One unique feature of the piano is the continual decay of the sound after a key is pressed until the finger is lifted from the key. In this process, the sound from the piano attenuates the level while slightly changing the frequency structure.<br> 　Further, when the pianist releases the key, but presses the forte pedal with the foot, the sound continues. The pianist can finely adjust the decay pattern that follows pressing a key by moving fingers and operating pedal. Thus, one of the major factors that can realize the expression of a brilliant piano performance is the decay of the sound itself, and the ability of pianist precisely control the characteristics of that decay to create a beautiful stream of sound. As shown in Fig.1, the dynamic characteristics of hearing greatly affect the perception of decaying sounds. The sound of a piano after a keystroke continues to attenuate, however, if the legato technique is performed with a finger held on a key or the forte pedal being pressed, the sounds played one after another will overlap. If dissonant sounds are generated over time, the sequence provides an unpleasant impression. If the decaying part of the preceding sound and the rising part of the following sound are superimposed, an acoustically dissonant sound is generated.<br> 　Reflecting on the dynamic characteristics of hearing, due to the overshoot of the following sound, the decaying part of the preceding sound is masked, dissonance is not heard, and the melody progresses smoothly. Thus, it can be said that the variety of expressions from a piano is large because piano sound is a decaying sound, and the pianist skillfully adjusts attenuation patterns by using the dynamic characteristics of hearing.<br> 　Dynamic characteristics of hearing can be investigated through psychophysical procedures under controlled conditions (Namba, S. 2013). However, it is not easy to describe the world of music in the heart, although this world is vividly expressed in excellent novels and essays. Research on the correspondence between the beauty of the sound of a piano in the world of the mind and as a precisely measured physical quantity is not highly remarkable. One reasons for this is the world of beauty in piano performances as depicted in novels is delicate, subtle and complex. Further, the physical quantities of piano sounds measured from various aspects are highly precise and complicated. Such complications are a barrier to finding psychophysical laws that reveal the relationship between the world of the mind and physical quantities.<br> 　Historically, psychophysical measurements have not been benefical for assessing complex events. This is because of the difficulty in ensuring the reliability of experiments or in accurately controlling stimulus conditions in psychophysical experiments. In the case of a one dimensional attribute such as loudness, even if the physical quantity of sound is complex, it can be controlled by psychophysical methods. However, the tone quality that is an important attribute in the evaluation of piano sounds is multidimensional, and the physical quantity that affects the tone quality is also multidimensional. Therefore, it is not easy to find the psychophysical law between the two.<br> 　If we can solve these problems, we should from the physical quantities, be able to correlate beautiful piano sounds that are described in novels. Not only that, it would be an important step on the path to elucidation the psychophysical relationships of complex events in daily life.<br> 　Many studies on tone quality have been conducted in which target sound sources were selected to limit factors affecting the tone quality. Verbal expressions of tone quality, which is a multidimensional attribute, have been addressed also by selecting only a small number of adjectives. Further, statistical methods such as factor analysis were used in the selection process.<br>(View PDF for the rest of the abstract.)