# Measurement of Head-Related Transfer Functions: A Review
## 3 Overview of HRTF Measurement Setups
### Multi-Loudspeaker Setups  
Figure 8 shows some examples of multi-loudspeaker-based HRTF measurement setups. The setups M\({}_{\text{A}}\) and M\({}_{\text{P}}\) contain multiple loudspeakers placed in a spatial or sphere layout. For the HRTF measurement, the subject should sit or stand in the measurement position, and the center of the subject's head is coincident with the center of the loudspeaker arrangement. Since the loudspeaker positions may already cover all desired measurement directions, there is no need to ask the subject to turn the head or body into other orientations. To obtain HRTF datasets with a high spatial density, it is still necessary to change the the subject's orientation.  
Figure 8: Examples of multi-loudspeaker-based HRTF measurement setups taken from [2,108,116,121,122,123,124,125,126,127,128,129,130,131,132,1334] (All pictures are taken from available publications, some of them are partly cropped to fit into the overall picture).  
The loudspeaker setups M\({}_{\rm B}\)-M\({}_{\rm I}\), M\({}_{\rm L}\)-M\({}_{\rm O}\) are widely applied for the HRTF measurement, where the loudspeakers are mounted on a vertical arc, a horizontal arc, or a circular arc. In order to cover different measurement directions, either the loudspeaker array (see setups M\({}_{\rm F}\), M\({}_{\rm L}\), M\({}_{\rm M}\) and M\({}_{\rm O}\)) or the subject (see setups M\({}_{\rm B}\)-M\({}_{\rm E}\), M\({}_{\rm G}\)-M\({}_{\rm I}\), and M\({}_{\rm N}\)) should rotate, where the rotation of subjects can be achieved by using a turnable chair or a turntable.  
The setup M\({}_{\rm K}\) is a two-arc-source-positioning (TASP) system consisting of two vertical arcs [129]. The arcs can freely rotate around the vertical axis, and two loudspeakers on arcs are allowed to move along the arcs to cover measurement points. In setups M\({}_{\rm J}\) and M\({}_{\rm Q}\), loudspeakers are mounted on a pivoting arc and a boom arm, respectively. HRTFs for different azimuth angles are measured by rotating the subject with a turntable, while HRTFs for elevation angles are measured by controlling the loudspeaker positions using the pivoting arc and the boom arm. Note that, in the setup M\({}_{\rm Q}\), two loudspeakers are used to measure HRTFs for two different distances. Though this setup contains two loudspeakers, the loudspeaker numbers do not help to speed up the measurement procedure. It can be considered as a single-loudspeaker setup [116].  
The loudspeaker setups M\({}_{\rm I}\) and M\({}_{\rm Q}\) (only the left loudspeaker in the M\({}_{\rm Q}\)) are specially applied for measuring near-field HRTFs. The setup M\({}_{\rm Q}\) only allows for measuring near-field HRTFs with a single distance, while the setup M\({}_{\rm I}\) enables the measurement of distance-dependent HRTFs. In the setup M\({}_{\rm I}\), multiple loudspeakers are mounted on a vertical locating loop with support rods. Various distances between the loudspeakers and the listener can be achieved by adjusting the length of support rods [127]. The details about loudspeakers in setups M\({}_{\rm I}\) and M\({}_{\rm Q}\) can be found in [116; 127].