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Author Topic: LR-4 crossover still allows lows in tweeter. Cap on negative output of tweeter?  (Read 21953 times)

Jeff Schoonover1

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I lay speakers on their side, and block the back side up to have speaker point perpendicular to mic on floor.  Makes for good on-axis.
It pays to just move mic around to make sure measurements aren't too mic position sensitive, needing averaging.
Wow, those are a lot closer than I would have thought.  Totally close enough for my purpose.
When you say that you prop it on its back, I don't understand.  Perpendicular  to the mic?  You want the mic pointing directly at the speaker...
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Tim McCulloch

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Jeff - we have a measurement forum here among the various parts of the LAB.  Measurement-specific questions would probably get more/faster answers there.

There are measurement techniques and stimuli that allow you to "window" a measurement but those become less effective/affect the quality of data as the frequency gets low.
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Mark Wilkinson

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Wow, those are a lot closer than I would have thought.  Totally close enough for my purpose.
When you say that you prop it on its back, I don't understand.  Perpendicular  to the mic?  You want the mic pointing directly at the speaker...

Maybe a picture helps explain what I was trying to say ...

2nd Tim's suggestion, you'll get more replies...and likely from folks with more experience than me !
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Jeff Schoonover1

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Maybe a picture helps explain what I was trying to say ...

2nd Tim's suggestion, you'll get more replies...and likely from folks with more experience than me !
Oh wow. You weren't kidding about putting it on the floor!  I'm amazed it gets nearly the response as outdoors!
 Yes, now that my initial issue is solved, I'll head over to the measurements forum.
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Ivan Beaver

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Oh wow. You weren't kidding about putting it on the floor!  I'm amazed it gets nearly the response as outdoors!
 Yes, now that my initial issue is solved, I'll head over to the measurements forum.
The idea is to avoid reflections.

Even if the mic element is 1/4" off of the floor, you can see the reflections that are caused by that "large distance" (at least at the upper freq.)

A good idea is to point the mic directly towards the floor, with the end about the thickness of a business card from the floor.  YES, that close.

It depends on how high you want to measure.  As the "upper freq of interest" gets lower, the larger the distance the mic can be away from the boundary, and still get good results.

As I say all the time, you must ALWAYS ask yourself "What am I here to do?".

There are all kinds of things you CAN measure, but is it important for what you are trying to do at the moment.

I often do all sorts of measurements that "violate the basic rules", but when you start to realize the things that matter (and don't matter), you can ignore certain parts of the measurements, while you focus on what you are trying to see/measure
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Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

Jeff Schoonover1

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The idea is to avoid reflections.

Even if the mic element is 1/4" off of the floor, you can see the reflections that are caused by that "large distance" (at least at the upper freq.)
Ok, that totally makes sense.  Sound bouncing around the room is the problem - but in high frequencies, this becomes no issue (for direct measuring, anyway) because the waves are so much shorter.
Good info, it's appreciated.
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Mac Kerr

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Re: How to measure
« Reply #36 on: March 25, 2018, 08:30:32 PM »

Ok, that totally makes sense.  Sound bouncing around the room is the problem - but in high frequencies, this becomes no issue (for direct measuring, anyway) because the waves are so much shorter.
Good info, it's appreciated.

No, just the opposite. At high frequencies it takes a small difference in path length between the direct sound and the reflected sound to create cancellations, with low frequencies (long wavelengths) it takes a much greater path length difference to have the same effect. You can eliminate many of the reflections off the floor by placing the mic element very close to the floor, making the path length differences very small. This also benefits longer wavelengths, but is critical for short wavelengths.

Mac
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Mark Wilkinson

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Maybe this will help illustrate what Ivan and Mac are saying about mic placement and short wavelengths.

Here are a couple of plots using the "1 meter, speaker and mic both on the floor technique" ......like previously pictured.
The first is a HF driver used from 6-7kHz up, the second is a mid driver used from 100-700Hz.   

On both, the green traces are with the mic pointing straight at the floor, almost touching the floor.
Reds are with the mic laying horizontal on the ground just as in the thread pict, which puts the capsule center about 3/8" off the floor.

Greens are more accurate because they have less floor bounce in them.
But note that the floor bounce only matters up high. 
Mid traces are nearly identical.
No smoothing on any to help see.

20 kHz has about 0.7" wavelength, so 3/8" off floor is a big proportion of that, with lots of interference.
And of course 3/8" means nothing to long wavelengths.....

Hope this helps..





 

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Chris Grimshaw

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Hey Mark,

Just wondering if you've got a comparison of the same cabinets measured outdoors?
If this way of measuring is close, then that'd solve some issues for me.

Cheers,
Chris
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Jeff Schoonover1

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I made a couple measurements with the mic flat on the floor.  I will try them with the 'business card' approach next.
I was pleasantly surprised, however.  While the unprocessed signal needs eq-ing, I didn't see any huge nulls or peaks. I didn't save anything because I wanted to be more systematic than I had time.  I'll try to get to it again today.

I'd still like to check how the phase looks between my passive x-over and the active one. and between drivers.  I have yet to find an easy instructional here or elsewhere but haven't looked super hard.

I checked out the measurements forum here.  I found lots of interesting stuff but had a hard time quickly finding things specifically relevant to my project.
On many forums, not just this one, a general topic will often have thousands of posts, with many thousands of replies to them.  It's simply too much noise for someone new to that forum to sort before finding anything useful to the overall topic.
If I could make a suggestion.  Several 'sticky' posts covering things like general measurement techniques, different measurements and what they mean, adjusting, posting graphs etc. would be very helpful over there.
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