depends on the turbo muffler. from what i remember Thrush has 2 turbo style models. one of the models has fiber glass and thoe other is just the bypass tubes I have never ran a turbo muffler but they are very mild in sound.
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inside a turbo muffler. Most turbo mufflers have 3 perforated tubes to control sound and air flow. some even have fiber glass or stainless steele wool to help with sound control