  When ultrasonic wave is irradiated on materials, a small static stress is
required to get materials yielding and flowing. This is called acoustic
softening effect, also known as Blaha effect for a long time. In the past, this
effect was explained by several continuum scale or meso-scale solid mechanics
theories such as stress superposition or energy superposition theory, or
crystal/dislocation plasticity. Due to a lot of microscopic complexities
happening inside the materials during ultrasonic vibration, fully understanding
of acoustic softening effect is not easy. In this paper, traditional solid
mechanics theory is expanded by introducing several concepts in semi-conductor
physics. Four new aspects were introduced to understand acoustic softening
effect. Firstly, contrary to most existed work in acoustic softening research
area which treats ultrasound as waves, it was treated as particles. Secondly,
crystal/dislocation plastic theory was simplified to a single equation.
Thirdly, concepts of photoelectric effect or photo-voltaic effect were
introduced. Analogy of electron movement due to light wave and defect movement
due to ultrasonic wave was illustrated. Particularly, as light wave is treated
as photon, ultrasonic wave is treated as phonon in this paper. Fourthly,
defects such as point defects or line defects are assumed to have certain
bonding energy. Their bonding energies are assumed to be quantized or
discontinuous. The band gap theory used in photo-voltaic theory is embraced to
understand defects movements in solid mechanics due to ultrasonic phonon.
