Measurement of transmission loss of symmetric systems by using a 2-microphone standing wave tube

Transmission loss is a key property for evaluating the sound insulation performance of acoustic systems in applications ranging from architectural acoustics to automotive noise control. In ASTM E2611, a 4-microphone impedance tube setup is standardized for measuring transmission loss based on transfer matrix theory. By comparison with the 2-microphone impedance procedure, the 4-microphone measurement requires a more expensive setup and a more complex testing procedure. In contrast, the 2-microphone setup standardized in ASTM E1050 is simpler and more efficient but measures only the surface normal impedance, which relates to sound absorption rather than sound insulation. To bridge this gap, for symmetric acoustic systems, a closed-form expression has been derived in the present work to compute transmission loss based on two surface normal impedance measurements with different backing conditions, thus offering a practical and efficient alternative to the 4-microphone procedure. The accuracy of this approach has been validated through comparisons with transmission loss predictions obtained from both simulations and real measurements. This method simplifies the transmission loss testing procedure and so expands the application of the existing 2-microphone impedance tube configuration.