sentences:  This lowers the resonant frequency of the drive unit, negating the need for a highly compliant device.Furthermore, the column of air provides greater force on the driver itself than a driver opening onto a large volume of air (in simple terms it provides more resistance to the driver's attempt to move it), so to control the movement of air requires an extremely rigid cone, to avoid deformation and consequent distortion.The introduction of the absorption materials reduces the velocity of sound through the line, as discovered by Bailey in his original work.