dynamic spanner help

[James E. Bailey]

I've not understood this for a while, and I figure I should probably  
ask about it. When I have dynamics in a variable, the text spanner  
shows up and I can't get rid of it.
\version "2.11.51"
\layout {
   \context {
      \type "Engraver_group"
      \name VocalDynamics
      \alias Voice
      \consists Text_engraver
      \consists "Dynamic_engraver"
      \consists "Text_engraver"
      \override TextScript #'font-shape = #'italic
   }
   \context {\Staff \accepts VocalDynamics}
}
Voice = \relative { e1 f g e a g f d }
VoiceDynamics = \new VocalDynamics {
   \override DynamicTextSpanner #'dash-period = #-1.0
   \dimTextDim
   s1*3\> s1\!
   \crescTextCresc
   s1*3\< s1\!
   \setHairpinDim
}

\score {
   \new Staff << \VoiceDynamics \Voice>>
   \layout {}
}

\relative c'' {
       \override DynamicTextSpanner #'dash-period = #-1.0
       \crescTextCresc
       c1\< | d | b | c\!
}