## 3 Experimental Setup  
Seven subjects (two females and five males, aged between 26 and 31) listened to the test stimuli presented over a pair of Sennheiser HD800 headphones. The effect of headphones was compensated according to [PERSON] and [PERSON] [30] by measuring the headphone transfer function (HpTF) on the dummy head (KEMAR 45 BC-12). As shown in Figure 7, seven loudspeakers were placed at the measurement positions (\(-90^{\circ}\), \(-60^{\circ}\), \(-30^{\circ}\), \(0^{\circ}\), \(30^{\circ}\), \(60^{\circ}\), and \(90^{\circ}\) azimuth angles relative to the listener) to serve as visual cues. A subjective rating scale with four possible externalization degrees from 0-3 was used to evaluate the degree of perceived externalization, which was the same that we used in our previous study [14] (Table 1). Subjects could rate each stimulus by using a slider with a step-size of 0.1 between 0 and 3. During the listening experiment, subjects were not allowed to rotate their heads, since the perceived externalization might by reduced if listeners moved their heads without head tracking [18]. It should be noted that these two kinds of BRIR modifications were not intended to simulate naturally-occurring conditions. Rather, these artificial modifications were made to find out the relevant cues that produce the perception of externalization.  
The stimulus used in the listening experiment was a truncated speech sentence (Track 50) with a length of 1.3 s taken from the European Broadcasting Union (EBU) Sound Quality Assessment Material (SQAM) [32]. For each direction, two different tests were completed by subjects, i.e., two kinds of BRIR modifications (smoothing the magnitude spectra in direct parts or removing the reverberant parts), and in each test, four audio sequences rendered by modified BRIRs, i.e., the \"SL\", \"SR\", \"SB\", and \"SN\" or \"RL\", \"RR\", \"RB\", and \"RN\" conditions, were presented in a randomized order. A repetition of each test was performed for each listener, and the externalization rating for  
\begin{table}
\begin{tabular}{c l} \hline \hline
**Degree** & **Meaning of the Degree** \\ \hline
3 & The sound is externalized and at the position of the loudspeaker. \\
2 & The sound is externalized, but not as far as the loudspeaker. \\
1 & The sound is externalized, but very close to me. \\
0 & The sound is in my head. \\ \hline \hline \end{tabular}
\end{table}
Table 1: A subjective rating scale to rate the degree of perceived externalization.  
Figure 7: A photograph of the experimental setup. Seven loudspeakers were positioned at \(-90^{\circ}\), \(-60^{\circ}\), \(-30^{\circ}\), \(0^{\circ}\), \(30^{\circ}\), \(60^{\circ}\), and \(90^{\circ}\) relative to the subject with a distance of 1.9 m. The subject evaluated the externalization ratings of test stimuli via a graphical user interface (GUI) implemented on the Max/MSP platform [31] on a Thinkpad laptop. The Fireface UFX+ audio interface can be seen on the left side of the laptop.  
each experimental condition was taken as the mean of the listener's two scores. In total, 112 audio sequences (including the repetition) should be rated by each subject. Before the formal listening test, each subject was asked to listen to all stimuli once to become familiar with the perception of these stimuli. The binaurally-rendered speech signal with the non-processed BRIR was presented at a level of 64 dBA over the headphones (measured on KEMAR). During the listening test, listeners were able to repeat every sequence. They could listen to the original stimulus played back through loudspeakers and were informed that such stimulus should act as a \"fully-externalized\" signal (externalization rating = 3). Each listener took about 55 min to complete the whole experiment. This experiment aimed to evaluate the degree of perceived externalization of presented stimuli. Therefore, other perceptual attributes, e.g., coloration [12], plausibility [33], the accuracy of source localization [34], etc., were not evaluated.