return tau exponent "c" for noise type.
            MVAR = prefactor * h_a * tau^c

def c_mvar(self):
        """ return tau exponent "c" for noise type.
            MVAR = prefactor * h_a * tau^c
        """
        if self.b == -4:
            return 1.0
        elif self.b == -3:
            return 0.0
        elif self.b == -2:
            return -1.0
        elif self.b == -1:
            return -2.0
        elif self.b == 0:
            return -3.0