Virtual SMAART training: Tell the crossover frequency and type

P

Phil Graham

Honorary PhD
Mar 10, 2011
651
1
18
Atlanta, GA
Hello everyone,

Attached is a SMAART screenshot from yesterday's small gig, before I applied any equalization. I had time for four quick measurements of each side of the PA. These were taken with the mic on a straight stand at audience head level. The four traces shown are the L and R averages, respectively, both with and without coherence weighting. Because the box's crossover point and slope is unusually clear from these measurements, I thought it would be a good learning experience to post the measurements, and let people take a stab at figuring out these values in the data. I scaled the image down to 1280x800, but it is still fairly large, so click on the thumbnail to view full size.

Again, looking for the XO frequency, order, and type. Also how you determined these values.

Place your bets! And those who know immediately, wait a bit before chiming in.

Explain the XO.jpg
 
Re: Virtual SMAART training: Tell the crossover frequency and type

Ales,

You are specifically one of the people that I was thinking of when I asked you to wait on replying for the benefit of others. :( This is supposed to be a learning thread for less experienced people. Could you erase your answer, for now?
 
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Re: Virtual SMAART training: Tell the crossover frequency and type

I bet that's just humility - he didn't think that comment applied to him. People like that absorb knowledge quicker than a knife fight in a phone booth.

I also bet the compression driver is about 4" deeper than the adjacent passband... About 210 degrees later at 2kHz?
 
Re: Virtual SMAART training: Tell the crossover frequency and type

A few thoughts/questions

A) Do you want an approximation of the curve of the loudspeaker PLUS the electrical slope or just the electrical slope (if there are electrical filters involved ;)?

B) I assume we are to assume that the funk around 200 is from the ground bounce (mic at audience head level) and not another xover point?

C) Is the answer supposed to be in the form of a question?

D) Can you elaborate on how this ability to accurately guess the slopes of the filters in a complete speaker system from graphs posted on the internet will help a sound dude? Is this about measurement or something else?

E) This makes me think of... "Bender: I mean three-thousand-eighteen rat kidneys."

 
Re: Virtual SMAART training: Tell the crossover frequency and type

drew gandy said:
A few thoughts/questions

A) Do you want an approximation of the curve of the loudspeaker PLUS the electrical slope or just the electrical slope (if there are electrical filters involved ;)?

B) I assume we are to assume that the funk around 200 is from the ground bounce (mic at audience head level) and not another xover point?

C) Is the answer supposed to be in the form of a question?

D) Can you elaborate on how this ability to accurately guess the slopes of the filters in a complete speaker system from graphs posted on the internet will help a sound dude? Is this about measurement or something else?

E) This makes me think of... "Bender: I mean three-thousand-eighteen rat kidneys."

Click to expand...

A) All I care/know about is the final acoustic target response. I have no idea the internal electrical filters. The box is a mid-range self-powered loudspeaker from a major manufacturer.

B) That is for you to decide, but that is not a bad assumption.

C) However you want to phrase it. I'm most interested in people expositing their thought process, as this is what we can talk about.

D) The more one understands how a loudspeaker could/should measure, the easier it is to make sense of the measurements one takes. There's no hidden trickery here.
 
Re: Virtual SMAART training: Tell the crossover frequency and type

I'm still not convinced there isn't a little trick in here. Like someone did some reconing and swapped + and - on all the low drivers.
 
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Re: Virtual SMAART training: Tell the crossover frequency and type

drew gandy said:
I'm still not convinced there isn't a little trick in here. Like someone did some reconing and swapped + and - on all the low drivers.
Click to expand...

Drew,

This is silly. I picked this example specifically because it is so straightforward, and can be a learning experience for those trying to understand measurement. Unnecessary obfuscation serves no purpose for that intent.
 
Re: Virtual SMAART training: Tell the crossover frequency and type

However you want to phrase it. I'm most interested in people expositing their thought process, as this is what we can talk abou
Hey Phil, Great post, hopefully most smart users (who have taking a class) would have a thought process some thing like
1. Data is meaning less with out context..caution, caution
2.Where is the coherence trace?
3.Where is the coherence trace?
4. Is there not supposed to be a coherence trace, even with averaged traces?
5. Why is the live impulse resonse not running, not viewing the full (or a fuller) picture, caution, caution

I generally think post like this are super great, and we all learn a lot from them so keep em coming.
Rasmus
 
Re: Virtual SMAART training: Tell the crossover frequency and type

Rasmus Rosenberg said:
However you want to phrase it. I'm most interested in people expositing their thought process, as this is what we can talk abou
Hey Phil, Great post, hopefully most smart users (who have taking a class) would have a thought process some thing like
1. Data is meaning less with out context..caution, caution
2.Where is the coherence trace?
3.Where is the coherence trace?
4. Is there not supposed to be a coherence trace, even with averaged traces?
5. Why is the live impulse resonse not running, not viewing the full (or a fuller) picture, caution, caution

I generally think post like this are super great, and we all learn a lot from them so keep em coming.
Rasmus
Click to expand...

1. You have plenty of context from my post. You know the number of measurements and how I measured
2-4. The coherence trace is built in, as this averaged response is shown both with coherence weighting, and without, on the graph.
5. I have no use for live impulse response in this context, it tells me nothing useful.
 
Re: Virtual SMAART training: Tell the crossover frequency and type

Bennett Prescott said:
I dunno, Ales. I haven't used coherence in years. I find it tells me nothing I cannot infer from the phase and magnitude traces (and perhaps from mic position, etc), and clutters up my screen.
Click to expand...

Agreed, I'll use coherence at 1/4 height in SMAART 7, and turn it off frequently (shortcut "C"). I understand mathematically what the software included and excludes in a measurement, so I've primarily replaced coherence with experience.

PS We discussed what a measurement includes and excludes in the other long thread that Helge started.
 
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Re: Virtual SMAART training: Tell the crossover frequency and type

I think I may be totally in the SWAG guessing game, but I am not really sure if the logic is reversible:

i.e. if A (an electronic crossover) creates B (an acoustic crossover), then by looking at B (an acoustic crossover) you should be able to determine A (an electronic crossover)

however:

1. either 90 degrees or 270 degrees of phase shift make me think odd order. I think I am more comfortable with 3rd order (18 db per octave) than 1st order.

2. The roughly-3 to -5 dip at the crossover in frequency makes me think gentle shoulders, i.e. Bessel filter

So my SWAG is 3rd order Bessel at 2000hz.

edit- and for the record, I don't think I have ever designed a crossover totally on my own, although I have tweaked factory recommendations.
 
Re: Virtual SMAART training: Tell the crossover frequency and type

Hey Phil!

I'm SUPER new to the tuning game, so a lot of this is over my head. I'll ask some question, but if they give away some of the answers for others, you can PM with your answers if you want.

My questions are this, as I'm new to all of this, I want to understand all sides of it. (Obviously this takes time)

What are the delays for on the DSP and Microphone for? 154ms seems crazy(173 feet?) so I don't know how that fits in to your calculations for the DSP side of it.

What am I really looking for here? From what I know of SMAART and it's displays, it seems that that the lower end of this box is more in phase than the higher end? Especially around 2k. Is this indicative of a lows box in the PA?

As far as the orders go, and what 1st order, 2nd order and all that is, I am embarrassed to say that I don't know what that is, so I can't take a stab at what those are. What does one look for to see the difference between a passive and active crossover?

Lots of questions, I know, but that's how to learn in situations like this, right Thanks for the challenge!
 
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