  Low temperature studies of the behavior of the sound velocity and attenuation
of acoustic modes have been performed on a single crystal NdFe_3(BO_3)_4.
Transitions of the magnetic subsystem to the antiferromagnetically ordered
state at T_N \approx 30.6 K have been revealed in the temperature behavior of
the elastic characteristics. The features in the temperature behavior of
elastic characteristics of the neodymium ferroborate and its behavior in the
external magnetic field, applied in the basic plane of the crystal, permit us
to suppose that the transition to an incommensurate spiral phase is realized in
the system. This phase transition behaves as the first order one. H-T phase
diagrams for the cases H \parallel a and H \parallel b have been constructed.
The phenomenological theory, which explains observed features, has been
developed.
