Advanced control for modifying the acoustic impedance at the diaphragm of a loudspeaker

The control of low-frequency sound fields can be addressed efficiently through acoustic impedance matching. Basically, the diaphragm of electroacoustic transducers are used as refracting surface that controls the reaction of the boundaries in any surrounding sound fields. The general idea is to absorb the incident sound energy or to contain it, simply by altering the transducer dynamics in a controlled fashion. Usual techniques operate either by feedback control of acoustic variables (sound pressure or velocity) or by connecting some electrical load at the transducer terminals. The paper focuses on how to transform an electrodynamic loudspeaker in an active electroacoustic resonator through the use of sensor and controller. It is discussed how to achieve broadband sound absorption at the transducer diaphragm. Phase compensation technique are also introduced as a convenient way to overcome a practical issue that may arise in some cases, taking the form of an over-reflective behavior of the diaphragm. For illustrative purposes, computed results and measurements obtained in impedance tube are provided to show the performance of a controlled loudspeaker in terms of acoustic absorption capability and stability.