Re: [Harp-L] Progression

[Vern <jevern@xxxxxxx>]

On Oct 7, 2013, at 12:30 PM, Winslow Yerxa <winslowyerxa@xxxxxxxxx> wrote:

> I'm doubtful that the air direction has much to do with how good the note sounds.

Fluid flow theory supports your doubt.   At the pressure ratios very close to 1 that occur in a harmonica, the air velocities are very low.  With low velocity and a small passage, the Reynolds number (the ratio of inertial to viscous forces) is very low.  The direction of the inertial forces would affect the pressure at the reed slot only if the entering velocity were very high.  

IF you hold a harmonica out of the window of a car, it will start to sound at about 25 or 30 mph.  This is very low compared to the speed of sound which is about 750 mph. 

See http://en.wikipedia.org/wiki/Reynolds_number 

Air molecules in a reed chamber are like people in a crowded room.  Every time a few people enter via the mouthpiece, the crowding gets worse and a few people leave via the reed slot.  Because the speed of the people entering is low, their direction doesn't affect the direction or number of people leaving. The people leaving don't really know which direction the entrants came from, they just know that it is more crowded and take the path of least resistance out.

Vern