The main goal of limiting in mid side mixing is the prevention of signal clipping. In case the audio has been normalized to 0 dB, processing level will increase. After the processing the signal level will inevitably rise as well. It will bring on some overload. By including a limiter into the chain at the initial stage of processing you will prevent the audio from it.
The range of frequencies heard by humans is from 20 to 20 000 Hz. In spite of it, there can be such sounds in any audio that go beyond it, especially at low frequencies. As a rule, they are non-music sounds: interferences, pickups etc. The presence of such interferences can later have a negative impact on an overall level of sound. Low frequency interferences only load up loudspeakers. It often causes their rapid wear. The noises which go beyond the range of the initially recorded signal can interfere with its perception. Therefore mastering engineers often use filtering to eliminate frequencies below 20–40 Hz and above 18–20 kHz.
Equalization is one of the most significant and responsible steps of music mastering. This process involves audio processing via an equalizer and implies correction of the amplitude and alteration of the frequency balance. Equalization is used quite often and is conditionally divided into two kinds: aesthetic and technical. The target of technical equalization is to reach a precise tonal balance. Aesthetic equalization makes sound more saturated, vivid and clear.
While working, mastering engineers use two kinds of equalizers: graphic and parametric. Graphic equalizers usually have 31 bands for each channel and are often equipped with spectrum analyzers. It turns them into more convenient devices. At the same time parametric equalizers give more opportunities to correct frequency response as each band of this equalizer has at least three adjustable parameters. It allows engineers to choose the necessary frequency more precisely and to adjust it more accurately.