Final answer:
The loudness of sound decreases with distance due to the spreading out of energy and absorption by objects and conversion into heat. Additionally, the Doppler effect influences the perceived frequency as the distance between the source and the observer changes.
Step-by-step explanation:
The loudness of a sound decreases as you get farther from the source because the amplitude of a sound wave decreases with distance. The primary reason for this is that the energy of the wave is spread over a larger area as it travels, leading to a reduction in the amount of energy that reaches a given point. Additionally, the sound wave energy is absorbed by objects it encounters, like walls, and converted into other forms such as thermal energy due to the air's viscosity. This absorption and conversion of energy cause the sound's loudness to diminish further.
The phenomena of heat transfer from compression to rarefaction also play a role in reducing a sound wave's amplitude, and thus its loudness, by dispersing organized energy into random thermal motions. This is related to the second law of thermodynamics. Moreover, the Doppler effect affects the observed frequency of a sound as an observer moves toward or away from the source, with frequency and thus pitch appearing higher as the source approaches and lower as it moves away.