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Guitar feedback INFINITE sustain? Does it really have an infinite sustain?

  1. Mar 25, 2012 #1
    Now we can get many devices that can make guitar sustain longer, even sustaining it indefinitely or to be more precise, sustaining forever. For example, the EBow,(http://www.ebow.com/home.php) the Fernandes Sustainer and the Sustainiac. But I believe, even when the battery power will not getting low, the sustain will decay from times to times but without we noticing it. What's the physics behind this? Does it really sustain the sound forever?
     
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  3. Mar 25, 2012 #2

    rcgldr

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    The feedback is synthesized (except in the case that the speaker is close enough and loud enough to drive the guitar strings). Imagine that it's a synthesizer that samples the original sound, then loops outputing the sampled sound indefinately. It may deliberately degrade the sound to some simple harmonic wave form, but the volume will be sustained indefinately. It would be like holding down a key on a synthesizer, the sound level will not fade unless it's programmed to fade.
     
  4. Mar 25, 2012 #3
    How about in the case if we played a guitar closed enough in front of a very loud speakers to drive the string infinitely? Does it sustain forever?
     
  5. Mar 25, 2012 #4
    No. Somebody would shoot the guitarist and pull the plug. This result was derived in the late 70's.
     
  6. Mar 25, 2012 #5
    Wow, you're incredible.
     
  7. Mar 25, 2012 #6

    rcgldr

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    Unless there's a volume limiter feedback circuit in the amp, the volume will initially increase until it reaches the max that the amplifier can output and you end up with overdriven harmonics of the amplifier circuitry instead of the original guitar sound. This is what happened with those old amps. The "feedback effect" on a synthesizer can reproduce a similar sound but at any volume.
     
  8. Mar 25, 2012 #7
    Hmm, so can an infinite energy exist?
     
  9. Mar 25, 2012 #8

    rcgldr

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    No, the amplifier get's it's energy from an electrical outlet, which gets it's power from some generating station probably hundreds of kilometers away.
     
  10. Mar 25, 2012 #9

    sophiecentaur

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    To maintain the amount of feedback so that the sustain is constant (without building up and going unstable or dying down) is 'knife edge' and not achievable without a trick.
    This trick involves applying some (long term) feedback control to maintain the sound level by controlling the gain carefully. You could do it with the gain knob if you really wanted to for a short while but it would get out of hand.
     
  11. Mar 25, 2012 #10
    Can a human, esp. a very talented singer, etc. sustain his/her vocal indefinitely? (Maybe with a technique?)
     
  12. Mar 26, 2012 #11
    Make a hole in the chest and pump in air?
     
  13. Mar 26, 2012 #12

    sophiecentaur

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    'Circular breathing' can be used to sustain a note on some wind instruments and the bagpipes achieve a sustain with the bag that it topped up with breath or bellows.
     
  14. Mar 26, 2012 #13

    Can we used 'circular breathing' technique for our vocal?
     
  15. Mar 26, 2012 #14

    sophiecentaur

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    Try it.
    I think your vocal chords require air to be moving outwards past them - which rather conflicts with breathing in.
    I can whistle both ways, though, so it's almost sustained.
     
  16. Mar 26, 2012 #15

    rcgldr

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    Not with his/her own voice, but there are these amplifiers and/or synthesizers that output sound through a tube where the other end is directed at the open mouth of the singer, who can then just mouth words (or just make different sounds). I haven't seen this done in a while since microphones can be fed into a synthesizer to get a similar, but not quite as good or versatile effect. Howeve with the sound tube setup, breathing doesn't affect the sound (at least not noticably). An example old song using this device was Peter Frampton's - do you feel like we do - a bit past the half point of the live version song (plenty of youtube versions of this). In this case, a guitar amp fed the sound tube. Funk bands did this also.
     
    Last edited: Mar 26, 2012
  17. Mar 28, 2012 #16

    Guitar feedback is self-limiting. The excursion of the string is limited. The sound reaches a maximum and stays there until someone shuts it off.
     
  18. Mar 28, 2012 #17

    sophiecentaur

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    Makes sense. This must be because, somewhere in that feedback loop, the gain limits. That could be due to the Pickup / String interaction, which is pretty non-linear aamof.
     
  19. Apr 8, 2012 #18
    Oh I see, then after sometime the so-called infinite sustain will be started decaying. Right?
     
  20. Apr 9, 2012 #19

    sophiecentaur

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    I don't think that follows. If gain increases slightly as level falls, the sustain should continue.
     
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