Sound waves are vibrations in the air that travel to our ears. When these waves hit our ear drums, they cause them to vibrate, which sends signals to our brains that are interpreted as sound. The amplitude of a sound wave is how much the air pressure changes as the wave passes through, which in turn determines how much the ear drum vibrates. Higher amplitudes cause greater vibrations and therefore louder sounds.
In musical notation, volume is typically indicated by dynamic markings, such as "forte" (loud), "piano" (soft), and "mezzo forte" (moderately loud). These markings can be used to specify the overall volume of a piece or section, as well as to create contrast and shape within the music.
The human ear is able to hear a wide range of volumes, from very quiet whispers to loud noises that can cause pain. The softest sound that the average human ear can hear is about 20 decibels (dB), while the threshold of pain is around 120 dB.