HRTFs: Measurement of Head-Related Transfer Functions

Objective

Head-related transfer functions (HRTFs) describe sound transmission from the free field to a place in the ear canal in terms of linear time-invariant systems. They contain spectral and temporal features that vary according to the sound direction. Differences among subjects requires the measuring of subjects' individual HRTFs for studies on localization in virtual environments. In this project, a system for HRTF measurement was developed and installed in a semi-anechoic room at the Austrian Academy of Sciences.

Measurement of an HRTF is considered a system identification of the electro-acoustic chain: sound source-room-HRTF-microphone. The sounds in the ear canals are captured using in-ear microphones. The direction of the sound source is varied horizontally by rotating the subject on a turntable, and vertically by accessing one of the 22 loudspeakers positioned in the median plane. An optimized form of system identification with sweeps, the multiple exponential sweep method (MESM), is used for the measurement of transfer functions with satisfactory signal-to-noise ratios occurring within a reasonable amount of time. Subjects' positions are tracked during the measurement to ensure sufficient measurement accuracy. Measurement of headphone transfer functions are included in the HRTF measurement procedure for a potential equalization of the headphones.

ARI Database

Since 2005, listeners' HRTFs are continuously measured, extending the ARI database of HRTFs. The ARI database is publicly available at http://sofacoustics.org/data/database/ari/. The HRTFs can be used to create virtual stimuli and present them binaurally via headphones. Over the years, small modifications to the measurement procedure have been applied. A single letter encodes the modifications: 

  • no letter: Orginal setting, as described in Majdak et al. (2010). 
  • "b": The lowest frequency in the signal processing was moved from 300 Hz to 50 Hz. This results in HRTFs passing through more low-frequency energy and thus stimuli having more "bass" and being better suitable for hi-fi applications. 
  • "c": The starting azimuth angle of the measurement was moved from 0° to 270° because of more convenient placing of the listener in the measurement setup and of a continuous  measurement procedure when crossing the most frontal directions. We believe that this has no effect on the measurement, but who knows...
  • "d": We moved to another location and updated the setup: 
    • Sound absorbing material was replaced by a better one.
    • Loudspeakers were covered with absorbing material reducing the reflections from the oposite loudspeakers. 
    • The loudspeaker amplifiers were replaced.
    • The height of the booth was extended by 50 cm, having potential effect on the ceiling reflection. 
    • The listeners sit 25 cm hight, having potential effect on the floor reflection. 

Publications

The following publications are related with this project: 

  • Majdak P., Balazs P., Laback B. (2007). “Multiple Exponential Sweep Method for Fast Measurement of Head Related Transfer Functions,” J Audio Eng Soc 55, 623-637. Permalink: http://www.aes.org/e-lib/browse.cfm?elib=14190.
  • Majdak, P., Goupell, M. J., and Laback, B. (2010). "3-D localization of virtual sound sources: effects of visual environment, pointing method, and training," Atten Percept Psychophys 72, 454-469. 
  • Goupell, M. J., Majdak, P., and Laback, B. (2010). "Median-plane sound localization as a function of the number of spectral channels using a channel vocoder," J Acoust Soc Am 127, 990-1001.
  • Majdak, P., Carpentier, T., Nicol, R., Roginska, A., Suzuki, Y., Watanabe, K., Wierstorf, H., Ziegelwanger, H., and Noisternig, M. (2013). "Spatially oriented format for acoustics: a data exchange format representing head-related transfer functions," in proceedings of the 134th Convention of the Audio Engineering Society (AES), Roma, Italy, Convention Paper 8880.
  • Majdak, P., Walder, T., and Laback, B. (2013). "Effect of long-term training on sound localization performance with spectrally warped and band-limited head-related transfer functions," J Acoust Soc Am 134, 2148-2159. 
  • Majdak, P., Masiero, B., and Fels, J. (2013). “Sound localization in individualized and non-individualized crosstalk cancellation systems,” J Acoust Soc Am 133, 2055-2068.
  • Ziegelwanger, H., and Majdak, P. (2014). “Modeling the direction-continuous time-of-arrival in head-related transfer functions”, J Acoust Soc Am, 135, 1278-1293. DOI: 10.1121/1.4863196.
  • Marelli, D., Baumgartner, R., and Majdak, P. (2015). “Efficient Approximation of Head-Related Transfer Functions in Subbands for Accurate Sound Localization,” in the IEEE Trans Audio Speech Lang Proc, 23, 1130-1143. DOI: 10.1109/TASLP.2015.2425219.
  • Ziegelwanger, H., Majdak, P., and Kreuzer, W. (2015). “Numerical calculation of listener-specific head-related transfer functions and sound localization: Microphone model and mesh discretization,” J Acoust Soc Am, 138, 208-222. DOI: 10.1121/1.4922518.
  • Harder, S., Paulsen, R. R., Larsen, M., Laugesen, S., Mihocic, M., and Majdak, P. (2015). “Reliability in Measuring Head Related Transfer Functions of Hearing Aids,” Acta Acustica united with Acustica, 101, 1064–1066. DOI: 10.3813/AAA.918900.
  • Majdak, P., Zotter, F., Brinkmann, F., De Muynke, J., Mihocic, M., and Noisternig, M. (2022). “Spatially Oriented Format for Acoustics 2.1: Introduction and Recent Advances,” J Audio Eng Soc 70, 565-584. DOI: 10.17743/jaes.2022.0026.

 

Hearing

Team