A Comparison on the Localization Performance of Static and Dynamic Binaural Ambisonics Reproduction with Different Order

Ambisonics is a series of spatial sound reproduction system based on spatial harmonics decomposition and each order approximation of sound field.Ambisonics signals are originally intended for loudspeakers reproduction.By using head-related transfer functions(HRTFs)filters,binaural Ambisonics converts the Ambisonics signals for static or dynamic headphone reproduction.In present work,the performances of static and dynamic binaural Ambisonics reproduction are evaluated and compared.The mean binaural pressure errors across target source directions are first analyzed.Then a virtual source localization experiment is conducted,and the localization performances are evaluated by analyzing the percentages of front-back and up-down confusion,the mean angle error and discreteness in the localization results.The results indicate that binaural Ambsonics reproduction with insufficiently high order(for example,5-10 order)is unable to recreate correct high-frequency magnitude spectra in binaural pressures,resulting in degradation in localization for static reproduction.Because dynamic localization cue is included,dynamic binaural Ambisoncis reproduction yields obviously better localization performance than static reproduction with the same order.Even a 3-order dynamic binaural Ambisoncis reproduction exhibits appropriate localizations performance.

Multiple Scattering Between Adjacent Sound Sources in Head-Related Transfer Function Measurement System

To accelerate head-related transfer functions(HRTFs)measurement,two or more independent sound sources are usually employed in the measurement system.However,the multiple scattering between adjacent sound sources may influence the accuracy of measurement.On the other hand,the directivity of sound source could induce measurement error.Therefore,a model consisting of two spherical sound sources with approximate omni-directivity and a rigid-spherical head is proposed to evaluate the errors in HRTF measurement caused by multiple scattering between sources.An example of analysis using multipole re-expansion indicates that the error of ipsilateral HRTFs are within the bound of±1.0 dB below a frequency of 20 kHz,provided that the sound source radius does not exceed 0.025 m,the source distance relative to head center is not less than 0.5 m,and the angular interval between two adjacent sources is not less than 20 degrees.Similar conclusions under different conditions can also be analyzed and discussed by using this calculation method.Furthermore,the results are verified by measurements of HRTFs for a rigid sphere and a KEMAR artificial head.

Influence of the Number of Loudspeakers on the Timbre in Horizontal and Mixed-Order Ambisoncis Reproduction

Ambisonics is a series of flexible spatial sound reproduction systems based on spatial harmonics decomposition of sound field.Traditional horizontal and spatial Ambisonics reconstruct horizontal and spatial sound field with certain order of spatial harmonics,respectively.Both the Shannon-Nyquist spatial sampling frequency limit for accurately reconstructing sound field and the complexity of system increase with the increasing order of Ambisonics.Based on the fact that the horizontal localization resolution of human hearing is higher than vertical resolution,mixed-order Ambisonics(MOA)reconstructs horizontal sound field with higher order spatial harmonics,while reconstructs vertical sound field with lower order spatial harmonics,and thereby reaches a compromise between the perceptual performance and the complexity of system.For a given order horizontal Ambisoncis or MOA reproduction,the number of horizontal loudspeakers is flexible,providing that it exceeds some low limit.By using Moore’s revised loudness model,the present work analyzes the influence of the number of horizontal loudspeakers on timbre both in horizontal Ambisonics and MOA reproduction.The binaural loudness level spectra(BLLS)of Ambisoncis reproduction are calculated and then compared with those of target sound field.The results indicate that below the Shannon-Nyquist limit of spatial sampling,increasing the number of horizontal loudspeakers influence little on BLLS then timbre.Above the limit,however,the BLLS for Ambisoncis reproduction deviate from those of target sound field.The extent of deviation depends on both the direction of target sound field and the number of loudspeakers.Increasing the number of horizontal loudspeakers may increase the change of BLLS then timbre in some cases,but reduce the change in some other cases.For MOA,the influence of the number of horizontal loudspeakers on BLLS and timbre reduces when virtual source departs from horizontal plane to the high or low elevation.The subjective evaluation experiment also validates the analysis.