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Old 21st January 2013, 11:12 PM   #41
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Quote:
Originally Posted by tuxedocivic View Post
Is Australia a 60hz electricity country?
230V 50 Hz

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Old 21st January 2013, 11:20 PM   #42
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Hmm that is a bit perplexing, one thing that I noticed is that the measurements show a peak at ≈40Hz as well as a peak at ≈125Hz and somewhat, a dip at ≈70Hz, this mirrors the port tuning, are you sure there isn't some error in the wiring somewhere? It's as if you might be performing a 2 channel freq resp measurement but with a jig set up for impedance or something.
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Old 22nd January 2013, 01:56 AM   #43
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Aus is 50Hz. 5th you may be 100% correct!! I left the speaker connected to the Walin jig and didn't plug it direct to the amp! The jig was no doubt still in impedance setup! I think I ran the mic direct to the sound card input, but certainly if there was an 8 ohm resistor in series with the driver it would explain the fact that the sim with 10 ohms series resistance seems to match what I see!

DOH!! But thank you!!!

I will try again this evening after work

edit: It would also explain why my levels were so low!! I was cranking up the output much higher than normal and scratching my head as to why the levels were low. double doh. (hard to tell with the hearing protection on). Just looking for the jig schematic to find out if indeed it puts the 8 ohms in series with the driver in impedance testing mode...

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Last edited by wintermute; 22nd January 2013 at 02:05 AM. Reason: added bit on levels.
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Old 22nd January 2013, 10:30 AM   #44
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Default It was an ID10T error ;)

5th element wins the prize. It was my impedance jig stuffing things up :embarrased:

Tonights measurement is better. Still not perhaps quite what I thought it should be but better.

Definitely need to look at some sort of lining for the rear wall too, as I suspect that the big mess around 1Khz could be rear wall reflection, maybe not port resonance after all.

Blue is the woofer nearfield, red the port (still not sure how to get a long narrow port accurately) and orange is the two merged using speaker workshop.

edit: added sim vs nearfield. Might be time for me to try a groundplane measurement, haven't ever tried before... and third last nights vs tonights nearfield of the woofer (no port merging).

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File Type: png sb4_nearfield.png (49.7 KB, 405 views)
File Type: png nearfield_vs_sim.png (43.5 KB, 395 views)
File Type: png ugly_vs_better.png (47.3 KB, 389 views)
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Last edited by wintermute; 22nd January 2013 at 10:48 AM. Reason: add additional graph.
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Old 22nd January 2013, 11:22 AM   #45
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ok and with some 1" thick very low density open cell foam on the back panel. definitely smooths out that glitch at 1Khz. Not sure If I will try the good stuff

edit: also attached 30cm measurement gated (but still dodgy).

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File Type: png foam_vs_none.png (45.4 KB, 385 views)
File Type: png gated_30cm.png (73.3 KB, 379 views)
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Last edited by wintermute; 22nd January 2013 at 11:30 AM. Reason: add 30cm measurement.
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Old 22nd January 2013, 01:49 PM   #46
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That 1kHz glitch is port resonance. Note that it appears prominently in the port trace and only slightly in the driver trace. That kind of trace is typical of higher harmonics in MLTL's, but shouldn't appear in a BR.

Where are you gating your impulse? I think I see a bit of floor bounce in that trace. But hey, how much better do you want a gated trace to look? You a basically looking at the raw driver response and they aren't going to get much better. Do this: Smooth the gated trace to 1/3 octave. That's what your ears will hear. Now how do you like it?

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Old 22nd January 2013, 02:02 PM   #47
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It's nice to see that you managed to solve the problem. Small ported boxes can always be a little tricky, on the one hand you want minimal stuffing for the port, but on the other you want quite heavy stuffing for the mids. You definitely need to make gated farfield measurement to accurately see what these will do high up.
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Old 22nd January 2013, 09:17 PM   #48
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Hi Bob, Unibox calculates the port resonance as being around 1020Hz so it looks like it is. I thought that the fluctuations in frequency response around there looked very similar to theones I had with my 5" drivers in a sealed box where rear wall damping made a big difference. The foam seems to have lessened the effect without changing anything else though

I might need to put some damping material on the port shelf itself. Being only 6mm mdf it may be being excited by the port resonance and amplifying the effect.

5th I will need to work out where I can do some decent gated measurements. I used to do them in the living room at my old unit, or on the balcony. This new place is bigger overall but doesn't have any large rooms. I might need to drag the hulk of a computer downstairs and outside to do some good measurements. Anyway I think I'm happy enough with the bracing now to seal up the back. I might play around a little with the port to see what effect it has.

I'm still uncertain about taking measurements of the port. It's easy with round ones, just stick the mic in the centre. That's what I did for this but I figure it won't be getting anywhere near the full spl that the port is producing. perhaps three measurements accorss the lenghth of the port, merge them and then merge as three ports of 1/3 the port area each??

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Old 23rd January 2013, 09:11 AM   #49
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Default Calling Planet10 :)

Dave, this question is well and truly your domain

Is the deviation that I'm seeing in the measured nearfield response compared to my sim (using a round port) due to the high aspect ratio port?

The tuning is almost spot on compared to the sim at close to 70 hz but the spl curve is not.

my 70L 30 hz tuned boxes match the sim almost exactly, so I generally trust unibox to give me accurate results, however I'm now realising that I probably should have adjusted something for the different style port. The aspect ratio is close to 20:1 I read somewhere else that 8:1 or lower will model the same, however it didn't mention what changed when you went above that...

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Old 24th January 2013, 07:33 AM   #50
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The slot vent adds an R to the vent, you should see the FR roll off sooner and slower. When i do sims, once i get the curve i like the shape of, i overlay the sealed reponse and interpolate the real world result.

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