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

["geoff atkins" <geoffatkins@xxxxxxxxxxxxxx>]

Vern wrote to Geoff in reply to the [edited] statement

.>>They comprise pairs of opposed compressive and tensile 
>>stresses on the extreme surfaces which alternate
>>every half cycle. 
>
>How do you know this? 
I guess I picked up some information studying mechanics 
>What is your source of information?
We used books then, "Richards" was a good source.
>How would you measure such waves?
I probably wouldn't, it's a Schroedinger's cat thing.
An item applied to measure would possibly invalidate the result.
>What do you think is the frequency and length of such waves? 

We'll get to that in a minute.

There are many definitions of a wave, this one suits the harmonica 
reed  if "particles" are the composition of the metal.

"wave"
A disturbance, such as a pulse or vibration, which travels 
through an object, volume of matter, or space itself, without 
whatever it is travelling through moving as a bulk whole. 
Instead, the particles which comprise the object or volume 
of matter move individually, one after the other.

"forced wave" 
Any wave which is required to fit irregularities at 
the boundary of a system or satisfy some impressed force 
within the system; the forced wave will not in general be a
characteristic mode of oscillation of the system.

>I posit that such waves [ GA: within the reeds, not above definitions]
> are unlikely because...
>1.  http://en.wikipedia.org/wiki/Wave sez: 
> "a vibration is not necessarily a wave."

But a wave can be a vibration, as the above definition states.

>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.

Absolutely agree with the figures, but here the confusion occurs:
the reed is making the note; as it oscillates, it causes the forces
I described within the reed itself.
There is no resonant buildup , (I don't think that has been claimed),
and neither is there a standing wave (see definition of forced wave).
But the stresses exerted on the particles in the brass will be forced
to the frequency of the oscillation, which is in air. 
There could be latency also.

>3. The reason that reeds don't break at their tips is that 
there is no bending moment there and consequently no stress. 

Sorry, that was an interspersion of levity, my bad habit. :)

Best regards, 
Geoff Atkins