Due to the limited effectiveness of sound absorbing material at low frequencies the first (first few) acoustic mode(s) of enclosures, the size of automobile cabins, are very difficult to damp, resulting in the boom noise problem. The use of one or a bank of Helmholtz resonators for increasing the absorption in automobile cabins has been studied by different researchers. This approach is effective, but the large size of the resonators (due to the low frequencies they are tuned for) makes their use rather prohibitive. This has led to the use of active noise control, to add damping to the first (or first few) acoustic modes by nearly-collocating a microphone with a volumetric source (speaker) in the acoustic field and employing appropriate feedback compensators. Significant levels of low-frequency, global acoustic damping can be achieved in enclosures such as automobile cabins, with the proposed feedback control schemes.
