# Measurement of Head-Related Transfer Functions: A Review
## 3 Overview of HRTF Measurement Setups
### Single-Loudspeaker Setups  
Compared to multi-loudspeaker settings, single-loudspeaker setups can not rely on the number of loudspeakers to dramatically speed up the measurement process. Hence, these setups are often applied for measuring high-density HRTF datasets of dummy heads, when the measurement time is not a critical issue. In the case of measurements for human subjects, advanced algorithms or methods are required to reduce the measurement time.  
Figure 9 shows several single-loudspeaker-based HRTF measurement setups. As shown in the setup S\({}_{\rm D}\), a loudspeaker is placed at a fixed position and a dummy head is placed on a turntable. A set of HRTFs for different azimuth angles (1D HRTFs) can be measured by rotating the dummy head using the turntable. Except for the setup S\({}_{\rm D}\), other setups in Figure 9 are able to measure HRTFs on both azimuth and elevation planes (2D HRTFs). With setups S\({}_{\rm B}\), S\({}_{\rm C}\), and S\({}_{\rm E}\), different 2D measurement positions are covered by rotating the dummy head/subject with a turntable and changing the loudspeaker position. In the setup S\({}_{\rm F}\) or S\({}_{\rm G}\), a loudspeaker is mounted on a traverse arm and can be placed in any desired measurement position on the azimuth and elevation plane. By this means, the subject does not need to rotate during the HRTF measurement. The setup S\({}_{\rm A}\) shows another possibility, where a dummy head (KU-100, without torso) is mounted on a custom bracket, and a loudspeaker is placed at a fixed position. Through the control of the bracket, the dummy head can perform a 2D rotation to cover different measurement positions. A similar setup can be found in [135], where a self-designed head-and-torso-system has multiple degrees of freedom that allow the head to rotate horizontally and tilt vertically. The single-loudspeaker setups can simply be applied for measuring distance-dependent HRTFs: after the 2D HRTF measurement with a fixed source-listener distance, the sound source (setup S\({}_{\rm B}\)) or the dummy head (setups S\({}_{\rm A}\) and S\({}_{\rm D}\)) move to the next desired position for measuring 2D HRTFs with that distance. This procedure is repeated until all desired source-listener distances are covered.  
Setups \(\rm S_{H}\)-\(\rm S_{K}\) show novel measurement modes particularly designed for the fast HRTF measurement of human subjects. A loudspeaker is placed at a fixed position and continuously emits the excitation signal, while the subject is asked to rotate her/his head to cover different measurement directions. The head rotation is recorded using a head tracker device during the measurement. In addition, the movement pattern and desired measurement points are displayed on a video monitor (see the setup \(\rm S_{J}\)) or a head-mounted display (HMD, see setups \(\rm S_{I}\) and \  
data, are synchronized and further used to calculate the HRTF from each measurement direction. AR/MR headsets can not only record the head orientation, but also detect the source-listener distance with integrated depth cameras. Hence, it is also possible to fast measure distance-dependent HRTFs (3D HRTFs) by using AR/MR headsets (see Section 5).