Research On The Stability And Timbre Of Ambisonics Reproduction System

Ambisonics are a series of spatial sound recording reproduction system based onphysical sound field reconstruction. They are applicable to the fields of scientific research,engineering or technique, domestic sound reproduction and multi-media techniques. InAmbisonics, the directional information of sound field is represented by each order spatialharmonics which are serves as independent encoding signals. The encoding signals are linearmixed (decoded) into loudspeakers signals, so as to reconstruct target sound field withincertain frequency range and region. The accuracy of reconstructed sound field is controlled bythe order of spatial harmonics, and different order corresponds to different number ofindependent or encoding signals. Moreover, there are flexible and various loudspeakerconfiguration in Ambisonics reproduction.The analysis, design, realization as well as evaluation on the perceived characteristics ofAmbisonics are interesting topics. This dissertation focuses on far-field Ambisonics and itstimbre, including following several aspectsThe first work deals with the influence of loudspeaker configuration on the stability ofAmbisonics reproduction system. By analysis the condition number of the transfer matrix ofdifferent loudspeaker configurations, the sensitivity of Ambisonics reproduction againstvarious errors is evaluated. The result indicates that supplementing loudspeakers at top andbottom improve stability in reproduction. Considering the performance in reproduction andfeasibility in construction, a three-level configuration with28+1loudspeakers is suggested.The second work analyzes and compares the stability of different panning methods formulti-channel sound (including pair-wise, Ambisonics). The results indicate that, the stabilityof virtual source created by Ambisonics is better. Moreover, below the frequency of1.5kHz,the stability of binaural synthesized sound pressure created by3rdor5thorder Ambisonicswith8or12horizontal loudspeakers is better than that of pair-wise method.In the third work, based on theoretical analysis, an Ambisonics reproduction system with28+1loudspeakers is set up. The mechanical construction of system is simple and flexible; thehardware (sound card, power amplifiers) performances of system are appropriate. The systemis capable of reproducing the1stto5thorder horizontal Ambisonics and the1stto3rdspatial Ambisonics.In order to testify the constructed Ambisonics system, a virtual source localizationexperiment is carried in the fourth work. Results indicate that the system is suitable for1stto5thorder horizontal Ambisonics reproduction and1stto5thspatial Ambisonics reproduction,yielding reasonable localization performance. Moreover, higher order reproduction promisesdefinite and stable virtual source, and errors in reconstructed sound field above1.5kHzinfluence little to virtual source localization.Using Moore’s modified loudness model, the timbre in Ambisonics reproduction withideal independent encoding signals is analyzed in the fifth work. The binaural loudness levelspectra are used as criterion to evaluate the timbre in reproduction. The results indicate that,increasing of frequency or deviation from central listening position may break down thecondition of accurate reproduction, accordingly result in change in binaural loudness levelspectra and perceived timbre. Increasing order of system reduces the change in binauralloudness level spectra and improves the timbre. The subjective experiment yields consistentresults with those of binaural model, thus verified the effectiveness of the model analysis. Themethod has universal applicability in the analysis of the timbre.In the sixth work, the timbre in Ambisonics recording by rigid circular and sphericalmicrophone array is analyzed. The results indicate that, array recording influences little on thebinaural loudness level spectra and thus timbre in final reconstructed sound field providingthat the high-frequency limit of microphone array exceeds that of sound field reconstruction.And on the basis of timbre analysis, a comprehensive scheme for optimizing design ofrecording and reproduction of Ambisonics is also suggested.At last，some possible future works are prospected. It is also pointed out that the methodsin present dissertation can be used to analyze near-field or mixed-order Ambisonics,5.1channels, WFS and other multichannel sound system.