Comments on my first measurements? - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #31 of 73 Old 06-20-09, 04:14 PM
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Re: Comments on my first measurements?

Quote:
elee532 wrote: View Post
This is still confusing me. My pre-pro is set for 60Hz crossover. As the two measurements below show (red line shows the crossover setting on my sub set at about one-o'clock (120Hz?) - the dial on my sub ranges from 40Hz to 160Hz - and the gold line shows the crossover setting on the sub at about eleven-o'clock (80Hz?).)

This higher crossover setting on the sub shows a flatter response line. Doesn't this reflect that the sub is now producing output all the way up to 120Hz? Wouldn't this be contributing to my localization issue? If my pre-pro is set to 60hz crossover, shouldn't the sub only be getting signal at 60hz and below?
Crossovers provide a smooth transition, not a sharp cutoff. If you play with the target settings in REW you can see (in the target curve) what the crossover response looks like for various crossover frequencies for a subwoofer or non-full range speaker. For a 60Hz crossover the sub's output is down 6dB at 60Hz and down 25dB at 120Hz. By bringing the sub's own crossover into play as well you speed up the rolloff of the sub's output but create a dip in the transition between sub and mains.

The localisation issue may be a result of the distortion. The ETC shows significant harmonics from 2nd right up to 6th (so for a 50Hz input that means distortion products at 100, 150, 200, 250 and 300Hz), and these are at the output of the sub, they are not reduced by changing the crossover setting. It does seem odd, as high distortion would typically be seen in a sub being driven past its capabilities but from the extension you have your sub seems very capable - unless that extension is being achieved by boosting the sub's low end and that boosting is contributing to the distortion.
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post #32 of 73 Old 06-20-09, 05:30 PM
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Re: Comments on my first measurements?

OK, as if I'm not confused enough already...

It seems I am getting completely different results depending on the Start and End Frequency that I use for my sweep. The blue line is when I use 0 to 200Hz for my sweep. The purple line is when I use 0 to 15,000Hz for me sweep.

The results are obviously dramatically different up to about 20Hz. Any insight as to what's going on?

Thanks!
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post #33 of 73 Old 06-21-09, 04:14 AM
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Re: Comments on my first measurements?

That is very odd. You don't have a procesor that applies any sort of dynamic EQ or dynamic loudness do you?
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post #34 of 73 Old 06-21-09, 01:31 PM
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Re: Comments on my first measurements?

No dynamic EQ or loudness on my pre-pro (Emotiva LMC-1). When I take my readings, pre-pro reads "bypass mode, PCM48k." Sounds Effects are checked as disabled on the Enhancements tab for the playback devise in Windows. Here's another graph using various sweep settings.

Red = 0 to 200Hz
Gold - 0 to 500Hz
Green = 0 to 1,000hz
Aqua = - to 5,000Hz
Black = 0 to 15,000Hz

I have no idea what to make of this??
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post #35 of 73 Old 06-21-09, 01:41 PM
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Re: Comments on my first measurements?

The difference in the various sweeps is how long the signal spends at the lowest frequencies. If the sub is very non-linear below 20Hz changing the time spent there would affect the level of distortion products. You may get a sense of that from watching the driver as the sweep progresses. You might get more consistent results by changing the sweep start frequency to 10Hz or higher.
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post #36 of 73 Old 06-21-09, 07:48 PM
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Re: Comments on my first measurements?

JohnM, thank for the reply! I understand what you are saying re: the length of time at each frequency during the sweep might differ depending on the sweep range. However, I'm kind of lost after that (i.e. non-linear sub?). I'm pretty much an amateur at this stuff.

FYI, I did try starting my sweep at 10Hz instead of 0, but the results were the same as above.

What does all of this say about my sub? Which results, if any, results are accurate?

Thanks!!
post #37 of 73 Old 06-21-09, 08:29 PM
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Re: Comments on my first measurements?

It almost looks like your amp is switching through the different damping options. It looks similar to the last three graphs on this page:

http://www.rythmikaudio.com/eq.html



Is there a way to check the incoming signal response to the sub to eliminate it as the cause? This might help narrow down where the problem is coming from. You know, test something like:

1. Signal coming out of the computer before the pre-pro.
2. Signal coming out of the pre-pro before the sub.
etc.

brucek has mentioned similar tests to others. If I remember I did something like it to test if my pre-pro's xo was working properly.
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post #38 of 73 Old 06-21-09, 09:59 PM
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Re: Comments on my first measurements?

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Is there a way to check the incoming signal response to the sub to eliminate it as the cause?
Test the receivers response out the LFE port by using REW.

Just as you would test a loopback cable from line-out to line-in to get a flat response, simply include the receiver in that loop and take a measure. You have to fuss with the levels a bit when doing the Check Levels routine, but it's easy enough.

Connect soundcard line-out to receiver AUX-in, and receiver Sub-out to soundcard line-in, and measure.

Be sure the meter cal file is cleared and the soundcard cal file is loaded.

The response should be just like one of the REW targets, except it will roll off down low. Measure from 0Hz.

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post #39 of 73 Old 06-23-09, 06:54 PM
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Re: Comments on my first measurements?

brucek,

I'm think I am misunderstanding your instructions. When I follow what I think are the connection instructions you provided above, the output meter reads about -15 but the left/right input meters read nothing. Is the SPL meter supposed to be connected in the mix somewhere?

Thanks and sorry for my ignorance here!
post #40 of 73 Old 06-23-09, 08:15 PM
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Re: Comments on my first measurements?

Quote:
Is the SPL meter supposed to be connected in the mix somewhere?
No, you're measuring the electronic frequency response of your receiver by itself, just as if it was a loopback cable. No meters, no speakers - you're measuring the electronics.

Connect soundcard line-out right channel to receiver AUX-in right channel, and receiver Sub-out to soundcard line-in right channel. Run the Check Levels and Calibrate routine and Measure.

Quote:
but the left/right input meters
You'll only be looking at the right channel input meter. The left channel is not used.

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