Re: [Harp-L] Re: Re: Yellow Brass (was GM) (Vern) (geoff atkins)

["Doug" <mr4chnt@xxxxxxxxx>]

Hi Vern,

I have a rusty background in math and science (and a bit of engineering).  It's just enough for me to appreciate the straight-up-no-BS analysis you apply.  You're a great asset to the list.

I have question for you and the list regarding overtones, but I'll put it in a new thread.

Doug H

  ----- Original Message ----- 
  From: Vern 
  To: geoff atkins 
  Cc: harp-l@xxxxxxxxxx 
  Sent: Wednesday, March 10, 2010 6:49 PM
  Subject: Re: [Harp-L] Re: Re: Yellow Brass (was GM) (Vern) (geoff atkins)



  On Mar 10, 2010, at 7:48 AM, geoff atkins wrote:
  > 
  > Waves within reeds
  > 
  > Waves within the reed are generated as reactions to the oscillation of the
  > reed.
  > The "waves" within the reeds progress axially to the reed length, increasing
  > in magnitude as they approach the point of fixity (that's why reeds don't
  > usually break at the extreme tips). They comprise pairs of opposed
  > compressive and tensile stresses on the extreme surfaces which alternate
  > every half cycle. (i.e. like stretching a rubber cord *along* its length
  > then letting go).

  How do you know this? 
  What is your source of information?
  How would you measure such waves?
  What do you think is  the frequency and length of such waves? 

  I posit that such waves are unlikely because...
  1.  http://en.wikipedia.org/wiki/Wave sez: "a vibration is not necessarily a wave."
  2. The speed of sound in brass is 11,400 ft/sec, about ten times the speed of sound in air.   That would make the wavelength of A 440  =  11400 / 440  = 26 ft.  or 312 inches.  Thus, the length of an      "A" reed at  0.6 inches is only .002 wavelengths.  It follows that there will be no resonant buildup of energy in longitudinal standing waves.
  3. The reason that reeds don't break at their tips is that there is no bending moment there and consequently no stress. I has nothing to do with waves within the reed.
  4. Longitudinal waves within reeds isn't necessary to explain stress resulting from transverse reed vibration, stress concentrations at abrupt changes in cross section, metal fatigue, or reed failure.  

  The forces of compression and tension lessen and equate to zero at the
  > neutral axis, (unless the scoring locally changes the NA position)
  > They don't make the sound, they strain the metal.

  Yes, but that results from transverse vibrations, not waves.
  > 
  > The root of the milling is the weakness where the problem begins:
  > compressive wave cycle increases forces on the metal crystalline structure.
  > tensile forces tend to open the interstices of the crystals, eventually
  > forming a gap which we to our dismay experience as a cracked reed.

  Yes, fatigue usually occurs at a point of maximum stress which can be at an abrupt change of cross-section.
  > 
  > Sorry I thought it was clear the first time.

  The only thing that wasn't clear was the existence of waves in the reed.
  > 
  > In design, the filet is the curved shape of the junction of two planes,
  > irrespective of whether it is achieved by milling or by a "filet"
  > ("fill-it") weld.
  > (The latter is actually nearer an internal chamfer).
  > 
  > I also enjoy playing the harp.

  Me too.  ;o)

  Vern






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