Shunt Loudspeakers for Modal Control in Rooms

Engineers dealing with noise reduction in habitations close to transportation traffic or industrial facilities encounter several problems to decrease noise level in rooms at low frequencies. Passive materials and current building construction knowledge enable to avoid noise transmission in habitations at medium and high frequencies and the regulations based on the dBA scale can often be respected. But these regulations do not take into account the real annoyance of noise for the inhabitants who are still disturbed by low frequency noise. Because of the modal behavior of rooms, air-borne and structure-borne noise generate high sound pressure level at the first modal frequencies, even with small amount of energy. In order to lower the annoyance induced by low-frequency behavior in rooms, huge compliant wooden panels can be used, though with quite low performances in terms of resonances damping. An alternative can be found with bass-traps, comprising a mechanical resonator and a closedbox, with fixed acoustic performance at specific frequency. Enhanced performances can even be obtained with closed-box shunt loudspeakers, where the acoustic impedance of the loudspeaker's diaphragm around its resonance can be modified (in terms of magnitude and also in frequency) by way of a simple passive electrical device, providing the requested complementary complex acoustic impedance for perfect absorption at the first modal frequencies. The present work describes the design of a small array of low-frequency shunt loudspeakers, dedicated to damp a certain amount of modes within a known room. Assessment in laboratory conditions (impedance tube, reverberant chamber) will then be presented and compared to simulations, leading to concluding remarks on practical issues on the applicability of the concept of shunt loudspeakers for low-frequency noise reduction in habitations.