Sound Intensity

Introduction
Before we can speak about sound intensity, you have to understand that for there to be a certain amount of sound intensity, there must first be a gradient which in effect is the first sound that humans can hear. The formula of the so-called threshold of hearing in sound intesity terms is: http://upload.wikimedia.org/math/c/0/3/c03ecaf90dc50e9e82293fbefc7fcd48.png.

Formula of sound intensity
The amount of energy which is passed through a certain medium area during a given time is called the intensity of the sound wave. The greater the amplitude of the vibrations through a certain medium, the higher the energy of the particles is and thus, the intensity of the sound wave is higher. Intensity is the Energy/time/area, because energy/time is the same as the power, we can simply say that the sound intensity is the same as the power/area: http://upload.wikimedia.org/math/e/c/1/ec19a642c097935133baef1a8bb19e18.png.

Measuring
Sound intensity is measured in decibels (dB). This is a logarithmic scale, in which 0 dB the same is as the threshold of hearing with an intensity of 1*10^-12 W/m2. For every 10 decibels that the sound intensity is increased by, the intensity goes up by a gradient of 10. So if the sound intensity level is increased from 0 to 20 decibels, the intensity is increased by 10*10=100, meaning that the sound intensity is 1*10^-10 W/m2.

Distance
The further away from a sound source a person is the lower the sound intensity is. The mathematical relationship between the distance and sound intensity is known as an inverse squared relationship. This means, that as the distance from a source is doubled, the sound intensity is quartered.