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Author Topic: Case Study: setting up RCF TT22As & RCF 905AS with Sabine 4802, mic  (Read 8495 times)

Paul Tucci

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Re: Case Study: setting up RCF TT22As & RCF 905AS with Sabine 4802, mic
« Reply #10 on: May 06, 2012, 03:24:33 pm »

I guess I should run the math on this, but I'm thinking the height of the sub from ground, and the top-to-sub distance, and the relative ears of the audience are fine tuning things done at the venue, or maybe they are negligible in the context of room reflections, power alley,...


Out of curiousity I did the math.  If A squared + B squared still = C squared in a right triangle, the difference between a groundplane measurement at 60' away and a groundplane measurement at 60' away but with the main speaker raised 6'  would be neglible.

0 height                   squared       0
60 away on ground     squared       3600

Square root of 3600 = 60' path length



6' height                   squared        36
60 away on ground     squared      3600
                                                  3636

Square root of 3636                         60.3'

The difference in path length is .3' = 4".


At 100 Hz, one full wavelength is 1130/100 = 11.3' So we're talking a comparatively small difference between the two measurements.
How many of these differences are there in a total wavelength?    Approx 35.  4" * 35 = 140"     140" / 12" = 11.6' Close enough for this.
If you were predicting earthquakes I trust you were more scientifically exact than this.
Knowing there are 360 degrees of phase in a full rotation of a waveform we can see that 1/35th of a full rotation is a little more than 10 degrees of phase change.
There's enough slop factor to say your measurement with the top on the sub or raised up 6' vertically will be quite similar.

PT
« Last Edit: May 07, 2012, 06:34:29 am by Paul Tucci »
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Mark McFarlane

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Re: Case Study: setting up RCF TT22As & RCF 905AS with Sabine 4802, mic
« Reply #11 on: May 06, 2012, 04:00:51 pm »

...There's enough slop factor to say your measurement with the top on the sub or raised up 6' vertically will be quite similar.

That was my suspicion for 60' in the crowd. 20' out from the stack I get slightly less than a 1 foot difference in path length, say around 30 degrees out of phase at 100Hz (napkin calc, actually in-my-head calc).  Not great but manageable.  Inside 20' is problematic but if the tops are really 6' above the front row's ears we have other problems in this hypothetical situation.

I'm not going to worry about it.  Thanks for doing the math.

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Mark McFarlane
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Mark McFarlane

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Re: Case Study: setting up RCF TT22As & RCF 905AS with Sabine 4802, mic
« Reply #12 on: May 06, 2012, 04:27:54 pm »

Looks like a polarity switch and the user-selectable xover point of 80 or 120 is on the self powered box itself, nice.
Adding another xover in your processor for "safety" is the wrong thing to do. It will only add more phase shift.

PT

The 80-120 Xover on the sub appears to only control the signal sent to the OUTPUT jack on the sub, not the sub signal.  At least that is how I read the docs, but it doesn't really make much sense.  It appears the intended use is to allow you to send full range to the sub and daisy chain a high passed signal to the tops, without using an external crossover.

Page 4 of the full sub manual describes the switch function http://www.rcf.it/c/document_library/get_file?p_l_id=251487&folderId=22221&name=DLFE-921.pdf

The TT22A tops have an optional 24db/oct 90Hz high-pass filter which I did not plan to use. http://www.rcf.it/c/document_library/get_file?p_l_id=249233&folderId=22218&name=DLFE-914.pdf

Actually, this is making we now question the frequency response published for the sub.  The sub spec sheet says 40-120Hz response (without specifying if this is -10db or -3...) but the amplitude response on the spec sheet shows 120Hz as more than 40db down. The graph makes one think there is a high-cut filter applied to the sub signal (looks like -6db at 90Hz), but the documentation indicates the switch sets a high-pass only applied to the output. I guess when the speakers are delivered next week I can listen.

Hopefully these won't turn out to be 1-note subs.


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Mark McFarlane
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Doug Fowler

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Re: Case Study: setting up RCF TT22As & RCF 905AS with Sabine 4802, mic
« Reply #13 on: May 07, 2012, 01:33:33 pm »


You and your negative time.

I had an assistant on a tuning gig who made EQ changes and entered delay times.  Nothing was working as expected (for a good while, valuable time is being wasted).

I look over his shoulder and he is attempting to enter negative values in the "delay" field.

Reasoning: we need delay, so he entered negative time. 

Yeah, that will definitely affect your alignment job. 
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Mark McFarlane

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Re: Case Study: setting up RCF TT22As & RCF 905AS with Sabine 4802, mic
« Reply #14 on: May 08, 2012, 01:50:28 pm »

Step 2: Selecting the Crossover Point

I'm thinking about crossing over at 80Hz.  These speakers are plenty loud for my gigs and I'd like to push the crossover phase issues as low as possible,...  Then again,... is this too low?

So what do people think about setting the crossover at 80Hz for these TT22A's?  Too low?

FWIW, I ordered a TM1 mic today, and I'm heading to the airport in 5 minutes to catch a plane to the States to pick up all my new toys.  Life is good.  My thanks to Mike Pyle for loading up a truckload for me.
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Mark McFarlane
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Turn down what's too loud.
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