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Author Topic: What's a reasonable approach?  (Read 5276 times)

Adam Black

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Re: Re: What's a reasonable approach?
« Reply #10 on: May 22, 2013, 12:03:51 pm »

But I could not easily make out any hot frequencies that way.   I even created some feedback on a stage monitor while playing break music to see the results.  Well by the time I could really see the frequencies that were feeding back, the feedback was much to loud and I could have detected it more quickly by ear.  I could hear some of the ringing from FOH before it got loud but it was just not obvious in the spectrograph or the spectrum displays.  It was buried in the changing spectrum of the music.

Mike,

Basically a spectrograph rejects data that does not fall within a specified range, -70dBFS to -30dBFS for example. If the feedback is lost within the normal content, it sounds like the threshold level for the lower range is too low. Try increasing the lower threshold limit. This will reject more of the desired content and make it easier to spot feedback.

-A
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Doug Fowler

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Re: Re: What's a reasonable approach?
« Reply #11 on: May 22, 2013, 02:14:36 pm »

It seems like this thread has run its course so perhaps a little swerve is OK?

Jay that passage inspired me to try this so I set up Systune to look at the cue bus on my SL16.4.2.  I thought I could look at the monitor sends during a  live show and detect and/or head off feedback.

The set up worked OK, I could see whatever send I punched up.  I just looked at a single channel spectrograph or spectrum type measurement, not any kind of transfer function. 

But I could not easily make out any hot frequencies that way.   I even created some feedback on a stage monitor while playing break music to see the results.  Well by the time I could really see the frequencies that were feeding back, the feedback was much to loud and I could have detected it more quickly by ear.  I could hear some of the ringing from FOH before it got loud but it was just not obvious in the spectrograph or the spectrum displays.  It was buried in the changing spectrum of the music.

So I am wondering if there are some key points to your set up that make it work for you?  How do you see something starting to take off, especially if you have 4-6 monitor mixes that you would have to quickly check?


Mike -

Double click the color bar just above the SysTune spectrograph display.   You'll then be able to adjust the upper and lower SPL limits for the display.   Almost certainly, the default is too wide. Play with those values and you should get what you're looking for. 

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Merlijn van Veen

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Re: What's a reasonable approach?
« Reply #12 on: May 22, 2013, 08:01:48 pm »

Whatís reasonable and how much time I try to allow myself accordingly, is determined by my confidence in the circumstances.

Having the luxury of doing measurements at virtually every show Iíve done (both the production system but also house systems), convinced me of one thing. Good coherence results in faithful reproduction (coherence being an indicator of the integrity of a signal). I feel that poor system tuning, besides evident factors you have little or no control over like i.e. acoustics, results in poor coherence.

The possible reasons for bad coherence are beyond the scope of this post. However, itís my believe that good system tuning, requires a profound knowledge of the laws of physics considering sound. Specifically the relationship between level and time.

In my corner of the world, there are about 400 theaters, accommodating anything from approx. 200 to 1.600 visitors, most of them with respectable equipment. Iíve visited the majority of them at least two times or more. Unfortunately to often IMHO, the circumstances demonstrate a lack of insight in physics, ranging from poor speaker deployment, poor speaker aiming to poor time alignment to name a few.

I can only speculate about the reasons for this. Maybe some installations date from a time when such knowledge wasnít accessible or the means to properly tune the system werenít available. Then again, to often in the past Iíve been guaranteed, that a certain system was designed and tuned by a respectable engineer. Being intimidated by reputation and out of respect for these individuals, I quickly learned the hard way that most tunings have a certain application in mind, which isnít wrong by itself but not necessarily mine. Having learned a bit, Iíve found and measured on later returns i.e. different speakers with documented polarity reversal, being used together in an overlapping fashion. I assume a lot of engineers, including the system designer, must have noticed and suspected something was wrong but surprisingly nothing had changed in the meantime. Even when confronted with irrefutably evidence, the amount of indifference I sometimes run into is dazzling. Systems that actually have been tuned plausible might have the original DSP presets being overwritten by the sheer amount of passing productions. On other occasions, being warned in advance by house technicians for certain known but apparently ďunsolvableĒ issues, I ran into more elusive problems like polarity reversals in wiring, transducers or negligent rigging in paired traps resulting in severe comb filtering. Sometimes I witness so much overkill in equipment, resulting in questionable overlap from numerous sources, that commercial interest apparently overruled sensible system design. Lastly a lot of design decisions seem to be arbitrary.

But mostly I feel that audio is still something that is considered to be exclusively done with youíre ears, because thatís how itís always been done and why change that. Measuring still seems to have a questionable reputation. Maybe because everyone can do it, like operating a car. But the latter doesnít mean that you can drive. Interpreting the data is something that requires skill and experience. In the wrong hands, it could account for the former.

Excuse me if Iím stating the obvious
« Last Edit: May 22, 2013, 08:43:05 pm by Merlijn van Veen »
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Mike Karseboom

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Re: Re: What's a reasonable approach?
« Reply #13 on: May 22, 2013, 08:19:47 pm »


Adam and Doug - thanks for the setup tip.   I just got a chance to look at this in the shop. Setting a tighter scaling on the spectrograph display definitely makes it easier to see frequencies that look out of place.  Now I just need to put some more time in with it.
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Re: Re: What's a reasonable approach?
¬ę Reply #13 on: May 22, 2013, 08:19:47 pm ¬Ľ


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