Introduction of Human Auditory System and Psychoacoustics

1.1 Simple Introduction Of The Ear

The ear consists of three separate parts, the outer, middle, and inner ears as shown in Figure 1.

Figure 1.

Diagram of the Ear

The outer ear consists of the head, the pinna, and the external auditory canal. The main function of the pinna is to locate the source of the sound, especially at high frequencies. Auditory canal is where the sound travels through to hit the tympanic membrane. The outer ear offers frequency directivity by shadowing, shaping and diffraction. Different people localize sound differently due to considerable variations in the pinna. A generalized summary of such ability for average listener is modeled by the “Head Related Transfer Functions” (HRTF’s) or “Head Related Impulse Responses” (HRIR’s).

The air-filled middle ear is composed of the eardrum (tympanic membrane), the opening of the eustachian tube and the 3 small bones (ossicles), including the malleus (hammer), incus (anvil) and stapes (stirrup) (Bharitkar, et al. 2006). The sound vibrations in the ear canal are transmitted to the tympanic membrane, which causes movement of the malleus, incus and stapes. Then stapes footplate pushes on the oval window, causing the movement of the fluid within the cochlea in inner ear. Here the whole ossicles act as an amplifier, transmitting the sound vibrations and passing them through to the fluid-filled inner ear.

The inner ear is constituted of the cochlea, containing the organ of corti, two membranes (basilar membrane and tectoral membrane) and the associated fluids and spaces (Bharitkar, et al. 2006). The cochlea is lined with tiny hair cells, which create nerve signals when the sound reaches cochlea.

1.2 Properties Of The Human Auditory System

Psychoacoustic modeling is important in audio coding and watermarking to ensure changes to the original signal remain imperceptible. Compared to the human visual system (HVS), the HAS is much more sensitive, which makes the audio watermarking more challenging than image watermarking (Cox et al., 2002). The HAS can detect signal with a range of frequency greater than 10³:1 and with power greater than 10⁹:1 (Painter et al., 2000). Understanding how the HAS perceives sound important for the development of a successful audio watermarking system.

Although exact models of the HAS do not exist today, great progress has been made in the field of psychoacoustics to model human auditory perception as well as the time-frequency analysis capabilities of inner ear. The main property of audio perception lies in the masking phenomena which includes pre masking and post masking (explained later in section 1.4). Another issue in audio perception is the absolute threshold of hearing which is illustrated as follows.