New Room with Bad Bass - Please Help!!!

Discussion in 'Home Theater Projects' started by Jeff Elliott, Aug 11, 2004.

  1. Jeff Elliott

    Jeff Elliott Stunt Coordinator

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    Hi All - My room is nearing completion and I ran some audio experiments today with very disappointing results. At the prime listening position which is the front row, the sub sounds like it's not even on! Matter of fact, I asked a friend to toggle the sub on and off while seated in my listening position and I could tell very little difference. However, as I move around the room, I can clearly hear the sub get better but no position was worse than my seat! I can start in the front of the room near the screen and the bass is great. As I walk toward the back of the room toward my seating postion it still sounds great. When I arrive at my seating position, it's like the sub is turned off. As I continue to the back of the room, it gets better again - almost like you flipped the amp on.

    First, here's some facts about the room:

    - 8'6"H x 13'5"W x 21'5"D. Sofits are 14 x 18 and wrap around the room
    - Room within a room with double layers of 5/8 & 1/2 drywall. Completely sealed throughout.
    - Desired listening / viewing position is 14' from front wall and dead center.
    - Linacoustic is installed to a hight of 41". 8oz batting extends to the bottom of the sofit. The bottom of the sofit is lined with linacoustic. The stage area is completely lined with linacoustic.
    - Columns and carpeting are not yet installed. Concrete floor now.

    Next, here's what I've done and heard:

    - My target location for the subs was on the stage to the left and right of the screen. Today, I used one sub (AV-15 in 6ft^3 box, tuned to 20hz, 400W power amp).
    - I placed the sub in my desired listening position and walked around the room to find the optimal location for the sub. The best I could do was the rear left corner of the room which is not an option unless I build an infinite baffle with the rear wave venting into a hallway behind the theater.
    - When I placed the sub on the stage, it sounded completely dead at my listening position and great when I moved around the perimeter of the room. The best sound was in the second row (on the riser) near the corners.
    - With the sub on the stage, it was as if the bottom dropped out. No low bass, no punch, nothing.

    My questions:

    1) Is it possible that the sub is performing flawlessly on the stage and I should hear it "disappear"? When answering, remember that I could barely discern it if was on or off.

    2) Do my room dimensions and desired seating position defy the laws of physics to the point that I can't get good bass? Have I violated one of the so called "golden rules" of room design or otherwise offended the acoustics gods?

    3) Is it possible that the subs will sound better when the carpet and or columns are in? Could it make a huge difference?

    4) Any advice on how to proceed? I've been working on this room for a year and am just sick now that the bass stinks in my seat!

    Signed down and out in Charlotte!
    Jeff
     
  2. Allen Ross

    Allen Ross Supporting Actor

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    Have you messed around with phase? How about taking measurements, seeing where the room is sucking the sub out. It sounds like you have some acoustic treatment in the room so thats a plus, i am sure the carpet will only help your situation, so i would wait and see what it sounds like with carpet, maybe some more acoustic treatments.

    Also the easiest way to get rid of a bad room, BFD.
     
  3. ThomasW

    ThomasW Cinematographer

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  4. Ethan Winer

    Ethan Winer Stunt Coordinator

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    Thomas,

    > Pretty obvious that the listening position is located in a room null. Your options are, move the listening postion or move the sub. <

    I agree, and I suggested to Jeff in another forum that he try moving the sub. But I want to amplify your point about the significance of room nulls because so many people argue that nulls are not very important. Indeed they are, as Jeff's post proves!

    --Ethan
     
  5. ThomasW

    ThomasW Cinematographer

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    If people spent some time and energy measuring (mapping) the FR in the room, then -20dB nulls wouldn't be a surprise.

    Or if in advance of actually building the room; they were to use CARA, RPG Acoustics Room Optimizer program or even one of the freeware room mode programs, these nulls wouldn't be a surprise at all.... :wink:
     
  6. Bryan Michael

    Bryan Michael Supporting Actor

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    i have my sub in the middle of my side wall i dont get the benefits of corner loading but i get a real flat responce at the listning positon. take the sub put it where your sit and take mesurements around the primiter to find the loudest place and put sub there.
     
  7. Wayne A. Pflughaupt

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    Jeff, is what you’re seeing kind of like, as you move from the “dead” location towards a wall, the bass gets stronger and stronger, and it’s most strong in the corners? That’s the bizarre situation I’ve seen before in a smallish rectangular room like yours.

    Ethan, I “get it” now what you were talking about on the thread the other day where you mentioned how destructive nulls can be. Now, that one wasn’t exactly the same as this; it was merely a narrow dip in measured response at a specific frequency. Here we have what appears to be wholesale, across-the-board cancellation at his listening position.

    This seems to be a fairly common problem with rectangular rooms and I’m wondering, for the benefit of Jeff and others in his situation, what can be done about it? I’m generally a fan of equalizing bass problems, but I really doubt that would help this situation much – if at all.

    So - can bass traps, or your own broadband absorbers, smooth the response of the room like this – i.e., eliminate or reduce the central dead spot, and “tone down” the “build-up” of bass energy along the walls and corners? Would building the corners out at 45 degrees do anything for the problem?

    Regards,
    Wayne A. Pflughaupt
     
  8. ThomasW

    ThomasW Cinematographer

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  9. Ethan Winer

    Ethan Winer Stunt Coordinator

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    Wayne,

    > I “get it” now ... Here we have what appears to be wholesale, across-the-board cancellation <

    Yes, but it's just a variation on the same theme, where reflections from the room boundaries cancel some of the sound.

    > This seems to be a fairly common problem with rectangular rooms <

    Yes!

    > I really doubt that [EQ] would help this situation much – if at all. <

    Exactly.

    > can bass traps, or your own broadband absorbers, smooth the response of the room like this <

    Yes, absolutely. In this case the cancellations are severe, so Jeff will need a fair amount of bass trapping. But that is definitely a viable solution. In fact, short of tearing down walls I think it's the only solution.

    > Would building the corners out at 45 degrees do anything for the problem? <

    No, that would be a big mistake because it will remove the best places to put bass traps.

    --Ethan
     

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