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Old 12th August 2012, 10:50 PM   #1251
jdkJake is offline jdkJake  United States
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Default A new baseline, still many questions...

I decided I needed to understand the effect of damping material on the cabinet tuning. So, I broke down and removed ALL of the damping material from the cabinet. I left the port at the original length (~82mm). I then measured the impedance.

The results are pretty much the same as when I started this effort. The first two are the measurement with the port open. The third, with the port sealed. I cannot seem to get any significant tuning change (more than 2Hz or so) regardless of port length or cabinet damping.

As it stands now, it appears the overall volume of the cabinet is too large. That said, there are many things about this measurement I do not understand.

1) Can this measurement technique actually be used for this task? Joe implies this is the case in this post (The "Elsinore Project" Thread).

2) Why is the third peak present? I have checked and rechecked for leaks, most specifically forcibly sealing the tweeter opening and speakON jack opening to ensure they were not leaking any air. No change. Is this an artifact of the crossover itself? A non-issue? BTW, I am NOT referring to the blip at 158Hz, that is caused by standing waves in the undamped cabinet. I am referring to the peak at ~65Hz.

3) How can the cabinet volume be this far off? If anything, I am slightly short on the height (~5mm), but, the rest of the dimensions should be at least close if not dead-on to the design standard.

So, I am totally stymied at this point on how best to proceed. I can certainly fill and/or line the cabinet to reduce volume, but, is that the best way forward?

Has anyone else taken an impedance measurement of their Mark IV build? I would be curious to see if anyone else is having these types of results.

Comments are welcome and encouraged.
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Last edited by jdkJake; 12th August 2012 at 10:53 PM.
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Old 15th August 2012, 05:41 PM   #1252
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@jdkjake,

Looks like you are measuring impedance with the crossover hooked up.
Can't recall exactly, but I believe the three peak behavior is related to the woofer impedance compensation network.
To see box tuning trends you need to hook straight up to the woofers, no crossover parts.
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Old 18th August 2012, 01:02 PM   #1253
jdkJake is offline jdkJake  United States
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Originally Posted by bolserst View Post
@jdkjake,

Looks like you are measuring impedance with the crossover hooked up.
Can't recall exactly, but I believe the three peak behavior is related to the woofer impedance compensation network.
To see box tuning trends you need to hook straight up to the woofers, no crossover parts.
Thanks for the response bolserst. I did remove L4, R2 and C3 like Joe recommended, but, let the other crossover parts in the circuit. It appears the L4, R2 and C3 combo functions as the main impedance compensation for the lower woofers. That said, it is certainly easy to bypass the crossover altogether. I will give that a shot.

I am also reviewing all of my wiring just to be sure it was not something obvious. So far, everything looks correct. We shall see what things look like with the crossover out of the circuit.
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Old 19th August 2012, 09:49 PM   #1254
jdkJake is offline jdkJake  United States
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Default Mystery Solved!!!

I found a wiring error in one of my speakON harnesses. Turns out it was the one I was using to make all the measurements (DOH!!). Turns out that harness did not return negative from the lower drivers. As such, the lower drivers on the device under test were essentially out of the circuit. Not sure how that happened as I thought I had verified the wiring harness properly both in and out of the cabinet. Nevertheless, a mistake was found and corrected. I also TRIPLE rechecked everything. I am satisfied all is wired as it should be.

Lo and behold, with L2, R4 and C3 out of the circuit, the impedance measurement completes as expected. YEA!!

So, using the original port length as well as the cabinet that I did not mess with in terms of damping material, the port tuning is coming in right around 36Hz. Now I need to restore/fluff the damping material properly, measure again and then adjust the port lengths properly.

What an experience!! Man did I learn a lot tracking that down!! It's all good!

Oh yeah, they do sound much better with all the drivers functioning properly on BOTH speakers.
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Old 19th August 2012, 10:09 PM   #1255
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Quote:
Originally Posted by jdkJake View Post
I found a wiring error in one of my speakON harnesses. Turns out it was the one I was using to make all the measurements (DOH!!). Turns out that harness did not return negative from the lower drivers. As such, the lower drivers on the device under test were essentially out of the circuit.
Aaah, OK. The lower drivers would be acting like passive radiators, loading the upper woofers.
That explains the dip in the impedance curve shown in the 3rd pic from your posting #1251.

Congrats on tracking down the wiring error.
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Old 19th August 2012, 10:09 PM   #1256
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Good to see the results are very close, i,m sure we have all had our moments like that i know i have.

Whats that program you are using for the impedance charts.

Last edited by spoonted; 19th August 2012 at 10:12 PM. Reason: Spelling
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Old 19th August 2012, 11:28 PM   #1257
jdkJake is offline jdkJake  United States
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Originally Posted by spoonted View Post
Whats that program you are using for the impedance charts.
I have been using an iPad app from Studio Six Digital called AudioTools. I am coupling that with their iAudioInterface2 device. A really nice, compact and portable combination. I really like it. You have to rig and calibrate a custom cable to take the impedance measures, but, it is really simple and straightforward. Makes short work of impedance measurement, that much is for sure.

Do be aware, Some folks don't care for the ala carte nature of audioTools as purchasing a lot of the modules can add up quickly. Personally, I like buying only the units I need as I need them.

I also popped for a CM-125 calibrated mic kit. I have yet to use that extensively though. As I get better with it, I will publish some in-room acoustic measurements as well. Good stuff.
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Old 20th August 2012, 06:05 AM   #1258
irext is offline irext  Australia
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Glad to hear you solved the puzzle JK. It certainly prompted me to explore the damping material more fully which was beneficial. Now you get to listen to them essentially for the first time again.
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Old 20th August 2012, 07:30 AM   #1259
irext is offline irext  Australia
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After reading your post JK I disconnected my lower two LF drivers just to hear the difference. It makes the sound quite thin and forward. Interesting. I must admit that when I first fired mine up I had them out of phase. I only had about twenty minutes to listen to them before heading out (to my physio) probably as a result of lifting them. I thought they sounded a bit off but it was only whilst playing Dark Side of the Moon (yes I am a 70's rock tragic) a bit later, which has a bit on the second side with deliberately recorded out of phase laughter that the penny dropped as the laughter was heard planted right in the centre instead of the intended ethereal surround effect. I guess I was in too much of a hurry to fire them up. Live and learn.

Happy listening.
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Old 22nd August 2012, 08:01 AM   #1260
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[QUOTE=jdkJake;3132951]I have been using an iPad app from Studio Six Digital called AudioTools. I am coupling that with their iAudioInterface2 device. A really nice, compact and portable combination. I really like it.

Thanks for the info,

I had a look at the website and its a neat bit of gear alright never even new it existed.

I wonder what results you,ll get for the port tunning using the mike and a low frequency sweep.
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