Evaluation of the effect of head-mounted display on individualized
head-related transfer functions
Maria CUEVAS-RODRIGUEZ, David Lou ALON, Samuel W. CLAPP, Philip W. ROBINSON, Ravish MEHRA
Facebook Reality Labs, Facebook, 1 Hacker Way, Menlo Park, CA 94025, USA, Email: davidalon@fb.com

ABSTRACTHead Mounted Displays (HMDs) have become popular for acoustic experiments, such as perceptual comparisonof virtual and real sound sources, or Head Related Transfer Function (HRTF) measurement. However, wearing
an HMD while listening to real sound sources modifies the sound waves that reaches the listener’s ears. In thiswork, we study the effect of HMDs on individualized HRTFs. We have collected acoustic HRTF measurements,
with and without the HMD, from 24 human participants and a manikin head, over 612 directions (including
different azimuth and elevation angles). An objective evaluation indicates that, although there are frequencydependent directional regions where the distortion caused by the HMD is higher, other regions are less affectedby the HMD. A MUSHRA test with 15 subjects compares individualized HRTFs and a generic HRTF, measured
with and without HMD. The perceptual evaluation reveals a significant effect of the HMD on the measured
individualized HRTF, but it also shows that this effect is significantly smaller than the difference between individualized HRTFs and a generic HRTF, as indicated by the objective evaluation. This implies that, althoughdistorted by the presence of the HMD, some of the individualized HRTF characteristics might be preserved
when the HMD is worn.
Keywords: Head-related transfer function, 3D-audio

1

INTRODUCTION

The use of Head Mounted Display (HMDs) is becoming increasingly common in virtual and augmented reality
applications and experiments that involve auditory stimuli. Some examples are: natural listening to real soundsources while watching video on the HMD; Head-Related Transfer Function (HRTF) measurement processes
while wearing an HMD; or immersive audio experiences in mixed reality environments.
The HRTF represents how the listener’s head, pinna and torso filter the sound before it reaches his/her ears.
The HRTF is an individualized characteristic of the subject and plays an important role in the synthesis ofbinaural spatial sound [6]. In the case of sound localization of sources in the horizontal plane, the Interaural
Time Difference (ITD) provides one of the primary cues that determine the perceived location of the sound[12, 14]. However, for median plane localization, relevant information that allows the user to get elevation cues
is derived from the spectral features of the sound [3].
Wearing an HMD will modify the listener’s HRTF since it diffracts the acoustic waves that travel from thesound source to the listener’s ears. Some studies have been carried out to evaluate how an HMD affects the
HRTF. Gupta et al [9] presented an analysis of the effect of different HMDs, including the Oculus Rift™
(Rift), on the HRTF for a KEMAR manikin [5] over the horizontal plane. The objective and subjective analysis
revealed noticeable changes in the HRTF spectrum in the 1-16 kHz frequency range. In the case of Rift, themaximum error was observed at −60° azimuth. Another study that analyzes the distortion caused by HMDs
on the HRTF was presented by Genoveses et al [8]. This work is focused on headsets used for mixed andaugmented reality (more specifically the Microsoft Hololens™ and a Metavision Meta-2™), and presents theeffect of these headsets on a manikin’s HRTF over 200 azimuth and 6 elevation angles around the head. The
analysis shows that non-negligible relevant distortions caused by the HMD are mostly present at the contralateralanterior quadrant (azimuth 270°-360°) and ipsilateral posterior (azimuth 90°-180°), starting from approximately
3 kHz up to 8 kHz. The effect of other HMDs was very recently published for a Manikin head [7].

2635Regarding virtual spatial audio perception while wearing an HMD, Axel et al [2] have recently analyzed theeffect of the HMD on localization accuracy in loudspeaker-based virtual sound environments. Their results show
a reduction in the localization accuracy while wearing the HMD; however, once visual information was added
to the experiment the localization error induced by the HMD was negligible with respect to the localization
accuracy of the presented experiment. These studies show that the effect of the HMD in general, and, more
specifically the Rift, is not negligible and, for specific angles, this should be compensated for [8, 9]. However,the studied angles are mostly in the horizontal plane with a limited number of elevation angles (between 1to 6) and with a limited number of subjects. Moreover, in order to gain an insight into how substantial isthe effect of the HMD on an individualized HRTF, it is valuable to evaluate this effect with respect to thedifference between an individualized HRTF and the more commonly used generic (e.g. manikin head’s) HRTF.
Both generic and individualized HRTFs have to be evaluated using the same metrics and with the same system.
Such a comparison is missing in current literature.
In this paper we study the effect of the Rift HMD on acoustically measured individualized HRTFs for 24
subjects. The study compares the individual HRTF (without the effect of the Rift) with the same HRTF withthe addition of the Rift, and also these two individual HRTFs are compared with a generic HRTF. The 24
human subjects and one KEMAR manikin HRTFs were measured in an acoustically treated chamber, over
612 different directions, (36 azimuth angles ×17 elevations angles), without and with the Rift. The effect of
the HMD is objectively evaluated based on the Spectral Difference Error (SDE) and on discrepancies in the
ITD. Analysis of the SDE shows that the distortion of the HRTF magnitude is both frequency and directional
dependent, and bigger disturbances are introduced by the headset at the contralateral directions. Analysis of the
ITD reveals that ITD errors are usually larger around the front side of the head. Finally, a MUltiple Stimuli
with Hidden Reference and Anchor (MUSHRA) test [1] is used to perceptually evaluate the effect of the Rifton the measured HRTF based on the assessment of 15 subjects. In this test, subjects were asked to evaluate
four different HRTFs: the subject’s individualized HRTFs, without and with the Rift, and generic HRTFs, also
without and with the Rift. The results validate the objective evaluation and show that the effect of the Rift isperceptible and is significantly smaller than the difference between individualized and generic HRTFs.

2

HRTF MEASUREMENT WITH AND WITHOUT RIFT

HRTFs were measured using a rotating arc-shaped array of loudspeakers that is housed in an acoustically treatedchamber (anechoic down to a frequency of approximately 500 Hz). To conduct the HRTF measurements, thesubjects were positioned in the center of the array in a seated position. Wall-mounted lasers were used toaccurately position the subjects at the center of the apparatus, with the center of the head taken to be the midpoint of a line connecting the 2 ears. Knowles FG-23329 microphones were placed at the entrance to the ear
canal and embedded in foam earplugs. A chinrest mounted on the base plate of the apparatus was used to help
the subject remain still for the duration of the measurements. The entire platform can be electronically raised
and lowered to make sure that the subjects’ head is positioned at the correct height.
During the measurement, the subjects remained still, and a semi-circular arc mounted with 17 Meyer Sound
MM-4XP loudspeakers was moved manually around them. The loudspeakers range in elevation angle from−66° to +85° (with spacing of 9°-10° degrees between adjacent loudspeakers) and were sequentially positioned
every 10° in azimuth around the full sphere, yielding a total of 612 measurement directions. The distance fromthe center of the apparatus to the front of each loudspeaker was 1.53 meters. 1-second logarithmic sine sweeps
were played from each loudspeaker, ranging in frequency from 200 Hz to 20 kHz at a sampling frequencyof 48 kHz, with a level of 94 dB SPL at the center of the array. During the measurement, subjects wore a
wig cap with several attached infrared-reflective markers, which can be tracked by 10 OptiTrack™, Prime 17W,
cameras mounted on the room’s walls. The subjects’ head position and orientation were tracked during themeasurement, and a warning dialog box appeared in the measurement software (written in Matlab, communicating with OptiTrack’s Motive software for motion tracking) if the subject exceeded certain movement thresholds.
The measurement sessions described in this study used a three-stage process. Subjects’ HRTFs were measured
first without wearing the Rift. Then, the Rift was placed on the subject (while they tried to remain as still
as possible, keeping their chin on the chinrest), and alignment was checked again using the alignment lasers.
HRTFs were then measured while the subject was wearing the Rift. Finally, the Rift was removed, the subject’s
position was checked with the lasers once again, and HRTFs from all elevation angles at 0° azimuth were measured for validation. After the HRTF measurements were completed, the chair and chinrest were removed, andfree-field microphone measurements were taken from all loudspeakers in the array (positioned at 0° azimuth) toboth the left- and right-ear microphones, which were aligned to the center of the array with the alignment lasers
and supported with a microphone stand. In post-processing, these free-field sweep responses were deconvolved
from the measured head-related sweep responses to yield head-related impulse responses (HRIRs) and remove
the individual frequency response of the microphones and the loudspeakers. The HRIRs were then windowed
in time to 256 samples, corresponding to a length of 5.3 ms at a sampling frequency of 48 kHz.

2636

3

EFFECT OF RIFT ON HRTF - OBJECTIVE EVALUATIONThis section presents an objective evaluation of the effect of the Rift on the measured HRTF of 24 humansubjects and KEMAR. At the beginning of the section a validation of the measurements is presented. Then, theSDE and ITD error are used as objective metrics to compare the individual HRTFs, with and without the Rift,as well as with the generic HRTF. All the computations and graphs in this study use the coordinate systemdefined by the AES standard 69-2015 [4], where the azimuth angle ∈ (−180°, 180°], is the horizontal angle
in degrees measured counterclockwise from the listener look direction, the elevation angle ∈ [−90°, 90°] is thevertical angle in degrees measured from the horizontal plane.
3.1 Repeatability of HRTF measurement
As mentioned in the previous section, subject’s movement was limited by using a chinrest and alignment lasers.
To quantify the effect of subjects movement and the effect of the Rift on the measured HRTF, the HRTF without
the Rift (HRT FWO ) and the HRTF with the Rift (HRT FRi f t ) are compared with the second measurement of the
HRTF without the Rift (HRT FWO2 ), measured after the HRT FRi f t .
Figure1 shows the three compared HRTF conditions over the front direction for (a) KEMAR and (b) a humansubject. In Fig.1(a) it is noticeable that there is a remarkable effect of the Rift on the HRTF, and that thedifference between HRT FWO and HRT FWO2 is negligible, which implies that the measurement repeatability error
is relatively small. As expected with human subjects, in Fig.1(b) it is noticeable that the difference between
HRT FWO and HRT FWO2 is larger than in the previous case using KEMAR, but it is interesting to note that theeffect of movement seems smaller than the effect of wearing Rift.

Figure 1. HRTF magnitude over the front direction for (a)KEMAR and (b)human subject for the left ear.

3.2 Effect of the Rift on HRTF magnitude
To determine the differences in the HRTF magnitude for the cases without and with the Rift HMD, a comparison between the spectrum of both signals has been carried out. The SDE between two HRTFs can be computedas (Eq. 5.19 in [15]):|HRT FWO (Ω, f )|SDE WO−Ri f t (Ω, f ) = |20 log10|(1)|HRT FRi f t (Ω, f )|where Ω is the direction angle (azimuth and elevation) and f is the frequency. To study the effect of the
Rift across subjects, the SDEWO−Ri f t (Ω, f ) was computed for each subject and then averaged across subjects
and across frequencies, denoted by SDE WO−Ri f t (Ω). All the computations have been done for both two ears,
however, figures in the paper show only the left ear.
Figure 2 shows the SDE WO−Ri f t (Ω) averaged across a wide frequency bandwidth (0 − 16kHz). The horizontal
and vertical axis indicate the azimuth and elevation angles in degrees, respectively. The color map indicatesthe SDE WO−Ri f t (Ω) from 0 dB (denoted by the blue color) to 6 dB (denoted by the red color). A higher SDE
value implies larger differences between the HRT FWO and the HRT FRi f t . Figure 2(a) shows the average acrossall human subjects and Fig.2(b) shows the SDE for the KEMAR manikin. In these graphs larger errors can beobserved in contralateral directions. Also, comparing the two graphs on the contralateral side for low elevation
angles, larger errors are found for human subjects (Fig.2(a)) than for the KEMAR (Fig.2(b)), which might be
partly explained by subjects’ movement.
In order to analyze the SDE WO−Ri f t (Ω) across different azimuth and elevation angles, while taking into accountthat the SDE is also frequency dependent, the averaged SDE was calculated over three different frequency
bands. Figure 3 shows the averaged SDE across three frequency bands: low frequencies (lower than 1 kHz),mid frequencies (between 1 and 5 kHz) and high frequencies (higher than 5 kHz). In the low frequency band
the HRTF is less affected by the presence of the Rift, due to the fact that the wavelengths are large relative to
the Rift form factor. The effect of the Rift is larger at mid frequencies, and most dominant at high frequencies.
2637

Figure 2. SDE WO−Ri f t (Ω) averaged across frequency for (a) all human subjects and (b) KEMAR manikin.

Figure 3. SDE WO−Ri f t (Ω) averaged across all subjects and KEMAR for three different frequency bands (a) low
frequency, (b) mid frequency and (c) high frequency.
For mid and high frequencies, the differences between both sides of the listener’s head is easily appreciable. At
the ipsilateral ear (positive azimuth angles range for left HRTF) the errors are lower than at the contralateralear. This effect is caused because the sound wave will be obstructed by the headset to get to the contralateralear, which is not the case with the ipsilateral ear, where the direct path is not affected. A symmetrical effectalso occurs with the right-HRTF.
Different factors, such as the effect of the Rift’s presence, the effect of subject movements, and the measurement
system repeatabilily contribute to the SDE values that are presented in Fig. 3. In order to evaluate the part ofthe effect due to the Rift in this overall error, SDE WO−Ri f t is compared to SDE WO−WO2 .
Figure 4 shows the SDE averaged across all directions for a specific subject. The vertical axis shows theSDE value and the horizontal axis the frequency value. Even taking into account the errors introduced by the
movement of the subjects, the graph shows that the SDE WO−Ri f t is larger than the error caused by the subject
movements (SDE WO−WO2 ). In addition, the difference between the Individualized-HRT FWO and the GenericHRT FWO suggests that this difference is larger than the effect caused by the Rift. This implies that, although
the values of SDE WO−Ri f t are relatively large, it still preserves some of the individualized characteristics of the
HRTF. Similar results were obtained for other subjects as well.

Figure 4. SDE averaged across all direction for one subject. SDE was calculated for three pairs of HRTFs

3.3 Effect of Rift on ITD
ITD was calculated using the method presented by Katz [11], where it is estimated by determining the time
when the result of the correlation between the right and the left HRIRs reaches its maximum. For each subject,2638

the ITD error was calculated as the absolute value of the difference between the ITD of the HRTF without the
Rift and with the Rift for each direction. Then, to get an estimation of the effect across subjects, the average
was estimated as shown in Equation 2, where N is the number of subjects:
IT Derror (Ω) =1 N|IT DWOi (Ω) − IT DRi f ti (Ω)|.
N∑
i

(2)

Figure 5 shows the average ITD error for different azimuth angles (horizontal axis) and different elevations(vertical axis), where the color map shows the IT Derror (Ω) values in µs. The figure shows how the Rift

Figure 5. IT Derror averaged for all subjects and KEMAR
affects the ITD and how the distortion is larger for frontal directions, where IT Derror (Ω) is above 50 µs. Morespecifically, this error can be found in two regions, one on the front-left side of the listener’s head for angles
with azimuth between 30° and 80° and elevation between 0° and 30°, and the other on the symmetrical one on
the right-front. These regions correlate with the headset size and shape. There is also a non-negligible error forangles at −100° and −110° azimuth and 0° and −10° elevation. In addition, a symmetrical effect is evident on
the other side of the listener’s head. These discrepancies between the IT DWO and IT DRi f t can be explained, by
shadowing, diffraction and by reflections caused by the presence of the Rift.

4

PERCEPTUAL EVALUATION

To investigate the effect of the Rift on the perception of sound by a listener, a subjective evaluation was
performed. A MUSHRA test, defined in ITU-R BS.1534-3 recommendations for advanced sound systems [1],was carried out, where participants evaluated their individualized HRTF and the generic HRTF, both without andwith the Rift, denoted as: Individualized-HRT FWO , Individualized-HRT FRi f t , Generic-HRT FWO , and GenericHRT FRi f t . The Individualized-HRT FWO was chosen to be the reference (labeled and hidden reference) as it
is considered to produce the highest perceived quality. An anchor was not used in this study. The main
reason for this was to avoid the consequences of biasing or compressing the differences between test signals
due to an inappropriate anchor. The MUSHRA test was chosen because it allows the subjects to compare alltested conditions on the same trial, and provide feedback regarding their perceptible similarities and differences.
Moreover, this type of perceptual test is suitable for evaluation of intermediate audio quality [1].
4.1 Material and methods
The MUSHRA test recommendation specifies that the experiment population must be composed of experienced
listeners, due to their ability to detect subtle artifacts in different sounds. Hence, 15 experienced participants(13 males and 2 females) were chosen from the pool of 24 subjects whose HRTF was measured and analyzed
in the previous sections. Two subjects were excluded in a post-screening due to frequently rate of the hidden
reference as though it were significantly impaired [1]. The GUI used during the test was developed in Matlab
and the audio was played back via an RME Fireface and DT 990 PRO-250 OHM headphones (headphones
compensation filter was not applied).
4.1.1 Tested Conditions
The tested conditions in this perceptual evaluation include all combinations of three independent variables:
HRTF type, stimulus (i.e. audio material), and source direction. For each condition the sound quality wasevaluated based on two attributes (dependent variables).
The 4 tested HRTF types were: Individualized-HRT FWO , Individualized-HRT FRi f t , Generic-HRT FWO , and GenericHRT FRi f t . All were measured with the same system, described in section 2.
2639

In order to evaluate signals with different spectral content, 3 different stimuli were tested: female speech recording [10]; pink noise - a sequence of 4 broadband noise bursts with a length of 750 ms with 500 ms of silence
between them, each noise pulse was faded in and out with a 50 ms raised cosine window; and Guitar recording[10].
As the effect of the Rift on the HRTF was found to be direction dependent, 3 different directions were tested:[azimuth,elevation]=[60°, 0°], [60°, 30°], and [−15°, 45°]; these directions where chosen considering the directions with large and medium errors obtained in the objective evaluation (see Fig. 2).
Following the MUSHRA test recommendations[1], two attributes were evaluated: (1) Timbre quality, which isused to judge all detected differences in timbral impression, e.g. brightness, tone colour, coloration, clarity,
hardness, equalization, or richness between the reference and the evaluated condition; (2) Localization quality,
which is used to judge whether the perceived location of the virtual sound source is the same as the one of thereference.
4.1.2 Procedure
The MUSHRA test procedure consists of two sessions: a training session and the assessment session.
At the beginning of the test subjects perform a training session. In this session, all combinations of HRTF typeand stimulus that are experienced during the assessment session are exposed to the subjects. Subjects can spend
as much time as they wish in this part, until they feel confident with the signals. In this session, the four HRTF
type conditions for the three stimuli are presented, for direction [60°, 30°]. This direction was chosen because,
according to the objective evaluation, it is the one that shows the largest differences between the conditions.
Subjects were asked to modify the volume of the signals at the beginning of the test at their convenience.
The assessment session is divided into two parts, one part for each attribute, presented sequentially. During
the first part, the timbre quality attribute is evaluated and in the second part, the localization quality attributeis evaluated. In each MUSHRA-screen the differences between the four HRTF type conditions were evaluated,
for a single pair of stimulus and direction, ordered randomly. In total 18 MUSHRA-screens were evaluated byeach subject (2 different attributes, 3 different stimuli and 3 different source directions).
During each MUSHRA-screen, subjects are presented with the labeled reference and four HRTF type conditions: three test samples, and a hidden version of the reference, all of them presented in random order. The
labeled and hidden reference corresponds to the Individualized-HRT FWO , while the other three correspond to
the Individualized-HRT FRi f t , Generic-HRT FWO , and Generic-HRT FRi f t . Each subject is asked to provide ascore between 0 and 100 for the four conditions, according to how similar each condition is to the reference.
When assessing the conditions, subjects can switch instantaneously between conditions and the labeled reference
while listening.
To allow the subjects to become familiar with the test setup, a practice MUSHRA-screen test is carried out atthe beginning of each part, where the same format as the actual assessment part is presented. In this case, pinknoise placed at 60° azimuth and 30° elevation is used as the stimulus, since this signal aids the assessment ofsmall impairments and helps to find the hidden reference. Subjects are not able to pass this practice MUSHRAscreen until they identify the hidden reference by ranking it with a score of 100.
4.2 Results
In total, 36 conditions were statistically tested in a three factorial repeated measure multivariate ANOVA (analysis of variance) design, with factors: HRTF type (Individualized-HRT FWO , Individualized-HRT FRi f t , GenericHRT FWO , and Generic-HRT FRi f t ); stimulus (Female speech, Pink Noise, Guitar); and direction ([60°, 0°], [60°,30°], and [−15°, 45°]). These conditions were evaluated using two dependent variables, which are the tested
attributes (timbre quality and localization quality).
Figure 6 shows the listening test results for all tested conditions, for all subjects and for the two tested attributes
by means of box plot. Ratings reveal that, in most cases, subjects rate the Individualized-HRT FWO with 100(as expected for the hidden reference). The median rate for Individualized-HRT FRi f t is lower than the reference Individualized-HRT FWO for all tested conditions, which implies a detrimental effect of the Rift presenceon the individualized HRTF. In addition, it is interesting to note that the ratings for generic HRTF conditions (both Generic-HRT FWO and Generic-HRT FRi f t ) are consistently lower than the ratings for IndividualizedHRT FRi f t when comparing the median rating of the same direction-stimulus pair. This result may imply that
the Individualized-HRT FRi f t is perceived by subjects to be closer to the reference Individualized-HRT FWO than
the generic HRTF conditions. It is also noticeable that with the pink noise stimulus the differences in medianratings between Individualized-HRT FWO , Individualized-HRT FRi f t and the generic HRTF conditions were thegreatest.
To analyze the significance of the main effects and interaction of the tested factors on timbre quality and localization quality, the multivariate repeated measure ANOVA approach was used. The reason for not using theunivariate approach with Huynh-Feldt correction was that, for some factors, the Huynh-Feldt ε̃ was lower than2640

Figure 6. Ratings for all test conditions. Ratings are grouped by HRTF type and stimulus as indicated in
the labels. For each group three tested directions are presented: left to right - [60°, 0°], [60°, 30°], [−15°, 45°].
The box bounds the interquartile range (IQR) divided by the median, and Tukey-style whiskers extend to a
maximum of 1.5IQR beyond the box [13]. Ratings for (a) Timbre quality (b) Localization quality
the recommended threshold of ε̃ > 0.85 [1]. The multivariate tests show a highly significant (p < 0.001, Pillai’s
trace) main effect of the HRTF type and the stimulus, on both timbre quality and localization quality. The firstorder interaction between stimulus and HRTF type is also highly significant with (F(6,7) = 19.14, p = 0.001) and(F(6,8) = 7.57, p = 0.006) for timbre quality and localization quality, respectively. A significant effect was also
found for the direction (F(2,11) = 6.7, p = 0.013) and (F(2,12) = 9.4, p = 0.003) for timbre quality and localization
quality, respectively, but no significant effect for the interaction of direction with other factors, except for theinteraction between direction and HRTF type on localization quality (F(6,8) = 5.862, p = 0.013). Requirements
for normality were met for most of the factor combinations. In some cases with Individualized-HRT FRi f t asignificant deviation from normality was observed, which was probably due to a large number of high rankings(e.g. 100), which resulted in substantial negative skew of both the data and the residuals (extreme values were
skewness < −1.5, kurtosis > 4). Attempting to correct for this skewness by means of Log10 correction was
not successful (resulted in other conditions that did not meet the normality requirements); the raw data was,therefore, left unchanged. For a detailed analysis of the main effects, pairwise comparisons with Bonferroni
correction were performed.
The result of main interest is that the perceived difference between Individualized-HRT FRi f t and all other HRTF
types was highly significant (p ≤ 0.001) for both timbre quality and localization quality. These results may suggest that (1) the effect of the Rift on an individualized HRTF is perceived as a significant difference in source
timbre and source direction (with a score lower by more than 17 points than for the Individualized-HRT FWO );(2) although significant, the perceived effect of the Rift on an individualized HRTF is smaller compared to theperceived difference between and individualized HRTF and a generic HRTF (Individualized-HRT FRi f t received
a mean score that was higher than the generic HRTF conditions by more than 14 points). These results may
imply that, although the detrimental effect of the Rift on the individualized HRTF is significant, some characteristics of the individualized HRTF are still preserved, which makes the Individualized-HRT FRi f t to be perceived
closer to the true Individualized-HRT FWO than the Generic-HRT FWO .
Regarding the effect of the stimulus (type of audio material), it was found that when using pink noise theperceived quality of the tested conditions was significantly lower (p < 0.001) than when using the speech or
guitar stimuli. In addition, it was found that the interaction between stimulus and HRTF type was ordinal (i.e.
the main effect remains unambiguous) and that the perceived quality of the tested HRTF types were lower withpink noise than with the speech or guitar stimuli. The interaction between stimuli and HRTF type, and the factthat with pink noise scores were lower, can be explained by the fact that stimuli with richer frequency content
may assist in detecting differences in both timbre quality and localization quality.
The only direction that yielded significantly higher score than the other directions was [−15°, 45°] for localization quality. For the other two directions no significant difference was observed. This can be explained by thefact that ITD for directions [60°, 0°] and [60°, 30°] was found to be much larger than for [−15°, 45°]. It is
not possible to generalize this result to other directions, but it gives some indication of the relation between theobjective evaluation and the subjective results.
In addition to the MUSHRA test results, an informal questionnaire was filled by the subjects at the end of thetest. Subjects rated from 0 to 5 the two following questions: (a) when rating timbre quality, how dominant was
the difference in low, mid and high frequencies?; and (b) when rating localization quality, how dominant was
the lateralization, the elevation, and the externalization? Average rating for each question indicates that subjects
selected high frequency as dominant to the assessment of timbre quality (4.4), followed by mid frequency (3.5)
and low frequency with the lowest score (2.3), which fits the objective analysis, where the largest differenceswere observed at high frequencies. Regarding localization quality ratings, subjects found the lateralization to be
the most dominant cue (3.9), followed by the elevation (3.3), and finally, the externalization (2.3).

2641

CONCLUSIONSThis paper investigates the effect of an HMD, more specifically the Oculus Rift™, on individualized HRTFs
for multiple directions. The HRTFs were acoustically measured on 24 subjects and one KEMAR manikin.
The effect was studied, comparing the HRTF for each subject with and without the Rift. From the objective
evaluation we can map the directions where the effect of the Rift on the HRTF is larger, taking into accountthe ITD error and the SDE. A subjective evaluation validates the objective study and reveals that the effect of
the Rift on the individualized HRTF is significant in the tested directions. However, according to the subjects’
assessments, they find more similarities, in terms of the timbre and localization quality, between their individual
HRTF and their individual HRTF while wearing the Rift than between their individual HRTF and the generic
HRTF. This may imply that, although the effect of the Rift on an individual HRTF is significant, there are stillsome characteristics of the individual HRTF that are preserved.
These findings suggest that compensation for the effect of the Rift on an individual HRTF should be appliedin experiments that include a subject that is wearing a Rift and a real sound source (such as an external
loudspeaker). It would be very interesting to extend the subjective study with a comprehensive localization
experiment, to see how the distortion introduced by the Rift affects the perceived location of a source in specificdirections.

ACKNOWLEDGEMENTS
We thank our colleagues from Facebook Reality Labs, Alex Gustafson, Chris Hanson, Kamilah Uddin, Owen
Brimijoin, Pablo F. Hoffmann, Paul Solis and Vamsi Krishna Ithapu, who provided insight and expertise that
greatly assisted the research and improved the manuscript.

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