Chapter One: An Acoustics Primer

14. How does the ear work?

This chapter begins our study of psychoacoustics—how the human body and mind perceive the physics of sound discussed previously. We begin with our primary sensorium for sound, the human 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 outer ear also contains the auditory canal, which terminates in the eardrum, or tympanic membrane. The auditory canal behaves as a closed-tube resonator, amplifying frequencies in the 2-5 kHz range. Attached to the other side of the eardrum, in the middle ear (which is a small space of air), are three tiny bones or ossicles, the malleus, incus and stapes (or hammer, anvil and stirrup). The ossicles connect the ear drum to a fluid-filled inner ear structure called the cochlea 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. If you have ever had a middle ear infection, that air space can fill with fluid, impacting the way you hear. In addition, the Eustachian tube is designed to equalize outside pressure with the middle ear, so when we fly or otherwise rapidly change altitude, until the tube opens to equalize things, our hearing is also impacted.

In the cochlea, the vibrations transmitted from the ossicles to the oval window are then converted into fluid vibrations that move tiny hairs, which are then converted into electrical impulses sent along the auditory nerve (also called the cochlear nerve) to the brain explained in greater detail below. The flexible round window membrane (in the picture above) provides relief for the pressure waves that oscillate through the cochlea's two outer canals. The inner ear, which is surrounded by bone, also contains semicircular canals, which function more for purposes of equilibrium than hearing.

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 higher amplitude can't mask a lower frequency. 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 a later section. 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.

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.