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Author Topic: Learning to measure - what does it mean?  (Read 3370 times)

john lutz

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Re: Learning to measure - what does it mean?
« Reply #10 on: April 16, 2010, 11:22:07 am »

Evan Kirkendall wrote on Thu, 15 April 2010 16:24

john lutz wrote on Thu, 15 April 2010 16:37

Thanks, Evan for reply.

I attempted a ground plane measurement on a concrete driveway, with mic placed on axis, about 15' away with cab on the ground standing up but tilted so horn is pointed at mic.  I put tape marks down to get the 2 cabs close to the same spot.


John,
Is your mic pointed at the ground, or at the cab? The latter would result in some weird HF reflections off the ground. "Proper" ground plane measurement would have the mic capsule on the ground, maybe a 20 or so degree angle to the ground if your mic doesn't have good off axis pickup.


Evan


Evan, yes I have mic pointed at ground at about 45* angel and about 1/16 inch from concrete aimed at center axis of speaker.

I was actually thinking of (and looking for but couldn't find) the pics you had posted a while back of your own driveway test setup.
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Evan Kirkendall

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Re: Learning to measure - what does it mean?
« Reply #11 on: April 16, 2010, 11:45:46 am »

john lutz wrote on Fri, 16 April 2010 11:22

Evan Kirkendall wrote on Thu, 15 April 2010 16:24

john lutz wrote on Thu, 15 April 2010 16:37

Thanks, Evan for reply.

I attempted a ground plane measurement on a concrete driveway, with mic placed on axis, about 15' away with cab on the ground standing up but tilted so horn is pointed at mic.  I put tape marks down to get the 2 cabs close to the same spot.


John,
Is your mic pointed at the ground, or at the cab? The latter would result in some weird HF reflections off the ground. "Proper" ground plane measurement would have the mic capsule on the ground, maybe a 20 or so degree angle to the ground if your mic doesn't have good off axis pickup.


Evan


Evan, yes I have mic pointed at ground at about 45* angel and about 1/16 inch from concrete aimed at center axis of speaker.

I was actually thinking of (and looking for but couldn't find) the pics you had posted a while back of your own driveway test setup.


Here was my most recent driveway adventures:

index.php/fa/29505/0/


This was for alignments and EQ strictly below 250hz, so I wasn't too concerned about aiming the top.



Evan
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Not all change is good change.

john lutz

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Re: Learning to measure - what does it mean?
« Reply #12 on: April 16, 2010, 12:05:13 pm »

OK, I retested last night and I think I know whats going on even though I do not fully understand what the data shows me.  My measurements were consistent but my labels were screwy.

Using HolmImpulse to measure frequency and amplitude shows me one cab has slightly more HF output than the other.  In a music listening test at moderate volumes it is just noticeable as being brighter - more forward sounding.  Not a huge diff and no distortion.

The impedance sweep with WT3 shows the louder cab to have a peak of 43 ohms vs 50 ohms for the softer one.

Removing the HF drivers and testing them with WT3 shows the louder cabs HF driver to have an R(e) = 5.4 ohms and a peak of 36ohms at 500hz while the softer HF driver has an R(e) of 4.8 ohms and a peak of 18 ohms at 500hz.  Multimeter agreed with WT3.

So, measuring the whole cabs with WT3, the higher impedance peak measured cab yeilds less HF output.  That same HF driver measured out of the cab has a (much)lower peak impedance than the other.  Strange to me, seems backward somehow.

Doing a sine sweep listening test tells me I have some tired HF drivers.  Both are fine above 1k at modest power.  As I sweep down toward 800hz to 700hz even at low power (.4 volts) the distortion really kicks in.  I ab'd with some other drivers laying around and even my ancient Peavey 22As 500hz with out an issue.

Onward to the next adventure   Smile .
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Art Welter

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Re: Learning to measure - what does it mean?
« Reply #13 on: April 16, 2010, 01:40:33 pm »

The series pad is dropping more power on the lower impedance driver than the higher impedance driver.

Many people think you can just plop a diaphragm in and go.
Not so, low power alignment tests must be done at relatively low frequencies (<500Hz)  to make the problems show up.

Those buzzes, obvious well below the crossover frequency will sound like “hash” and distortion when run at normal levels, and may affect the impedence curve, messing with the frequency response.

The “next adventure” would be cleaning the magnetic  gaps, replacing ferro fluid (if used ), then aligning the diaphragms in the gap to get rid of the buzz.

If the DC resistance is different between the two diaphragms, they will still have differing responses because of the nature of passive crossovers, and the interaction in the magnetic gap.

Art Welter
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john lutz

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Re: Learning to measure - what does it mean?
« Reply #14 on: April 16, 2010, 03:45:22 pm »

Thanks again Art.  That makes sense.  The impedance curves I got off the raw drivers were quite different from each other.  

So if my local supplier will allow I should attempt to get closer matching DC resistance diaphragms?  

Can you point me to a tutorial on aligning the diaphragms in the gap?  I will do some searching for info - for some reason I thought these CD 5001s were self aligning.  The tolerances on those screw holes is pretty tight.  
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Art Welter

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Diaphragm Alignment
« Reply #15 on: April 17, 2010, 07:24:27 pm »

john lutz wrote on Fri, 16 April 2010 13:45

Thanks again Art.  That makes sense.  The impedance curves I got off the raw drivers were quite different from each other.  

So if my local supplier will allow I should attempt to get closer matching DC resistance diaphragms?  

Can you point me to a tutorial on aligning the diaphragms in the gap?  I will do some searching for info - for some reason I thought these CD 5001s were self aligning.  The tolerances on those screw holes is pretty tight.  

You  checked the impedance curves, did you check DC resistance?
The impedance curve difference could be the result of the voice coil drag which is indicated by the buzzing.

A quick Google check came up with the “clean the gap, drop in the self aligning diaphragm” fallacy.
Often, diaphragms do work with no problems plug and play, but not always.

Without a sweep test, the distortion from a dragging diaphragm is hard to hear. Sometimes the dragging won’t be heard in the range the driver is normally used, but will  change the  response.

To eliminate the buzzing, first you have to remove the diaphragm, and clean the gap with masking tape.
Small chips or debris in the gap may be causing the buzz.

Then check the gap  with a couple pieces of card stock to make sure it is even all the way around, if it pinches at some part of the gap, you will need to tap the top plate into position using a dead blow hammer. Clean the gap again if you have to move the top plate.

Put the diaphragm back in, screw the screws in just to the point where they won’t rattle, but will allow movement .

Using a sine wave generator, start with around 1000 Hz, level off. Bring the level up to about 90 dB, then slowly sweep down to around 300 Hz. As you sweep down, stop sweeping if you hear buzzing or distortion, then gently tap or push a corner of the diaphragm with your finger. The buzzing will usually get worse or better. If better, continue tapping in that direction until the noise goes away, if it gets worse, tap on the opposite side. By rotating around and tapping at various places you should be able to perfectly center the diaphragm and make the buzzing stop.

Once you have centered the diaphragm, tighten the screws, test again, sometimes the process of tightening the screws can change the position and you need to repeat the process.

Often the process takes hardly any time (when the diaphragm self aligns), sometimes it can take as much as a half hour to get it right.

Art Welter
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john lutz

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Re: Diaphragm Alignment
« Reply #16 on: April 19, 2010, 10:51:16 am »

And again, thank you Art.  When I opened up the drivers I was very surprised to find a fair amount of SAND in the gaps!   Confused  Can't figure that one - don't do many desert gigs.  Seriously though, I don't recall ever being in a windy, sandy environment.  Perhaps someone set some dirty something or others on the speakers when they were waiting to be covered or some such except there is no sand on the rest of the cabs/speakers anywhere - just the usual road dust. And the screen on the throat looks too fine for the particles.  Puzzelment.

Cleaned them all up.  Looks like a little scraping damage was done to one coil.  Maybe enough to short a few windings?  The CD5001s rely on a machined groove that mates with a raised ring for alignment so that part went easy.  They sweep good down to 300hz now.  Both drivers and diaphragms look to be the same vintage.  I've yet to do another impedance curve but the dc resistance is not a good match still.  5.5 and 4.5 ohms is what I'm getting.
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Art Welter

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Re: Diaphragm Alignment
« Reply #17 on: April 19, 2010, 02:01:50 pm »

A difference of 5.5 to 4.5 ohms is quite a bit, probably enough to account for the frequency response deviations.

I’d suggest labeling the cabinets Stage Left and Stage Right and eq them accordingly  Cool .
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