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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.