## Open acoustic measurements for validating edge diffraction simulation methods

D. Schröder¹, U. P. Svensson², M. Vorländer¹

¹ Institute of Technical Acoustics, RWTH Aachen University, Aachen, Germany

² Acoustics Group, Norwegian University of Science and Technology, Trondheim, Norway

Diffraction is a wave phenomenon that is experienced in mostly any daily life situation. Just imagine a walk through a corridor of an office building where sound sources are located in each room. The sound field does not change abruptly when an open door is passed. The transition is rather smooth which comes from sound energy that is bent around the door entrance's edges, i.e. sound diffraction. When a sound wave hits an obstacle, a frequency-dependent shadow zone occurs behind the object (related to the direction of sound propagation). If the object is small in comparison to the wave length, the incident wave remains unaffected. However, a shadow zone appears and grows clearer and sharper with decreasing wave length and increasing frequency, respectively. This shadow zone results from a total cancellation of the incident wave by the diffracted wave which is radiated from the edges or perimeter of the respective object.

Thus, sound diffraction must not be neglected in computer simulations, especially in indoor and large-scale urban scenarios. The development of noise barriers is a typical application where computer simulation methods of edge diffraction are taken in account in order to get detailed information about the barrier's efficiency in the respective scenario. Unfortunately, all known simulation methods, either based on Geometrical Acoustics or the numerical solving of the wave equation, are just approximations. They work fine for distinct simple test cases within a certain frequency range but none of them covers the effect of diffraction in its whole complexity. Consequently, huge effort is currently put into further improving these simulation models to enable a realistic prediction of edge diffraction even in complicated real-world situations. This demands for taking into account the influence of sound scattering and multiple wave diffraction, too. Therefore, appropriate test scenarios are required in order to validate these extended prediction models.

In this contribution various measurement series of a scaled noise barrier model are presented which aim to give developers and acoustic consultants the possibility to test their prediction methods. The model features three types of ground layers: an absorbing surface, a rigid surface and a highly scattering surface. The latter was realized by a self-constructed Skyline Diffuser so that the occurring sound scattering can be simulated in two ways, either deterministic or stochastic. The measurements were carried out in a full anechoic chamber and a turntable was used to rotate the scale-model during the measurements with a resolution of one degree. All measurements together with geometrical models of the scale-model (with/without diffuser), detailed information on sources and receivers, material data (absorption- and scattering coefficients) and helpful Matlab tools are freely available for download (www.openmeasurements.net).