UnderTone is an added feature on our main HydroGen line (HydroGen 5, HydroGen 6, HydroGen 7) which is an attached underlay pad that provides numerous advantages and benefits.Firstly, it helps provide comfort. By having a soft, cushioned pad so every step you take will feel extremely comfortable.Secondly, UnderTone is a silent feature that cuts the sound of footsteps down. It dampens vibrations resulting in a much quieter experience.