Maybe you find this helpful: https://www.akustik-messen.de/index.php/en/background-information/psycho-acoustics/psycho-acoustics-terms-and-definitions

It’s a complex topic, there is a lot to discover. In short, sounds (which are supposed to be relaxing) should not be too loud, too rough, too shrill, too impulsive. A lot of sounds can be used for sound baths, from harmonic-based to certain types of noise (Like pink noise which is associated with waterfalls or the sound of the sea).