Potential Sources of BRS Errors

Olive and his team (2007) identified the source of potential errors with the BRS system and defined them as:

1) Measurement errors – These are related to the repeatability and accuracy of BRS measurements and the playback of binaural signals.

2) Anatomical errors – These are related to the differences in the shape and size of the manikin’s head/torso/pinna versus listeners’.

Playback using Sennheiser HD 518 headphones and Logitech ultrasonic head-tracker.

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3) Positional errors – These are related to positional differences between the manikin’s head at the time of the BRS measurement, and the listener’s head observed in situ. 4) Cognitive-related errors – These are related to errors from inaccurate BRS reproduction of non-auditory (e.g. tactile and visual) stimuli.

These errors were categorized as directionally-dependent and directionally-independent with sub-categories of non-individualized and individualized, as shown in figure 4.2.

Figure 4.2: Source of BRS errors; derived from (Olive et al., 2007)

The directionally dependent errors are those measurement errors which change as the angle of the sound arriving at the binaural manikin's ears changes. These types of errors can all be classified as "individual". In contrast, directionally independent errors are not influenced by the location of the listener's ears or the sound source. Directionally independent errors can be further broken down into individualized and non-individualized errors.

4.4.1 Directionally Dependent Errors

The first two sources of directionally dependent errors are directly related to the physical differences between the listener and the manikin. One error is based on the differences in pinna / concha shape while the other is due to differences in head / torso shape and size. These errors change with each individual and therefore cannot be corrected through the use of a globally

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applied filter. The third source of directionally dependent errors stems from the BRS calibration process itself, which is further explained in section 4.5.2. This error is based on the idea that the calibration of the BRS system to one loudspeaker, room or headphone may not be appropriate to apply to other like objects. The reason that this error falls into the individual classification is that it will change from one object to another, such as calibrating to a particular set of headphones and then applying the calibration filter to a different set of headphones. The first error is difficult and time consuming to correct as it requires that each test subject's pinna / concha be measured and taken into consideration during calibration. There are also questions as to whether individual pinna measurements are required. Researchers have found that, when using head-tracked binaural playback, listening test results on localization accuracy are the same as when the listener sits in the manikin's position (Romigh et al., 2015), (Begault et al., 2001), (Horbach et al., 1999). Research has also shown that listeners tend to adapt to non- individualized binaural recording and, with repeated listening tests, further adaptation helped to resolve many of the initially perceived errors (Zahorik, 2002), (Minnaar et al., 2001). The second error has been identified as changes in arch-shaped notches in the mid-band frequencies that lessen as the frequencies get higher (Algazi et al., 2001). However, more perceptual research on this type of error needs to be conducted to determine if it is significant. In the case of the BRS experiments conducted for this thesis, the third error is of no consequence as the headphones utilized in the calibration procedure were the same ones used for the listening tests.

4.4.2 Directionally Independent Individualized Errors

The first source of directionally independent individualized error is caused by the differences in transfer function between the manikin's and the listeners' conchae. This error is somewhat directionally independent, with the non-directional (frequency response) part of this error being corrected during calibration. The second source of error is related to the change in frequency response of the headphones due to varying fit between listeners and repeated re-seatings of the headphones. Compensating for varying fit is difficult, as it once again requires measurements for each individual listener and there remains a question as to the magnitude and perception of this error. Informal listening tests by the author and her colleagues have indicated that this is not mandatory, although further research is needed. Attempts are made to correct for a degree

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of re-seating errors by undertaking the energy averaging of several re-seated measurements on the binaural manikin prior to calibration.

Errors caused by the difference in manikin and listener seating locations in these experiments were minimized by carefully measuring and marking the locations by using a hand-held laser distance measurer (Bosch GLM 30 MP). Though exact placements of the listeners can be difficult, it is believed that any residual error is probably unnoticeably small.

4.4.2 Directionally Independent Non-individualized Errors

Directionally independent non-individualized errors are typically the easiest and least costly to correct. Non-linear frequency responses from the manikin microphones, microphone pre-amps and headphones can all be corrected with a single filter set and in a one-step process, as is shown in section 4.5.2. Regularization and smoothing of the signals during the calibration process, as described in section 4.5.2, will also introduce some amount of error which is difficult to avoid but a necessary tradeoff.

The lack of tactile and/or visual cues can create another source of directionally independent non-individualized errors. These errors were present in the experiments reported in this thesis; notably the lack of tactile feedback from the subwoofers which were not present during BRS playback and the absence of visual feedback with respect to the rooms which were scanned. Concerns regarding visual errors brought on by a type of cognitive dissonance when comparing BRS and in situ playback have been shown to be unwarranted in similar tests for automotive audio (Postel et al., 2011), and loudspeaker auditioning in varying rooms (Olive and Martens, 2007). In their research, Gros and Chateau showed that environmental and/or visual cues have little influence on audio quality scores(Gros and Chateu, 2002). Beresford et al. further look at the significance of differences in listening environments when comparing a listening room to a car with final results of their listening tests broken into two groups: trained and untrained listeners. For untrained listeners, the lack of visual accord did not have a significant effect on quality scores. For trained listeners, there was an interaction between stimulus and visual context but it proved to be of a very small magnitude (η2 _{= 0.017) (Beresford et al., 2006a),}

(Beresford et al., 2006b). These studies also lacked the tactile feedback of a subwoofer, which did not appear to have an influence on listener's ratings. However, this issue warrants further research.

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