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Author Topic: QSC K10 tweeter polarity  (Read 17526 times)

Jay Barracato

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Re: QSC K10 tweeter polarity
« Reply #10 on: June 15, 2011, 09:55:17 pm »

As I pointed out, the definition of "on axis" is itself unclear with a speaker such as this. It is most common to use the HF horn axis for that definition. In the case of the K10, there is a pronounced amplitude notch at crossover on the axis of the HF horn with the stock reverse HF polarity. .

I am unable to reproduce this. At 1.5 m, one of my K10's basically measures flat through that range with the mic directly on axis with the HF horn. I think to compare, I would need a better definition of your setup including speaker and mic placement.

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Yes. That is true in both directions, above and below axis. The response is flatter over a wider range of listening positions. Pretty much what you're looking for in sound reinforcement applications. Or what you should be looking for.

Once again, I cannot reproduce similar data. My smoothest responses are all in locations with the smallestangles from directly on axis.

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The notion of "smoothness" of phase response is off base. What matters is the ability of a loudspeaker to preserve transient information. Reversing the polarity of one of the drivers always degrades this ability.


I never said anything about smoothness of the phase response. I refered to the slope of the phase response. I really don't understand what you mean by "preserve transient information" as a function of polarity. Earlier you said "transient response", which if it is used in the usual engineering sense meaning return to steady state, looks to be about the same based on your impulse response graphs.



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Jay Barracato

Dave Dermont

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Re: QSC K10 tweeter polarity
« Reply #11 on: June 15, 2011, 10:04:22 pm »

I am sure the products Mr. Mitchell is involved with are wonderful things. Just the same, whenever someone presents something prefaced with "Here's something you are probably too stupid to understand, but I'll say it anyway", red flags start to fly.

Then, with each succeeding post, I get more information about his vast experience, and how he is smarter than the rest of us.

OK. I get it. You know what you are doing. I don't dispute anyone's intelligence. I know a lot of people who are smarter than I am, yet I seem to do OK.

I really don't know how much more straightforward Bob Lee could have been with his answer on the QSC forum. If there is a notch in the 1.5kHz to 2kHz range, the tweeter polarity is reversed. When the polarity is correct, there is no notch. That's how the speaker was designed. It seems pretty reasonable to me.

If you dig placing frequency response notches in your speakers by reversing the polarity of one of the drivers, by all means, knock yourself out. It aint gonna hurt nobody. You can decide to second guess the factory guys because you are smarter than they are or you think some guy on the internet is smarter than they are. You won't be the first to do so, and you probably won't be the last. Hell, maybe you just like being the contrarian.

Everybody knows that chicks dig the bad boy.

I love pretty response plots as much as the next guy, but very few audience members ask to see the tech data before they decide whether or not they enjoyed the show.

Just sayin'.

Have a nice day.

Rock on!




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Dave Dermont

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Jay Barracato

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Re: QSC K10 tweeter polarity
« Reply #12 on: June 15, 2011, 10:21:18 pm »

Here are some of my quick and dirty measurements. The blue is directly on axis at 1.5 m, the red and the green are + 15 degrees and +30 degrees in the horizontal, and the last two which only the SMAART folks know the actual name for the color are +15, and +30 degrees in the vertical. I did not measure any negative vertical angles, but I would tend to attribute the greater cancelation in the vertical to a primary reflection from the floor.

I would not be uncomfortable sitting a paying customer anywhere in that range.
« Last Edit: June 15, 2011, 10:29:52 pm by Jay Barracato »
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Jay Barracato

Rich Grisier

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Re: QSC K10 tweeter polarity
« Reply #13 on: June 16, 2011, 12:09:52 am »

I love pretty response plots as much as the next guy, but very few audience members ask to see the tech data before they decide whether or not they enjoyed the show.

Which brings me back to my original question- wondering if anyone can confirm the sound quality improves with the change.

I'm a big theory guy and enjoy theoretical discussions... but sometimes I've been known to abandon what I believe to be theoretically correct in favor of what sounds best.  I ran into that situation just this weekend- we played in a bar that was shaped like an L with the stage being in the corner of the L.  I used to run subs center clustered in front of the stage.  It created decent coverage, but more than anything it created a bass pocket between the singer and I that would rattle our molars!  O opted to split the subs on either side of the stage.  Coverage on the dance floor was good but reduced to zero against the wall.  I had placed the sub tight up against the wall... so I went against what I thought to be theoretically correct and moved the sub 3' from the wall.  When I did this it got rid of the drop-off in sub level near the wall.  Coverage was great across the entire dance floor.  Everything I've ever calculated or read suggests to never put a sub 3' from a wall... but because putting a sub against the wall didn't work, I went the complete opposite route and was pleased with the result.

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Jay Mitchell

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Re: QSC K10 tweeter polarity
« Reply #14 on: June 16, 2011, 09:06:19 am »

Here are some of my quick and dirty measurements.
Here is my response: whenever a device the size of a K10 appears to have substantial directivity at 200 Hz and below - as it does in your data - then something in the measurement is not indicative of the actual device's behavior. The wavelength of 200 Hz is just over 5 1/2 feet. The largest dimension of the K10 is much smaller than this, so it cannot possibly be directional at this low a frequency. That's just the laws of physics at work.

It is essential in acoustics work to separate the characteristics of the source from those of the acoustic environment. Otherwise, you will be unable to draw meaningful conclusions about either. The data I provided in the FAS forum are indicative of the source behavior and intentionally exclude environmental reflections. One major advantage TDS has over other measurement techniques is the ability to establish a user-determined time window.  When acquiring loudspeaker data, you want to set a time window that is small enough to include only direct sound. When you are fully aware of the behavior of the sources you are using, you are much better-equipped to address any undesirable effects due to a specific acoustic environment. That is a basic, noncontroversial principle of sound system design and commissioning.

In the future, when you want to acquire loudspeaker data, I suggest you investigate the ground plane measurement technique.

@Rich - there's one way to find out if the correct HF polarity sounds better to you: try it. It's easily reversible if you don't like it.
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Jay Mitchell

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Re: QSC K10 tweeter polarity
« Reply #15 on: June 16, 2011, 09:16:41 am »

I really don't know how much more straightforward Bob Lee could have been with his answer on the QSC forum. If there is a notch in the 1.5kHz to 2kHz range, the tweeter polarity is reversed.
Here's the dealbreaking problem with that answer: there are notches in the crossover frequency range with either choice of HF polarity. They are in different positions, but in both cases the notches fall well within the designed coverage of the speaker. The result is that some listeners will always be subjected to those notches. The assumption that there is only one "frequency response" that matters - whatever the manufacturer decides to designate as "on axis" - is in error.
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Peter Morris

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Re: QSC K10 tweeter polarity
« Reply #16 on: June 16, 2011, 09:33:54 am »

I read recently HERE that the sound quality of the QSC K10 can be improved by changing the polatiry on tweeter.

I have a pair of K10 and thought they've always sounded great as is.  Just wondering if anyone has done this and can confirm that the sound quality improves.

Hi everybody,

Read this, its about one of the tricks now used in the K series - Intrinsic Correction™,
http://www.qscaudio.com/products/dsp/SC28/intrinsic_correction_whitepaper_2007.pdf
… and then tell me the guys that wrote this don’t understand how to  hook up the HF driver …   :o

Peter
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Jay Mitchell

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Re: QSC K10 tweeter polarity
« Reply #17 on: June 16, 2011, 09:35:33 am »

HOWEVER-I know Jay-and he knows of what he talks and does.
Hey Ivan. How goes these days?

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ALSO-I do wonder how the driver got wired out of polarity.
Lots of manufacturers do that intentionally, either to "fix" an amplitude notch caused by the crossover filters themselves (second-order Butterworth filters do this, which is the original reason for JBL's reversing their woofer polarity marking convention from the rest of the world) or to Band-Aid misaligned transducers. Many coaxial loudspeakers - including some well-known studio monitors - have reversed-polarity HF drivers. When you place an HF driver several inches behind the woofer cone, the misalignment you create is almost exactly half a wavelength in the range of viable crossover frequencies for this type of device. The solution to the cause of the problem is to delay the signal to the woofer by the appropriate amount, but this is not an option with a passive crossover.

For anyone who is interested in learning more about crossover summation behaviors, I highly recommend Richard Small's 1971 AES paper, "Constant Voltage Crossover Network Design."
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Jay Mitchell

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Re: QSC K10 tweeter polarity
« Reply #18 on: June 16, 2011, 09:50:07 am »

Read this, its about one of the tricks now used in the K series - Intrinsic Correction™,
http://www.qscaudio.com/products/dsp/SC28/intrinsic_correction_whitepaper_2007.pdf
… and then tell me the guys that wrote this don’t understand how to  hook up the HF driver …   :o
A few observations:
1. Mark Engebretson - the author of the white paper - knows what he's doing. Nobody has said or implied otherwise.
2. The white paper in question has nothing to do with the subject of this thread.
3. Intentional polarity reversal of one element in a two-way loudspeaker is very common practice. When that is the practice you've adopted - as QSC has done here - it ill-serves any legitimate purpose to assert that the tweeter is in polarity. That's exactly what Bob Lee did in the QSC forum. It is not in polarity, and you can establish that with a simple clicker. Whether the speaker works better with the polarity corrected is arguable, as I acknowledge in the thread in the FAS forum (http://forum.fractalaudio.com/amps-cabs/37266-qcs-k10-tips.html). Whether it is actually in polarity is not a matter of interpretation or opinion, however.

ps - You should hear the story Don Davis tells - and Mark Gander confirms - about Mark Engebretson and acquiring loudspeaker response data back in the early 1960s.
« Last Edit: June 16, 2011, 10:14:18 am by Jay Mitchell »
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Jay Mitchell

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Re: QSC K10 tweeter polarity
« Reply #19 on: June 16, 2011, 10:19:29 am »

Here are some of my quick and dirty measurements.
One more observation: your phase plots clearly indicate a reverse-polarity tweeter, as do mine. This establishes that at least two K10s came from the factory this way.

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ProSoundWeb Community

Re: QSC K10 tweeter polarity
« Reply #19 on: June 16, 2011, 10:19:29 am »


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