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Author Topic: Out of Polarity HF?  (Read 2393 times)

Rory Maguire

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Out of Polarity HF?
« on: June 18, 2014, 04:25:10 pm »

Hello all!

I was out on a show yesterday with a bunch of QSC K10s as delays. So I pulled out my SMAART rig to measure some delay times and came up with this result from four different speakers.

Now to me this looks a lot like an out of polarity HF driver. Large phase deviation of roughly 180ish degrees with an amplitude dip to match. But obviously these are powered speakers with no user serviceable parts.

The owner assured me that they hadn't been opened or anything, so they must have come from the factory like this.

Now granted these measurements were in no way scientific, but they were reasonably near field with a delay of <10ms. However they were measured in place, on stands, with seating and what not around.

The perplexing thing is that all four measured this way in their own locations, making me think it could be an issue with my measurement rig. (Earthworks M23, Focusrite Scarlett, Avid Profile)

Any thoughts team?
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Jay Barracato

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Re: Out of Polarity HF?
« Reply #1 on: June 18, 2014, 05:36:48 pm »

I believe you measured the way they are. I have done the same. I went round and round with another speaker designer about this on the boards a couple of years ago.

He couldn't or wouldn't explain why he thought this was such a problem except finally falling back on " the flipped polarity has worse impulse response" supported only by "everyone knows that".

I guess I am not everyone because I am not convinced the absolute polarity means much at frequencies where 10's of waves are going to pass in the shortest duration we can hear.

According to his TEF measurements on his boxes, and matched by my SMAART measurements on my boxes:

1. The HF is 180 in polarity from the LF

2. In this configuration the amplitude response is reasonably smooth through the crossover both on axis and off axis.

3. Inverting the HF to match the polarity created a hole in the response from about plus or minus 20 degrees from on axis and was similar from 20 to 45 degrees. Beyond 45 degrees ( which matches the stated pattern of the horn) flipping the polarity made the response marginally better but at that point you are at or beyond the extremes of the pattern.

So unless you plan on listening to the sides of the speakers I would leave them stock.

Sent from my DROID RAZR HD using Tapatalk

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Jay Barracato

Malcolm Macgregor

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Re: Out of Polarity HF?
« Reply #2 on: June 18, 2014, 05:44:09 pm »

What happens when you move you're mic closer to either the HF or LF (by moving the mic up and down) ?
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Rory Maguire

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Re: Out of Polarity HF?
« Reply #3 on: June 18, 2014, 06:59:55 pm »

Well in measuring the different speakers I did use non identical measuring systems, so different heights, different distances and got very similar results regardless!

I am hoping to be able to perform some more scientific tests in a few days... So I will keep you posted!

It does seem like a ~8dB dip through the crossover region is less than ideal however, that particular measurement was ground plane, on axis so you'd hope it would give a pretty good representation of the box as a whole!
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Jay Barracato

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Re: Out of Polarity HF?
« Reply #4 on: June 18, 2014, 09:42:22 pm »

Well in measuring the different speakers I did use non identical measuring systems, so different heights, different distances and got very similar results regardless!

I am hoping to be able to perform some more scientific tests in a few days... So I will keep you posted!

It does seem like a ~8dB dip through the crossover region is less than ideal however, that particular measurement was ground plane, on axis so you'd hope it would give a pretty good representation of the box as a whole!

Well the dip in amplitude does correspond to a region with a dip in the coherence which indicates some measurement issue. I am also not sure how the setup was as you refer to both ground plane placement of the mic and the speakers on stands. I would not call the distance you were working at, based on the delay, "on axis" to the horn if the speaker was 6+ feet in the air and the mic was at the ground.
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Jay Barracato

Rory Maguire

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Re: Out of Polarity HF?
« Reply #5 on: June 18, 2014, 10:06:15 pm »

Apologies!

Just looked at the screenshot more closely! The delay reading was for a later measurement where the speaker was stand mounted 4 feet from the speaker.

The magnitude and phase curves displayed were for a measurement taken with the mic on a road case lid sitting on a seating block in line (vertical plane) with the speaker. As much of a ground plane measurement as I could get in the situation!

Both measurements however did exhibit very similar phase shift and dip in magnitude.
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Matthew Knischewsky

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Re: Out of Polarity HF?
« Reply #6 on: June 19, 2014, 08:12:29 pm »

your measurement looks normal to me- like all of the K10s I have measured.

Matt
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Ivan Beaver

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Re: Out of Polarity HF?
« Reply #7 on: June 22, 2014, 10:56:36 am »

It may be as good as they could get.

Lots of factors affect the overall phase and freq response-including the spacing of the HF driver to the low freq driver.

Of course the best way to tell would be to take a measurement as is-then don't move anything and reverse the polarity of either the LF driver (this is just for testing so as to leave the phase on the top end the same) and see what happens.

If it gets better overall-then I would put the LF back to normal and reverse the HF.

But as others have said-it may be what it is and you can't get it any better with the current design.
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Re: Out of Polarity HF?
« Reply #7 on: June 22, 2014, 10:56:36 am »


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