Currently, the Brazilian fleet is estimated at 40.0 million automotive vehicles. In the urban centers, as a consequence of this fact, the humans are being exposed daily to continuous and intermittent unwanted noises emitted by these vehicles. The prolonged exposure to noise can cause irreversible damage to human health. As a general rule, the automotive noises are low frequency and the mufflers used to attenuate these noises are made by uniting several kinds of acoustic filters. There are two types of acoustic filters: reactive and dissipative. The first type is made by the expansion chambers, resonators, labyrinths, perforated tubes and other types, by the addition of absorbent material to its inner part. The absorbent materials are fibrous and they have the purpose of increasing the efficiency of acoustic filters. The mufflers built with these two types of filters are called hybrids. There are many works that involve purely reactive mufflers without absorbent material. However, there are few studies that depict the acoustic behavior of mufflers with absorbent material. The difficulty in studying this type of filter is to characterize the acoustic properties of the absorbing material. The properties required for the evaluation of dissipative mufflers are: characteristic acoustic impedance and complex wave number of the traveling wave. This work evaluates the efficiency of a hybrid muffler with the Finite Element Method in terms of its sound transmission loss. The absorbent material is characterized by an inverse methodology. The computational results are compared with an experimental analysis and show a good agreement.