What is usually the reason for a mpc having a bit of a dirty sound...is 
it in the baffle work?  Its sort of an edgy dirty sound...something 
operating on top or along side the edge.

This can be from a number of sources depending on the sound.  I have not
done any structured A vs B testing so these are just my impressions.  

Sharp edges in the chamber/baffle contribute.  This can be molding or
casting seams.  Also, a sharp wedge baffle.  Bullets seem less "dirty".

A blunt wall at the base of the window "U".

A flat section in the facing.  The reed slaps it like a beaver tail on
water.

More open tips and/or resistant facings require you to use softer reeds. 
Some players hear the additional reed buzz as this "dirty" sound.

If the facing flips up a lot at the tip, there will ba a significant airy
sound.  This can happen with sharp edges too.


		
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