An in-ear earphone housing has a speaker driver installed therein. The driver has a diaphragm with a front face and a rear face, and a motor to vibrate the diaphragm in accordance with an audio signal. A back volume chamber is positioned behind the driver within the earphone housing. The diaphragm is part of a wall of the back volume chamber. An acoustic mass loading membrane that is part of a wall of the back volume chamber, and that is to vibrate in response to acoustic waves produced by vibration of the diaphragm impinging on a front face of the membrane, is provided. Other embodiments are also described and claimed, including a polymer production process for an elastic material.