The human ear is a complex organ, capable of collecting, amplifying, and transmitting audio signals to the brain. Following the path of the sound wave, the vibration enters through the outer part of the ear, the auricle, which is responsible for collecting and directing the sound inward through the ear canal, which amplifies low-level sounds and protects against high-level sounds by producing earwax.
At the end of the ear canal begins the middle ear. The eardrum is a vibrating membrane in contact with a chain of ossicles (the malleus, incus, and stapes). The Eustachian tube is the organ responsible for equalizing ambient pressure with that of the middle ear, and its function is noticeable, for example, underwater or during airplane trips, when atmospheric pressure changes more rapidly.
The chain of ossicles is an acousto-mechanical transducer responsible for moving the fluid inside the cochlea, a spiral-shaped organ in the inner ear. The movement of this fluid produces a wave on a membrane, the basilar membrane, which supports the organ of Corti, made up of thousands of receptor cells (about 24,000). Depending on the wave formed in the basilar membrane, more or fewer cells will be excited. These cells, connected to neurons, transmit information to the brain. These cells do not regenerate; very loud sounds, injuries, age, etc. cause the death of some of them and the subsequent hearing loss.
Masking
The physiology of the ear and the brain's processing of the audio signal explain the masking phenomenon.
Temporal masking occurs when two sounds are received, one with a higher amplitude, close in time. The lower amplitude sound may become inaudible.
Frequency masking occurs when two sounds of different frequencies are received simultaneously. There is usually a decrease in loudness around the highest frequency.
This phenomenon is important in acoustics. It influences, for example, the way some audio encoders are designed (which eliminate information that would be masked), or the configuration and adjustment of public address systems that must ensure signal-to-noise ratios and signal equalization that guarantee final intelligibility.
