Saxophone Reed's vibrations

Discussion in 'General Acoustics Discussion' started by jbtsax, Dec 29, 2015.

  1. jbtsax

    jbtsax Distinguished Member Distinguished Member

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    It is well established in the acoustics literature that the resonance of the body tube to the first opening causes the reed to vibrate at that frequency. This raises the question of whether the reed is also vibrating at the frequencies of the harmonics of that tone as well, or whether those frequencies are generated past the reed by the interaction of the mouthpiece and the standing wave itself.
     
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  2. Mojo

    Mojo

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    Videos of reed motion confirm that it's motion is not sinusoidal. So it has other frequencies present in the motion. Harmonics and non-harmonics. I would expect it to be the same ones as seen in a frequency spectrum. The mouthpiece shape changes the frequency spectrum.
     
  3. jbtsax

    jbtsax Distinguished Member Distinguished Member

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    Thanks for that information. Is there any literature out there that addresses how the interior of the mouthpiece affects the harmonics present in the sound?
     
  4. Mojo

    Mojo

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    You probably have seen what I have seen. The Wyman thesis is the best empirical study I have seen. The Scavone thesis has a look at short fat mp vs a long thin mp. But it does not get into real mouthpiece shape comparisons.

    The Ferron book, The Sax is My Voice, has his thoughts on what a mouthpiece baffle does. It has some sound vector diagrams in it that I thought were cool until a discussion on the web convinced me they were one man's fantasy and not based on science.
     
  5. kymarto

    kymarto Content Expert/Moderator Staff Member CE/Moderator

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    It is one man's fantasy. The wave does not bounce from wall to wall. It is a compression wave.
     
  6. kymarto

    kymarto Content Expert/Moderator Staff Member CE/Moderator

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    The way I understand it is this: any perturbation in the cone changes the impedances in the tube and throws the partials off from their integer relationships to the fundamental. Since any sounding note MUST have the partials in integer relationship (mode locking) what happens is that what Benade calls the "regime of oscillation" is formed in which all the partials find their relationship to form a mode-locked note with all of them lined up mathematically. Each get pulled off their natural impedance frequency to some extent, depending on their relative strength, and lose energy the farther off their natural frequency they are. This all has to do with wave timing in the tube.

    A perturbation (deviation in ideal shape) lower in the tube will affect lower partials; higher in the tube affects higher partials (read Scavone). The big perturbation in the ideal shape of the tube near the top of the cone is the mouthpiece, which has nothing like the ideal conical shape of the rest of the tube. This will not affect the lowest partials too much, but it will definitely yank the higher partials out of their integer relationship with the fundamental, and will thus weaken them. The farther from a conic shape the mouthpiece is, the more the upper partials will be weakened. Scavone thinks this is why a short fat mouthpiece plays darker than a long thinner one. The actual modeling is very complex and not easily done, which is why Scavone doesn't get into it.

    For a good paper on the complexities of what happens in a single reed mpc read this:

    https://hal.archives-ouvertes.fr/jpa-00252796/document


    Benade has written a clear explanation of the function of the baffle, and it has to do with Bernoulli forces on the reed as it closes. By increasing Bernoulli force on the almost closed reed, a high baffle forces the reed to close more quickly at the end of the cycle, which has the effect of clipping the signal, so to speak, which attenuates the lower partials.

    Just found this--I'll have to read it more closely, but it might be of interest:

    http://viennatalk2015.mdw.ac.at/proceedings/ViennaTalk2015_submission_27.pdf
     
  7. Helen

    Helen Content Expert Saxophones Staff Member Administrator

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    Hey there, a kymarto sighting! Nice to have you back. I was wondering where you'd wandered off to. ;) I'm glad we didn't have a search team off to look for you. :)
     
  8. jbtsax

    jbtsax Distinguished Member Distinguished Member

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    A bit of the scent of acoustics draws him back every time. :) Ditto to what Helen said. Nice to have you back.
     
  9. kymarto

    kymarto Content Expert/Moderator Staff Member CE/Moderator

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    Been sorta busy--since October I have traveled all over the place filming. As to this acoustics business: the reed vibrates at the frequency of the fundamental. It is a non-linear generator that gets its cue from the strongest tube impedance, generally speaking. Partials are potentials in the tube, so to speak. They are affected by the reed timing, but they are not generated by the reed.
     

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