2. Amplitude
Amplitude is the physical equivalent to what we perceive as loudness. It is the maximum displacement of any particle from its normally position. The greater the amplitude, the greater the volume of sound.
This diagram shows that Wave A has twice the amplitude of wave B. This means that wave A will sound louder than wave B. But - just like frequency and pitch, amplitude and loudness are not linear. Twice the amplitude does not mean twice as loud. There are a number of ways that amplitude can be measured but the one which is most predominant is the unit of dBA. This unit refers to the way the human ear perceives volume but there are many different units relating relating to other subjects.
The human ear can hear anything as quiet as a leaf dropping (1 dBA) and anything as loud as a jet plane at 20m (120 - 135 dBA). These values are known as the Threshold of Hearing (ATH) and Threshold of Pain (THP). If the human ear is exposed to a 147dBA sound for anything longer that 20minutes at short distances, it can cause permanent hearing loss, usually attributed to the imploding of the inner ear.