Low Density:
If the density inside the rear housing is low, some of the pressure of the air inside the rear housing is absorbed by the material when the diaphragm moves, reducing the air rebound. This means that the spring of the air inside the rear housing becomes weaker.
Therefore, the diaphragm moves gently and slightly. This kind of diaphragm movement makes it easier to produce soft and gentle sounds. As a result, the sound pressure tends to be subdued.
