[en] The Raman and infrared absorption spectra (up to 30 cm^-1) of β-Li Yb(WO₄)₂₉ and a spectrum of an isotope-substituted, with respect to lithium, sample have been measured. Frequencies and shapes of normal vibration of two non-equivalent crystal directions have been calculated in a polymer approximation. The system of force constants has been determined. The frequency branches of the vibrations have been calculated when a phase vector changes from S=1 to 100 and atoms displacements. The calculation results have shown that a considerable contribution to the vibrations of end W-O bonds (the region of 900-930 cm^-1) is made by the coordinates of the n-O bonds (where M is Li or Yb) elongation and by deformation of the WOM and OMO angles