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Critical Thinking

* Should MP3 players be manufactured so they can't be turned up high enough to damage hearing? Why or why not?

* What other loud noises might endanger hearing? (engine noise, rock concerts) How might people protect themselves from those noises?

National Science Education Standards

* Personal health: injury prevention

* Structure and function in living systems: sensory organs

Internet Link

* San Francisco Chronicle: Play it loud, and you may pay for it: www.sfgate.com/cgi-bin/ article.cgi?f=/c/a/2005/09/22/ DDGKUER3M323.DTL

LIZ Mashburn listens to rap and hip-hop on her new green iPod for about two hours a day. "It's awesome," says the 14-year-old student from the Christian Fellowship School in Denver.

Liz bought her iPod several months ago with money she had saved doing family chores. Like many teens around the country, she has plugged into a hugely popular new craze. In October, Apple Computer, the maker of the iPod, reported that it had had the best year in its history. In just three months, the company had sold almost 6.5 million iPods and predicted total sales of 35 million by year-end.

There's a downside to listening to iPods or any other type of digital audio player, however. At high-intensity levels, they can harm the hearing.

Without knowing it, many young people who listen to loud music are permanently damaging their ears. "We're starting to see hearing loss in young adults that we expect to see in middle-aged adults," Robert Novak, director of clinical education in audiology at Purdue University, told Current Science. He says audiology clinics across the country have seen an increase in noise-induced hearing damage in younger people during the last decade.

DIGITAL TECHNOLOGY

The iPod is the most popular type of Moving Picture Experts Group Audio Layer III (MP3) player. Introduced in 1998, MP3 players store, organize, and play digital music files. A digital music file contains music that has been converted into the language of computers--digits and stored in a stream of 8-digit packages called bytes. MP3 technology compresses files and enables players to hold huge amounts of information.

MP3 players can hold far more music--hundreds or even thousands of songs--than a tape or a CD can. As a consequence, says Novak, "listening to MP3 players is becoming more of a full-day experience." Liz says she would listen to her iPod more than two hours a day if she weren't so busy with school and sports.

DAMAGED HAIRS

MP3 players can be hazardous to the hearing when their volume is turned up too high. Any loud sound can damage nerve endings called hair cells in the inner ear (see "Have You Heard?"). The hair cells that respond to high pitches are especially vulnerable.

Sometimes the damage to the hair cells is temporary, says Novak. The ears are never truly at rest; their hair cells are vibrating, even during sleep. But quiet times allow hair cells to vibrate to nondamaging levels of sound. During those times, the hair cells can repair any temporary noise damage they might have suffered.

If a sound is intense enough, however, it can shake the membrane on which the hair cells sit--"like an earthquake," says Novak. That vibration can break or even uproot the hair cells. "When that happens, the hair cells are finished," he says.

Human ears cannot regrow hair cells. No pill or surgery can bring them back, either. A certain amount of hearing loss seems to be inevitable with age in modern society and &ten becomes noticeable in middle age. But hearing loss induced by portable music players is putting "older ears on younger bodies," says Novak.

MP3 players are particularly hazardous to the hearing because they have brought with them a change in headphones. Old-style headphones rest on the outer ears. The headphones, or earbuds, sold with many MP3 players are inserted into the auditory canals, the passageways that lead into the skull. Because earbuds are closer to the eardrum, they are capable of delivering even higher sound intensities to the inner ear.

TOO HIGH

Some electronics manufacturers warn customers against listening to loud music but don't indicate the volume levels that are unsafe on their portable music players. According to studies, sound levels below 80 decibels (dB) are harmless. A decibel is the measure of the loudness of a sound. Many MP3 players can reach levels above 100 dB.

How can you find a safe volume level on your personal stereo? Novak suggests setting it to a comfortable volume in a quiet room. From then on, don't turn the volume above that level no matter where you are. "You should be able to hear someone talking to you at a normal conversational level from a distance of 3 feet," says Novak. If that someone can tell what music you're listening to--that's another sign the volume is too high.

Liz Mashburn tries not to play her music too loud, but she isn't sure that her friends and classmates are equally careful. "Most teens like their music really loud because they think it is cool," she says.

"Hearing is the one sense that enables humans to most easily use language and develop speech and build relationships," says Novak. "So we need to protect that very special sense."

Have You Heard?

This is how the ear hears: Sound waves travel down the outer ear's auditory canal and strike the tympanic membrane (eardrum), causing it to vibrate. The vibrations are transmitted through the middle ear by three ossicles (tiny bones). The third ossicle, the stapes, sends waves through a fluid inside the cochlea, a snail-shaped organ in the inner ear. The cochlea contains about 15,000 hair cells, which respond to the waves. The hair cells relay signals by way of the auditory nerve to the brain, which interprets the signals as sounds. No sound is heard until it reaches the brain.

SKILLBUILDERS

Multiple Choice

Choose the response that best completes the statement or answers the question. Write the letter of the response in the blank provided.

--1. The eardrum is also called the (A) auditory membrane. (B) cellular membrane. (C) tympanic membrane.

--2. No sound is ever heard until it reaches the (A) brain. (B) ears. (C) stapes.

--3. The language of computers is written in (A) digits. (B) hieroglyphics. (C) letters.

--4. Loud sounds can damage or destroy nerve endings in the ear called (A) hair cells. (B) earbuds. (C) headphones.

--5. The loudness of a sound is measured in (A) decibels. (B) frequencies. (C) hertz.

--6. How many digits are in a byte? (A) 2 (B) 4 (C) 8

--7. The middle ear contains three tiny bones called (A) ossolotches. (B) oscillators. (C) ossicles.

--8. The cochlea is shaped like a (A) doughnut. (B) snail. (C) wishbone.

--9. Sound travels through the air in (A) pulses. (B) a stream of packages. (C) waves.

--10. The inner ear is connected to the brain by way of which nerve? (A) auditory (B) gustatory (C) olfactory

ANSWER KEY

1. C, 2. A, 3. A, 4. A, 5. A, 6. C, 7. C, 8. B, 9. C, 10. A