ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 102 (2016) 129 – 140 DOI 10.3813/AAA.918930 Investigation of Perceptual Interaural Time Difference Evaluation Protocols in a Binaural Context L. S. R. Simon, A. Andreopoulou, B. F. G. Katz Audio Acoustics group, LIMSI, CNRS, Université Paris-Saclay, Orsay, France laurent.simon@atiam.fr, areti.andreopoulou@limsi.fr, brian.katz@limsi.fr Summary We report the advantages and drawbacks of three protocols (adjustment, AFC-yes/no, and AFC-left/right dis- crimination protocols) evaluating the estimation of the interaural time differences (ITDs) for correct lateraliza- tion perception. The protocols were compared with respect to reliability of the perceived ITD and just noticeable difference (JND) estimations, robustness to participants’ errors, effect of the number of tested blocks of trials, and test duration. Binaural stimuli were employed, including all spatial cues for sound sources placed at hori- zontal positions of 30 ◦ and 90 ◦ . All three protocols yielded comparable perceived ITD but different JNDs at 30 ◦ . At 90 ◦ , both the AFC-left/right and the adjustment protocols were more problematic than the yes/no protocol. Overall, only the yes/no protocol fulfilled the requirements of this study, i.e. a quick protocol that can be used for all angles. PACS no. 43.66.Pn, 43.66.Qp 1. Introduction 1.1. Interaural Time Differences: a binaural local- ization cue Spatial auditory perception, i.e. the ability of humans to interpret a complex auditory scene and to draw conclu- sions regarding the apparent locations of its sound ob- jects is a very complicated task. In a natural environment, where humans can dynamically interact with their sur- roundings, this involves a combination of acoustical and non-acoustical cues. Acoustical cues are Head-Related Transfer Functions [HRTF], comprising Interaural Time Differences [ITD], Interaural Intensity Differences [IID], and spectral information; non-acoustical cues are related to the familiarity with the timbre of the sound sources, as well as visual and haptic information. In this article, we use the term “HRTF” to refer to a full spatial set of binau- ral filters measured on a single individual. Localization perception was explored as early as the end of the 19 th century by the British physicist Lord Rayleigh [1]. His research resulted in the introduction of the “Du- plex Theory” of sound localization which has been the ba- sis for modern localization and lateralization studies[2, 3]. Auditory acuity in localization has been extensively stud- ied. The “Minimum Audible Angle” (MAA), defined as the smallest detectable difference in the position of a sound source, was introduced in the mid 20 th century by A.W. Received 29 October 2014, accepted 04 December 2015. Mills [4]. Early studies were conducted in anechoic or sound treated spaces, equipped with one or multiple iden- tical sound sources surrounding the participants. The two main testing principles involved either the audition of a single tone, for which participants had to report whether it appeared from the left, right, or center [4], or the audition of two sound stimuli, for which participants had to indi- cate whether the second (target) stimulus appeared to the left or to the right of the first (reference) [5]. Both were forced-choice tasks, requiring a response even in cases of uncertainty. These principles resulted in the two main experimental protocol designs dominating spatial acuity studies over the following decades. The first describes an absolute localiza- tion task where participants are asked to indicate the appar- ent position of a sound source in space. This method mea- sures the absolute localization precision in human auditory perception. The second describes a relative precision task where participants are asked to indicate whether or not two sounds originate from the same direction and measures the relative resolution of auditory localization, also known as the “Just Noticeable Difference” (JND) [6]. Variations of these two protocols have been used in both free-field [7, 8] and headphone conditions [5, 9, 10, 11, 12, 13]. According to Mills [4], the just noticeable change for a reference azimuth of 0 ◦ on the horizontal plane was 1 ◦ . Makous and Middlebrooks reported that the average az- imuthal resolution for locations directly in the front was approximately 2 ◦ , decreasing to approximately 20 ◦ for pe- ripheral locations [7]. © S. Hirzel Verlag · EAA 129