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In this paper the measurement, in situ or under free-field conditions, of the surface impedance and absorption coefficient is investigated. Numerical simulations of the measurement of impedance above al ocally reactive surface is performed with the Boundary Element Method (BEM). Experiments are also made in asemi-anechoic chamber and in ar egular office room. Three calculation methods used to obtain the surface impedance of an absorbent surface are described and compared, twoo ft hem being iterative.T he first, referred to herein as the "q-term", relies on an exact description of the sound field above an infinite locally-reactive plane. The second, the "F-term", relies on an approximation for large values of the argument k | r 2 | in its equations. The third, the "Plane Wave Approximation (PWA )",isasimplification of the spherical wave reflection which considers that the reflected wave has its amplitude and phase changed by asimple planar reflection coefficient. The "F-term" and the "Plane Wave Approximation" methods also assume an infinite locally-reactive plane. The three calculation methods are compared, the differences in the found results are discussed. The three methods are compared mainly for small sound-source to sensor distances (| r 2 | = 0.3m)and it is seen that theytend to convergeasthis distance increase. This comparison is relevant to in situ impedance measurements, since abad choice of the calculation method may lead to apoor result. PACS no. 43.58.Bh, 43.20.El, 43.55.Ev Received2May 2011, accepted 18 September 2011.the sound pressure at twol ocations close to the surface of the absorber.T he particle velocity wasc alculated by the gradient of the sound pressure. The authors assumed the acoustic field to be formed by plane waves. The in situ measurements were compared with impedance tube results. Substantial deviation wasf ound below5 00 Hz, which can be attributed to an insufficient spacing between the microphones and to the assumption of plane waves. Li and Hodgson [2] also used twom icrophones, assuming that the reflected spherical wave fronts have its amplitude and phase changed by aplanar reflection coefficient. Despite the assumption regarding the reflection, asurface impedance closer to that obtained with the phenomenological model wasestimated. The results in reference [2] were compared with those in reference [1]. The assumption in reference [2] is the same as that described in section 3.3.Ac ommon weakness of these methods is the lack of a good description of the sound field above the surface. The ©S.Hirzel Verlag · EAA 1025 ACTA ACUSTICA UNITED WITH ACUSTICA Brandão et al.:M easurement of free field surface impedance Vol. 97 (2011)

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With PU probes the sound pressure and acoustic particle velocity can be measured directly. Over recent years, the in situ surface impedance method, making use of such a probe, has proven to be an alternative to Kundt's tube measurements for product development type of work. The in situ method can also be used for the purpose of quality control on the acoustic material, be it during manufacturing or assembly, ensuring the best possible way to monitor the practical effectiveness of the acoustic package designed earlier on. In order to assess the variance of the acoustic package material leaving the assembly line, a relevant number of samples were taken over time. The quality of both the headliners, and the passenger seats were measured, of 25 cars of the same type. The robustness of the measurement method will be discussed, and the results will be presented.

The pressure-particle velocity (PU) impedance measurement technique is an experimental method used to measure the surface impedance and the absorption coefficient of acoustic samples in situ or under free-field conditions. In this paper, the measurement uncertainty of the the absorption coefficient determined using the PU technique is explored applying the Monte Carlo method. It is shown that because of the uncertainty, it is particularly difficult to measure samples with low absorption and that difficulties associated with the localization of the acoustic centers of the sound source and the PU sensor affect the quality of the measurement roughly to the same extent as the errors in the transfer function between pressure and particle velocity do.