u/CalligrapherDry891

I'm working on optimising a muffler using FEM and have started by simply modelling a resonance chamber (diameter 200mm, length 300mm), with an inlet and outlet directly across from one another (diameter 40mm, length 40mm). When calculating the acoustic pressure I obtain 3 peak values, and when calculating TL through the resonator I also get 3 negative peaks at the same frequencies. Most sources however say that TL cannot be negative? However I would expect it to be negative at these 'problem frequencies' as this is where the resonance chamber amplifies sound?