Interior quietness has been improved over the years until it has now become an important fundamental performance requirement of vehicles. Simple approaches to assuring quietness include the addition of sound isolation materials or the provision of sound-deadening space. These approaches, however, have the undesirable effects of increasing the vehicle weight or sacrificing interior space. Previously, we investigated the effect of the fiber composition on sound isolation performance, focusing on polyester fiber material (nonwoven fabric), and researched sound-isolating materials that would not involve a weight increase. That work led to the development of a polyester fiber material with an optimized fiber composition. The material has been applied to a dash silencer that simultaneously achieves improved sound isolation performance and a lighter weight compared with previous materials. That technology provided the basis for developing a polyester fiber carpet pad. Besides sound isolation, a carpet pad must also provide reactive force and durability against loading as well as good formability and form adaptability because of its complicated shape. These requirements pose a higher technical hurdle than in the case of the dash silencer. The new carpet pad features an optimized fiber composition consisting of side-by-side conjugate polyester matrix fiber and crystal-line polyester binder fiber. The composition satisfies the requirements mentioned above with only half the density of previous materials. Compared with the previous carpet pad made of foamed urethane, the newly developed carpet pad improves the sound isolation performance of the floor and also achieves a weight saving of 15-40%. Furthermore, the new carpet pad can be returned to polyester staple fiber by a fiber opener (that is a recycle machine from nonwoven to staple fiber). This recycled polyester staple fiber can be used again for a carpet pad or other pad use.