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         Chladni Patterns

Mandola:  Top is Carpathian Spruce and the back is Tasmanian Oak.

A few things to note.  Note the changes in frequencies after the top is braced, particularly mode 2.  Mode 2 jumps way up in frequency after bracing because it stiffens the top where the maximum movement occurs (i.e. it crosses the nodal lines).   The X brace follows the nodal lines of mode 1, so the increase in frequency is relatively small.  Mode 2 is usually close in frequency to mode 1 in an unbraced top or back, and can be higher in frequency or lower than mode 1, depending on each individual piece of wood.  Mode 2 is often difficult to measure if it is very close to mode 1 (commonly observed).  Mode 4 is often referred to as the "ring" mode in other instruments, and is the most important mode to observe.  Note that mode 4 of the top and back have been closely matched.

 

 

Top mode 1, 162Hz

Top mode2, 151 Hz

 

Top mode 3, 254Hz

 

              Top mode 4, 360Hz

 

Top braced, mode 1, 175Hz

 

 

   Top braced, mode 2, 246Hz

 

 

Top braced, mode 3, 283Hz

 

 

Top braced, mode 4, 383Hz

 

 

Back mode1, 188Hz

 

 

Back mode2, 185Hz

 

 

Back mode 3, 290Hz

 

Back mode 4, 382Hz

 

Mode 4 after the ribs have been glued to the top , 612Hz.  What I am looking for here is a ring pattern and the saw dust to jump up high where the bridge will be.