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Ground coupling? Or decoupling?

Discussion in 'Amps and Cabs [BG]' started by amos, Dec 3, 2007.

  1. Dec 3, 2007 #1
    amos

    amos

    Oct 23, 2003
    SE Portland Oregon
    In the current issue of Bass Player, the review for the new Trace Elliott 1048H cabinet does not have casters or even a place for casters because [according to Trace Elliott?] "ground decoupling reduces low=end response" How many TB'ers out there decouple their speaker cabinets from the floor or stage, either with an Aurelex GRAMMA pad, casters, or a stand? How many prefer to keep it on the floor? Why?

    I know in our tiny practice space, when I put my two 1x12s on the floor, the whole room, as well as my drummer's snare, vibrates a whole hell of a lot more. So mine is elevated on an OnStage stand in the practice room, for now anyway. I used to have a GRAMMA pad but it got stolen. Live is a different scenario, unless it's a hallow stage or boomy room, I have not had much reason to elevate, though I have not played many shows without the GRAMMA yet.

    Pretty sucky: My GRAMMA got stolen along with a crate practice amp, monster bass cable, furman power strip, and tape recorder :mad: It all happened while I was in the hospital too.

    Man that Trace AH500-7 SS head looks like a beaut...getting off track here...what do you guys prefer?
     
  2. Dec 3, 2007 #2
    billfitzmaurice

    billfitzmaurice Guest Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    There two distinct and independant forces at work here. One is the physical vibration of the floor, the bulk of which is created by the sound coming from your speaker, not by physical vibrations transferred from the cabinet to the floor. Nothing short of adding a few more 2x10s to the floor supports will fix that. Isolation pads can help with that component which is the direct product of a vibrating cab, but what they mainly do is to isolate the cab from the vibrations of the floor created by the soundwaves from the cab. Clear as mud? But as for physical coupling adding low end, even bolting the cab to the floor won't result in more bass output.
    The other force is boundary reinforcement, which occurs when the cab baffle is less than 1/4 wavelength from the floor. Lifting the box as much as 2.8 feet will have no effect below 100 Hz, what is considered 'bass', as it's still within 1/4 wavelength, so de-coupling doesn't reduce low-end response. However, above 100 Hz response is reduced, and that can be very beneficial in reducing 'boom', as boom is mostly sourced not in the bass below 100Hz but in the mid-bass from roughly 120-350 Hz. That's probably the case in your practice room.
     
  3. Dec 3, 2007 #3
    Vic Winters

    Vic Winters

    Apr 20, 2006
    Western NY
    I keep mine off the ground via the casters at the bottom. If I didn't I'd rattle the hell out of the drums that are right next to me at practice.
     
  4. Dec 3, 2007 #4
    amos

    amos

    Oct 23, 2003
    SE Portland Oregon
    Thank you much for that explanation Bill. I had to read it a few times but it makes sense to me now. I kind of push my speakers a bit hard to get enough volume with the cabs elevated, the boundary reinforcement probably the reason. If I EQ'd in more mids @ 250hz I image that would help a bit.

    Can you explain the word "baffle"? Is that the space between the soundwave and the floor/wall?

    The other thing is, if an isolation pad isolates the cab from the "vibrations from the floor created by the soundwaves from the speaker cab" what, exactly, does this accomplish in terms of volume, tone, etc.?

    I was younger and dumber when I bought my rig and read about GRAMMA pads on here and bought one, without actually knowing what it did. I am learning a lot on Talkbass these days.

    -David
     
  5. Dec 3, 2007 #5
    billfitzmaurice

    billfitzmaurice Guest Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    It's the board on the front of your cab that the drivers are attached to.

    It mainly prevents sympathic vibrations between the floor and cab. In some cases those vibrations can add undesirable overtones.