Chapter One: An Acoustics Primer
13. How does the ear work?
This chapter begins our study of psychoacoustics, or how the human body and mind perceive the physics of sound discussed above.
Image courtesy of NASA
Obviously, our primary organ used to sense sound is the ear (although sitting next to a subwoofer at a rock concert might convince you otherwise). The structure of the ear can be divided into three main parts: the outer ear or pinna, the middle ear, and the inner ear. The outer structure of the ear is responsible, in part, for helping us to place the original location of a sound, be it ahead or behind, above or below us. It also helps to funnel and focus sound waves on their way to the middle ear and auditory canal. The middle ear contains the auditory canal, which terminates in the eardrum, or tympanic membrane. Attached to the other side of the eardrum, in a small space of air, are three tiny bones or ossicles, the malleus, incus and stapes (or hammer, anvil and stirrup) which then attach to a fluid-filled structure, called the cochlea, of the inner ear at a point called the oval window. The fulcrum/lever design of the ossicles actually amplify the movement transmitted from the ear drum to the oval window of the cochlea by a factor of 15 times or so. It is in the cochlea that the vibrations transmitted from the eardrum through the tiny bones are converted into electrical impulses sent along the auditory nerve to the brain. The inner ear, which is surrounded by bone, also contains semicircular canals, which function more for purposes of equilibrium than hearing.
One of the difficulties above in determining a universal threshold of pain is that the chain of ossicles can be stiffened or muted by a contraction of the stapedius muscle. This provides a form of protection against loud sustained sounds, but not of sharp, sudden ones, such as a gunshot. This reflex is far less efficient in older people, which along with differing tastes, may explain their lower tolerance for louder music, as well as an increased risk level for hearing loss.
The most fascinating aspect of perception takes place in an area of the cochlea called the basilar membrane. The cochlea is a tapered tube, which circles around itself like the scroll on a violin. The basilar membrane divides the tube lengthwise into two fluid-filled canals, which are joined at the tapered end. The ossicles transmit the vibration to the cochlea where they attach at the oval window. The resultant waves travel down the basilar membrane where they are sensed by the approximately 16-20,000 hair cells (cilia) attached to it. These hair cells poke up from a third canal called the organ of Corti. It is the organ of Corti that transforms the stimulated hair cells into nerve impulses. Because of the tapered design of the cochlea, waveforms traveling down the basilar membrane peak in amplitude at differing spots along the way according to their frequency. Higher frequencies peak out at a shorter distance down the tube than lower frequencies. The hair cells at that peak point give us a sense of that particular frequency. It is thought that a single musical pitch is perceived by 10-12 hair cells. Due to the tapered shape of the cochlea, the distance between frequencies follows the same logarithmic distance as our perception of pitch (e.g., the placement of octaves is equidistant). This arrangement is responsible for the fact that a lower frequency at an equal or higher amplitude can mask a higher frequency, but under most circumstances a higher frequency of equal or lower amplitude can't mask a lower one (masking is actually a very complex phenomenon influenced by the pitch and amplitude relationship of the tones inside or outside the critical band, discussed in the next chapter). The masking phenomenon is used to advantage in reducing the amount of bandwidth and digital memory necessary to produce near-high-fidelity music in compressed perceptually-coded audio formats such as mp3's. Frequencies that would normally be masked are eliminated from the coding altogether, since theoretically they would not be heard or missed.