# Measurement of Head-Related Transfer Functions: A Review
## 2 Principle of HRTF Measurements
### Sound Sources  
According to the definition in [1], a point sound source should be used for measuring HRTFs. Electro-acoustic devices, such as loudspeakers, are generally used as sound sources for measuring HRTFs. An ideal loudspeaker should have a flat frequency response, low nonlinearity, and omni-directional directivity (within the measurement region) across frequencies of interest. In practice, a compromise should be made, since no loudspeaker can fulfill all of these characteristics.  
To ensure a flat frequency response, two or more drivers are usually working together in one loudspeaker, and each driver is responsible for reproducing sounds in one specific frequency range. For a two-way loudspeaker, the driver with a large size is used for reproducing sounds between low and mid frequencies, while the small-size driver is responsible for sounds between mid and high frequencies. A typical N-way loudspeaker with different radiation centers can not be considered as a point sound source. Alternatively, N-way coaxial loudspeakers or small single-driver speakers may be more suitable for approximating point sound sources, but sufficient performance at low frequencies can not be guaranteed [35]. The choice of the sound source may not be critical for measuring far-field HRTFs with a large distance, and many laboratories use commercially available two or three-way loudspeakers for far-field HRTF measurements because of their flat frequency responses (\"direct measurement method\"). Alternatively, some researchers build their own loudspeakers towards an optimal sound source for the measurement [107,108].  
In the case of near-field HRTF measurements, the sound source is particularly important. The directivity of the sound source should be nearly omni-directional (approximation of the characteristic of an acoustic point source) at least within the main measurement region between the sound source and the subject's head. Moreover, with a close distance, multiple reflections/scatteringbetween subject and sound source may influence measurement results. This problem is more serious when measuring near-field HRTFs with multi-channel systems due to multiple reflections/scattering among sound sources [109]. Hence, a small-size sound source is required for the measurement of near-field HRTFs. In the literature, some sound sources have been proposed to approximate ideal point sound sources, e.g., a probe tube-type source, consisting of an electrodynamic horn driver and a 3 m-long section of Tygon tubing [16], a spark noise generated by an electrical discharge with a transformer and electrodes [110], a micro-dodecahedral loudspeaker with piezoelectric ceramic devices [111; 112]. However, most of them have poor SNRs below 1 kHz. To improve the SNR at low frequencies, [PERSON] et al. [113] designed a micro-dodecahedral loudspeaker system consisting of electrodynamic speaker units. Moreover, [PERSON] et al. [114] used a spark gap (Type BDMS1-040528) to measure near-field HRTFs, and the property of impulse signals generated by the spark gap was very close to a point sound source. However, this particular sound source is not often used in most acoustic laboratories. [PERSON] et al. [115] numerically analyzed influences of the sound source size on near-field HRTFs caused by multiple reflections/scattering. The simulation results suggested that, in order to guarantee the spectral distortions within 1 dB at a source-subject distance of 0.2 m (or 0.15 m), the source radius should be smaller than 0.05 m (or 0.03 m). Otherwise, some absorption material around or on the source surface is required [109]. In addition to the special sound sources mentioned above, some researchers prefer to use custom-made loudspeakers (broadband drivers with specially designed enclosures) for near-field HRTF measurements [79; 116; 117].