How does sound intensity decrease as a listener moves farther from a source?

As a listener moves farther from a sound source, the intensity of the sound decreases primarily due to geometric spreading, a principle known as the inverse square law.

Geometric Attenuation

For a spherical wave, energy is distributed over an increasingly larger surface area as the wave propagates outward. Because the surface area of a sphere is proportional to the square of its radius, the intensity of the sound is inversely proportional to the square of the distance from the source. Consequently, each time the distance from the source doubles, the intensity decreases by a factor of four, which corresponds to a reduction of six decibels.1

Additional Factors

Beyond this geometric decrease, sound intensity is further reduced by sound absorption. This occurs as energy is lost to the medium through:
* Heat conduction: The conversion of coherent molecular motion into incoherent molecular motion.
* Viscosity: Internal friction within the fluid medium (such as air or liquid).

These processes cause classical attenuation, which is proportional to the square of the sound wave's frequency. Additionally, environmental factors like temperature and humidity can influence how much sound is absorbed as it travels.2