In the last decades, the high pressure sodium (HPS) lamp has been developed to be supplied in high frequency in order to increase the lighting efficiency of the system lamp/ballast. However, at some given frequencies, standing acoustic waves, namely acoustic resonance (AR), might develop in the burner and cause lamp luminous fluctuation, extinction and destruction in the most serious case. Two main characteristics of the acoustic resonances in a 150 W HPS lamp are presented in this paper to seek for a control method to avoid the lamp AR. The first one is the characteristic of the lamp AR threshold power, and the last one is the differences between forward and backward frequency scanning effects. Both characteristics allow us to further understand the AR and to better control the lamp.