Noise reduction door are generally made from very dense, stiff materials. These dense, stiff materials also work well at reflecting sound back to its source.
A door’s ability to reduce noise is called its sound transmission loss (TL) effectiveness. The higher the TL, the better the result.
Sound transmission class (STC) ratings solve that problem by giving a single value to acoustical performance for a door. The higher the STC value, the better the rating and the better the performance.