There are two essential issues concerning the physics of hearing.
Frequency response is the range of frequencies which a given individual can hear.
Since intensity is power per unit area, we can change the measured db by either changing the power or changing the distance from the source. So far we have only expressed the objective quantity intensity in relation to a standard subjective threshold. Hence the measurement of loudness is an extremely subjective effort. The next section is on refraction and reflection . 1996, Kenneth R. Koehler.