Examination of Candidate Calibration Samples for Normal Incidence Impedance Measurements

Several ASTM and ISO standards describe the experimental procedures for measuring the normal incident acoustic impedance of samples in a standing wave tube, but they do not describe a standard sample to evaluate the performance of these experiments. Such a standard would be desirable for calibration and validation of impedance measurement systems. Ideally, such a standard would be stable, repeatable, easy to construct with readily available components, and have predictable performance. In this paper, several candidate concepts meeting the above criteria were identified, constructed, and tested. The performance of these concepts and an examination of some factors that control their performance are discussed. The impact of errors in the measurement of ambient temperature, barometric pressure, and sample placement on measurements of acoustic impedance is evaluated. Candidate concepts for potential acoustic standards are identified and discussed.