When validating systems that use headphones to synthesize virtual sound sources, a direct comparison between virtual and real sources is sometimes needed. This paper considers the passive influence of headphones on the sound transmission and perception of external loudspeaker sources, for which physical measurements and behavioral data have been obtained. Physical measurements of the effect of a number of headphone models are given and analyzed using an auditory filter bank and binaural cue extraction. These highlighted that all of the headphones had an effect on localization cues and repositioning had a measurable effect. A localization test was undertaken using one of the best performing headphones from the measurements. It was found that the presence of the headphones caused a small increase in localization error and that the process of judging source location was different, highlighting a possible increase in the complexity of the localization task. INTRODUCTIONThe use of binaural rendering is popular in a number of audio applications-from hearing research [1-3] to entertainment [4,5]. In each application, the specific requirements for the performance of a binaural system will be slightly different although generally, the aim is to induce the perception of intended auditory events as accurately as possible. Designing an assessment methodology that validates a binaural system within its intended application is often a difficult task. A common metric for a binaural system is the ability to produce a virtual sound source that is indistinguishable from a real sound source. Indirect comparisons have been investigated, for example, by Minnaar et al. [6] and Møller et al. [7,8] in which non-dynamic binaural simulation and real loudspeaker localization tasks were considered in separated experiments. However, for direct comparisons where real and virtual loudspeakers are presented simultaneously, the validation of headphone-based binaural systems against a real loudspeaker reference can be problematic. The listener must wear the headphones throughout the experiment, which will affect the sound transmission from the external loudspeakers. A number of discrimination studies have involved direct comparison of real sources with headphone-delivered virtual sources [9-13] as well as some recent localization tests [14,15] and loudness equalization studies [16,17]. The passive use of headphones may have a significant effect on the perception of the external loudspeaker and therefore cause an unknown and possibly directionally dependent bias. Hartmann and Wittenberg [10] noted that wearing headphones appeared to affect the listeners' ability to distinguish between front and back, although they also state that they were not aware of its effect on experiments in the azimuthal plane. To highlight the importance of the problem, Erbes et al.[18] presented work on the development of an advanced headphone system specifically for the field of binaural reproduction.This study investigates whether headphones mounted on a listener will ...