Many engineering applications call for the acoustic properties of materials to be known for the frequency range of 125 to 4000 Hz. According to current Australian and international standards, the present impedance tube apparatus in the Department of Mechanical Engineering laboratory at the University of Queensland is only able to produce results that are valid up to 1900 Hz. This thesis is an investigation to determine if measurements obtained in the 2000 to 4000 Hz range are valid. The investigation was successful in that it determined that in some cases the standard unnecessarily restricts the upper frequency limit. However, it was not possible to establish prior identification of the cases for which the frequency range could be extended. Therefore any experimental data obtained from experiments above the current limit require an investigation into the lateral pressure distribution inside the tube. Such a study is essential for validating any results.