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Old 10th May 2012, 11:17 PM   #21
gedlee is offline gedlee  United States
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Originally Posted by JMB View Post
Earl, thanks for jumping in. Is there any way to calculate this out using your method in such a way that anyone would be able to identify a relative error at different points with different drivers? If not, is my approach within a reasonable error range? If not, any suggestions as to how I might make it so?

Jay
Thats a good question, but I am not sure how one would do that, or if it even makes sense. The reason is that I would expect the "errors" to be different for every case, thus making a simple approach kind of meanigless. I might try and do a far field plot and then show what the plot looks like using the nearfield data that would show the errors for that case, but I don't think that you could generalize the situation.

For example, the monopole mode has a very short nearfield and basically any measurement of it is a good measurement. On the other side is the 16th mode which will have a very long nearfield and one needs to be fairly far away to get good far field accuracy (if you don;t do what I do) of this one. So it is easy to presume that a very wide directivity speaker would be easier to measure at close distances than one with a very high directivity. This simple example does not seem to fall out from what you are showing or discussing since the errors do not seem to depend on the sources directivity and they will.
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Old 10th May 2012, 11:31 PM   #22
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Thanks for the curves Bolserst. Of course this does remind us of why we don't use 12" woofers to 57,000 Hz!

These look similar to my line array simulations except mine showed the periodic wiggles and a 3dB per doubling slope since I was approaching a line rather than a plane.

Still, I think a ka of 5, or at most 10 is as high as we would go with any radiator (multiway assumed), and at those frequencies the low frequency curve isn't far off.

David
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Old 11th May 2012, 01:25 AM   #23
JMB is offline JMB  United States
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...so this all begs the question, "how much error are we experiencing (realizing that each is a case specific situation) when we measure drivers or loudspeaker systems at 1 meter?"

The directivity issue also seems to address my concern earlier about the enclosure dimensions not necessarily contributing as much to the signature as the actual driver.

Jay
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Old 11th May 2012, 10:30 AM   #24
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I know from experience that medium or small sized systems, if you pick the axis carefully, can be measured at 1meter with reasonable accuracy. There is a very real trade off with farther distances meaning more room effect, more anechoic chamber contribution (at LF), or a smaller time window for gated measurements, fighting against any potential near field inaccuracy.

David S.
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Old 11th May 2012, 01:09 PM   #25
JMB is offline JMB  United States
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OK, so I decided to do a very rough experiment. I measured a loudspeaker in an enclosure at varying distances and calculated the anticipated drop from each distance to the next if we were to be in the farfield (20*LOG(Distance a/Distance b)). I compared this to the actual drop measured. This was done with a calibrated mic and data below about 350 hz is windowed out but there are some visible patterns. In addition, the measurement distance has an innate error as it is measured to the baffle and not to the acoustic center (wherever that truly is). Additional information includes: The diameter of the woofer including the surround: 15.1 cm; without the surround; 12 cm; The enclosure is 20.7 cm by 34 cm; The tweeter is 2.3 cm in diameter to the outside of the surround and 1.9 cm on the inside of the surround though it is behind a waveguide. Both drivers are centered on the baffle with the top of the tweeter 6 cm and the top of the woofer 12.7 cm from the top of the baffle.

Admittedly, this is a crude experiment and I did not measure beyond 112 cm. In addition, this is a two driver system with a crossover in place so it will also impact things (I did not take the time this morning to disconnect one driver, that will be my next experiment)...but this does show some interesting information, regardless. Certainly, the degree of error is minimal by the time one gets to a meter away.

Attached are the actual measurement curves and then a graph of the error at each measurement distance.

For some reason, I can only upload one file at a time. I will upload the measurement error in the next reply.

Jay
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File Type: jpg Farfield Experiment JPG01 Apr. 27 17.49.jpg (102.4 KB, 136 views)
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Old 11th May 2012, 01:14 PM   #26
JMB is offline JMB  United States
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Here is the other graph. Jay
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File Type: jpg FF experiment.jpg (73.9 KB, 130 views)
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Old 11th May 2012, 01:38 PM   #27
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Your curve progression looks exactly as we would expect. The LF varies because the farther curves include the 4pi to 2pi transition, the nearer curves simply show the 2pi response (see Keele) Plus the nearest curves start to show the high ka effects at high frequencies.

Science works!

Where is the acoustic center? As a circular arguement you could define it as the depth location that makes the nearfield draw away curve follow the ideal. (As you get closer, every 6.02dB rise must be cutting the distance to somewhere in half.

David S.
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Old 11th May 2012, 01:56 PM   #28
JMB is offline JMB  United States
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The original curves were done when I started thinking this through. I have been building and measuring speakers for about a decade now and had just blindly followed the 1 meter rule but never really understood it. As I began to look into it, I found that there were many definitions of farfield and that there seem to be many approximations (that are often promoted as definitions). As an ex-Medical researcher and now clinician, I am generally hesitent to accept research that I do not or cannot understand how they got from point A to point B. I am hoping that others who might see this thread will also ask questions and learn as I have along the way. It is evident that there is more to understanding farfield than meets the eye; Earl's take on it suggests that there are inaccuracies in our estimations and perhaps his method is more exacting (only time and scrutiny will tell) but for now, I think that some guidance can be found in this thread.

One final evaluation that I plan to do is to compare the error identified in my measurements to the spreadsheet at higher frequencies. I expect to see some discrepancies due to the measurements not being from the Acoustic Center and also due to the woofer influences on the tweeter, particularly closer to the xover point. But looking a couple of octaves above the crossover, I anticipate a fairly close correlation. I will report here after I examine the data.

Thanks, Jay
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Old 11th May 2012, 02:03 PM   #29
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Hi JMB
How much error you get (and how important is it) at one meter depends on the frequency, cabinet size and design as well as what you want to do with the data.
The link I posted earlier is one set of criteria which is based on collecting data for a model of the speakers 3d radiation model, which is then used in EASE and other programs to predict far field behavior in room acoustics / design. It is from an independent acoustic measurement company we hire to measure our cabinets at work partly because I can’t take full spherical measurements and don’t have a very large indoor space to measure in. Measuring every 2.5 or 5 degrees around a sphere is not the definition of a fun time, those guys have a robotic system to do it. In the case of a speaker like an SH-50 with a 28 inch square mouth, I believe they measured at 7 .4 meters mic to source distance. That may be in part due to the source of radiation being a few inches forward of the cabinet’s rear wall and not right behind the grill.

As “everything gets worse” the larger the room and greater the distance from the source, that level of concern may be totally UN-necessary in the home but doing it that way gives measured results in the far field which closely match the predictions (real important in commercial sound).
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Tom
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Old 11th May 2012, 03:49 PM   #30
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Originally Posted by speaker dave View Post
Thanks for the curves Bolserst. Of course this does remind us of why we don't use 12" woofers to 57,000 Hz!

Still, I think a ka of 5, or at most 10 is as high as we would go with any radiator (multiway assumed), and at those frequencies the low frequency curve isn't far off.
Ha! 57kHz
Yeah, the higher ka curves are really only relevent for line arrays of small drivers or ESLs where bigger dimensions are involved.
I'd agree that ka < 10 for single cone drivers in multiway is a pretty safe bet.
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