The object of this paper is both to investigate the required sound or preferable sound in each circumstance and to find out the characteristics of the sound. The adequacy of the sound corresponding to the circumstance is investigated. Then the characteristics of the sound are specified. Furthermore, the evaluation of sound quality is attempted on the basis of the concept of the soundscape. The sound from the outdoor unit of an air conditioner is used as an example of mechanical sound, and the pure-tone level is calculated under consideration of the critical bandwidth which is a hearing characteristic of human beings. The results of the pure-tone level are consistent with those of sound quality evaluation by the SD method. Then the usefulness of the evaluation method by use of the pure-tone level is confirmed.