Three of the processes that mastering engineers use are Compression, Expansion and Limiting. All of these shape the sound in a different manner. Sound amplitude is measure on the dB scale. A quiet office is around 30dB while jets taking off are around 115dB. Typical Rock Concerts sometimes are above 100dB.
The Dynamic Range refers to the softest to the loudest sound. A recording studio can be as quiet as 10dB when it’s empty or as loud as 115dB when a rock group is recording. This is a dynamic range of 105dB. Tape is the best way to record the widest dynamic range, which is about 70dB.
You can see right away that there is a problem. How do you record a dynamic range of 105dB when the best recording media has only a 70dB range? You compress the signal. Compression takes the entire dynamic range and cuts off the loud parts so the meter never goes into the red or above a preselected dB level.
The difference between a compressor and a limiter is the compressor limits the loudness gently, inversely proportional to the input level. A limiter does not affect the signal until it reaches a preset level and the will not pass anything above that setting. This is ‘hard limiting’.
The third process that affects the dynamic range is expansion. Expansion limits how soft the sound can be. A symphony orchestra’s dynamic range may be as much as 100dB. A 70dB recording range may be achieved by compressing the loud passages and expanding the softest.
Handle’s ‘Messiah’ was originally written to include actual cannon shots, rather than the tympani shots we hear on the recordings. That is real dynamic range.
Have you every wondered why commercials on radio and television seem much louder that the music or the movie? The answer is compression and expansion. Broadcast are restricted by the FCC to a maximum dB level. That dB level is determined by measuring the total dynamic range of the program material. Commercials narrow the dynamic range and boost the amplitude. That is how broadcasters get that ‘In Your Face’ sound we have all come to apprerciate and enjoy.
Please see the following link for more detailed information http://tinyurl.com/Compression-01
Building a Recording Studio 