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Old 7th April 2014, 11:19 PM   #311
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Opps... forgot to add the updated polar plot for the Vifa...

Only thing I can think off for the slight variance in +/- data is down to room furniture. Also double checked the IR of the 4FE35 and there is the same Initial impulse followed by increasing distortion at 2.5ms... Caused by the baffle edge maybe?
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File Type: jpg polar plot 2_inch cut.jpg (229.5 KB, 193 views)
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Last edited by BuildMeSomething; 7th April 2014 at 11:28 PM.
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Old 7th April 2014, 11:51 PM   #312
xrk971 is offline xrk971  United States
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BMS,
Thanks for the measurements - great stuff there! The polars up to 4khz show very good dipole shape.
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Old 8th April 2014, 03:13 PM   #313
Rudolf is online now Rudolf  Germany
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Hallo BMS,

is this still your measurement situation:

Click the image to open in full size.

The couch is much too near to speaker and microfone imho. Your response graphs in post #308 suggest that you've got a problem there. If you can't change that you may measure to one side of the speaker only. Just look at your IR graphs, which side has the later onset of reflections - a wider reflection free zone.
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Old 8th April 2014, 03:39 PM   #314
Rudolf is online now Rudolf  Germany
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Quote:
Originally Posted by BuildMeSomething View Post
Here's the 0 to 90 and 0 to -90 plots from REW, 6.5ms right gate and 1/12 smoothing...
REW polar 2.gif

It "features" all bad habits of a (too) wide open baffle: Lots of dipole notches (red circles - there must be even more) and a "midrange", where the directivity widens to only about 6 dB difference between 0 and 90 (red arrow). Compare again to the Linkwitz diagram with b=8a.
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BMS,
Those polars are kind of ugly I think that cutout idea is not my cup of tea.
Those polars have nothing to do with the cutout. They are ugly because of the large baffle. They do indicate that the cutout does not work as wanted in this stage.
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BMS,
Thanks for the measurements - great stuff there! The polars up to 4khz show very good dipole shape.
No, they don't show it! Look how the yellow 2000 Hz curve is only about 6 dB down at 80. You can call it OB, but it's not a good dipole. At least not compared to the standards that can be achieved with a true 2 or 3-way system.
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Old 8th April 2014, 03:59 PM   #315
xrk971 is offline xrk971  United States
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Thanks for clarifying why it looks ugly and that a true dipole needs to go to zero at 90 deg. I did not realize a wide baffle causes so many problems. I made a measurement in a 24 in x 36 in baffle once and do not recall all these nasty dips.
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Old 8th April 2014, 07:35 PM   #316
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Got to say the results look nicer if you don't gate the measurements...

I changed ends of the room this morning so the speaker is sited roughly where the mic was, and the mic where the speaker was... Still not perfect situation, as there's still room furniture that can't be moved.

Saying that, for two sets of measures done this morning, the IR looks a lot cleaner. With a more defined first reflection at ~6.3ms... +/- 80 & 90 start to suffer from an earlier reflection off a kitchen side unit, however the initial impulse looks less distorted than with the first set of results posted.

Set one: 20" wide baffle, no cut out.
Set two: 20" wide baffle with 2" U-shape cut [again]

Furthermore, increased the cut out to 2.5" ready for the next set of measures.

Again... just about to start pulling the data across to the spread sheet.
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Old 8th April 2014, 09:15 PM   #317
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Ahhh... the 0 to 90 are good, the 0 to -90 not so good, as before... the couch & kitchen side unit strike once more...

With the data I have and going forward, I'll just measure 0 to 90 and mirror the data to the negative side...

I'll run through as much as possible this week and over the weekend. Then post an update early next week, so it's not given in drips and drabs
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Old 8th April 2014, 09:24 PM   #318
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Quote:
Originally Posted by xrk971 View Post
Thanks for clarifying why it looks ugly and that a true dipole needs to go to zero at 90 deg. I did not realize a wide baffle causes so many problems. I made a measurement in a 24 in x 36 in baffle once and do not recall all these nasty dips.
A while ago in either this or another OB/dipole thread, I stated that a wide OB has absolutely nothing to do with the speaker being a dipole and did not get a single rebuttal. Theoretically, a dipole has zero width and depth. A practical dipole would be simply a bare driver with as little baffle as necessary to hold the "speaker" together. Any additional width will start to screw up the dipole figure-8.

But, a very narrow OB has no bass. So we widen the baffle to get the bass down to where it can be conveniently passed to a sub. Of course, the ultimate end of this process is a true infinite baffle -- a speaker mounted in a wall with an empty room behind it.

Beside the fact that I didn't like the sound, I went down the OB road for a small stretch. Quite small OB tops over H-baffle subs. I am back to boxed speakers -- MLTL's or BR's over sealed subs.

To each his own.

Bob
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Old 8th April 2014, 10:28 PM   #319
Rudolf is online now Rudolf  Germany
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Quote:
Originally Posted by Bob Brines View Post
A practical dipole would be simply a bare driver with as little baffle as necessary to hold the "speaker" together. Any additional width will start to screw up the dipole figure-8.
This holds true if you ask for a "full range" dipole with a single driver. And since this is the Full Range part of the forum ...

Which leaves the question how much screwing would be allowed before you really have screwed it up.

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Old 8th April 2014, 10:35 PM   #320
Rudolf is online now Rudolf  Germany
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Originally Posted by BuildMeSomething View Post
I'll just measure 0 to 90 and mirror the data to the negative side...

I'll run through as much as possible this week and over the weekend. Then post an update early next week, so it's not given in drips and drabs
Good strategy! I'm looking forward to what you can achieve.
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