Princeton University  

Head-Related Transfer Function Measurements

This work was sponsored in part by the Sony Corporation of America


Introduction


An individual's head-related transfer functions (HRTFs) describe the idiosyncratic filtering of incident sound waves by the individual's morphology. HRTFs are widely used to synthesize binaural signals for spatial audio reproduction, and are generally acquired either through acoustical measurements or by modeling from morphological data. Acoustically measured HRTFs:

  • are accurate but difficult to acquire,
  • serve as benchmarks for validating modeled HRTFs,
  • and serve as training data for developing HRTF models.

Publicly available databases provide measured HRTFs for many human subjects and mannequins. However, few such databases also include corresponding morphological data. Consequently, the 3D3A Lab is measuring HRTFs and corresponding 3D morphological scans of subjects, which we provide as a freely available database. Below, the measurement and processing procedures are described for the HRTFs and morphological scans.

As this is an ongoing project, the database is continually evolving as we accumulate more data. Click here to download the latest version of the database as a .zip file, containing all measured impulse responses (in .sofa format) and morphological scans (in .ply format). (1.53 GB, last updated August 13th, 2018)


HRTFs


The figure below illustrates the HRTF measurement setup in the anechoic chamber. Briefly, the subject is seated in the chamber in front of a vertical arc, which holds 9 loudspeakers. Binaural microphones are inserted into the subject's ears, and binaural impulse responses (BIRs) are measured for each loudspeaker. The subject is then rotated in 5° increments using a computer-controlled turntable upon which the seat is affixed. The HRTF measurement procedures are described generally in this video by Michael Lavorgna, and in more detail in this paper.

HRTF Measurement Setup

The table below summarizes information about the HRTF measurements.

Measurement room

Anechoic chamber

Measurement positions

Radius: 76 cm
72 azimuths: [0°, 5°, 10°, …, 355°]
9 elevations: [–57°, –30°, –15°, 0°, 15°, 30°, 45°, 60°, 75°]
(72 × 9 = 648 positions in total)

Turntable

Outline ET-250 3D

Loudspeakers

9 × Genelec 8010A

Binaural microphones

Theoretica Applied Physics BACCH-BM Pro

Excitation signal

Multiple exponential sine sweeps (Majdak et al., 2007)
Sweep type: phase-controlled (Vetter and di Rosario, 2011)
Sweep duration: 500 ms (200 ms inter-sweep delay)
Frequency range: 20 Hz to 48 kHz

Sampling rate

96 kHz

Data export format

Spatially-oriented format for acoustics (SOFA)

To acquire the subject's HRTFs, the measured BIRs must be equalized by reference impulse responses (RIRs), in order to remove the loudspeakers' and microphones' responses. The RIRs are measured for each loudspeaker using the same binaural microphones, which are placed at the origin of the measurement arc (i.e., at the position of the center of the subject's head). Inverse filters for the measured RIR's are then designed and applied to the BIRs, yielding the equalized HRTFs. The HRTF processing is described in more detail in this paper.


Morphological Scans


The diagram below illustrates the morphological scanning setup. Briefly, the subject is seated, puts on a wig-cap, and has colorful adhesive markers placed around the subject's face. The subject's head and torso are then scanned using a “consumer-grade” 3D scanner, followed by a high-resolution scan of the individual pinnae using a state-of-the-art, structured-light, 3D scanner. The morphological scanning procedures are described in more detail in this paper.

Morphology Scanning Setup

The table below summarizes information about the morphological scans.

Head and torso scanner

PrimeSense Carmine 1.09

Pinna scanner

Artec Space Spider

Scanning software

Skanect Pro; Artec Studio 12 Pro

Data export format

Polygon file format (PLY)

The consumer-grade head-and-torso scans are then converted into watertight meshes and aligned such that the subject's interaural axis coincides with the y-axis of the scan. A copy of each of these scans is then made, following which the “consumer-grade” pinnae scans are manually replaced by the corresponding high-resolution pinnae scans, yielding the “reference-grade” scan. The morphological scan processing procedures are described in more detail in this paper.

Important Note: If any of the data provided in this database are used for research resulting in published academic work, please cite the publication listed below.


Publications:

R. Sridhar, J. G. Tylka, and E. Y. Choueiri. A database of head-related transfer function and morphological measurements. In Audio Engineering Society Convention 143, October 2017. (pdf, poster)