Pitch and loudness are both characteristics of sound, which are not physical but perceptual. Students often confuse these two terms, wrongly believing they are interchangeable. Pitch is basically the response of our ear to the frequency of the sound, where frequency is defined as the number of waves that move past a point in unit time. The closely packed the waves are, the higher the pitch.
Loudness, on the other hand, depends on the energy and amplitude of the sound wave, where amplitude is the height of the wave. A wave with greater energy and greater amplitude will sound louder.
The basic difference between pitch and loudness is that the former is quantitative and can be high or low in the register, while the latter merely affects the volume of the sound. The highness or lowness of pitch is actually responsible for creation of melodies, whereas the loudness of a sound wave has nothing to do with that. High pitched sounds are more penetrating, but not necessarily louder, which is why it is easier to hear a melody as compared to a harmony.
The pitch of a sound has nothing to do with how loud it is. An inaudible pitch, can be made audible to the listener by increasing the wave amplitude. Furthermore, the pitch of a sound needs to be high enough to be heard, while loudness does not need to be high or low.
The highness or lowness of the pitch affects the character and quality of a melody. Almost every musical instrument sounds different on high notes as compared to low notes. The loudness on the other hand, does not affect the character of the melody, it just makes the music more audible.
In music and filmmaking, loudness is used to create suspenseful climaxes and to put the audience into a state of heightened consciousness. Pitch, on the other hand, is used to create moments of dread and fear and to convey horror.