All waves carry energy, including light, sound, infrared, microwaves, x-rays and water. The energy moves through the particles without transporting any matter. Amplitude is the measurement of the energy carried by any wave. It is the furthest distance that the particles move from the wave’s undisturbed position, or when the wave is flat, due to the energy passing through it.
The greater the amplitude of the wave, the higher the level of energy that is carried by the wave. With a sound wave, this means that the sound will be louder. With light, the bigger the amplitude means the light will be brighter. In water, the bigger the amplitude of the waves, the higher they will be.
For waves in a medium like water, amplitude measures displacement, or a change in position of a particle. Imagine dropping a pebble into a calm pool where the water is flat and still. Before the pebble hits the water, the water particles are at rest, relatively speaking, but after the pebble falls into the water, waves radiate from the point of impact out. The particles of water move up and down due to the energy imparted by the pebble hitting the water. The amplitude is measured from the center of the wave to the peak or through of the wave, basically the height of the wave.
With sound waves, amplitude measures the amount of positive or negative change in atmospheric pressure caused by the wave. If the sound wave has a greater amplitude, it will cause a greater change in atmospheric pressure. This can be changed by changing the amount of force of the current. For example, a horn blower can create a greater sound by blowing harder into a horn or providing a greater force of air in the horn.
The measurements are taken as the amount of force over an area and are directly related to the acoustic energy or intensity of a sound. The unit of measurement for sound amplitude is Newtons per square meter (N/m2) or pascals (Pa). The amount of force it takes to accelerate a one kilogram object by one meter per second is equal to one Newton. The smallest amplitude that can be heard by humans is about 0.00002 N/m2 for a tone of 1 kHz. The threshold of pain for sound is thought to be about 60 N/m2, although this can vary depending on the individual and the age of the subject.