# Measurement of Head-Related Transfer Functions: A Review
## 3 Overview of HRTF Measurement Setups
### Setups for HRTF Measurements in Non-Anechoic Environments  
The HRTF measurement is normally performed in an anechoic and low-noise environment, such as an anechoic chamber, to avoid noticeable reflections and ambient noises during the measurement. For some reasons, e.g., lack of anechoic chambers, avoidance of long-term measurements on human subjects in anechoic environments, some studies measured HRTFs in non-anechoic but controlled acoustic environments, e.g., listening rooms [127; 146; 147; 148], and some studies even performed measurements in ordinary rooms [149; 150; 151; 152]. A typical method for eliminating reflections is to truncate the measured impulse responses with a window function in either the time [149; 150; 151] or the frequency domain [153] (see Section 6.1). As an alternative, [PERSON] [152] represented the measured impulse responses with spatial principal components analysis (SPCA), and truncated the weight coefficients of principal components to remove reflections. Regardless of the excitation signal used, a common way to suppress the background noise in the measurement results is to repeat the measurement several times (or use repeated excitation signals) [149; 150]. However, the difficulty in eliminating reflections and background noises depends on the acoustics of the measurement environment. Two recent preliminary studies proposed novel concepts to suppress the background noise [154] and reflections [155] in the measurement results by analyzing acoustics of the environment with additional microphones (see Figure 11). [PERSON] et al. [154] proposed to use an ambisonic microphone (Sennheiser AMBEO, above the dummy head in the setup E\({}_{\text{A}}\)) to capture the sound field for measuring HRIRs. The existing sound field in the current room was recorded by the ambisonic microphone and further used to obtain the sound source signal, the ambisonic energy, the diffuseness, and the source direction. For each desired measurement direction, HRTFs could be calculated with the division of recorded ear signals by the sound source signal in the frequency domain (deconvolution method, see Section 2.1) when the diffuseness was minimum, the ambisonic energy was highest, or the ear signal energy was highest. Unfortunately that study presented only some simulation results. A similar approach can be found in [156], where the acoustic signal was recorded by a mono microphone and further used as a reference signal for the deconvolution process. Both studies show the possibility to use passively recorded natural acoustic sounds as excitation signals instead of actively emitting measurement signals, but the frequency range of measured HRIRs depends on the stimuli recorded. The major difference between these two studies is the microphone used, which leads to different possibilities in the choice of time frames for the deconvolution process. [PERSON] et al. [155] proposed a method to cancel reflections in the measurement result by analyzing the reflection pattern prior to the HRTF measurement. The right panel in Figure 11 (setup E\({}_{\text{B}}\)) shows the loudspeaker setup for measuring HRTFs in an ordinary room, consisting of 72 loudspeakers arranged on a circular array on the horizontal plane (the height of listener's ears), and two circular arrays each  
Figure 11: Examples of system setups for HRTF measurements in non-anechoic environments [154; 155] (The pictures taken from publications are partly modified to highlight measurement setups). with eight loudspeakers suspended from the ceiling and placed on the floor, respectively. Prior to the HRTF measurement, a custom-made spherical microphone array was placed in the middle of the loudspeaker setup where the listener would be, to measure impulse responses from loudspeakers to microphone arrays. Each impulse response measured was decomposed in different directions using the plane wave decomposition (PWD) method to detect the reflection pattern.