Toward adapting spatial audio displays for use with bone conduction: the cancellation of bone-conducted and air-conducted sound waves.

Virtual three-dimensional (3D) auditory displays utilize signal-processing techniques to alter sounds presented through headphones so that they seem to originate from specific spatial locations. A full function of shift values could be used to adapt virtual 3D auditory displays for use with bone-conduction headsets (bonephones). This study provided anchor points for a function of shift values. The shift values were established by having participants adjust phase and amplitude of two waves in order to cancel out the signal and thus produce silence. These adjustments occurred in a listening environment consisting of air-conducted and bone-conducted tones, as well as air-conducted masking. Performance in the calibration condition suggested that participants understood the task, and could do this task with reasonable accuracy. In the bone-to-air listening conditions, the data produced a clear set of anchor points for an amplitude shift function. The data did not reveal, however, anchor points for a phase shift function – the data for phase were highly variable and inconsistent. Application of shifts, as well as future research to establish full functions and better understand phase are discussed, in addition to validation and followup studies. Sound Wave Cancellation Through Bone and Air 3 Toward Adapting Spatial Audio Displays For Use With Bone Conduction: The Interaction of Bone-conducted Waves and Air-conducted Waves There are a variety of reasons for using sound to convey information to a listener. These include conveying speech signals, as well as conveying information to a person who’s eyes are busy or to a person who is visually impaired. Regardless of the application, auditory stimuli (sounds) are typically presented to a listener through air, using loudspeakers or headphones. Headphones allow private presentation of highfidelity dichotic (stereo) sounds to a listener, without the perception changing as a person moves and turns, all in a portable package. On the other hand, there are problems with headphones. Covering the ears with headphones deteriorates detection and localization of ambient sounds in the environment. Those external sounds may be of particular interest in augmented reality and tactical situations, as well as for visually impaired users who rely on environmental audio cues as their primary sense of orientation. Furthermore, headphones do not allow auditory display to occur simultaneously with most types of hearing protection. These situations would benefit from an alternative to headphones. Because the auditory system is also sensitive to pressure waves transmitted through the bones in the skull (Békésy, 1960; Tonndorff, 1972), bone conduction may lead to an acceptable solution. Although bone conduction of sounds occurs naturally in listening to one’s own voice and to loud external sounds, it can also be directly transmitted through the skull via mechanical transducers. Presenting auditory information to listeners through bone conduction by placing vibrators on the skull can afford the same privacy and perceptual constancy that standard headphones offer, yet leave the ear canal and pinna uncovered. This may facilitate improvement in the detection and localization of Sound Wave Cancellation Through Bone and Air 4 environmental sounds, and allows the display of auditory information even when hearing protection is inserted into the ear canal. Bone-conduction devices also cater to the preferences of users who would rather not have their ears occluded (Walker, Stanley, & Lindsay, 2005b). The use of bone conduction transducers to deliver sound is not new. Because bone-conducted sound bypasses the middle ear and directly stimulates the cochlea, bone conduction is typically used in clinical audiology settings to assess the locus of hearing damage in patients. Developed for such clinical purposes, most bone-conduction transducers in production are not suitable for use in an auditory display: They typically consist of a single transducer, which is bulky and requires special equipment to drive it. Recently, compact binaural bone-conduction headsets have become available. Due to their potential for stereo presentation of sounds, their small size, comfort, and standardized input jack, these “bonephones” are much more suitable for implementation in auditory displays. The transducers of the very latest bonephones rest on the mastoid, which is the raised portion of the temporal bone located directly behind the pinna. The mastoid is a preferable transducer location relative to the forehead or temple (used in previous bone-conduction devices) because it contains the inner ear, is relatively immune to the interference associated with muscle tissue operating the jaw, and allows dichotic presentation of sounds. Bone Conduction and Spatial Audio Most of the psychoacoustics research and virtually all of the human factors research on auditory displays has assumed the conduction of sound through air (i.e., from speakers or headphones), and thus has overlooked the alternative acoustic pathway of Sound Wave Cancellation Through Bone and Air 5 bone conduction. Because sound design guidelines established for air conduction will not necessarily apply to bone conduction, auditory display design needs to be re-evaluated for bone conduction. One type of auditory display that requires extensive research to implement with bone-conducted audio is a virtual three-dimensional (3D) auditory display. In this type of display, sounds are typically presented through headphones, after being processed to make them sound like they are originating from specific spatial locations outside the head (i.e., they are “spatialized”). Virtual 3D audio displays have gained recent popularity, due to their ability to increase detectability of signals amidst distracters and noise (e.g., Brungart & Simpson, 2002), as well as provide orientation cues in cases of vision loss (e.g., Walker & Lindsay, 2006). Spatializing audio signals for virtual 3D auditory displays is a complex process, based on considerable psychophysical research investigating how to manipulate acoustic cues to produce a reliable percept of sounds originating from different locations (see Blauert, 1983). Because spatialized audio is typically delivered through air-conduction via traditional headphones (which cover the ears), the perception of environmental sounds is deteriorated. As a result, a tradeoff must occur between hearing spatialized audio and hearing external sounds when using regular headphones. This tradeoff is a problem when spatialized audio and sounds in the environment are both important for the user’s task, such as with audio navigation systems like the System for Wearable Audio Navigation (SWAN) (Walker & Lindsay, 2006). The SWAN is just one example of a system that could benefit from presentation of 3D audio via bone conduction. However, there is little research on whether bonephones can effectively replace headphones for the display of spatial audio, and how the audio would need to be processed to produce virtual sound source locations. Sound Wave Cancellation Through Bone and Air 6 One approach to evaluating the potential effectiveness of bonephones for spatial audio is to just replace headphones with a pair of bonephones, use standard spatial audio filters developed for headphones, and just see how well people can perform the spatial audio task. Although this approach has shown that bonephones can produce some degree of spatial audio (Walker & Lindsay, 2005), higher performance and greater perceptual fidelity may be achieved if the processing applied to sounds for spatialization is customized for the bonephones. A substantial difference in optimal acoustic parameters between air and bone conduction is likely, given the very different mechanisms through which those pathways transmit sound to the cochlea. Air-conducted signals are filtered by the pinna and ear canal, as well as by the workings of the tympanic membrane (eardrum), and the ossicles in the middle ear. The ossicles connect to the cochlea at the oval window; this results in standing waves on the basilar membrane, which are converted into neural impulses by the hair cells (Sekuler, 2002). For the bone-conducted signal, however, the majority of the perception results from the signal traveling directly through the skull and shaking the cochlea to set up standing waves on the basilar membrane (Békésy, 1960; Yost, 1994). Further, the bone-conduction pathway does not need to accomplish the impedance-matching that air-conduction does (Tonndorff, 1972). The goal of the research presented in this document is to begin to identify techniques for processing sounds presented through headphones can be customized for bonephones. With this information, spatial audio displays can be tuned to be more effective with bonephones. Sound Wave Cancellation Through Bone and Air 7 Relevant Research: Threshold Measurement and Related Discussion A formal evaluation of using bonephones to present spatial audio requires understanding the basic properties of the bone-conduction hearing mechanisms, spatial audio cues, and how the virtual 3D audio is created. As with air-conduction hearing, some basic information about thresholds of audibility has been gathered for bone conduction. In particular, most research on bone conduction has focused on establishing threshold norms for clinical testing of middle ear disorders. This clinical research has yielded threshold curves such as those shown in Figure 1. The methods and implications of this research can inform the design of research aimed at using bonephones in spatial audio displays. * values estimated from graph Figure 1. Bone-conduction thresholds from several researchers. The y-axis is in units that are the bone-conducted equivalent to the decibel measurement used for air-conduction (see text). These thresholds are used to define “normal” hearing in order to screen people for whether