I am a Physics student who also love music and learned a little bit about Medical Physics. I know that the basilar membrane in the cochlear duct enables us to distinguish different frequencies. The different parts of the basilar membrane have different resonance frequencies and this leads to frequency dispersion.
I've also learned that the pitch of a tone is determined by the fundamental frequency, and the overtones just add timbre to the sound, and are difficult for common people to hear separately.
I think the above implies our ears are more sensitive to lower frequencies. However, this is obviously not true when we listen to a symphony, in which I think most people are more "sensitive" to the higher parts. And I think this is the reason while the melody is usually in the highest part in a song.
So I am confused. Are our ears more sensitive to high frequencies or lower frequencies? Is there a biological explanation of why we usually tend to hear the highest-pitch part of a piece of music?