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If 24" is still good for bass coupling (with floor) what happens at 32"

Discussion in 'Amps and Cabs [BG]' started by Jim C, Apr 25, 2015.


  1. Jim C

    Jim C Is that what you meant to play or is this jazz? Supporting Member

    Nov 29, 2008
    Bethesda, MD
    As per previous posts, the math says that 0 -approx. 24" off the floor has no ill effect on bass response.
    What happens at 32" - 36"?

    Due to a new built in set of books shelves, I don't have as much space in the finished section of the basement and had to stack my 2x12" on top of a spare cabinet. This is much easier to hear than 24" off of the floor with this particular cabinet. Does anyone know the math to determine what happens at 8" - 12" higher than optimum?

    Also without the increased elevation, does playing on the floor (or concrete sidewalk at an art festival tomorrow) vs. a high stage have an effect on bass coupling assuming the stage has decent depth?
    TIA
     
  2. A bass wave of 1/4 wavelength gets reflected from the floor exactly out of phase with the original. It had to travel to the floor and back to catch up ie half a wavelength. Either side of the 1/4 wavelength frequency the effect is less.

    Velocity = frequency x wavelength. Use 1100ft/second, solve frequency for wavelength being 4x(distance in inches)/12.
     
  3. Foz

    Foz

    Jul 26, 2008
    Jax FL USA
    calc the quarter wave... you'll lose coupling at the quarter wave distance = speed of sound in feet per second divided by (distant to coupled surface in feet x 4) - thus 1130 fps divided by 12 [3'x4] = 94 HZ - everything below the quarter wave will be coupled to the subject suraface [provided it is of sufficient size to reflect the wave].

    But keep in mind real spaces are not typically governed by such simplistic models = there are other large flat surfaces often close enough to gain/lose coupling inside the bandwidth of frequencies interest by the moving of the cab which may interfere constructively or destructively with the change in floor loading, as well a number of other acoustic boundary phenomena such as corner loading effects, modal behavior of the room, Speaker Boundary Interference Response, et cetera - some of which can have as much to do with listening position as the altered cab location
     
  4. Foz

    Foz

    Jul 26, 2008
    Jax FL USA
    In short - a lot easier to move the cab and adjust the EQ that pretend you know what will happen for any given listening position.
     
  5. Go back to school Foz. You can't run away from a 1/4 wave boundary cancellation. It follows you everywhere.

    Neither can you fix it with EQ. You'll blow the speaker trying.
     
  6. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    That is assuming that you're playing flat-out through your amp that's rated for more wattage than the speakers. Nobody does that unless they're in a doom band.
     
  7. The blowing up bit, sure, can't be done without the power, but the rest is solid. You can't fix boundary cancellation with EQ, doesn't matter if it is jazz or doom.
     
    wcriley likes this.
  8. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    What you get is cancellations AND summations that repeat at the multiples of ~1/4-wave intervals (and 3/4-wave). The result is called a comb filter with many narrow peaks and valleys.
     
  9. Omega Monkey

    Omega Monkey

    Mar 8, 2015
    Shouldn't it depend more on listening position than cabinet position? Reflections combining with the original wave happens at points, not in the whole space simultaneously. And you'll only get 180 degrees out of phase with a 1/4 wave distance if you measure at the lip of the speaker, right? But nobody listens there.

    In the real world, the sound is going to reach you directly from the speaker, as well as after reflecting off the floor, ceiling, or any other hard surface. Meanwhile, anything that's been reflected by definition has lost energy (ie it's quieter). It's more about a reflected wave being half a wavelength behind the original WHEN IT HITS YOUR EARS.

    So in other words I wouldn't worry too much about it.
     
  10. How many different ways are there to say it? If the path via the floor to you is half a wavelenth longer than the direct path it's directly out of phase.
     
  11. Real world test for the disbelievers. Pull your cab out from a long wall so the front is 3 1/2 ft from it. Stand at the end of your lead and tell me your low E or F didn't lose oomph.
     

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