Impulse changes, when no changes made. - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 10 Old 07-14-16, 09:05 AM Thread Starter
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Impulse changes, when no changes made.

I did notice something strange today when doing some test measurements on my new subwoofer that I am trying to add to my system. The impulse response changed between measurements even if I did not change anything, just run the measurements after each other.

So which of the measurements is correct in time? I am using the acoustic timing reference when doing the measurements, using my center speaker as reference. I am also using UMIK-1 as microphone. Version of REW is 5.16.

I am attaching a mdat-file for review.
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post #2 of 10 Old 07-14-16, 10:30 AM
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Re: Impulse changes, when no changes made.

I don't know any way to confirm which is wrong. This shouldn't happen with 'Wait for timing reference' selected. I see your timing reference level is reported at -38 dB in your file. Possibly that level is a little too low for reliable timing?
I usually take several repeated measurements initially to confirm the repeatability of any given setup. I have occasionally run into an odd measurement, but it is pretty rare with good settings.
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post #3 of 10 Old 07-14-16, 02:59 PM
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Re: Impulse changes, when no changes made.

Quite strange. What does your center speaker measurement look like from the same measurement position, could there be a very strong reflection in it?
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post #4 of 10 Old 07-14-16, 03:36 PM Thread Starter
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Re: Impulse changes, when no changes made.

Quote:
jtalden wrote: View Post
I don't know any way to confirm which is wrong. This shouldn't happen with 'Wait for timing reference' selected. I see your timing reference level is reported at -38 dB in your file. Possibly that level is a little too low for reliable timing?
I usually take several repeated measurements initially to confirm the repeatability of any given setup. I have occasionally run into an odd measurement, but it is pretty rare with good settings.
So the problem could be one of volume. I have been doing the measurement at about 72-73dB. I will dial it up some to say 75dB. Also I did notice there was a new ASIO4ALL driver so have installed that to be safe.

And where do I find what timing reference level a graph has?

Quote:
JohnM wrote: View Post
Quite strange. What does your center speaker measurement look like from the same measurement position, could there be a very strong reflection in it?
I have tried to replicate the previous measurement as close as I could to take a reading of the center speaker with and without the sub, file is attached. There is also one full range graph included.

Hope this will help and I will try to do the graphs at 75dB from now on and see if that solves my problem.
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post #5 of 10 Old 07-14-16, 04:04 PM
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Re: Impulse changes, when no changes made.

There are some very large reflections at 5ms and 6ms on the centre speaker, that might be an issue. Could try using a different channel as the reference.
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post #6 of 10 Old 07-14-16, 04:18 PM Thread Starter
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Re: Impulse changes, when no changes made.

Thanks John, I will try that.
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post #7 of 10 Old 07-14-16, 04:26 PM
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Re: Impulse changes, when no changes made.

Fredrik74,
I was referring to the Center Channel timing signal. There is a volume control on that also on the top of the measurement panel. The SW level is fine around 75 dB.
Note that, your center channel has very little output of the TW where the timing signal sent. The low overall level and the very low high freq output of the center channel may well be the problem (chart below). You should easily hear the high freq timing signal sweep from the timing channel before the measurement channel sweep starts.
JohnM's suggestion is a good one until you determine why the high freq output is so low on the center speaker - bad Tweeter?
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post #8 of 10 Old 07-14-16, 05:09 PM Thread Starter
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Re: Impulse changes, when no changes made.

This is strange, is this from the latest .mdat I posted with the Center fullrange measurement? I can not see this in rew when reviewing that center measurement from the last posted .mdat or am I missing something important?

And I will checkout the volume setting in measurement dialog.
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post #9 of 10 Old 07-14-16, 07:57 PM
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Re: Impulse changes, when no changes made.

Sorry! I just open the file again and it look fine just as your chart does.
The reference time must have still been set positive and thus clipped the HF response. I did have it shifted about +12ms for the first time on the previous file I was working on. It didn't occur to me that a new file would open with that same shift. Anyway I somehow had my settings wrong... Nevermind...
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post #10 of 10 Old 07-14-16, 08:30 PM Thread Starter
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Re: Impulse changes, when no changes made.

Thats fine jtalden, I have gotten som good advice I will try out. So many thanks for your help
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