[Harp-L] Overblow goes UP in pitch - WHY? (was: Bending Stress in reeds (was Manji Reed Quality))



In the interest of "morphing" this topic (and because I'm "crazy"), I'm going to 
hazard a "guess" for the reason that overblown notes go up almost a halftone.
(As my older brother says, "Where angels fear to tread, there go the Cobles!")

Short answer: it is the difference between the action of a closing reed (normal)
and opening reed (overblow).

Long answer (probably full of hot air): When a reed is sounding in the "normal"
direction (i.e., closing), it moves from one side of the reed plate through the slot
and out a certain distance on the opposite side of the reed plate. Let's agree to
refer to that as Sound NORMAL. It doesn't matter whether it is blown or drawn;
the process that the reed goes through is the same. The size (especially length)
and shape of the reed is optimized to produce a specific pitch when producing
Sound NORMAL. The breath direction pushes the reed into the slot, and the
nature of the metal causes the reed to return toward its "at rest" position. But
the breath then stops it (before it gets to the "full rest" position) and forces it
back through the reed slot again, repeatedly.

On the other hand, when a reed is overblown (i.e., opening), it moves AWAY
from the reed plate under breath pressure and the nature of the reed causes it
to rebound (for want of a better term) back to the "at rest" position and then
overshoot through the reed slot to the other side. The amount of time spent
rebounding through the reed slot and then being forced back through the slot 
causes Sound ABNORMAL to be higher in pitch (by almost a halftone) than 
Sound NORMAL.

Both sounds are produced by the traversal of the tip of the reed past the reed
plate; nothing else. Sound NORMAL has the breath energy forcing the reed
through the slot and metal tension causing it to return toward the "rest" position. 
Sound ABNORMAL has the breath energy forcing the reed away from the slot
and the metal tension causing it to return through the slot. My conjecture is that
this difference in range of travel is what causes the overblow (Sound ABNORMAL) 
to be higher in pitch than the normal sound of the reed (Sound NORMAL).

Let's pick arbitrary values for the sweep of the reed through the slot. Imagine
(like I'm doing) that Sound NORMAL oscillates (from the "at rest" position to the
other extreme on the other side of the reed slot) a total distance of 1 cm. My
hypothesis is that the total distance when producing Sound ABNORMAL is less
than this; let's guess at approximately 0.94cm. Since the distance traveled when
traversing the reed slot is slightly less for Sound ABNORMAL, I speculate that this 
produces a sound which is almost (but not quite) a half tone higher in pitch. The 
same energy is going into the reed but the distance traveled is not the same, 
causing the difference. Since the distance traveled while traversing the reed slot
is shorter for Sound ABNORMAL, this produces a higher pitch.

I eagerly await a(ny) correction on my conjecture from someone (anyone!) who
really knows what is happening!

Crazy (and yes, I know I'm also full of --it!) Bob
 		 	   		  


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