Re: [Harp-L] Re: Are different overtones available by changing reedshape?

["Vern Smith" <jevern@xxxxxxx>]

The basic sound spectrum of the vibrating reed arises from the non-linear 
variation of flow area as the reed swings through the slot.  The variation 
of area as a function of displacement is gradual near the extremes but very 
rapid as  the reed enters the slot. Also, the speed of the reed tip is 
increasing as it approaches the slot. There can be a "mirror image" as the 
reed exits the other side of the plate and on the return trip. This extreme 
non-linearity produces a lot of sound energy in overtones.  The shape (e.g. 
rounded tip) can be expected to have only a secondary effect.

The static pressure on the reed is not what excites the vibration.  If there 
were not another effect, the reed would deflect intil the spring tension 
balanced the static pressure and it would just stay there. These forces are 
in phase with reed displacement and thus cannot cause vibration.

The forces that cause vibration must be out of phase with reed displacement. 
They arise from the lift (as in an airplane wing) of flow along the under 
surface of the reed.  That is why a gap is necessary to start a reed 
vibrating...it allows initial airflow along the under surface.   This 
airflow urges the reed toward the plate at both extremes of the reed 
displacement but is shut off when the reed is in the slot.   In contrast, 
the static pressure remains constant.  It works like pushing a child in a 
swing.  You give a pulse of force  toward the bottom of the arc when the 
swing is past max displacement.  Then you release it for the rest of the 
cycle.
If you act like static pressure and follow the swing, pushing in the same 
direction, You wind up stopped with the child above your head.

The resonance of your embouchure is more important than reed shape. 
Jonathan points out that breathing through your lips on the reedplate can 
produce a full-bodied and loud harmonica tone.  This illustrates that the 
function of the comb and covers is only to protect the reeds and direct your 
breath to the selected reed.  For this reason, the comb and covers have 
hardly any effect on the sound.

With only minor exceptions, "A reed is a reed is a reed!"

Vern
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