In recent years, Japanese companies are focusing on enhancing the knowledge creative activities of office workers, and the way of working in the office is shifting from the conventional divisional routine work to collaborative and creative work. On the other hand, office spaces are becoming quiet, and the number of extremely quiet them with noise levels below 40 dB is increasing. Previous studies have reported that a sound environment that is too quiet gives the worker the impression that it is difficult to have a conversation, further accumulation of research results is desired for the construction of a sound environment that enhances knowledge creative activities. Therefore, in this study, focusing on the relationship between sound environment and intellectual productivity, we investigated a sound environment suitable for knowledge creation activities by multiple people. Psychoacoustic experiments were conducted to examine the effects of sound pressure level (signal-to-noise ratio), type of sound and reverberation time of meeting room on the impression of good conversation. Furthermore, using the psychological evaluation data of the experimental participants, the causal model of psychological evaluation of good conversation was examined by multiple regression analysis, and the psychological factors that contribute to the impression of it was clarified.