The speed of sound in conventional linear elasticity is determined only by
the elastic modulus and the density of the medium. In actuality, however, t
he speed of sound depends on the stress and this dependency becomes nonline
ar as the stress increases. This paper explains such phenomena by introduci
ng the nonlinear elastic modulus. Additionally, the relationship between no
nlinear elastic modulus up to the fourth-order and the internal stress is d
iscussed through computer simulations and experiments for an aluminum speci
men. In the simulation, it is shown that the third-order elastic constant c
ontributes to the slope of the sound speed vs stress curve and the fourth-o
rder one determines the curvature. Experimental results shows good agreemen
t with the expected result and the ratio of third- and fourth-order elastic
constants present significant changes in magnitude and with sign inversion
after the internal stress, becomes larger than the yielding stress. These
results show that the measurement of nonlinear elastic constants may enable
internal stress evaluation.