Abstract
The linearized hydrodynamics of 3He-A in high magnetic fields is derived from broken symmetries, conservation laws and thermodynamics. Only the transverse part (with respect to the magnetic field) of the spin order parameter is treated as a hydrodynamic variable in a strict sense. A field dependence is found of first sound and a coupling of the longitudinal spin waves (longitudinal NMR resonance) to second and fourth sound. The magnetic dipole-dipole interaction influences the orbit 'wave' spectrum, one branch of which is definitely damped for k=0. The transverse NMR shift remains field-independent, but is no longer equal to the longitudinal NMR.