Sound reflection in an acoustic impedance tube terminated with a loudspeaker shunted by a negative impedance converter

A system for the absorption of sound at the termination of an acoustic tube using an electrodynamic loudspeaker connected to a shunt circuit is presented. A theoretical model of the electrodynamic loudspeaker in the acoustic tube is used for the calculation of the frequency dependence of the ideal shunt circuit impedance, which yields perfect sound absorption in broad frequency range. It is shown that both the real and imaginary parts of the shunt circuit impedance must be negative in the considered system. The required negative values of the shunt circuit impedance are achieved using a negative impedance converter. Frequency dependences of the reflection coefficient were measured in the acoustic impedance tube using the two-microphone transfer function method. Greatly reduced values of sound absorption coefficient were achieved in a narrow frequency range. The stability, applicability of the sound absorption system, and broadening its frequency range are discussed.