HRTF Error

Behind-the-ear microphone

Omnidirectional microphones were positioned above the pinna, mounted in a casing of a hearing-aid device (Tempo, MED-EL).

NH10 Left Ear

NH10 Left Ear

In-ear microphone

Blocker-ear-canal technique with the in-the-ear microphones (KE-4-211-2, Sennheiser). The microphones were fully inserted in the ear canal, see figure to the right.

NH10 Left Ear

NH10 Left Ear

This documentation may help you to interact with the HRTFs.

Download

Description

How to use the HRTF results Results from the post processing procedure (pdf, also included in ExpSuite/AMTatARI Installer)
ARI HRTF format
Description of the HRTF ARI format (pdf, also included in ExpSuite/AMTatARI Installer)
(Description of the old HRTF ARI format v1: ARI HRTF format v1)

These tools may help you to interact with the HRTFs. We recommend to read the documentation before using HRTF data.

Download

Description

ExpSuite Installer This is a full package of ExpSuite and contains:
  • AMTatARI, a viewer and measurement tool for HRTFs saved in the ARI format. It is for MS Windows and requires MATLAB.
  • VSP, a player for virtual sound positions, using ARI HRTF data to filter sound samples. It is for MS Windows and requires MATLAB.
AMTatARI Test Setting  Use this settings file in AMTatARI to interact with the HRTFs. Short how-to:
  • Save target file to your computer
  • Open this settings file in AMTatARI
  • Set the "work directory" in View/Settings to the directory with your HRTFs
  • Connect to MATLAB with Run/Connect
  • Execute the Post-Processing Toolbox (right-bottom part of the main screen)
  • Load the HRTFs with "Load *M from..."
  • Plot in the median plane by using "Plot hM", Loading "HRTF: median, pcolor", and clicking on "Plot"
Further details can be found in AMTatARI documentation
VSP Test Setting  Use this settings file in VSP to play virtual sound positions, filtered with the HRTFs. Short how-to:
  • Save target file to your computer
  • Open this settings file in VSP
  • In View/Settings, set the data directory "Sound files" to the directory with your sound files
  • In View/Settings, set the data directory "HRTF Set files" to the directory with your HRTFs
  • In View/Settings -> Variables, set positions (Azimuth, Elevation) and sound file(s)
  • Connect to MATLAB with Run/Connect
  • Create List; when asked if you want a uniform distribution press "No"
  • Select any item in item list and press "Stimulate Selected"
Further details can be found in VSP documentation
Hrtf2DtfCtf Calculates directional transfer functions (DTFs) and the common transfer function (CTF) for the HRTFs in ARI format according to Middlebrooks (1999) and Majdak et al. (2010).
ARI2STx Convert ARI HRTFs to STx format
ARI2CIPIC Convert ARI HRTFs to CIPIC format (ARI and CIPIC database)
CIPIC2ARI Convert CIPIC HRTFs to ARI format (ARI and CIPIC database)
hor2geo Convert coordinates from the horizontal-polar system to the geodetic system ([Lat, Pol] -> [Azi, Ele])
geo2horpolar Convert coordinates from the geodetic system to the horizontal-polar system ([Azi, Ele] -> [Lat, Pol])

Individual HRTFs were measured in a semi-anechoic chamber. Twenty-two loudspeakers (custom-made boxes with VIFA 10 BGS as drivers; the variation in the frequency response was ±4 dB in the range from 200 to 16000 Hz) were mounted at fixed elevations from -30° to 80°. They were driven by amplifiers adapted from Edirol MA-5D active loudspeaker systems. The loudspeakers and the arc were covered with acoustic damping material to reduce the intensity of reflections. The total harmonic distortion of the loudspeaker-amplifier systems was on average 0.19 % (at 63-dB SPL and 1 kHz). The subject was seated in the center of the arc and had in-ear-microphones (Sennheiser KE-4-211-2) placed in his/her ear canals. The microphones were connected via amplifiers (RDL FP-MP1) to the digital audio interface. A 1728.8-ms exponential frequency sweep beginning at 50 Hz and ending at 20 kHz was used to measure each HRTF. The HRTFs were measured for one azimuth and several elevations at once by playing the sweeps and recording the signals at the microphones. Then the subject was rotated by 2.5° to measure HRTFs for the next azimuth. In the horizontal interaural plane, the HRTFs were measured with 2.5° spacing within the azimuth range of ± 45° and with 5° spacing outside this range. The positions of the HRTFs were distributed with a constant spherical angle, which means that the number of measured HRTFs in a horizontal plane decreased with increasing elevation. For example, at the elevation of 80°, only 18 HRTFs were measured. In total, 1550 HRTFs were measured for each listener. To decrease the total time required to measure the HRTFs, the multiple exponential sweep method (MESM) was applied (Majdak et al., 2007, JAES). This method allows for a subsequent sweep to be played before the end of a previous sweep, but still reconstructs HRTFs without artifacts. The MESM uses two mechanisms, interleaving and overlapping and both depend on the acoustic measurement conditions (for more details see Majdak et al., 2007). Our facilities allowed the interleaving of three sweeps and overlapping of eight groups of the interleaved sweeps. During the HRTF measurement, the head position and orientation were monitored with the same tracker as used in the experiments. If the head was outside the valid range, the measurements for that particular azimuth were repeated immediately. The valid ranges were set to 2.5 cm for the position, 2.5° for the azimuth, and 5° for the elevation and roll. On average, measurements for three azimuths were repeated per subject and the measurement procedure lasted for approximately 20 minutes. Equipment transfer functions were measured. They were derived from a reference measurement in which the in-ear microphones were placed in the center of the arc and the room impulse response was measured for all loudspeakers. The room impulse responses showed a reverberation time of approximately 55 ms. The level of the largest reflection (floor reflection) was at least 20 dB below the level of the direct sound and delayed by at least 6.9 ms. The equipment transfer functions were cepstrally smoothed and their phase spectrum was set to the minimum phase. The resulting minimum-phase equipment transfer functions were removed from the HRTFs by spectral division. We assume that after this equalization, the room reflections and the equipment had only a negligible effect on the fidelity of HRTFs.

Directional transfer functions (DTFs) were calculated using a method similar to the procedure of Middlebrooks (1999). The magnitude of the common transfer function (CTF) was calculated by averaging the log-amplitude spectra of all HRTFs for each subject. The phase spectrum of the CTF was set to the minimum phase corresponding to the amplitude spectrum of the CTF. The DTFs were the result of filtering the HRTFs with the inverse complex CTF. Finally, the impulse responses of all DTFs were windowed with an asymmetric Tukey window (fade in of 0.25 ms and fade out of 1 ms) to a 5.33-ms duration.

Measurements are done with AMTatARI, a software based on the ExpSuite framework developed in our institute.

HRTF positionsHRTF positions

The HRTF positions are not the same in the ARI and CIPIC format. For example: The polar angles in ARI format cover a range from -30 to 80 deg, the range in CIPIC is from -45 to 90 deg (and 230.625 in the back). Even though the ARI format provides a flexible grid, the HRTF positions in the CIPIC format are hard-coded. During the conversion to CIPIC, the HRTF positions are adapted to the CIPIC grid, which may result in errors. Below the errors for the lateral and polar angles are shown, together with spheres showing the ARI and CIPIC positions.The plots can be re-produced with ARI2CIPIC for an arbitrary ARI grid.

Click to enlarge Click to enlarge
Click to enlarge Click to enlarge

 

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