PROGRAMM - DAGA 2012

Programm DAGA 2012 219Mi. 16:55 hassium 3.02 Akustische Messtechnik 1Correction Errors in Acoustic Measurements Caused by TemperatureVarianceX. Wang und M. VorländerInstitut für Technische Akustik, RWTH AachenThe impulse response and its associated transfer function are the mostimportant properties of linear and time invariant acoustic systems. If themeasurement is performed under low SNR conditions, e.g. when measuringsound barriers in the open air or calibrating microphones by reciprocalmethods, hundreds of synchronous averages have to be implemented.During the long period of averaging, a slight temperature driftwhich results in the change of sound speed can lead to changes in theimpulse response, and consequently causes wrong averaging results.In order to perform the accurate averaging, the change of the impulseresponse from the temperature variance must be corrected. In fact,corresponding to the temperature and sound speed shift, the impulse responsevaries with a time-stretching process. The time-stretching factorcan be estimated by maximizing the cross correlation between the measuredimpulse responses. In this paper, the averaging of the acousticmeasurement is performed after modifying the temperature-dependentimpulse responses to constant-temperature impulse responses, and theSNR is correctly improved. The stability of this time-stretching averagemethod is also discussed.Mi. 17:20 hassium 3.02 Akustische Messtechnik 1In situ sound absorption measurement: investigations on obliqueincidenceE. Kuipers und Y. WijnantUniversity of Twente, Chair of Structural Dynamics and AcousticsA novel method for the in situ measurement of sound absorption hasbeen developed. By assuming that, in a single point, the sound fieldconsists of an incident- and a reflected plane wave, the locally incidentandreflected intensities can be determined. To this purpose, the activeintensity and the sum of the potential and the kinetic energy density(only the particle velocity component in the direction of interest is used)are measured. The main advantage of the method that its applicationis not restricted to locally-reacting surfaces. For normal incidence, themethod was validated. Here, the method is applied to cases with obliquesound incidence. The influence of the angle of incidence and thenon-planarity of the incident and reflected waves on the accuracy of thesound absorption coefficient are investigated. Analytical models as wellas FE-simulations were applied to describe a few typical cases. Finally,parameter bounds for some applications will be given.