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post #1 of 9 Old 06-29-2013, 01:29 AM - Thread Starter
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Hi folks, I would like to discuss with you the following phenomenon.
I am using an electronic crossover from Accuphase (F-20) for the main speakers (Soundlab A-1 electrostatics). The (high pass) crossover frequency is set to 70Hz with a slope of 12dB/octave.
Four Rythmic Audio subwoofers are responsible for the frequencies below 70Hz.
I did some REW RTA measurements and found that the near field response of the A-1 (without subwoofers) didn't show the crossover effect (there was no ramping down of the amplitude at 70Hz) --> see graph.
I have also measured the output at about 4 m from the speakers (subwoofers out). The graph still shows the lack of effect of the crossover at 70Hz.
The crossover itself has been checked and appeared to function normally.

What could be the reason that there seems no measurable crossover effect in the RTA?
Is it some sort of artifact induced by the microphone itself or the interaction between the microphone and the objects or acoustical bounderies in the vincinity of the microphone?
Or is it a sort of acoustical reinforcement of the lower frequencies (below 70Hz)? I'm clueless and confused.




Chris
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post #2 of 9 Old 06-29-2013, 06:17 AM
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You need to run the crossover with a resistive load and a signal generator input and find out if it is even working.
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post #3 of 9 Old 06-29-2013, 06:24 AM
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Your levels are very low. To assure your way up out of the noise of the room, I'd recommend at least 20dB higher. Also, is this measuring rig a known quantity? Have you established trustworthy measurements prior to now?

That said, although I love large dipole panels, I've no good experience measuring them in the nearfield or close mic'd. Surely there's a very complex interference pattern associated with close mic measuring of the large panel. However, I don't believe that's the artifacts I'm seeing here.

I'd be more apt to trust measuring back at the LP, with the filtering engaged, then dis-engaged. Then form an opinion from that. Or, if you had another more typical box loudspeaker to experiment with, or even utilize your subs to test the Accuphase piece, and the efficacy of the high-pass functionality.



Good luck

btw; Sound Lab + multiple Rythmik subs = cool.gif

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post #4 of 9 Old 06-29-2013, 11:06 AM - Thread Starter
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Thank you for your response.
Commsysman, the Accuphase unit has been checked and measured at the service shop and it is functioning properly.
FOH, the first graph is a near field measurement, but the second graph is from a measurement at about 4 m from the Soundlabs (listening position).
I have very limited experience doing measurements like these, so it can be a lot of artifact generated by inadequate measurement techniques.

Chris
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post #5 of 9 Old 06-29-2013, 01:19 PM
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Quote:
Originally Posted by dazzdax View Post

FOH, the first graph is a near field measurement, but the second graph is from a measurement at about 4 m from the Soundlabs (listening position).
I have very limited experience doing measurements like these, so it can be a lot of artifact generated by inadequate measurement techniques.

Chris


Hey Chris,

You've got to get the signal up out of the noise.

I understand you're limited in experience, no problem.

The levels used are just too low. Bass levels in the 40dB-60dB range aren't fully out of the noise floor of the room. As long as you're within the linear envelope of the system, the higher the acoustic output of the loudspeaker, the better the measurement.

Again, remember you can experiment with the high pass filter with the Subs too. I remember your images of your system now, very nice space/system.

Best of luck

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post #6 of 9 Old 06-29-2013, 03:14 PM
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Can you overlay two graphs: with and without the crossover? That is what we need to figure out what is happening. Put the mic where you normally sit.

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post #7 of 9 Old 06-30-2013, 05:24 AM
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Quote:
Originally Posted by dazzdax View Post

Hi folks, I would like to discuss with you the following phenomenon.
I am using an electronic crossover from Accuphase (F-20) for the main speakers (Soundlab A-1 electrostatics). The (high pass) crossover frequency is set to 70Hz with a slope of 12dB/octave.
Four Rythmic Audio subwoofers are responsible for the frequencies below 70Hz.
I did some REW RTA measurements and found that the near field response of the A-1 (without subwoofers) didn't show the crossover effect (there was no ramping down of the amplitude at 70Hz) --> see graph.
I have also measured the output at about 4 m from the speakers (subwoofers out). The graph still shows the lack of effect of the crossover at 70Hz.
The crossover itself has been checked and appeared to function normally.

What could be the reason that there seems no measurable crossover effect in the RTA?
Is it some sort of artifact induced by the microphone itself or the interaction between the microphone and the objects or acoustical bounderies in the vincinity of the microphone?
Or is it a sort of acoustical reinforcement of the lower frequencies (below 70Hz)? I'm clueless and confused.




What is not clear is whether the subwoofers are turned up high enough to make a difference. Most modern subwoofer crossovers use higher slopes which makes their effect more pronounced.

Some fairly robust floor-standers, especially if set to large, with a room resonance around 50 Hz and a laid back gain setting for the L & R speakers could give you the results you observe.

I can't find an online technical review of the Soundlab A1s, but a review of the A3s says:

http://www.stereophile.com/content/sound-lab-3-loudspeaker-j-gordon-holt-january-1992

"Most obvious was the extended bass range, which went from its previous 40Hz effective limit to an astonishing 33Hz, "
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post #8 of 9 Old 07-01-2013, 03:25 PM - Thread Starter
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Hi Arnold, both graphs depict the output of the Soundlab without subwoofers but with the crossover engaged at 70Hz, 12dB/octave slope.
I will repeat the measurements of the Soundlabs with and without the crossover.

Chris
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post #9 of 9 Old 07-08-2013, 02:00 PM - Thread Starter
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I measured the output of one of the Soundlab with and without crossover. Purple = with crossover, blue = without crossover. Crossover frequency is 90Hz with 12dB/octave slope (in both cases the subwoofers were switched off). Distance to microphone = 1 foot.



Chris
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