Amount of bass gain from room acoustics?

Discussion in 'Archived Threads 2001-2004' started by VinhT, Feb 21, 2002.

  1. VinhT

    VinhT Second Unit

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    I'm doing preliminary calculations for a starter DIY subwoofer. About how much gain can be expected from room acoustics in terms of frequency range and dB increase?

    My project calls for a sealed enclosure(for ease and greater margin of error), and I'm using a 10" driver. The predicted F3 is about 46Hz, F6 is about 35Hz, and the Qtc is .7, so the drop-off is very graceful. Now, would the gain from room acoustics bring up the faltering end for a more flat response?

    I ran some calculations using a ported enclosure for the same driver, which gives an enticing F3 of 26Hz or so. However, I've heard that a flat subwoofer ultimately sounds boomy because of room gain.

    So will the seal enclosure be good enough? Or should I dive in and attempt a ported enclosure?
     
  2. an F3 of 46 hz is a bit high for a subwoofer. With sealed, I would want atleast f3=35hz with a Q=0.707...but that is just me.
    most medium to small rooms will get between 6-9db room gain @ 20hz. large rooms may bet only a few db.
    www.adireaudio.com has a technical paper graphing a typical room gain.
     
  3. Mark Seaton

    Mark Seaton Supporting Actor

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    Mark Seaton
    The frequency at which it starts, and the amount of room gain is dictated by the dimensions, construction, and openings in your room. With these, you can make a reasonable estimate of what to plan for. As one good starting point, calculate the longest 3D dimension in your room, typically diagonal floor to ceiling. Now determine what that dimension corresponds to in terms of wavelength. As a rule of thumb, the room gain will be starting around the frequency where this dimension is equal to 1/2 wavelength. The closer to a cube your room is, the more accurate this figure is. You can also make some interesting realizations if you consider the dimensions of your room individually.
     
  4. AjayM

    AjayM Screenwriter

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    Well, I have a large room (T shaped, about 26 x 26 at most, listening area is on a 26ft wall and I'm 14ft from equipment, 20ft ceiling in most of the room) with high ceilings, I also have a sealed "lower" Q (around .63) Tempest, LSPCad's predicted roll-off had me with an F3 around 33-35Hz, in room and measured and I am flat to 18Hz, and probably lower except the rumble filter starts to kick in.

    I don't know how an F3 of 46Hz would behave, but I would think you could at least have good response down into the mid 30's.

    Andrew
     
  5. Richard Greene

    Richard Greene Stunt Coordinator

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    In a rectangular room bass will ramp up at approximately

    12db/octave below the lowest frequency standing wave.

    That standing wave frequency in Hz. equals 565/largest room dimension in feet (usually the length) measured from wall to wall.

    If your room has large openings into other rooms,

    the subwoofer may "see" a larger room (so the 12dB/octave bass ramp up will start at a lower frequency than you expect).
     
  6. Ken Woodrow

    Ken Woodrow Stunt Coordinator

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