When the volume or loudness of a sound increases or decreases, it is primarily due to changes in the amplitude or intensity of the sound wave. The behavior of sound particles, or more precisely, the air particles that carry the sound wave, remains essentially the same.
In a sound wave, particles in the medium (such as air molecules) vibrate back and forth, transferring energy from one particle to another. These particles do not change in terms of their density or number when the volume of the sound changes. Instead, the key difference lies in the magnitude of the particles' displacement from their equilibrium positions as the sound wave passes through the medium.
When a sound is louder, the amplitude of the sound wave is greater. This means that the air particles are displaced from their equilibrium positions to a larger extent, resulting in more vigorous back-and-forth vibrations. As a result, the compressions and rarefactions of the sound wave, which correspond to regions of higher and lower pressure respectively, become more pronounced.
Conversely, when a sound is softer, the amplitude of the sound wave is smaller. This leads to smaller displacements of the air particles from their equilibrium positions, resulting in less vigorous vibrations and smaller variations in pressure.
To summarize, changes in the volume or loudness of a sound are primarily associated with changes in the amplitude or intensity of the sound wave, rather than alterations in the density, number, or thickness of the sound particles (air particles).