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Old 4th February 2009, 05:29 PM   #601
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Quote:
Originally posted by gedlee



I really believe that some LF absorption is essential to good bass. The combination of good LF absorption and multiple subs creates, IMO, and ideal bass situation. If you can do one and not the other then do multiple subs, but absorption adds a lot to the modal interactions and smoothness of the response.

Some "bass traps" are more absorptive at mid to HFs than they are at LFs, these must be avoided. What I have found works well is very heavy drapes behind the speaks, since in that location even HF absorption is positive. But HF absorption anywhere forward of the speakers should be avoided.

Dr Geddes,
To this point, I have several 4'x2', 4" thick, 8# rockwool panels from a previous "bass trap" experiment in a small room. Is there a material that you could recommend that I could use cover the faces of these so that they would reflect the mid & high freq and leave the bass (<150hz) range of absorption? I guess it might be something that is refered to as a "limp mass membrane" -not really sure if that would correct.


I have seen some comments of yours regarding the placing of panels like these against the wall as being less than optimal.


-Apologies if you have covered this elsewhere, I have not found it.

Appreciate any insights you care to share.

-Tony
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Old 4th February 2009, 05:54 PM   #602
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Default Sub configurations

Quote:
Originally posted by gedlee



If done correctly eight subs would yield a smooth response in any simulation. Its the real world that counts.

What do you think Todd, what could you do with eight subs?

I think the best passive configuration (as far as sseat to seat consistency) I have seen is what I'll call the "Floyd Toole" configuration, since he suggested I try it. It is 4 subs, located where the 1/4 room dimension lines would intersect. This puts the subs out in the room, so perhaps not as practical as in the corners, but it is very flat and consistent. I would like to try this with ceiling mounted subs, though dont know when i will get the chance. If it were me, I would probably double them up at the four wall midpoints (more practical than Floyd's idea).

However the four midpoints is nearly as good. The eight in all eight corners looks about the same, but probably would be better in terms of standing versus sitting ear heights (which are not simulated here. See attached. And Earl, I know you will have something to say about the regular seating grid used!

See attached
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File Type: jpg floydsidea.jpg (32.3 KB, 382 views)
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Old 4th February 2009, 06:09 PM   #603
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Todd, we still see variations bigger than 40dB (!) which is unacceptable to me. Look what a real world DBA is able to deliver: http://www.avsforum.com/avs-vb/showthread.php?t=837744

Best, Markus
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Old 4th February 2009, 06:10 PM   #604
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Default DC mode

John, you got some 'splainen to do. What do you mean:

"Additionally, since the DBA does not cancel the DC mode the response has a nice linear boost as the frequency drops. "

Do you mean the "0th" mode? For that mode, I believe the room acts as a lumped parameter system, so you have 2 sources out of phase and delayed. Sounds like a comb filter, with complete cancelation at/near DC.

What am I missing...?
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Old 4th February 2009, 06:12 PM   #605
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Quote:
Originally posted by markus76
Todd, we still see variations bigger than 40dB (!) which is unacceptable to me. Look what a real world DBA is able to deliver: http://www.avsforum.com/avs-vb/showthread.php?t=837744

Best, Markus
Sorry, I should have explained the plots.

Look at the lower plot, that is the one with "direct" sound included. Also these are including a nearfield measurement of an Entre subwoofer.

And I'm sure your not refering to the dip below 10 Hz...?!?
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Old 4th February 2009, 06:16 PM   #606
gedlee is offline gedlee  United States
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Todd

I mostly see any situation that requires specific locations as being impractical. I setup sound systems in real rooms, although I have done (many years ago) a tremendous amount of simulations in order to get an idea of what to do. When I put these simulations into practice I found that they worked very well and that obsessing about specific locations, which is almost never possible (I can't do DBA in my room or any room that I have built). Hence I no longer worry about "what is ideal?", etc. etc. Floyd's suggestion would never be acceptable to any of my clients.

What I have found is that you get "very good results" with a few simple and easily adopted rules (I won't won't go into those again), but it requires some measurements and parameter setup. Can the ideal improve upon this (in a real room)? Maybe and maybe not, it will be a long time before that is proven in the general case, if there even is a single answer. I would suggest that any setup in a real room with real walls and furniture is going to need measurements to verify it, so there is no "setup free" approach to be had.

As to the seating locations, what you show is unaffected by this. Its when you do a statistical analysis on the data. It will be biased by the symmetrical sample spacing in a symmetrical room. Statistics assume independence of the samples, but your points are not statistically independent when laid out that way in that room. If the room were not symmetrical or the sample set was not symmetrical then this would go away. But when both are symmetrical, it is an issue.
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Old 4th February 2009, 06:22 PM   #607
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Todd, that's still 20dB deviation? Still not good enough for me. That's the frequency response at my listening position from 20Hz to 100Hz (3 subs):

Click the image to open in full size.

Best, Markus
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Old 4th February 2009, 06:24 PM   #608
gedlee is offline gedlee  United States
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Quote:
Originally posted by TRADERXFAN



Dr Geddes,
To this point, I have several 4'x2', 4" thick, 8# rockwool panels from a previous "bass trap" experiment in a small room. Is there a material that you could recommend that I could use cover the faces of these so that they would reflect the mid & high freq and leave the bass (<150hz) range of absorption? I guess it might be something that is refered to as a "limp mass membrane" -not really sure if that would correct.


I have seen some comments of yours regarding the placing of panels like these against the wall as being less than optimal.


-Apologies if you have covered this elsewhere, I have not found it.

Appreciate any insights you care to share.

-Tony
Hi Tony

I would put those panels behind the speakers. They would work better there. I wouldn't try and cover them as I can't think of anything that would work very well. When block damping material like that is placed against a wall it cuts its effectiveness about in half compared with placing it away from the wall several inches. The more inches away, the lower the frequency at which it will work - has to do with wavelength away from a wall which is always a velocity node.
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Old 4th February 2009, 06:28 PM   #609
gedlee is offline gedlee  United States
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Quote:
Originally posted by markus76
That's the frequency response at my listening position from 20Hz to 100Hz (3 subs):

Best, Markus
Markus

Thanks for that. What we need to see is some more real world data. The simulations are good for judging the right approach, but lets face it real rooms are not simulations. They are never that simple. I'll see if I can't find some data for my room.
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Old 4th February 2009, 06:36 PM   #610
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Quote:
Originally posted by markus76
Todd, that's still 20dB deviation? Still not good enough for me. That's the frequency response at my listening position from 20Hz to 100Hz (3 subs):

Best, Markus

Put even a single modest 10 dB paramteric filter on it, and the response is pretty darn flat, especially when you compare to most rooms which are considered quite acceptable and have easily 20 dB variations (but without a single big boomy one).

In any case I really dont understand this obsession with visually flat responses. Do you listen with your eyeballs? Do you think you could express a consistent preference for a response that was reasonably flat to one that was ruler flat? I doubt you could in a controlled test.

If you are interested in fidelity, the desireability of any room resposne characteristic has to take into account the conditions the music you listen to was created under. Do you think the response in the control room was anywhere near ruler flat when the music you listen to was mixed?!?

There - now I've done it!
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