In this paper, we present and discuss some phenomena in an undergraduate experiment for the measurement of the speed of sound in air. A square wave distorts when connected to a piezoelectric transducer. Moreover, the amplitude of the receiving signal varies with the driving frequency. Comparing with the Gibbs phenomenon, these phenomena can be explained by the theory of Fourier series. In addition, we also measure the characteristics of the piezoelectric transducer and numerically simulate the experimental results. (Contains 2 tables and 8 figures.)