STC = Sound Transmission Class
This single number rating applies to the frequency range from 125Hz to 4kHz. A higher STC rating is desired especially for keeping your sound inside and the neighbors noise, outside!
These ratings are used for all wall assemblies and give the person developing a wall for a high rated STC, an understanding of what building materials and building methods have to be used to achieve a specific STC rating.
The average difference between the sound level(in decibels) on one side of a wall panel and the sound level on the opposite side of the same panel determines the final rating of the field STC.
What is an acceptable STC rating for your walls? There is no one simple answer. Your surrounding external noise levels, interior noise levels including at what level you and your musical equipment will be playing at determine the actually number you require by law, first, and isolation requirements, second.
STC: The ability of a material or wall assembly to stop noise from transmitting through it.