Lecture 1 Sound - Volume

Decibel
A decibel is a measurement of volume. That starts at 0 which is sound that can barely be heard and goes up in levels of 10, so 10 decibels is 10 times the start level, 20 is 10 times 10 decibels so 100 times the start, 30 is 10 times more so is 1000 times the original level. Which caps off for the human ear at about 120 decibels before your ear drum can take no more and can break if pushed high enough.

The Inverse-square law

"The intensity of the sound received varies inversely as the square of the distance R from the source i.e. as 1/R²" 
Traveling through air sound decays, so if the person or microphone receiving the sound was 3 meters away from the source it would be nine times less intense than if  it was 1 meter away from the source.
So if the person was 6 meters away from the source it would be 1/6² - 1/36th of the intensity from 1m.

Though this doesn't always happen in rooms and so on because of surfaces that reflect sound. This is why we have Echos and reverberations. The difference between these is if a sound comes back in under 100ms the brain cant destinguish it as a different sound and it adds on to the sound you've already heard making it a richer sound overall but if the delay is over 100ms the brain hears 2 seperate noises and it is heard as an echo.

Though reflection of sound can be completely removed in an Acoustic Anechoic Chamber.

The chamber is made up of walls normally out of foam and in shaped that deflect the sound instead of sending it back towards the source. These are useful tool for sound recording as it removes all reverberations or echo's.