It is very easy to get the wrong acoustic materials for the job. It is not easy knowing which materials are best for the job, and getting enough of the proper material , installed correctly, to solve the problem.
There are two main types of acoustic materials. The first is most common, and it is material which allows sound energy to pass through allowing less sound to come back into the environment, for instance, acoustical wall panels and acoustic ceiling tiles. Often, these materials are misused trying to block sound from one area to another. They are mostly used to improve sound quality and speech intelligibility within a space. These materials are NRC rated (Noise Reduction Coefficient). NRC is based on an average percentage of sound absorption across common frequencies. It appears at a decimal like NRC 0.85 which equates to roughly 85% average absorption across common frequencies (not to be confused with decibel).
The second most common type of acoustic materials is material which is used to block or isolate sound from one area to another. This material is STC rated (Sound Transmission Class), which is given in number of average decibel reduction across common frequencies. STC 28 equates to roughly 28 decibel reduction on average across common frequencies.