DAFX: Digital Audio Effects, Second Edition

5.2 Concepts of Spatial Hearing

As already mentioned, using only hearing, humans can localize sound sources, and they are also able to perceive some properties of the space they are in. This section considers both physical and perceptual issues which are related to such perception of spatial sound.

5.2.1 Head-related Transfer Functions

As a sound signal travels from a sound source to the ear canals of the listener, the signals in both ear canals will be different from the original sound signal and from each other. The transfer functions from a sound source to the ear canals are called the head-related transfer functions (HRTF) [Bla97]. They are dependent on the direction of a sound source related to the listener, and they yield temporal and spectral differences between left and right ear canals. Due to the fact that the ears are located on different sides of the skull, the arrival times of a sound signal vary with direction. Also, the skull casts an acoustic shadow that causes the contralateral ear signal to be attenuated. The shadowing is most prominent at frequencies above about 2 kHz, and does not exist when the frequency is below about 800 Hz. The pinna and other parts of the body may also change the sound signal. In some cases, it is advantageous to think about these filtering effects in the time domain, thus considering them head-related impulse responses (HRIR). Several authors have measured HRTFs by means of manikins or human subjects. A popular collection of measurements ...