title: The influence of different BRIR modification techniques on externalization and sound quality
Creators: name: Giller, Peter Maximilian
name: Wendt, Florian
name: Holdrich, Robert
Publisher: name: EAA Spatial Audio Signal Processing Symposium
Year: 2019
Subjects: externalization
binaural
reverberation
head-related transfer function

description: Binaural synthesis aims to produce well externalized sound images, i.e.,auditory events are located outside the listener's head.Various studies have examined perceptual and technical aspects of the phenomenon.Studies showed that the presence of reverberation can increase the degree of externalization.For binaural rendering, however, it is usually desired to preserve the room impression and sound color of the original recording.We distinguish head-related impulse responses (HRIRs), which capture the influence of reflections at the pinnae, head, and torso on the direct sound impinging at the ear canals from a certain direction, and binaural room impulse responses (BRIRs).The latter consist of the direct part, which is identical with the HRIR, followed by early reflections and diffuse reverberation of the measurement room from all directions, convolved with the HRIRs for the respective directions.Thus, the goal is to add as little reverberation as necessary to HRIRs in order to increase externalization, or to reduce the reverberation of BRIRs as much as possible in order to avoid sound colorations.The influence of BRIR truncation as the simplest method to reduce reverberation on externalization was investigated in several studies [1-4].It was found that a minimum BRIR length of 80-100 ms is sufficient to yield externalized sound images.However, this truncation leads to short, but rather artificially sounding BRIRs.Another method investigated in the context of the room divergence effect [5] is to modify the direct-to-reverberant energy ratio, yielding more natural sounding BRIRs while preserving externalization.Inspired and motivated by these approaches, we investigate the influence of modifications of the reverberant part systematically in order to determine the optimal method with regard to the achieved sound quality.Three basic manipulations to reduce the reverberation of BRIRs are studied, which are: (i) truncating the impulse response length (temporal modification), (ii) manipulating the reverberant-to-direct ratio by weighting the reverberant part with a constant factor (modification of level), (iii) and altering the reverberation time by weighting the reverberant part with an exponential decay function (temporal modification of level).We have compared the three approaches in informal listening experiments and are currently working on a formal experiment.In the first part of the experiment, participants rate the degree of externalization of anechoic speech signals convolved with manipulated BRIRs presented via headphones in comparison to a loudspeaker reference.We determine externalization scores for each manipulation method as a function of the varied parameter, i.e., truncation time, reverberation time, reverberant-to-direct ratio.Obviously, the three parameters are not suitable for direct comparison.An objective comparison of the three methods is achieved by the energy percentiles of manipulated impulse responses, i.e., the interval of time which contains a certain percentage of the total energy of the impulse response.In the second part of the experiment, we evaluate the same conditions regarding the perceived amount of reverberation and sound colorations.Our findings indicate strong differences regarding the amount of perceived reverberation and sound colorations between the different manipulation methods.Manipulating the reverberation time followed by truncation appears to be a reasonable tradeoff to reduce reverberation and sound colorations while preserving externalization and still yielding short impulse responses.Literature:[1] Catic, Jasmina, Sebastien Santurette, and Torsten Dau. "The role of reverberation-related binaural cues in the externalization of speech." JASA (2015).[2] Begault, Durand R., Elizabeth M. Wenzel, and Mark R. Anderson. "Direct comparison of the impact of head tracking, reverberation, and individualized head-related transfer functions on the spatial perception of a virtual speech source." J. Audio Eng. Soc. (2001).[3] Li, Song, Roman Schlieper, and Jurgen Peissig. "The effect of variation of reverberation parameters in contralateral versus ipsilateral ear signals on perceived externalization of a lateral sound source in a listening room." J. Audio Eng. Soc. (2018).[4] Crawford-Emery, Ryan, and Hyunkook Lee. "The Subjective Effect of BRIR Length on Perceived Headphone Sound Externalization and Tonal Coloration." Audio Eng. Soc. Conv. 136. AES, 2014.[5] Klein, Florian, Stephan Werner, and Thomas Mayenfels. "Influences of training on externalization of binaural synthesis in situations of room divergence." J. Audio Eng. Soc. (2017).