close to the boxed unit. As one moves away, you can see the standard 6 dB drop per doubling of distance. So, like the paper that you reference, Sydney, there is the HF issue of when Farfield truly begins and it is certainly frequency dependent.

    ...but what about low frequency? We can get nearfield measurements and then model the diffraction effects and fit it to our farfield measurements; but can we truly get farfield measurements on low frequency signals without being several wavelengths away from the source? It does not seem so from what I have read. Historically, we have used (as the lowest reliable frequency), the time window based upon the direct measurement until the initiation of the geometric calculation of the nearest reflection - but this does not take into account that a 300 Hz signal is over a meter long and would therefore require a measurement at least a couple of meters away to truly capture a far field signal.

    This is what I am trying to wrap my mind around. I guess the real issue here is, perhaps we have been splicing our anechoic measurements too low? Perhaps we should be attempting to splice our signals closer to 500-600 Hz (nearfield allowing)?

    Jay
    Jay
    Our greatest glory lies not in never falling, but in rising each time we fall.

  13. #13
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    Default Re: The Real Farfield Measurement Distance

    Jay:
    I feel like I telling you what you already know here.
    Practically speaking - far field is where the energy drops inversely (-6db ). Closer in is the interference field.
    Of course small wavelength high frequency energy is subject to attenuation by humidity ( which can be noted and accounted for ).
    I believe you know the reason for using windowing, and what would need to happen to have the window "opened longer" so to speak.
    There are numerous approaches to near field measurement methods, like other testing methodology in other domains they have limitations or compromises depending on what is being quantified.
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

  14. #14
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    Default Re: The Real Farfiedl Measurement Distance

    Quote Originally Posted by jmb View Post
    this does not take into account that a 300 Hz signal is over a meter long and would therefore require a measurement at least a couple of meters away to truly capture a far field signal.
    You're confusing the issue. There are two 'nearfields'. One is 'very close', as opposed to 'far'. The other is where a source differentiates from point to line source radiation characteristics. That definition relates to the size of the radiating plane versus frequency plus distance, not distance alone. Yes, a 300Hz signal is a meter long, but the source must be approximately three times that in height to be a line source. So if you're measuring a ten foot high speaker and want to do so within the farfield zone you need to be well away. With a two foot high speaker a few meters will do.

  15. #15

    Default Re: The Real Farfield Measurement Distance

    I think you guys may be looking at this incorrectly. Here is a figure of the near to far field transition for a flat, circular piston source:



    Far field is where the response drops 6dB per doubling of distance. Notice that regardless of wave length, as indicated by ka, (ka = 2Pi x a / wavelength where a is the radius of the source), the response is pretty much dropping by 6dB/doubling at distances of r/a = 3 or 4. Small ka indicates low frequency, long wave length, relative to the source size. Notice that it is r/a, distance from source divided by source radius, that matters, not distance divided by wave length. For a 12" woofer, regardless of frequency, the far field transition is complete by about 2 feet (r/a = 4). To be super cautious, use r/a = 10, or about 5 feet. For a 6" midrange the far field is at about 1 ft of a little more.


    The size of the baffle should probably be figured in, but for the most part 1 M measurements are far field for a single source.
    John k.... Music and Design NaO Dipole Loudspeakers.

  16. #16

    Default Re: The Real Farfiedl Measurement Distance

    Quote Originally Posted by jmb View Post
    Hi Pete,
    Keele's discussion really focusses on Nearfield and does not really explain the farfield distance rationale from what I can tell.

    This is what I am trying to wrap my mind around. I guess the real issue here is, perhaps we have been splicing our anechoic measurements too low? Perhaps we should be attempting to splice our signals closer to 500-600 Hz (nearfield allowing)?

    Jay
    If you don't want to read the theory in Keele's paper then look at the measurements which he provides for 4.5", 8", and 15" drivers. The validity of the nearfield measurement depends on frequency/wavelength as compared to the size of the driver so how can you make a blanket suggestion of 500-600 Hz? Note in Keele's paper Figs. 11 and 12 where notching is seen in the near field measurement, clearly there is gross error in the measured response at such a high frequency and the measurement is only valid within about 1 dB to about .3 times that frequency as can be seen in Fig. 2. Figure 2 shows that the error increases quickly above the 1 dB error point, by .4 it is about 3 dB and by .7 about 5 dB.

    Also, if you look at Fig. 10 in Keele's paper, notice that the near field is nearly perfect, over the valid frequency range, as compared to the farfield half space measurement, however baffle step considerations must be accounted for if you wish to determine the full space frequency response. Is your goal for your measurement to determine the far field full or half space response?

  17. #17
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    Default Re: The Real Farfield Measurement Distance

    Hi Pete,
    Please don't misunderstand. I have read and re-read Keele's paper but for some reason I was not absorbing the farfield analysis. Thank you for re-directing me to it. If I would have been more careful, I would not have missed it.

    John, as always, I appreciate your input and it does help to clarify things even more.

    Bill and Sydney, thanks for your input, as well.

    I think that my confusion started with my reading of
    AES2-1984 (r2003), where, it is suggested that the measuring distance be the largest of:
    1) at least four times the largest dimension of the effective radiating surface or
    2) two times the square of the largest dimension of the radiator divided by the shortest wavelength to be measured.

    Part 2 brought in wavelength and got me to thinking about how one could accurately measure longer wavelengths if one was not far enough away. In Colloms' book (I have the 3rd Edition) he even states that one would need to be a few wavelengths away for farfield - this further confused the issue for me...but John's diagram and my measurements both indicate that the issue is more one of complex interference of shorter wavelengths than having anything to do with longer wavelengths (the article that Sydney referenced also helps to clear that up).

    At this point, I feel that I can wrap my mind around this and I really appreciate all of the input.
    Last edited by jmb; 04-30-2012 at 03:31 PM. Reason: typo
    Jay
    Our greatest glory lies not in never falling, but in rising each time we fall.

  18. #18
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    Default Re: The Real Farfield Measurement Distance

    You're welcome Jay:
    It isn't always easy to convey concepts in sound.
    I studied optics long before acoustics and the visual depiction in optical experiments and wave tanks demonstrations makes it easier to understand unseen sound phenomena.
    It's amazes me that individuals like Huygens were able to do the work they did by observation and mathematics.
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

  19. #19
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    Default Re: The Real Farfield Measurement Distance

    Okay, I messed around with the far field question some more and have come up with what I believe to be a more exact understanding of this. I just did not feel right following cookbook estimates. I am attaching a spreadsheet for any other OCD types who really want to understand what they are getting when they measure far field. There is always some degree of error in this measurement.

    To use this spreadsheet, you enter data in the yellow boxes only. Enter the highest frequency of interest and the radius of your driver in question (in cm). The ka is then calculated and a list of various ka's are then filled in below in column A. Column B then calculates the actual equivilent frequency that the data in the table is representing.

    If you enter an allowable error pct, the table will highlight which distances and ka's (frequencies) are within your limits. The r/a mulitplier allows you to modify the r/a ratios to see more extended ratios. The actual measurement distance that the r/a ratio is representing is calculated above.

    This allows you to see the error based upon the expectation that in the farfield there should be a loss of 6.02 dB for every doubling of distance. It is based upon the solution of the Rayleigh Integral.

    I believe my calculations to be solid but if someone sees any error, feel free to jump in. I hope that this hopes some other poor obsessive soul.

    Jay
    Jay
    Our greatest glory lies not in never falling, but in rising each time we fall.

  20. #20
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    Default Re: The Real Farfield Measurement Distance

    oops. did not attach, it appears that this forum will not allow spreadsheets. If anyone would be so kind as to host it, I would happily email it to you.

    Thanks,
    Jay
    Jay
    Our greatest glory lies not in never falling, but in rising each time we fall.

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