Phonons in magnon superfluid and symmetry breaking field

Recent experiments [1, 2] which measured the spectrum of the Goldstone collective mode of coherently precessing state in ${}^3$He-B are discussed using the presentation of the coherent spin precession in terms of the Bose-Einstein condensation of magnons. The mass in the spectrum of the Goldstone boson – phonon in the superfluid magnon liquid – is induced by the symmetry breaking field, which is played by the RF magnetic field.