## 1 Introduction  
After the sound wave reaches the listener, the size and shape of the ear changes the spectrum of sound reaching the eardrum. These spectral distortions act as a unique feature of the human body to help us understand the location of the sound source. This phenomenon is called the head-related transfer function (HRTF)--a response describing how we receive sound waves from different points in space. Other parts of the human body such as the head, shoulders, and torso also change the sound spectrum, but only frequencies up to 3 kHz are affected in this way. Frequencies above 3 kHz are shaped by the individual anatomy of the external ear [1]. Hair can also affect the HRTF spectrum, especially at high frequencies, but studies have shown that this should not affect the user's ability to accurately localize a sound source [2]. The pinna is one of the most individual parts of the human body, even variations of a few millimeters in its geometry have a strong effect on the HRTF filter [3]. When analyzing the entire human population, different pinna shapes result in changes of up to 20 dB in the HRTF spectrum above 4 kHz [4]. However, it has been shown that not all parts of the pinna are equally important for the HRTF spectrum. The cavum conchae, fossa triangularis, and scapha have the greatest influence, while the posterior half of the helix changes the spectrum the least [5]. There are two main factors that allow a human listener to locate a sound source. These are the interaural time difference (ITD) and the interaural level difference (ILD). An ITD occurs when the sound source is at a different distance from the two ears. Studies show that humans can detect ITDs as small as 10 us, which corresponds to differences in distance of about 0.3 mm [6]. The ILD is the result of the acoustic shadow created by the head, which acts as a barrier to high frequencies. For example, a wavelength of 2000 Hz is shorter than the distance between human ears. Below 2000 Hz, ILD becomes ineffective because the lower frequencies are able to bypass the detected obstacle [7]. The spatial resolution of an HRTF is determined by the number of positions of the sound source distributed over the surface of the virtual sphere in a certain interval of degrees. It has been shown that this resolution may not be the same for sounds coming from the front and from the side. The minimum audible angle (MAA) is the smallest angular deviation perceptible to a person; on the horizontal axis, it is 1 degree for frontal sounds and proportionally increases to 10 degrees for lateral sounds. On the vertical axis, a person feels a change in frontal sounds from 4 degrees, and for sounds coming from the side, the MAA increases up to 20 degrees [8]. Individual HRTFs are important for improving the user experience in virtual reality. Only by using it can a person hear the virtual sound through headphones exactly as they would hear the sound if the source existed in the natural environment. Use cases range from entertainment games to assistive systems for the blind. The user can be navigated by voice, giving them the right direction and eliminating the need to look at a map [9]. Online work meetings that have become popular during the COVID-19 pandemic could be more realistic, with participants hearing immersive virtual acoustics. Everyone who joined the conference could sit in a different part of the virtual room and hear the information being said by the other participants from the side corresponding to their position. Most of the virtual reality products available today still use a common HRTF, which is the average of several measured subjects, but averaging over individuals does not produce good perceptual results [10]. Virtual source localization experiments have shown that the use of a common HRTF has a very high possibility of up-down and front-back confusion, while the use of an individual HRTF significantly reduces the occurrence of this effect [11].