There are two acoustical phenomena of interest with regard to sandwich panels: sound transmission loss and sound absorption.
The first relates to the use of honeycomb sandwich as sound barrier that is to prevent sound from being transmitted, while the second refers to its capacity as a sound absorber; in other words, to cut down reflected sound inside the room.
|Enclosure||Transmission loss requirements||Absorption requirements|
A honeycomb panel is an excellent sound barrier for very low frequencies (the range of structural borne vibrations), is fair to poor at somewhat higher frequencies (the range of engine noise), and is very poor at still higher frequencies (aerodynamic noises and human speech).
A panel whose front facing is perforated and whose back skin is impervious will be a good absorber over a very narrow frequency band. The mechanism of absorption in this case is by the Helmholtz resonator effect. This has to do with the volume of trapped air in the cell and the size of the hole in the facing.