In most buildings, walls and floors have an interior support structure of wood, steel studs or joists. These are covered with some form of rigid sheeting like plasterboard or fibre cement sheeting , leaving an air space between the walls, floors and ceilings. This air space allows for the easy transfer of sound waves with very little solid mass for the sound to vibrate through or slow its transfer, and may even act to decrease sound dampening.
One of the methods used to reduce the transfer of this sound is to insulate the wall, floor or ceiling with one of several types of insulation. However, although this technique may reduce the transfer of sound vibrations through the air space, it does nothing to stop the sound transferred through studs, plasterboards, joists or flooring via vibration.
Additionally, with the conventional method of insulation, it’s also important to understand that all sound frequencies are not the same. Therefore some low frequency sounds may not be stopped by insulation and equally, some high frequency sounds may not be reduced significantly. Consequently, insulating the air space in a building is only effective in deadening mid-range frequency sound.
Many people have found this out to their cost by having sound deadening insulation installed only to find that the bass beats come through almost unaffected, as if no insulation was present. High pitched sounds also often travel through uninterrupted.