Binaural hearing allows for localizing the source of the sound, suppressing noise, example to better understand speech. To localize sound there is an important aspect of auditory perception that allows us to adjust to the room, namely spatial hearing. There are two processes in localizing sounds in humans, monaural cues and different cues.
- Monaural Cues
Monaural cues are how each ear translates the captured sound signal. Monaural cues are the result of a convolution of sound sources with head-related transfer function (HRTF) impulses. Head-Related Transfer Function (HRTFs) is a form of transformation of sound wave propagation from the source to the ear or Head-Related Impulse Response (HRIR). HRTF is also defined as a form of modification of a sound from a certain direction that reaches the ear. This transformation involves diffraction and reflection from the anatomy of the ear. HRTF also depends on the location of the sound source relative to the listener so that it can determine the sound source.
- Difference Cues
Difference cues are how the difference between two ears translates to sound signals. These differences cues contain information on International Time Difference (ITD) and Interaural Level Difference (ILD). ITD is the difference in the arrival time of the left and right ear sound waves while ILD is the difference in pressure level between the left and right ears. Based on Duplex Theory, ITD values are used for localizing sounds at low frequencies, which is below 1.5 kHz while ILD is used for localizing sounds at high frequencies, which is above 1.5 kHz. Environmental sounds are in the range of low frequency and high frequency so that the human auditory system uses ITD and ILD.
The basic principles in ITD are illustrated in Figure 1
When the sound source is sound waves with low frequency, the propagation of sound waves will reach both ears without decreasing the sound pressure level. This is because the wavelength of sound is smaller than the dimensions of the head. However, there is a time difference received between the two ears. Therefore, sound waves at low frequencies are related to ITD.
The basic principles of ILD are illustrated in Figure 2. The ILD value is influenced by the size of the head and for sources that are very close to the head. When the sound source is in the high-frequency range where the wavelength of the sound is smaller than the dimensions of the head, the sound will reach the ears closer to the sound source. When will reach the other ear, the sound will be held up or there is a failure of propagation of sound waves for a while, this phenomenon is called an acoustic shadow. The sound that finally reaches the other ear will experience a decrease in the level of sound pressure caused by the phenomenon of acoustic shadow.
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