The loudness of a sound is most directly affected by its:

The intensity or loudness of a sound is primarily determined by its amplitude. Amplitude refers to the maximum extent of a vibration or oscillation in a wave. In the context of sound waves, greater amplitude corresponds to a higher amount of energy in the wave, which translates to a louder sound perception.

When amplitude increases, the pressure changes that the sound wave creates in the air become larger, leading to higher sound levels that we hear as increased loudness. A sound with high amplitude will be perceived as much louder compared to a sound with lower amplitude, even if both sounds have the same frequency.

While frequency and pitch are closely related concepts, they pertain to the perceived highness or lowness of a sound, not its loudness. Duration refers to how long a sound lasts but does not influence its loudness directly. Thus, amplitude is the key factor that affects how loud a sound is perceived.