# Measurement of Head-Related Transfer Functions: A Review
###### Abstract  
A head-related transfer function (HRTF) describes an acoustic transfer function between a point sound source in the free-field and a defined position in the listener's ear canal, and plays an essential role in creating immersive virtual acoustic environments (VAEs) reproduced over headphones or loudspeakers. HRTFs are highly individual, and depend on directions and distances (near-field HRTFs). However, the measurement of high-density HRTF datasets is usually time-consuming, especially for human subjects. Over the years, various novel measurement setups and methods have been proposed for the fast acquisition of individual HRTFs while maintaining high measurement accuracy. This review paper provides an overview of various HRTF measurement systems and some insights into trends in individual HRTF measurements.  
head-related transfer function; head-related impulse response; HRTF measurement system; individual HRTF measurement +
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to interpolate and extrapolate (distance or direction) sparse HRTF sets to obtain a high-density HRTF dataset [17; 18; 19; 20; 21; 22; 23; 24; 25; 26]. Although these interpolation/extrapolation approaches can reduce the HRTF measurement points, the required number of measurements is still high [27; 28]. Over the years, different measurement systems have been proposed for the fast acquisition of individual HRTFs. In addition to the acoustic measurement solution, other approaches, such as individualization/selection of HRTFs from non-individual HRTF datasets, calculation of HRTFs from scanned/simulated head models, can alternatively be applied to obtain personalized HRTFs. These solutions exceed the scope of this review article, and for more information please refer to [15; 29; 30; 31; 32; 33; 34].  
There exist various HRTF measurement systems including measurement setups and methods. The latest summaries of HRTF measurement systems are given in [35; 36]. [PERSON] [35] provided a detailed introduction to HRTF measurement principles and several examples of measurement systems. [PERSON] et al. [36] presented some rapid measurement systems for recording far-field HRTFs and showed trends in acquisition of HRTFs, but focused mainly on continuous measurement methods which they proposed at that time. This article provides an overview of the state-of-the-art in HRTF measurement systems and some insights into the trends in individual HRTF measurements.  
As shown in Figure 1, the rest of this paper is structured as follows. Section 2 describes the basic principle of HRTF measurements. Afterwards, various measurement setups for the acquisition of high-density HRTF datasets are outlined in Section 3. Depending on the measurement setups used, different fast HRTF measurement methods are reviewed in Sections 4 and 5. Then, Section 6 presents some relevant post-processing steps/methods for the measured HRTF data to eliminate the influence of measurement setups/systems and environments. The measurement uncertainties, HRTF evaluation methods, HRTF storage formats, and considerations and trends in HRTF measurements are discussed in Section 7. Finally, a conclusion of this review paper is drawn in Section 8.