Sound Pressure Level and Decibels
This is the amount of air pressure fluctuation a sound source creates. We perceive this pressure in terms of loudness. A simple example is a drum, when you hit it gently the surface doesn't move very far and so because the of this the pressure is lower than hitting it hard.
The Sound Pressure Level is also effected by where the listener is from the source and the environment they are in. A drum hit hard in a small bathroom will sound a lot louder than a drum in a cathedral because many of the discrete reflections will never reach the listener due to the expanse of the space. This is even less in an open field where there is nothing to reflect off in the sky.
Sound Pressure Level is usually expressed as pascals (Pa). Usual conversation pressure is 0.02 Pa and a petrol lawn mower is around 1 Pa and sound becomes painful at around 20 Pa, otherwise known as the threshold of pain. So a healthy range is around 0.00002 to 20 Pa.
The Sound Pressure Level range offer such a wide range of scale that we use the decibel scale instead. This is because it compresses the scale into a manageable range. This conversion from sound pressure to the decibel scale is called sound pressure level. 0dB is 0.00002 Pa and this is the start of the scale. This is logarithmic which means every 6dB is a doubling of Sound Pressure Level in terms of intensity.
How much bigger is a sound at 40dB compared to 0dB? = 10,000 times. How much bigger is a sound at 110dB compared to 50dB? = 1,000,000 times.
This is redrawn on paper due to copyright and is still an accurate reference.