Proceedings of ICAD 04- Tenth Meeting of the International Conference on Auditory Display, Sydney, Australia, July 6-9, 2004 THE INTERACTION BETWEEN HEAD-TRACKER LATENCY, SOURCE DURATION, AND RESPONSE TIME IN THE LOCALIZATION OF VIRTUAL SOUND SOURCES Douglas S. Brungart Brian D. Simpson Richard L. McKinley Air Force Research Laboratory 2610 Seventh Street WPAFB, OH 45433 douglas.brungart@wpafb.af.mil brian.simpson@wpafb.af.mil richard.mckinley@wpafb.af.mil Alexander J. Kordik Ronald C. Dallman David A. Ovenshire Sytronics, Inc General Dynamics Dayton, OH alex.kordik@wpafb.af.mil ron.dallman@wpafb.af.mil david.ovenshire@wpafb.af.mil ABSTRACT One of the fundamental limitations on the fidelity of interactive virtual audio display systems is the delay that occurs between the time a listener changes his or her head position and the the time the display changes its audio output to reflect the corresponding change in the relative location of the sound source. In this ex- periment, we examined the impact that six difference headtracker latency values (12, 20, 38, 73, 145 and 243 ms) had on the local- ization of broadband sound sources in the horizontal plane. In the first part of the experiment, listeners were allowed to take all the time they needed to point their heads in the direction of a continu- ous sound source and press a response switch. In the second part of the experiment, the stimuli were gated to one of eight different durations (64, 125, 250, 375, 500, 750, 1000 and 2000 ms) and the listeners were required to make their head-pointing responses within two seconds after the onset of the stimulus. In the open- ended response condition, the results showed that latencies as long as 243 ms had no impact on localization accuracy, but that there was an increase in response time when then latency was longer than 73 ms. In contrast, the data from the time-limited response conditions showed that latencies that exceeded 73 ms had no im- pact on response time but that they significantly increased the an- gular localization error and the number of front back confusions. Together with the results of earlier studies, these results suggest that headtracker latency values of less than 70 ms are adequate to obtain acceptable levels of localization accuracy in virtual audio displays. 1. INTRODUCTION A fundamental requirement of all interactive virtual audio display systems is the ability to quickly update the virtual sound field in response to the movements of a listener’s head. These exploratory head movements play a number of critical roles in human sound lo- calization. They help listeners distinguish between sound sources located at equivalent lateral positions in the front and rear hemi- spheres [1, 2]. They influence the perception of elevation, particu- larly for low frequency sounds [3]. They allow listeners to increase their spatial acuity by orienting themselves directly towards the sound source [4]. And they also play a crucial role in increasing the realism and immersiveness of virtual audio simulations [5]. However, because of the limitations inherent in virtual audio display systems, it is not always clear that the users of these sys- tems are obtaining as much useful information from exploratory head motions as they would if they were listening in the real world with their own ears. In the real world, there is no time delay be- tween the movement of a listener’s head and the corresponding change this movement produces in the sounds reaching the two ears. Unfortunately, this kind of instantaneous responsiveness is not feasible with the current generation (or possibly any future generation) of virtual audio displays. All current display systems introduce some delay between the time the head is moved to the time the sound field is updated. These delays come from a num- ber of sources, including the latency of the actual tracking device, the communications delay between that device and the audio dis- play, the time required to select the appropriate head-related trans- fer function (HRTF) and switch to that HRTF, the processing time required for the HRTF filtering, and any output buffering that oc- curs between the digital filtering of the sound and its eventual pre- sentation to the listener over headphones [6]. Additional complications can occur when head-coupled vir- tual audio display systems are integrated into larger, more com- plex systems that may include several subsystems that all require headtracking information at the same time. In an aircraft cockpit, for example, headtracking information might be used by an audio display, a head-mounted visual display, and also for other purposes such as target cuing. When this occurs, it is not always clear how to prioritize the routing of the headtracker information to the different competing components within the system. It might be necessary to have the headtracker directly coupled with one system, such as the visual display, and then have this intermediate system pass the information on to other subsystems through a separate communi- cations channel. In these complex systems, there may be important tradeoffs between total system cost and the headtracker latency of the virtual audio display. Thus, the impact that headtracking de- lays have on the virtual audio display performance is a question of great theoretical and practical interest for the designers of spatial audio systems. Although a number of researchers have examined the effects of headtracker latency on sound localization, the results have not been completely consistent. Some researchers have reported that ICAD01-1