Enhancement of acoustic absorption coefficient of foams using active feedback control

The absorption coefficient (AC) measures the fraction of acoustic power absorbed by foam sample. AC in foams is usually small at low frequencies, where the sound wavelength is larger than the foam thickness. This shortcoming is addressed in the current study by increasing the low-frequency AC in foams using active feedback control. The control strategy is to increase foam AC by setting zero acoustic pressure boundary condition at the backside of foam sample inside an impedance tube. The frequency-domain loop shaping method is used to design the loop gain. A speaker and microphone are positioned at the backside of the sample. The plant transfer function is experimentally determined as the speaker-to-microphone response, fitted with poles and zeros, and the controller is derived using this fit and loop gain. A one-dimensional model of the active feedback control applied to the foam sample in an impedance tube is also developed to analyze the control strategy and gain insights. The pressure field leakage from the control speaker to the microphones used in the impedance tube, and its impact on AC, is analyzed using the model. The proposed system is demonstrated to show AC increase from about 0.4 to 0.7 over 200-400 Hz.