This paper introduces a non-classical nonlinear acoustic theory for microcrack detection in materials, comparing contact nonlinearity with material nonlinearity. The paper’s main work concentrates on the experimental and numerical verification of the effectivity of contact nonlinear acoustic detection by using the contact nonlinear parameter β′, which can be represented by the ratio of the second-harmonic amplitude to the square of the first-harmonic amplitude. Both experiments and numerical tests are performed. The results show that β′ is sensitive to the initiation of microcracks and varies with the development of the microcracks. The numerical test illustrates the decline of β′ when microcracks penetrate each other.