splicing nearfield to farfield, does this look right? - diyAudio
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Old 9th January 2011, 12:26 PM   #1
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Default splicing nearfield to farfield, does this look right?

OK so I decided that to properly do the active crossover for my Sub to MTM's I needed some decent measurements of the "SUB's". I've put subs in quotes because they are actually my old three-ways with the wiring wired direct to the woofer and crossover detached.

I did nearfiled measurements summing the woofer and port response, performance was MUCH better than I expected. Bit of a problem with port resonance at around 700Hz, but since I'll be building new boxes (eventually) I'll address that then. Note that the nearfield measurement is the raw measurement, no smoothing or gating applied.

I also did a "farfiled" measurement at 1.5M Speaker around 1M off the ground and well away from walls at the rear. Was on the balcony and positioned to ensure the reflections went off into free air.

Matching up the nearfield and farfield responses seems to correlate nicely between around 300 and 500 Hz. so I spliced at 400Hz (which is also where I set the gating for the farfield) with 100Hz overlap.

The thing that is bugging me is that the spliced response looks too bass heavy (no signs of baffle step at all) tapering off from around 100Hz down to around 1500Hz (about 5db drop, which is the exact opposite to what I would have expected) I'm not sure if my methodology was flawed or not (should probably re-read D'appolito).

I've attached the individual measurements the overlapped ones and the final spliced one.

It does look like the response drops as frequency increases above 300Hz. Howver F3 Baffle step freq should theoretically be at 239Hz (480 mm baffle width) so I'm a little concerned that what I've got is not valid for putting into my crossover model for working out the active filter slopes.

Anyway here are the measurements: any feedback would be appreciated


Images in order, nearfield, 1.5M overlayed (levels adjusted to fit at 400Hz splice point) and finally the spiced results.

Tony.
Attached Images
File Type: png subs_nearfield.png (51.7 KB, 479 views)
File Type: png subs_1.5M.png (52.4 KB, 462 views)
File Type: png overlayed.png (58.4 KB, 455 views)
File Type: png spliced.png (52.1 KB, 451 views)
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Last edited by wintermute; 9th January 2011 at 12:32 PM.
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Old 9th January 2011, 12:47 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

AFAIK baffle step will not show up on nearfield measurements.

After that the issues get tricky, see PM's articles on measurement at Stereophile.

Also see :
FRD Consortium tools guide
http://www.rjbaudio.com/Audiofiles/F...0example2.html

rgds, sreten.
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Old 9th January 2011, 12:49 PM   #3
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Curve splicing is only an approximate technique and there is always some arbitrariness to the result.

Remember what you have: a nearfield curve represents the 2 Pi response. If you flush mount the system in an infinite baffle, then the nearfield and the farfield curves would be the same. Keele showed this in his original paper on the subject. But with a conventional system then the nearfield and farfield curves are different and the difference is the baffle effect or the 4pi to 2pi conversion curve (4pi at LF and 2pi at higher Fs).

For splicing to be accurate you need to splice at frequencies below the transition range. The difficulty is that, if you could get nice curves accurate at those frequencies, you wouldn't need splicing! You can see though, that lower splicing frequencies would get the combined curve a little bit more downshelved at LF, closer to what you suspect the true response is.

Be careful that your impulse truncation doesn't warp your 1.5m response (try smoothing rather than truncation) and splice lower, and I think you will have a more accurate response.

David S.
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Old 9th January 2011, 08:33 PM   #4
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Thanks Sreten and David I'll check out the FRD tools a bit later, just posting before heading for work.

I tried different splicing just now.

3rd octave smoothed looks pretty ordinary (and lesser smoothing a lot worse). I gated at 200Hz as well which is clearly including reflections but I think it was better. the three measurements attached.

Tony.
Attached Images
File Type: png 3rd_octave.png (49.6 KB, 434 views)
File Type: png gated200Hz.png (52.2 KB, 52 views)
File Type: png splice200Hz.png (50.9 KB, 50 views)
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Old 9th January 2011, 10:01 PM   #5
Shaun is online now Shaun  South Africa
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What woofer are you using? You could model baffle step using the BDS, which should give you an idea of what to expect.
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Old 9th January 2011, 10:34 PM   #6
DavidL is offline DavidL  United States
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See David Ralph's two-way build here for an in depth study about your problem.

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Old 10th January 2011, 02:00 AM   #7
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Good Idea Shaun! I've got BDS (and it is on the FRD site that Sreten linked to as well) Brain not engaging obviously!! It's a Vifa M26WR-09-08. I actually downloaded a new BDS type spreadsheet from Jeff Bagby only a couple of days ago as well... Loudspeaker Design Software

Thanks for the Link David I will try and digest that tonight! Very comprehensive write up I must say!!

[temporarily suspend reality] I was thinking this morning on the way to work, I need to take the measurement outside with the speaker at the top of a steep hill (sloping away at an angle that the reflections cant come back up to the mic) with nothing around at all, and a boom going up and out and back down again to position the mic.... also would need to be quiet, no wind and fine wheather.. not too much to ask is it Of course would also need a power source (perhaps a 12V battery and an inverter)... [restore reality]

edit: I've just done a baffle sim in Jeff's new sim (it is a bit simpler than BDS to get something done!) and attached here. It does look like the 1.5M measurement is following the curve expected in the sim. The port resonance problem at 700Hz is showing as a deviation but other than that, it looks to be fairly close.

Tony.
Attached Images
File Type: gif Baffle1.gif (18.5 KB, 59 views)
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Last edited by wintermute; 10th January 2011 at 02:28 AM. Reason: attach baffle sim
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Old 10th January 2011, 03:21 AM   #8
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Hi Tony, could you detail how you summed the near and port please. It is a tricky area!

Terry
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Old 10th January 2011, 06:54 AM   #9
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Hi Terry, I have to admit I just followed my nose!

I did the measurements with Holmimpulse

Woofer about 5mm from the centre of the dustcap, thought OMG that looks terrible...

I then measured each of the two ports with the mic about 5mm from the plane of the baffle in the centre of the port. This was an area I wasn't sure about as I was uncertain whether there may be phase issues since the port exit is further forward than the position of the mic for the woofer measurement.

I then simply put the woofer measurement and first port measurement into slots A AND B in holm-impulse and used the built in SUM function to sum to slot C. I then put that into slot A and the second port response into Slot B and then did the sum again, the result looked feasible (but better than I expected) so I figured it must have worked, as I didn't think that if I stuffed up that I would get something that resembled what the models showed.

The bit that surprised me was it being flat down to around 30Hz... I expected the -3db point to be around 36Hz so something could well be amiss. (I can't remember the exact tuning of this box, and I'm unsure if I have the original model, but I should be able to work it out based on the port lengths, and the enclosure volume (or alternatively an impedance plot).

I'll post the individual measurements when I get home, as I'm on the train on the way home at the moment and the measurements are on my desktop

Tony.
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Old 10th January 2011, 07:05 AM   #10
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In Speaker Workshop there is a port-nearfiled summing wizard that seems to work. It uses port diameter as one of its input requirements.

In the old days, guys used to sum nearfield measurements with the simulated box low end response. Don't see much of that these days.
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