Abstract: For the study of detecting sonar signals with complex time-frequency compositions under a noise-background by human auditory system, the problem of auditory response to the transition between signal frequencies has been examined through a number of tests. According to the results of measurement within the frequency region of 850-1130 Hz, an equation (τΔF=1-2) to define the ambiguity region of responding to frequency transitions by human anditory system is found, where ΔF and τ are the difference and duration of the transition between two frequencies, respectively. When τ·ΔF≤1, the human auditory system hardly can sense the transition between two frequencies, however when τ·ΔF≥2,not only can clearly sense the transition but the diffrent tonalities of two frequencies as well.