Yow-yow-yowing bass strings

[Robert Scott]

Here is my guess.  The Yow-yow-yowing is a false beat at a high 
partial.  As the partial goes through its false beat, the apparent balance 
between the partials seems to change, thus giving the impression of 
changing voicing, or yow-yow-yow.

So what can cause a false beat at a high partial?  I suspect that it is a 
difference between vertical inharmonicity and horizontal 
inharmonicity.  Remember that inharmonicity is caused by the stiffness of 
the string - its resistance to bending.  But suppose that a string has more 
resistance to bending in, say, the vertical direction than it does in the 
horizontal direction?  Now recall that when a string is struck, it begins 
precessing into an oval-shaped pattern, moving in both the up/down 
direction and in the side-to-side direction.  If the string has a different 
amount of stiffness depending on the direction of bending, then the effect 
of inharmonicity will produce a different resonant frequency in each of 
these directions.  That causes the string to vibrate at a slightly 
different rate up and down than it does side to side.  These two rates of 
vibration occur simultaneously and beat against each other, giving the 
Yow-yow-yowing.

Let's speculate even further about what might cause a difference in 
stiffness in the horizontal and the vertical bending direction - especially 
in bass strings.  Some bass strings are wound on hexagonal cores.  If the 
machine that makes the hexagonal core is imperfect, then the hexagon might 
come out flattened a little in one direction.  That would certainly make it 
easier to bend in the flat direction and harder to bend in the 
perpendicular direction, which would lead to yow-yow-yowing.  And even if 
the core is just a round string, it may be that somewhere in the winding 
machine, the core string goes over a hard surface that crushes the 
cross-section a little, making it oval instead of round.  That, too, would 
lead to a difference in stiffness that depends on the direction of 
bending.  Finally there is the copper winding itself.  It contributes only 
a little to the overall stiffness of the bass string, but it does 
contribute some.  Suppose the winding machine were mis-adjusted so that the 
copper wrappings were pressed tightly against each other on two opposing 
sides of the cross-section, but were a little less tight in the directions 
that are perpendicular to the tight directions.  This could give the 
completed string a difference in stiffness in one direction vs another.  So 
my guess is that it is the fault of the machine that makes the bass strings.

-Bob Scott
  Ypsilanti, Michigan