23 rd International Congress on Sound & Vibration Athens, Greece 10-14 July 2016 ICSV23 WASHING MACHINE SOUND QUALITY Ercan Altinsoy Technische Universitaet Dresden, Chair of Acoustic & Haptic Engineering, 01062, Dresden, Germany email: ercan.altinsoy@tu-dresden.de Metin Gül and Aleks Kuyumcuoglu Arcelik A.S., Cayirova, Istanbul, Turkey Sound quality gains importance in our daily life. One of the important household appliances which has a long operational duration is washing machine. The washing machine sounds have mostly instationary character. Two main operational stages, which are washing and spin, cause totally different sounds. Although, the sound of the washing part is relatively quiet, machines emits very loud sounds during the spin cycle. In this study, we concentrated on front-loading washing machines. The aims of this study are to evaluate the perceived overall quality of various washing machine sounds and to identify the perceptually important features of the washing machine sound which are relevant to the quality impression. To systematically approach to these aims, first the washing machine sounds were recorded and the recorded stimuli were modified via digital filtering in frequency and time domains. Then the recorded signals were analyzed and their psychoacoustical metrics were calculated, in parallel a listening test to evaluate the sound quality was conducted. The results of the psychoacoustic analyses and the jury test were correlated and a sound quality index was developed. 1. Introduction Sound quality of household appliances becomes more and more important. Most of the sound quality research studies concentrate on the vehicle sounds. The obtained experiences are very useful for other products such as, household products, medical products, etc. The focus of recent sound quality research is on refrigerator, vacuum cleaner and dishwasher sound quality [1, 2, 3, 4]. In this study, we conduct some investigations on the washing machine sound quality. Acceptability and overall impression of washing machine sounds was the subject of a former study conducted by [5, 6]. The results of the study show that the sound pressure level of different components with different weightings depending on the operational stage have an influence on the acceptability. The flow noise, the valve closure sounds, circulation pump sound, motor noise or clothes flopping sound are important sound sources for the perception of washing machine sound [5]. Another research study concentrated on the water, motor, drop (falling clothes) and circulation pump noise of the washing machines [6]. They found that drop noise has an important influence on the perception of the washing machine sounds during the wash cycle. However the water noise is not very important for the perceived quality. The results of the study shows that particularly kurtosis and SIL (Sound level of frequency range that affect in conversation) can be useful to describe the perceived annoyance. The main aim of our study is to develop a sound quality index which is based on psychoacoustical properties of the washing machine sounds.. 1
2. Perceptual Evaluation of the Washing Machine Sounds 2.1 Stimuli 9 different washing machine sounds were recorded for this study. The recordings were conducted using a microphone (Brüel & Kjær, Type 2671). The microphone was placed 1.5 m away from the washing machine (front). The height of the microphone position was 1.65 m. All washing machines are front-loading. The max rotating speed at spin was 1200 rpm. The recordings were conducted with half and full load. The load has an important influence on the balance and accordingly on spin sounds. The analysis of the original sounds was useful to generate virtual washing machine sounds, which are possible variations of existing washing machine sounds. The sound of washing machines changes its character during the operation (approximately 2 hours). Therefore characteristic time spans of each washing machine sounds were selected. Then additional sound stimuli were generated by filtering important frequency components (e.g. tonal components using band pass filters, high or low frequency ranges using high or low pass filters) of these time spans. The procedure was same as in the studies [7]. The modified sounds are still perceived as usual washing machine sounds. A total of 216 sound stimuli was used in this study. 2.2 Subjects Eight men and twelve women (altogether twenty) subjects, aged between twenty-two and sixtyone years, participated in the experiment. The subjects had no specific knowledge regarding acoustics or vibrations. All of the subjects were paid for their participation on an hourly basis. 2.3 Experimental Setup and Design Using a loudspeaker, the recorded sounds and their modifications were presented to the participants. The sound pressure level of the sound presentation was calibrated. The experiments were conducted in a sound-attenuating room. A random order was used for the presentation of the stimuli. The procedure of the experiment was same as our former studies [3, 9, 10]. The subjects were asked to evaluate the annoyance of the sounds on a quasi-continuous scale, for which Rohrmann had tested the equidistance of neighbouring categories (not at all, slightly, moderately, very, and extremely) (Rohrmann, 1978 [8]). The length of the slider was 100 mm with a resolution of 1 mm. The score on this scale was equal to the distance (mm) from the left end of the bar. A graphical user interface in MATLAB was implemented for the evaluation experiments. In the training phase, all of the participants were presented with different combinations of stimuli from across the full stimulus range, and they were then familiarized with the procedure of the experiment. 3. Results of the listening test The jury test data (section 2), which was collected in the experiment, was analyzed statistically. The mean values of the annoyance judgments were determined and some of them (exemplary) are presented in Figure 1. The perceived annoyance ratings of the water, pump, friction and spin noises (dark blue) are presented separately. 2 ICSV23, Athens (Greece), 10-14 July 2016
Perceived annoyance rating (100: very annoying, 0: not annoying) Various original and virtual washing machine sounds (ordered according to their annoyance ratings) Figure 1: The mean annoyance ratings of different washing machine sounds. The results show that the perceptual annoyance ratings of the washing machine sounds vary strongly. The water sounds causes much more less annoyance than spinning sounds. Pump noises are important for the perceived annoyance, but they are not as much as important as spin sounds. According to the results, it was not possible to say that half load or full load is much more annoying. 4. Psychoacoustical analysis of the washing machine sounds The signal analysis of the washing machine sounds and listening test results show that loudness, sharpness, roughness and tonality are important psychoacoustical parameters to describe the perception of the washing machine sounds. Although wash cycle is very quiet, the spin cycle is very loud and annoying. Therefore the loudness is one of the important parameters. In some cases the sounds have dominant high frequency components which causes annoyance. These components can be described by sharpness. Washing machine sounds contain modulated and tonal sound components, which can be described using roughness and tonality. 5. Sound Quality Index for Washing machine Sounds Taking into account the results of the listening test and psychoacoustical analysis, an index was developed. This index contains the above mentioned psychoacoustical parameters, such as loudness, sharpness, roughness and tonality. The loudness calculation is based on Zwicker model (ISO532B [11]), the sharpnes and roughness calculations are based on Aures models (Aures, 1985a [12], 1985b [13]), the tonality calculation is based on Terhardt model (Terhardt, 1968 [14]). A regression analysis between the developed index and annoyance ratings resulted in a correlation coefficient of r 2 = 0.82 (Figure 2). ICSV23, Athens (Greece), 10-14 July 2016 3
Figure 2: The correlation between the annoyance ratings and the weighted combinations of the psychoacoustic properties as index. 6. Conclusions A sound quality index for washing machines was developed in this study. The most of the wash cycle sounds are very quiet, however the spin cycle sounds are very loud and annoying. Taking into account the results of the listening test and signal/psychoacoustical analysis, an index, which contains the weighted combinations of loudness, sharpness, roughness and tonality, was defined. The results of the study show that water sounds are not dominant for the annoyance perception. This result is in line with former studies [6]. 7. Acknowledgments This work was supported by EUREKA with project number 8684 and TUBITAK with project number 9130072. REFERENCES 1 Sato, S., You, J. and Jeon, J. Y. Sound quality characteristics of refrigerator noise in real living environments with relation to psychoacoustical and autocorrelation function parameters,.j Acoust Soc Am. 122 (1), 314-25, (2007). 2 Altinsoy, M.E. The Sounds of Household Appliances and its Relationship with the Quality of Life in Proceeding of the Internoise 2015, San Francisco, USA, (2015). 4 ICSV23, Athens (Greece), 10-14 July 2016
3 Altinsoy, M.E., Gül, M. and Kuyumcuoglu, A. Sound Quality of Household Appliances for Life Quality An Investigation on Tumble Dryer Sound Quality in Proceedings of the ICSV 2015, Florence, Italy, (2015). 4 Altinsoy, E., Kanca, G. and Belek, H.T. A Comparative Study on the Sound Quality of Wet-and-dry Type Vacuum Cleaners in Proc. of Sixth ICSV, Lyngby, Denmark, 3079-3086 (1999). 5 Bowen, D. L. Sound Quality Studies of Front-Loading Washing Machines. Sound and Vibration (2010). 6 Jeong, U. et al. Development of a sound quality index for the wash cycle process of front-loading washing machines considering the impacts of individual noise sources. Applied Acoustics 87, 183 189 (2015). 7 Altinsoy, M.E., Knocking Sound as Quality Sign for Household Appliances and the Evaluation of the Audio-haptic Interaction, in C. Magnusson, D. Szymczak, and S. Brewster (Eds.): Haptic and Audio Interaction Design (HAID) 2012, LNCS 7468, 121 130, Berlin: Springer, (2012). 8 Rohrmann, B., Empirische Studien zur Entwicklung von Antwortskalen für die sozialwissenschaftliche Forschung, Zeitschrift für Sozialpsychologie, 9, (1978). 9 Altinsoy, M.E., Identification of Quality Attributes of Automotive Idle Sounds and Whole-Body Vibrations, International Journal of Vehicle Noise and Vibration, 9 (1/2), 4-27, (2013). 10 Altinsoy, M.E. and Jekosch, U., The semantic space of vehicle sounds: developing a semantic differential with regard to customer perception, Journal of the Audio Engineering Society, 60 (1/2), 13 20, (2012). 11 ISO 532, Method for calculating loudness level, (1975). 12 Aures, W., Ein Berechnungsverfahren der Rauhigkeit, Acustica, 58 (5), 268 280, (1985a). 13 Aures, W., Berechnungsverfahren für den sensorischen Wohlklang beliebiger Schallsignale, Acustica, 59 (2), pp.130 141, (1985b). 14 Terhardt, E., Über akustische Rauhigkeit und Schwankungstärke, Acustica, 20 (1), 215 224, (1968). ICSV23, Athens (Greece), 10-14 July 2016 5