  The relationship between acoustic parameters and the microstructure of a
Cu30Zn brass plate subjected to plastic deformation was evaluated. The plate,
previously annealed at 550 {\deg}C for 30 minutes, was cold rolled to
reductions in the 10-70\% range. Using the pulse-echo method, linear ultrasonic
measurements were performed on each of the nine specimens, corresponding to the
nine different reductions, recording the wave times of flight of longitudinal
wave along the thickness axis. Subsequently, acoustic measurements were
performed to determine the nonlinear parameter ($\beta$) through the second
harmonic generation. X-ray diffraction analysis revealed a steady increase and
subsequent saturation of deformation twins at 40\% thickness reduction. At
higher deformations, the microstructure revealed the generation and
proliferation of shear bands, which coincided with a decrease in the twinning
structure and an increase in dislocation density rate. Longitudinal wave
velocity exhibited a 0.9\% decrease at 20\% deformation, followed by a
continuous increase of 2\% beyond this point. These results can be rationalized
as a competition between a proliferation of dislocations, which tends to
decrease the linear sound velocity, and a decrease in average grain size, which
tends to increase it. These variations are in agreement with the values
obtained with XRD, Vickers hardness and metallography measurements. The
nonlinear parameter $\beta$ shows a significant maximum, at the factor of 8
level, at 40\% deformation. This maximum correlates well with a similar
maximum, at a factor of ten level and also at 40\% deformation, of the twinning
fault probability.
