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Sub Alignment
PostPosted: Sun Oct 18, 2009 3:57 pm Reply with quote
M.JOHAN
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Hi, someone have a way to realign sub's during the show? indeed we can't mute sub's and then top's.

Thanks
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PostPosted: Mon Oct 19, 2009 3:59 am Reply with quote
Langston Holland
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Are you really saying that you have a need to change the delay between the subs and the adjacent bandpass during a show? Even large temperature variations would not cause a need for this given the wavelengths in question.

Secondly, I've never seen a situation where one had access to bandpass delay adjustment but no access to a mute function, but possibly you're referring to the fact that you wouldn't want to given that the show would be in progress?

Maybe if you rephrase your question or provide more detail with the goal you have in mind we could be more helpful.

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PostPosted: Mon Oct 19, 2009 7:29 am Reply with quote
M.JOHAN
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actually my English is approximate, Frenchy forces. You Right i'm reffering to the fact that i won't mute separetely the PA during the show. For the delay adjustement of sub's was where I am asked to increase the levels of sub, so the frequencie of the crossover change, consequently the alignment is wrong.

I hope to be more understandable

Thanks
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PostPosted: Mon Oct 19, 2009 6:18 pm Reply with quote
Doug Fowler
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Quote:
For the delay adjustement of sub's was where I am asked to increase the levels of sub, so the frequencie of the crossover change, consequently the alignment is wrong.


Hi -

When you increase the level of the subs, you raise the acoustic crossover point. This is not the same as changing the alignment. To change the alignment, you must change the time relationship between the subs and the LF band of the rest of the PA system.

If you must increase the level of the subs during the show and you are concerned you are also raising the acoustic crossover point, you can also lower the low pass frequency for the subs. This will make a gap between the subs and the LF and will help compensate for the additional energy at the crossover point, which was increased when you added gain to the subs.

Does this make sense?

-doug
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PostPosted: Mon Oct 19, 2009 7:19 pm Reply with quote
Langston Holland
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I understand - thanks. :)

You are using an aux fed sub arrangement and are concerned about the summation of the sub to the low bandpass throughout the crossover region. The fact is that raising and lowering one of the bandpasses relative to the other doesn't cause an alignment problem, so there's nothing to correct for with that specific issue.

The summation of two sources at similar frequencies is determined by the time of arrival which we generally measure in phase. Most folks are happy with an alignment that keeps this time of arrival, or phase, within 90 degrees throughout the crossover region where the two sources have a reasonable amount of output to add to one another. The region of interest IMO is at least 12dB down from the acoustic crossover point. Keep in mind that all of this discussion concerns the actual acoustic result, not just the pretty little curves you see coming out of a loudspeaker processor in the electrical domain.

The fact that the phase does not change with level changes can be considered with a thought experiment: Consider the sub and low bandpasses by themselves - you mute one and measure the other at a given SPL. Now increase or decrease that level, does the phase curve change? No. Therefore, the combined phase curve of the two bandpasses will not change with level changes as long as you operate them within their linear (not overloaded) regions.

If you have phase curves with the same slope within 90 degrees of each other throughout the crossover region, you have done well, particularly for one-off live sound applications. As you mentioned, the acoustic crossover point will move and that will result in an audible difference - but you'll have to be the judge of whether that difference is better or worse. If you end up with the sub bandpass 6dB (or more) higher than the upper frequencies, as many find pleasing with contemporary music, you will want to reduce the overlap at the crossover region to keep that area from sounding "muddy". This is all subjective at this point and you have to do this to taste. In my one-off live sound applications I use a 4th order Linkwitz Riley crossover between subs and lows. I then separate the -6dB acoustic crossover points by about 1/2 octave. Thus, with a 75Hz sub lowpass crossover point, I'll highpass the tops at about 110Hz and then do the alignment.

Edit: Just read what Doug said and it's a perfect summary. I'm just trying to bait you into learning more - attending the AFMG classes would be a great start. :)

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PostPosted: Wed Oct 21, 2009 12:20 pm Reply with quote
Charlie
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Good comments from both Doug & Langston. I have written an article on subwoofer alignment with full-range systems and recently given a presentation at the 127th AES Convention. Both the article and the presentation are available on my website at http://www.excelsior-audio.com/Publications.html.

I will agree that, as Doug & Langston have said, changing the level of the subs relative to the tops will not change the phase response of the combined system. However, if one moves the low pass filter corner frequency for the subs to compensate for the increased output from the subs as the acoustical crossover point in effect raised, then the alignment does changes. This is due to the phase response of the low pass filter changing as its corner frequency is changed.

I like Langston’s approach of separating the low pass and high pass filter corner frequencies and then doing the alignment. This is very similar to what I detailed in the AES presentation a couple of weeks ago.

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PostPosted: Wed Oct 21, 2009 4:59 pm Reply with quote
M.JOHAN
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Thanks for your answers clear and relevant.
But the initial question was the band is playing on stage, how can i measure the correct alignment of my system Top's, Sub's ?

Thanks
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PostPosted: Wed Oct 21, 2009 5:16 pm Reply with quote
Doug Fowler
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Quote:
But the initial question was the band is playing on stage, how can i measure the correct alignment of my system Top's, Sub's ?


That is a hard one, but SysTune allows one to band-limit the IR measurement.

Set the IR band for 125Hz, which is currently as low as you can go, and look for peaks. You will not be able to do a phase alignment, but you can at least look at the octave centered on 125Hz.

This may be useful, maybe not :-)

Good luck!
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PostPosted: Wed Oct 21, 2009 5:33 pm Reply with quote
Bruce
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Doug Fowler wrote:
Quote:
But the initial question was the band is playing on stage, how can i measure the correct alignment of my system Top's, Sub's ?


That is a hard one, but SysTune allows one to band-limit the IR measurement.


In addition to what Doug said you can also go to the Tools tab and restrict the bandwidth using the Use Band-Pass Filter checkbox. This allows you to look at any arbitrary pass-band while the IR/ETC Display is set to Broadband. It also shows the Band-Pass used in the Mag/Phase graphs.

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PostPosted: Wed Oct 21, 2009 10:05 pm Reply with quote
Charlie
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I think that what M. Johan is asking is how can he measure his PA system with the band playing on stage. In other words, in the presence of the noise from the stage, how can the PA be measured to align things.

If this is the case use log sweeps and lots of averages to try to increase the signal-to-noise ratio of the measurement.

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PostPosted: Thu Oct 22, 2009 9:35 am Reply with quote
M.JOHAN
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Location: RENNES, FR




Charlie wrote:
I think that what M. Johan is asking is how can he measure his PA system with the band playing on stage. In other words, in the presence of the noise from the stage, how can the PA be measured to align things.


No charlie, Doug and bruce are in thrue. Apparently there is no solution. So I'll continue unscientific ways. I take an overlay magintude at time t and i move discretly upward and forward the delay of sub's to see if the magnitude is better... Confused
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