What do I do with this? - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 6 Old 10-12-09, 04:53 PM Thread Starter
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Jarrod
 
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What do I do with this?

Thanks to the members of this forum, my subwoofer measurements are working and the setup has come a long way. I was able to identify a null at the listening position at 38Hz when the sub is placed along the left wall (the only place it can go and pass WAF), and subsequently fill it in reasonably well by integrating a second, small subwoofer next to the center channel.

Feeling pretty good about that, I decided to take some full-range measurements to see what that looks like. Boy, did that open up a can of worms! Below is a graph of subs plus center channel from 15-20k (black) and the L+R mains (magenta). I also measured L and R separately and they both overlay onto the magenta L+R graph (data not shown).

What do I do with this?-fullrange.jpg

As you can see, my previously identified mode of 38-40Hz appears to also rear its ugly head at 10 times the frequency. I think I was happier when I was somewhat ignorant of the problem. I have known for some time that something was going on - just had never quantified it.

So I played a 40Hz test tone and walked around the room with my SPL meter. There is a huge standing wave across the width of the entire room, with its null at the listening position, and a peak along each L/R wall. Spatially, it is one giant cosine wave with the left wall representing x=0 degrees and the right wall representing x=360 degrees. It doesn't matter where you stand in the length dimension - you can walk the length of the room in a straight line along the null or one of the peaks and the sound meter doesn't change.


Last edited by Jarrod; 10-12-09 at 05:04 PM.
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post #2 of 6 Old 10-12-09, 05:10 PM
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Re: What do I do with this?

First of all can you redo the graph limits so they only show 20-200Hz this will help us see what is really going on. Dont worry too much about the graph above 2K as its not going to be very accurate unless your using the ECM8000 mic. There is a nasty dip at 400Hz what is going on there? I would be more concerned with that dip over the dip at 40Hz.

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post #3 of 6 Old 10-12-09, 06:37 PM
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Re: What do I do with this?


He has an ECM8000, Tony.

Quote:
Jarrod wrote: View Post
As you can see, my previously identified mode of 38-40Hz appears to also rear its ugly head at 10 times the frequency. I think I was happier when I was somewhat ignorant of the problem. I have known for some time that something was going on - just had never quantified it.
Ouch. That 400 Hz thing’s going to be a tough one, Jarrod

Quote:
So I played a 40Hz test tone and walked around the room with my SPL meter. There is a huge standing wave across the width of the entire room, with its null at the listening position, and a peak along each L/R wall. Spatially, it is one giant cosine wave with the left wall representing x=0 degrees and the right wall representing x=360 degrees. It doesn't matter where you stand in the length dimension - you can walk the length of the room in a straight line along the null or one of the peaks and the sound meter doesn't change.
I’m going to hazard a guess that this is a “shoebox” room? This kind is thing is not unusual for them. Basically, the bass “chases” the boundaries, and the further you move from one to the center for the room, the more bass levels decrease, with the dead center of the room being the worst. This is one reason I like my open family room.

Regards,
Wayne




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post #4 of 6 Old 10-12-09, 07:11 PM Thread Starter
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Re: What do I do with this?

Tony: the 20-200 Hz graphs are posted in my other thread. As Wayne already pointed out, I'm using an ECM8000 with the ECM8000_CS.cal calibration from the download area.

Quote:
Ouch. That 400 Hz thing’s going to be a tough one, Jarrod
That's what I was afraid of. The room is shaped kind of like a barn with the back torn off - it's open to the kitchen and the rest of the house. The ceiling is vaulted, but symmetrical. The right wall has a fireplace and two windows so I can't do much there. The left wall is solid sheetrock - I could put some absorptive devices there but I know that 400Hz is close to the lower limit of what typical absorptive panels (like OC703) can handle.

What about diffusors? Is this hopeless?
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post #5 of 6 Old 10-12-09, 07:34 PM
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Re: What do I do with this?


We're mainly EQ guys here. For questions on acoustical treatments, we have informed parties at separate Forum for that. Scroll down our main index page and you'll find it.

Regards,
Wayne




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post #6 of 6 Old 10-12-09, 07:54 PM Thread Starter
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Re: What do I do with this?

Roger that. I had looked for an acoustics forum before I posted it here - not sure how I missed it!

Thanks.
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