What primarily distinguishes infrasound from audible sound?

The defining characteristic that distinguishes infrasound from audible sound is the frequency range. Infrasound refers to sound waves that are at frequencies below the range of human hearing, typically below 20 Hz. This means that infrasound consists of very low frequencies that cannot be detected by the average human ear, which can generally hear sounds in the range of 20 Hz to 20,000 Hz.

Understanding frequency is crucial in differentiating types of sound. While volume level does influence how we perceive sound, both infrasound and audible sound can be measured with varying volume levels. Waveform shape pertains to the specific characteristics of sound waves but does not define the distinction between infrasound and audible sound. Similarly, complexity of sound relates to the harmonic content or richness of a sound but does not address the fundamental difference in frequency ranges that separates infrasound from what we can hear. Thus, frequency range is the correct element that clearly distinguishes these two types of sound.