An electroacoustic sound transmission system that is stable in any (dissipative) acoustic environment: An application of sound portholes

Active electroacoustic systems are commonly employed to transmit sound from one location to another. For example, consider the following configuration: a person talks into a microphone, which produces an electrically amplified signal for driving a loudspeaker, and the loudspeaker induces an output acoustic signal allowing someone else at a more distant location to hear the person talking. Unfortunately, such system designs typically do not consider acoustic feedback, which can destabilize the system and result in “howling.” In contrast, a feedback control system can transmit sound from one location to another without the risk of howling using sound portholes, which are collocated microphone/loudspeaker transducers. We design feedback controllers to transmit sound between sound portholes. These controllers model physical analog systems, such as a spring or a gyrator. The spring controller essentially binds the diaphragms of the two sound portholes together. We relate the spring controller to the string connecting two tin cans in the classical tin can telephone. Measurements are performed on a real feedback control system with two sound portholes. Because the feedback controller models a passive system, it is theoretically stable in any (dissipative) acoustic environment.