A loudspeaker cabinet has a number of pairs of microphones, each pair includes the same internal microphone and a different external microphone. For each pair of microphones, a process (i) receives a first audio signal of sound captured by the internal microphone and a second audio signal of sound captured by the different external microphone, (ii) estimates, using first and second audio signals, a radiation impedance, and (iii) computes a detection value based on the radiation impedance in a frequency band. A difference between (i) a currently computed detection value associated with a given pair of microphones and (ii) a previously computed detection value associated with said given pair, is computed. The sound produced by the cabinet is adjusted, in response to the computed difference meeting a threshold. Other embodiments are also described and claimed.