Shock waves

Discussion in 'Trumpet Discussion' started by gordonfurr1, Jul 23, 2015.

  1. gordonfurr1

    gordonfurr1 Forte User

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    I realize this is old news to many of you, but I still found it interesting...Visualization of shock waves emanating from a trumpet bell interacting with various materials and angles.

    https://www.youtube.com/watch?v=adJEumpIN9M
     
  2. Michael T. Doublec

    Michael T. Doublec Pianissimo User

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    Super cool video. Thanks for the post and link!
     
  3. Sethoflagos

    Sethoflagos Utimate User

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    I love this sort of stuff! Really helps visualise what is going on in the normally invisible world.

    But I do wish they didn't use the term 'shock wave'. I know hyperbole is to be expected these days, and they want to grab the audience's attention. But the images are just normal pressure wave pulses. True shock waves would pass through solid objects in their path.
     
  4. rowuk

    rowuk Moderator Staff Member

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    Actually, they do. The standing wave in the horn does not only leak out of front of the bell, it also leaks through the brass in the bell. That "resonance" helps to give the player important feedback. A lightweight trumpet outdoors helps alot because an appreciable amount of energy leaks making life easier in a difficult acoustic.

     
  5. gordonfurr1

    gordonfurr1 Forte User

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    I would like to see a similar test with the emanations coming from the surfaces of the horn...that the player would hear....especially one that is known for great feedback (say, a Jaeger) and another that seems to provide very little feedback.
    Then, same but with various mute designs, or various techniques for potentially ADDING a feedback reflex to the player.
     
  6. Sethoflagos

    Sethoflagos Utimate User

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    Wondered if you'd call me out on that one! ;-)

    Okay. In normal pressure waves, the pressure varies smoothly like riding a bicycle over a hump-backed bridge; whereas a shock wave features a sudden pressure discontinuity like riding a bicycle into into the side of a house.

    Some pretty pictures of bell vibrations in the pdf downloadable from [url="http://iwk.mdw.ac.at/lit_db_iwk/auswahl_anzeigen_detail.php?id_string=16267&lan=eng&[email protected]]here[/url].
     
  7. rowuk

    rowuk Moderator Staff Member

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    And I was anxiously awaiting YOUR response. I think we have been here once before in a discussion of transversal vs longitudinal wave propagation....... You most certainly are far more knowledgeable in this respect.

    In this case, I am somewhere else. The lips open and close like a switch, feeding the resonant system with puffs of air (could this be defined as a "pulse" or "shock"?). A standing wave is set up in the horn and is repeatedly interrupted by our articulation. This standing wave is no longer puffs or pulses, it has a more or less steady state waveform.

    What I am referring to however is the "color" of the bell resonance which interestingly enough has its own resonant modes and does not simply pass most frequencies played. The coloration can be heard by "pinging" the bell. The bell is relatively stiff compared to the acoustic power for the frequencies emitted, so there has to be some kind of process that causes the bell ringing/passing energy while playing. My hunch is that it has something to do with the fundamental that we don't really hear because the bell is too small to couple those frequencies with any type of efficiency to the air. Maybe we do have something like a pulse?

    The bicycle analogy is very good except one thing is missing. It only works up to mezzoforte. As we play more loudly, the waveform distorts and we have more harmonics compared to the fundamental. As this occurs acoustically without changing the played pitch, it must be pressure based. The more that we cap a sine wave (for instance), the closer it gets to a square wave (with infinite harmonics). Could it be that the louder that we play, the closer the resonant system becomes to producing pulsed waves?


     
  8. Sethoflagos

    Sethoflagos Utimate User

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    Oh Gosh! Well here goes.......

    Shock waves in gases at least, are phenomena associated with ultrasonic speeds, such as aircraft or other projectiles exceeding mach 1, detonations, steam pressure relief valves etc. The energy cannot escape forwards as it's already exceeding sonic velocity and tends to concentrate on the leading face of what we might call a blast front.

    I don't think we ever get there with trumpets. Potentially close in the backbore when playing really loud but I don't believe we're physically capable of generating the necessary pressure to go 'supercritical'.

    The switch analogy, as with all analogies, is useful to a point. But we cannot generate a true square wave or anything else with a vertical gradient as this would require infinite acceleration of air and aperture, and hence infinite energy. Depending on the scaling of the graph paper, we can seem to get quite close to vertical, but this deceptive. Stuff is happening very quickly on a human time scale and gives the illusion of being instantaneous. But compared to say the quantum world, all the changes would be seen as slow, gradual and ordered.

    When we do detailed analysis of the behaviour of pressure waves in long liquid or gas pipelines, we usually have to abandon the simple 'rigid pipe theory' and make allowance for that fact that the pipe wall will flex in response to the pressure fluctuations going on inside it. This makes the mathematics a lot more 'interesting' and yields some points relevant to us as trumpet players.

    In thin walled pipe in particular, quite a high proportion of the energy flow is travelling in the pipe metal, rather than though the pipe contents. And because the speed of sound in metals is generally much higher than most fluids (especially low pressure gases) the flexing of the wall is transmitted ahead of the initiating pulse, both speeding up the pressure wave and making detectable alterations to the shape of the waveform. The position of pipeline anchoring plays a critical role in either damping or amplifying particular frequencies (obvious parallel to bracing) as does the amount of concrete in the anchor foundations (heavy mouthpieces, caps etc).

    One irony here is that the flexing of the pipe induces some additional transverse flow and corresponding pressure waves in the fluid. You could, I suppose argue that these were 'transverse waves' ;-). I'd prefer to think of them subsidiary longitudinal waves in a perpendicular axis. Doesn't make my head hurt so much. (Have we roasted that old chestnut now?)

    The bicycle analogy still holds I think, though the road is a little bumpier.

    A while back I was looking at how the total waveform developed from the spectra I was getting from Audacity. One big proviso here: I took the simplistic assumption that all harmonics were in phase. That being said, even with the first couple of harmonics, the waveform peaks were becoming sharper and moving closer to the nodes. One typical pattern I was seeing was a steady attenuation of say -3dB per harmonic. Graphed out over 30 harmonics I was seeing transformations like this:

    [​IMG]

    So if my assumptions were true, it would seem that richer, louder tones can tend towards a unit pulse with relatively little happening between the pulses.
     
  9. gordonfurr1

    gordonfurr1 Forte User

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    FASCINATING discussion.
    I'm rereading several times to help my tired brain absorb, for it is information like this that, once set securely into the back of the brain, is secretly cogitated upon and springs back out at a later time...chiming...in a more focused pulse (if you will) fostering a creative event.
    I love the explanations, very much helps visualize what actually might be occurring hidden inside those tubes.
    Thank you.
    This chestnut is now more roasted.
     
  10. redintheface

    redintheface Pianissimo User

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