Exploration into the sources of error in the two-microphone transfer function impedance tube method

Abstract

The two-microphone transfer function method has become the most widely used method of impedance tube testing. Due to its measurement speed and ease of implementation, it has surpassed the standing-wave ratio method in popularity despite inherent frequency limitations due to tube geometry. Currently, the two-microphone technique is described in test standards ASTM E1050 and ISO 10534-2 to ensure accurate measurement. However, while detailed for correct test execution, the standards contain vague recommendations for a variety of measurement parameters. For instance, it is only stated in ASTM E1050 that “tube construction shall be massive so sound transmission through the tube wall is negligible.” To quantify this requirement, damping of the tube was varied to determine how different loss factor values effect measured absorption coefficient values. Additional sources of error explored are the amount of required absorbing material within the tube for reflective material measurements, additional calibration methods needed for test of excessive reflective materials, and alternate methods of combating microphone phase error and tube attenuation.