resonance

Hello, I have measured one living room of an L shape. It is ordinary living room with kitchen corner, very sparse acustic tretment. On SPL measurement there is one clear resonance at around 50 Hz. The resonance is clearly seen also at spectrogram graph. however when I try to "see" the resonance...

Hi guys, I have been working on room mode for some time, my room is about 6m x 4m x 2.4m, and having first problematic mode at ~52hz, ~60z and ~90hz, and I almost tired every passive method, eg. >4in absorbent on wall, membrane type bass trap on wall, corner bass trap, but do not really help me...