It was clear that my right surround LX55 is defective, it always sounds dark or recessed compared with the other speakers.
Using REW, and my UMIK-1 at a distance of about 10 inches with attempted (but not achieved) consistency in placement, I tested 3 LX55's, the defective one (1), another unit with damaged woofer surround (2, actually I call this "unit 4" because it was the original left surround until I damaged the surround of the woofer slightly), and a NOS LX55 I recently acquired (3).
The defective unit showed a large drop above 5kHz, from there fairly flat to 10kHz, then down to zilch.
Number two showed a more gradual from 3-10khz, but 10kHz was equally down.
Number 3 showed only a minor loss around 6kHz, otherwise fairly flat to 10kHz, then sharply down.
At first I figured the tweeter in unit 1 may have been fried, what I was seeing was the woofer response. I tested that theory by disconnecting the woofer. Now the tweeter response was clearly visible, and, strangely, fairly flat from 5kHz to 10kHz.
Next I tried reversing the tweeter polarity on unit 1. This made it somewhat similar to unit 2 (making me suspect unit 2 already has the tweeter phase reversal modification). But it did nothing the bring the upper highs back up around 10kHz, and therefore still showing considerable rolloff compared to unit 3.
This looks like the difference between units 1 and 2 might be tweeter phase reversal. It's possible that unit 2 already got this modification. But the extension to 10kHz is apparently something different. The NOS unit 3 has that because, possibly, it hasn't been used as much. Or perhaps there is just considerable unit-to-unit variation, and unit 3 is the lucky one that came out best.
As good as unit 3 is, I wonder if it would be even better with the tweeter polarity change. I suspect it would fill in the 6kHz suckout, but it would also probably add more bulge around 3kHz. I'm almost inclined just to take unit 3 to replace unit 1, with or without the phase reversal, but then how would it compare with my other units???
THURSDAY UPDATE
Interpreting these graphs, despite their "objectivity", is full of ambiguities. Despite using REW for the first time, which was a major accomplishment for me (requiring the new DAC, etc), I was unable to control exact microphone and speaker positions. I started out planning to mark positions with masking tape, but as all was being done on on a placement on the marble coffee table, the placement pulled up and all was lost. Everything was unavoidably moved on each trial, and with a mere 10 inches from microphone to speaker, which itself is not good at all, but it also made it difficult to assess all the angles precisely eatch time. So I plan on doing a full set of greatly improved precisely positioned measurements of all my LX55 speakers today (Thursday).
However one pattern seems pretty clear. And that is that the new LX55 has much more extended response, almost flat at 10kHz, whereas the other two speakers are down 10dB at that point, and this was true regardless of the tweeter phase reversal on unit 1. That reversal did fix the 5-10kHz suckout, but exchanged it for a gradual rolloff down to 10kHz.
John Atkinson measured several LX5 units, and also found big differences similar to what I show (loss of about 10dB at 10kHz in the badly performing unit). He determined however (via unspecified means) that all drivers were connected the same in both speakers regardless of measurment: the woofers were in polarity and the tweeters out of polarity. Thus, in his evaluation, miswired polarity did not play a role in these differences. Instead, he suggested quality control of the tweeter was an issue.
I have often been skeptical, thinking perhaps he merely checked the wires in the speaker, and perhaps the wires themselves were reversed within the tweeters themselves. But the truth is he does not say how he did this driver polarity assessment, and he could have used a dirac or similar test signal, which someone like him (and me now actually) would have right at hand. (For me it's not just the test signal, it's getting one of my scopes to trigger properly on it...)
However the findings shown here suggest that Atkinson nailed it, at least that it is a driver quality control issue. While the defective unit is somewhat better wired in different polarity, it does not fully make it like the good unit. Notably, the total loss at 10khz is unchanged.
But thinking about this over and over, and looking about my pretty good measurment of the tweeter itself, I'm thinking perhaps the tweeter is not the source of these differences.
Perhaps they are largely determined by the dustcap resonance of the woofers. That resonance may change as a result of the history of heat and humidity it has experienced. In some it has a broad 5-10kHz resonance, and in others the resonance is mostly gone. And, following this theory, it is the better measuring speakers that have the most dustcap resonance. Further, following this theory the tweeter itself has fairly flat response...but it is adjusted fairly low in level so that the woofer dominates the response up to 10kHz.
But perhaps it only appears this way because of sloppy measurements. Testing the theory fully requires a fair amount of soldering and resoldering, which I try to avoid, and I want to assay all my surround speakers too.
Using REW, and my UMIK-1 at a distance of about 10 inches with attempted (but not achieved) consistency in placement, I tested 3 LX55's, the defective one (1), another unit with damaged woofer surround (2, actually I call this "unit 4" because it was the original left surround until I damaged the surround of the woofer slightly), and a NOS LX55 I recently acquired (3).
The defective unit showed a large drop above 5kHz, from there fairly flat to 10kHz, then down to zilch.
Number two showed a more gradual from 3-10khz, but 10kHz was equally down.
Number 3 showed only a minor loss around 6kHz, otherwise fairly flat to 10kHz, then sharply down.
At first I figured the tweeter in unit 1 may have been fried, what I was seeing was the woofer response. I tested that theory by disconnecting the woofer. Now the tweeter response was clearly visible, and, strangely, fairly flat from 5kHz to 10kHz.
Next I tried reversing the tweeter polarity on unit 1. This made it somewhat similar to unit 2 (making me suspect unit 2 already has the tweeter phase reversal modification). But it did nothing the bring the upper highs back up around 10kHz, and therefore still showing considerable rolloff compared to unit 3.
This looks like the difference between units 1 and 2 might be tweeter phase reversal. It's possible that unit 2 already got this modification. But the extension to 10kHz is apparently something different. The NOS unit 3 has that because, possibly, it hasn't been used as much. Or perhaps there is just considerable unit-to-unit variation, and unit 3 is the lucky one that came out best.
As good as unit 3 is, I wonder if it would be even better with the tweeter polarity change. I suspect it would fill in the 6kHz suckout, but it would also probably add more bulge around 3kHz. I'm almost inclined just to take unit 3 to replace unit 1, with or without the phase reversal, but then how would it compare with my other units???
THURSDAY UPDATE
Interpreting these graphs, despite their "objectivity", is full of ambiguities. Despite using REW for the first time, which was a major accomplishment for me (requiring the new DAC, etc), I was unable to control exact microphone and speaker positions. I started out planning to mark positions with masking tape, but as all was being done on on a placement on the marble coffee table, the placement pulled up and all was lost. Everything was unavoidably moved on each trial, and with a mere 10 inches from microphone to speaker, which itself is not good at all, but it also made it difficult to assess all the angles precisely eatch time. So I plan on doing a full set of greatly improved precisely positioned measurements of all my LX55 speakers today (Thursday).
However one pattern seems pretty clear. And that is that the new LX55 has much more extended response, almost flat at 10kHz, whereas the other two speakers are down 10dB at that point, and this was true regardless of the tweeter phase reversal on unit 1. That reversal did fix the 5-10kHz suckout, but exchanged it for a gradual rolloff down to 10kHz.
John Atkinson measured several LX5 units, and also found big differences similar to what I show (loss of about 10dB at 10kHz in the badly performing unit). He determined however (via unspecified means) that all drivers were connected the same in both speakers regardless of measurment: the woofers were in polarity and the tweeters out of polarity. Thus, in his evaluation, miswired polarity did not play a role in these differences. Instead, he suggested quality control of the tweeter was an issue.
I have often been skeptical, thinking perhaps he merely checked the wires in the speaker, and perhaps the wires themselves were reversed within the tweeters themselves. But the truth is he does not say how he did this driver polarity assessment, and he could have used a dirac or similar test signal, which someone like him (and me now actually) would have right at hand. (For me it's not just the test signal, it's getting one of my scopes to trigger properly on it...)
However the findings shown here suggest that Atkinson nailed it, at least that it is a driver quality control issue. While the defective unit is somewhat better wired in different polarity, it does not fully make it like the good unit. Notably, the total loss at 10khz is unchanged.
But thinking about this over and over, and looking about my pretty good measurment of the tweeter itself, I'm thinking perhaps the tweeter is not the source of these differences.
Perhaps they are largely determined by the dustcap resonance of the woofers. That resonance may change as a result of the history of heat and humidity it has experienced. In some it has a broad 5-10kHz resonance, and in others the resonance is mostly gone. And, following this theory, it is the better measuring speakers that have the most dustcap resonance. Further, following this theory the tweeter itself has fairly flat response...but it is adjusted fairly low in level so that the woofer dominates the response up to 10kHz.
But perhaps it only appears this way because of sloppy measurements. Testing the theory fully requires a fair amount of soldering and resoldering, which I try to avoid, and I want to assay all my surround speakers too.
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