Measurement and Simulation Results Comparing the Binaural Acoustics of Various Direct Radiators

The binaural acoustics of conventional, dipolar and panel loudspeakers are compared, using both anechoic measurements with a dummy head, and numerical simulation. The results are compared both objectively and psychoacoustically using a binaural localisation model. 0 Introduction Loudspeakers are routinely characterised by single, on-axis microphone measurements. For a conventional monopole-like source with a reasonably wide directivity that only changes slowly with listening angle and frequency, it is assumed that this measurement is reasonably representative of the signal reaching the ears-at least under near anechoic conditions. A previous paper [1] has shown that this approach in not necessarily valid, especially for complex acoustic sources such bending-wave transducers. This paper will demonstrate that the on-axis response is actually of limited use as acoustic measurement, in that merely reflects the driving force and the size of the baffle. Exploratory measurements were made of direct radiating loudspeakers using both microphone and dummy head under anechoic conditions. Comparisons between left-ear and right-ear responses showed that even with relatively simple sources, there were noticeable differences. Slight asymmetries in the loudspeaker, such as the placement of a reflex port, resulted in disturbances in the acoustic field produced by the loudspeaker, the frequencies affected having a simple wavelength relationship to the scale of asymmetry. The binaural dummy head measurements were presented to the Lindemann-Gaik binaural Iocalisation model [2][3]. Despite being small and distant from the dummy head, a conventional loudspeaker was shown to produce a systematic localisation error at higher frequencies. In contrast, the image of a much larger panel loudspeaker sbowed less error and was more consistent across the frequency range. The loudspeaker microphone responses were co-equalised to give identical on-axis measurements, and the equalisations then applied to the binaural model. The spatially complex nature of the acoustic field meant that the localisation results were not equalised by this procedure. These early results encouraged the authors to investigate the issues more fully, using both measurement and simulation. Numerical simulation using a commercial Finite Element Analysis package [4] was employed, in both the time and frequency domains, to help understand the formation and evolution of the complex acoustic fields generated by loudspeakers interacting with a head. 1 Layout of Paper The paper begins in section 2 by introducing the loudspeaker types. It then discusses aspects of numerical modelling in sections 3 and 4, presenting results in section 5. Sections 6 and 7 cover the measurement of actual loudspeakers. …