This paper is one of the efforts where the classical passive controls of compressor instability, such as casing treatment, high pressure bleeding, steady jet injection, and so on, are given a fresh look in the context of unsteady flow and system dynamics. Understanding the underlying mechanism that these passive control schemes use to improve the stability will help engineers design compressors that are robust to external disturbances. In this paper, the unsteady tip flows in a high speed, single-stage, and transonic axial compressor are experimentally investigated. The pressure signals obtained from the sensors located on the casing of the rotor are studied with a one-dimensional, continuous wavelet transform. Practical issues are discussed so that one can decode the information in the wave spectrograms correctly. The characteristics of wavelet transform of unsteady traveling signals are studied. Using this wavelet tool, it is found that disturbances in tip flows exist over a thousand rotor revolutions before the onset of stall. They are characterized by the interconnected frequency/scale components in wavelet spectrograms. The time instants of their appearances can also be identified. However, they do not directly lead to stall because the development of such disturbances is intermittent. The link that relates such tip flow disturbances to stall is missing, or more precisely, casing measurement alone is not sufficient for detecting the true cause of stall.

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