If the rear housing material is soft:
If the rear housing material is soft, the material of the housing is easily deformed by the pressure of the air inside the rear housing when the diaphragm moves, so the air rebounds slowly and gently.
Consequently, the diaphragm moves slowly and broadly. This kind of diaphragm movement tends to produce thicker sounds, particularly enriching the mid to low range. However, compared to hard materials, the contours of the sound might be less sharp, giving a slightly softer impression.
