If you have ever tried to draw someone's ear, you will have noticed that our ears are as individual as our fingerprints. They are small or large, simple or convoluted, smooth or fuzzy, or even hairy, but all healthy ears have the same parts.

The structures of the ear (Fig. 50-2) enable us to collect sound waves, which are then converted into nerve impulses. The outer ear is a sound-gathering funnel. The outer ear in humans is a more efficient sound gatherer than the nonexistent external ear in many reptiles and birds, but lacks the collecting and focusing capacity of a cat's ear. Sound travels from the outer ear through the auditory canal, which is also called the external ear canal, and into the middle ear.

Within the middle ear, sound waves set the eardrum (tympanic membrane) in motion. At the threshold of hearing, the displacement of air molecules on the eardrum and the eardrum movement is equal to about the diameter of an atom. If our ears were one order of magnitude more sensitive, we could hear thermal noise. The sensitivity of our ears is close to the practical limit for sound reception. The average human ear can withstand the loudest sounds of nature, yet be able to detect the tiny pressures of barely audible sounds.

The middle ear is an air-filled space surrounded by bone and bounded by two membranes, the eardrum on the outer side and a flexible membrane separating it from the inner ear on the inner side. The main job of the middle ear is amplification. The vibrations from the eardrum are transmitted across the middle ear space by three tiny levers of bone to the extraordinarily sensitive inner ear mechanism (cochlea). These three bones, called the ossicles, are the malleu (hammer), incus (anvil), and stapes (stirrup). The movement of each bone increases the amplification of sound. The footplate of the stirrup bone is attached to a flexible membrane that covers an opening into the inner ear called the oval window. Moving back and forth like a piston, the stirrup bone sets in motion the fluids of the inner ear. In the short but intricate journey from eardrum to inner ear, the sound wave is amplified as much as 25 times.

The inner ear is where the sound vibrations are converted to electrical nerve impulses for interpretation by the brain. The rhythmic waves in the inner ear fluid are set in motion by the stirrup bone's pressure on the oval window. They excite a highly delicate organ that is at the heart of sound reception. Coiled like a snail shell

(its name, cochlea, is from the Latin word for snail), it is sometimes described as a spiral piano keyboard. Hair cells at one end of the keyboard respond to sounds at high frequencies, up to 20,000 cycles per second; those at the opposite end respond to low ones, down to 16 cycles per second. The receptors for low tones are at the innermost turn of the spiral. The basilar membrane in the cochlea resonates at one end at a frequency of 20 Hz (Hertz), and at the other end at 20 kHz (kilohertz). This range of 20 Hz to 20 kHz establishes the range of frequencies that the human ear can hear.

As the vibrations of the hammer shake the hair cells of the cochlea, anvil, and stirrup, they initiate an electrical impulse that is transmitted to the nerve fibers, which then merge into the auditory nerve. These impulses are carried into the central auditory pathways of the brain and ultimately to the cerebral cortex, where their pattern is interpreted as sound.

If you are young and your ears are in excellent physical condition, you can hear sounds in the 20 to 20,000 Hz range. You will be most sensitive to frequencies in the 3000 to 4000 Hz range. Very high frequencies may be uncomfortable for young listeners, who may be very sensitive to the sound of high-speed dental drills. Our ability to hear upper frequencies decreases with age. By middle age, the typical upper limit is around 10 kHz to 12 kHz. Upper range hearing loss is usually more pronounced in men than in women.

Our ears can pick out specific sounds to which we want to pay attention, but more frequently it combines sounds distinct from each other in frequency and phase, as chords in music, for example. Most sounds are actually complex combinations of frequencies. Musical tones combine fundamental frequencies with harmonics (overtones). A trained conductor can pick out one single instrument in a 120-piece orchestra. Amazingly, we have the ability to pick out one voice in background noise much louder than that voice, a phenomenon known as the cocktail party effect.

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