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Dealing with Resonant Frequencies

Discussion in 'Amps and Cabs [BG]' started by Frankie Fender, Apr 24, 2009.


  1. Frankie Fender

    Frankie Fender Supporting Member

    Jul 13, 2004
    Central Massachusetts
    I was auditioning for a band last night in a new unfamiliar room. I set up my cab (BE15L-D) on the wooden floor of the large room we were in and went to town. I had a new head to break in and really put the rig through its paces. Nothing too loud, just good overall volume and the tone was sweet. During certain songs I noticed a big dropout whenever I played a C, it was most obvious on the A string. I think I found a dead spot of the floor, or is it called the resonant frequency? Anyway, my C on the A string was practically non existant. It was like all the power was being sucked from that note. If I did not know my bass and my equipment I would have thought it was a dead spot on my bass, but no, it was definitely the floor or the room.

    I have experienced some rooms that emphasize certain notes and tend to bloom, but here it was very obvious something was absorbing the power of that frequency. Very weird. It did not matter whether I turned up or boosted that frequency, in fact it was counterproductive. I have a Gramma pad and of course I did not bring it and it *may* have helped - I'm not sure. The problem was that bad. So I just basically ignored it all night.

    My question is this. How do you guys deal with minimizing a dead note when on a gig and what are some of the things you do to mitigate them?

    I have played a lot of different gear and set ups in different clubs, yet still experience this from time to time. I would appreciate your input and discussion on the nature of this phenomenom.

    If only our bandmates knew what kind of crap we bass players contend with in our quest for that elusive tone.:crying:

    I guess that is one of the drawbacks of playing with our ears open.
     
  2. jokerjkny

    jokerjkny

    Jan 19, 2002
    NY / NJ / PHL
    i had the same trouble in certain venues too (mostly bars w/ makeshift "stages"), but bill fitzmaurice explained that sometimes you just hit a node, and the rug gets pulled out from underneath you.

    usually, simply moving around and the note miraculously reappears. other times you have to totally move your rig around.
     
  3. Rick Auricchio

    Rick Auricchio Registered Bass Offender

    There's almost nothing you can do to improve a dead note. With a wolf tone, you can cut EQ to reduce it. But no affordable EQ can deal with a single note, so EQ is often not good enough.

    You can try moving the rig. Or move your listening position. Otherwise, you have to rebuild the room. There is no easy solution for boundary conditions and room modes.
     
  4. Frankie Fender

    Frankie Fender Supporting Member

    Jul 13, 2004
    Central Massachusetts
    Thanks guys, those have been my conclusions too. I do know the sound is coming out of the cab when I stoop down and put my face right in front, it sounds normal, but when I stand up and my listening position changes, Whammo! the sound is gone. Very interesting and strange to behold.

    Is it that the room is tuned to the same frequency and absorbing the energy of the sound cancelling it out? Next time it happens I will try moving the rig around to another spot and see if that helps a little.
     
  5. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    Room interaction produces a peak-and-dip pattern in the bass region at any given location within the room, with some peaks and dips worse than others. Moving the sound source will shift this peak-and-dip pattern (though not eliminate it), so that's probably your best bet if you happen to be set up where there's a strong cancellation notch. Even a small movement of less than a foot can be beneficial in some cases.

    Let's look at just one reflection to get an idea of how this works. Suppose the cabinet is 3 feet out from the wall, and you are standing 2 feet in front of ithe cabinet (so you are 5 feet out from the wall). We have two sound path lengths: From the cabinet directly to you, and from the cabinet to the wall and then bouncing back to you. When the path length difference is equal to 1/2 wavelength (188 Hz in this case), or an odd-number multiple thereof, the wall bounce energy will arrive 180 degrees out-of-phase with the direct energy and you will get cancellation. This happens in other directions as well, and when the cancellations stack atop one another at the same frequency you get a deep notch.

    Let's suppose you're getting cancellation from the ceiling bounce and from two or more nearby wall bounces at the same frequency, so the result is a deep notch. By moving the cabinet diagonally a foot or so, you'll shift the cancellation frequencies due to the wall bounces. Even though the ceiling bounce remains unaffected at least now the wall bounce cancellations aren't all stacked on top of it, nor atop one another. So the cancellations get spread around somewhat, and you don't have that annoying deep notch where you are standing.

    If you are using two cabs, you might try spreading them apart so that each is a different distance from nearby room boundaries as much as is practical. This way, each of your bass sources will be producing a different peak-and-dip pattern at any given location within the room, and the average of the two will be smoother than either one alone would have been. I use a variation on this techinique in a product designed for another market, and it works quite well. The tradeoff is a couple dB reduction in maximum SPL, as the output from your two sources will no longer be combining in-phase.
     
  6. I occasionally experience the same phenomenon. Sometimes I can fix it--or at least improve it--by playing the same note at a different position (five frets up or down) on an adjacent string. I realize that's not always possible due to fingering patterns, speed of the bass part, etc.; but when it is possible, it's worth a try.

    I've also had better success with this technique on 5-strings rather than 4-strings, simply because a 5-string offers more options for shifting the entire part--or at least the troublesome passage--to a different neck position.

    Bluesy Soul :cool:
     
  7. seamonkey

    seamonkey

    Aug 6, 2004
  8. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    seamonkey, in the real world cancellation and reinforcement due to room interaction depend not only on the source location but also on the listener location, so the actual in-room situation is unfortunately a lot more complex that what those charts depict.
     
  9. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    True, but if you start off with the cab front 3 feet from the rear wall, giving a 24dB null at 94 Hz, you're in trouble before you hit a note. Unless it's a boomy room, of course, and then that null can be beneficial. Knowing how distances to boundaries affect your tone and output should be second nature to a bassplayer, since one does have to deal with it at every gig.
     
  10. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    Bill, I'd be surprised by a 24 dB deep global (rather than localized) null caused by the reflection off the rear wall. The frequency and/or depth of the null would change with angle and distance, I would think. And unless other cancellations modes are stacked on top of that wall bounce, I don't see how the null could be so deep. The wall bounce would have to be almost as loud as the direct sound. Of course, I don't have your wealth of real-world experience. Is this something you've observed and measured? If so... very interesting, to say the least!
     
  11. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    The problem is, though, most of the time there's just nothing you can do about it. Most stages lock you into certain positions. You do what you can, I guess.

    Here's a question...at one certain gig I do, when I use my 60's B-15 stack, I always get a cancellation on the open D note, which at 73.4 hz translates according to this cancellation chart somewhere around the 4 ft. mark. Doesn't happen with the SVT. But I put my B-15 a foot away from the curtained back wall at the most. I've tried moving it a little (don't have a lot of room but I have some) and it doesn't seem to affect it. So what would cause that particular cancellation? Could it be the drum riser or the guitarist's amp being a few feet in front of mine and off to the side? We set up with the drums on a 2 ft. riser in the middle, our guitarist's Marshall 210 combo on the stage floor to the left of the front left corner of the riser, and my amp next to his left but in line with the drummer's seat.

    I don't know what the exact distances are, but a 4 ft. distance from either is a possibility. The top speaker clears the amp and riser and hits me good in the back of the head, though, so I would think that it wouldn't be so prone to cancellation from them. Am I wrong?
     
  12. It's not entirely "global", but it will be very wide in the forward direction. Far off-axis, the null won't be there, but neither will you or the audience.

    The wall bounce is only about 6 dB down at 6 feet (two-way distance), so a 24 dB null is not unrealistic. Some say 6 feet is still near-field, so it may not be 6 dB down...
     
  13. HAd SAME experiences MANY times- some rooms are just SHOCKING!!! One I played in a few months ago ANY notte around the Bb area would just BOOM out!!! TWICE as loud as most other notes, & this was ANY note near it- Bb-WORST, B, A, all bad - even up the octave etc..... Tried moving the rig, as much as poss. angling it in a diff. direction.. Did a little better, but still was a VERY noticeable
    B(L)OOM!!!!!! Tried to EQ it out- No go really, t'was a HARD nite! :-(
     
  14. Stumbo

    Stumbo Wherever you go, there you are. Supporting Member Commercial User

    Feb 11, 2008
    the Cali Intergalctic Mind Space
    Song Surgeon slow downer software- full 4 hour demo
  15. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Bump for the speaker geniuses on here who actually sleep at night like they're supposed to, as opposed to me, who barely sleeps at night and shoots his day in the foot because of it ;)
     

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