## 1 Introduction  
Home appliances have a significant influence on the living conditions of consumers. The vibration of home appliances can cause interior noise in the home and make people uncomfortable. Consumers need to remove the irritating feeling by reducing such noise and vibration. Although air conditioner noise is low level, there are still a lot of complaints from consumers about the noise, particularly from indoor units of air conditioners, which affects their purchasing decisions. Air conditioner noise is known to be generated mainly due to the refrigerant flowing in air conditioning systems. In an air conditioner, the refrigerant is a medium for conveying heat. For heat transfer, the refrigerant repeats evaporation, compression, condensation and expansion, during which two-phase flow occurs due to nonlinear phase changes of the refrigerant. As a result, the flow pattern varies and a wide frequency range of noise is generated. In general, research on refrigerants has been mainly done from a thermodynamic perspective [1; 2]. [PERSON] and [PERSON] classified two-phase flows into some regimes: slug flow, churn flow and annular flow [3; 4]. The study of air conditioning refrigerant noise mainly focuses on theoretical and experimental identification of noise from pipes. However, the characteristics of refrigerant flow noise from air conditioners have not been identified, and there has been no method suggested for classification of the noise. [PERSON] et al. identified the noise sources on a residential split-system air conditioner using sound intensity measurements. However, this was not the refrigerant flow noise, but rather was the inlet and exhaust noise [5]. [PERSON] et al. conducted an experimental and numerical study on axial flow fan noise of a room air conditioner [6]. Therefore, the study of the identification of air conditioning refrigerant noise is needed. To do this, refrigerant noise should first be classified. In general, a jury test, used to classify the noise, or sound quality index through regression method, is widely used [7]. [PERSON] et al. conducted sound quality evaluation of air conditioner noise based on autocorrelation [8], and [PERSON] et al. characterized the sound quality of air conditioning noise [9]. [PERSON] et al. developed a technique for evaluating the sound quality of an air conditioner [10]. However, the results of evaluations using this method can vary according to different listeners and it is difficult to identify when the noises occur. Therefore, refrigerant flow noise needs to be accurately classified for the reduction of such noise. In order to achieve this purpose, the start/end time and frequency characteristics of air conditioner refrigerant flow noise should be investigated and identified to ensure accurate classification of the refrigerant flow noise. In identifying the start/end time of the flow noise--the characteristics of which vary with time--it is important to find singularities in which changes occur due to the generation of refrigerant flow noise in the signal. Because the refrigerant flow noises are abnormal signals, it is possible to detect the time of occurrence by using the continuous wavelet transform. The continuous wavelet transform (CWT) is widely used to determine when rapid changes occur in frequency characteristics or when signal singularities appear. Signal singularities are the points in which the stead -state changes to other states. The CWT is also used to detect abnormal signals generated by earthquakes, shock or engine acceleration or deceleration [11; 12; 13; 14]. In other words, abnormal signals are detected with large changes in the CWT coefficient at signal singularities. The frequency characteristics of abnormal signals can be analyzed by using a time-frequency analysis method, such as short-time Fourier transform (STFT) or CWT. STFT has a fixed window size and a low frequency resolution which depends on low- and high-frequency domains.