PERSONAL HEALTH : Headsets for Kids Strike a Sour Note

About 23 million headset radios and tape players are sold in the United States every year, and a look at toy store racks confirms that there’s a new target audience for them: the grade-school crowd.

Painted in bold primary colors or cute pinks and purples, the personal stereos range from sturdily built $50 models to bare-basics versions obtainable for less than $10. What they all have in common is the ability to create sound so loud it can cause an insidious, cumulative hearing loss.

Studies have shown sound levels from the machines can reach 115 decibels or more. At that level, permanent hearing damage could occur after just 15 minutes.

“I think that there are a lot of kids who get an awful lot of hearing damage because I’m sure that they never use the headset at 15 minutes a day, but a great deal more than that,” said Dr. Jack Vernon, otolaryngology professor at Oregon Health Sciences University in Portland.

Noise-induced hearing loss is cumulative and is a function of sound intensity and duration. So listening to even moderately loud music for longer than 15 minutes or listening day after day can cause permanent damage.

And the earlier a child begins using a headset to listen to loud music, the more years over which damage can accumulate.

Consequently, parents need to be careful about supervising their children’s use of these powerful little sound machines, hearing experts say.

But that is easier said than done because, unlike other situations where young people encounter loud music, headset use is less likely to provide clues that the sound is too loud.

For example, a teen-ager who has just attended a rock concert may experience muffled hearing or ringing in the ears, clear signals that the sound was too loud. Such symptoms--although they often disappear after a few days--mean some permanent hearing loss has occurred at high sound frequencies, many experts agree.

And a child who turns up the volume on his bedroom stereo too high will soon have angry parents at his door, which can guard against damage.

But a headphone-wearing child is difficult for a parent to monitor. As a rule of thumb, experts say, headset sound that is loud enough to be heard by passers-by can cause damage if used more than two hours a day. However, individuals can be more sensitive than that to damage.

Record producer Jeff Baxter is serving as a celebrity spokesman for the “Hearing Is Priceless” education campaign of the House Ear Institute of Los Angeles. When talking to teen-agers about saving their hearing, he uses analogies he thinks young people will understand.

“You like to swim . . . you like to be in the water, but you’re not interested in drowning,” he tells them. “So why would you fill your ears with debilitating sound when what you really want to do is feel the music with your body?”

Baxter was noted for blocking out the booming sound of on-stage amplifiers with an earmuff-style headset when he was a guitarist in the 1970s with the rock groups Steely Dan and the Doobie Brothers. He credits this with maintaining his hearing so that he is able to produce records today.

He says young people can enjoy the experience of being surrounded by sound at a very loud concert but still protect their hearing by wearing foam earplugs. These let sound pass through but cut down on the volume.

At clinics such as the one associated with the House Ear Institute, it is common to see young men in their 30s who have constant ringing in their ears, called tinnitus, or who are having trouble understanding speech because of hearing damage from rock concerts two decades earlier.

If a similar phenomenon occurs with headset use--which has enjoyed its boom only since the early ‘80s--hearing specialists don’t expect it to emerge for another decade.

One of the early studies on headset effects was conducted in 1984 at the University of Iowa. Researchers asked 16 headset-stereo owners to listen to music for three hours at the loudness setting they normally used.

The study found that the nine volunteers who listened to music at an average of 90 decibels--equivalent to noise from a vacuum cleaner--showed little or no hearing loss.

But the seven subjects who listened at 94 to 104 decibels--roughly equivalent to the noise from a blender at the low end and a chain saw at the high end--temporarily lost an average of 10 decibels of hearing at high frequencies.

With a permanent hearing loss of this degree, conversational speech would have to be 10 decibels louder for the person to hear the difference between “feet” and “heat.” That is because consonant sounds are of higher audio frequency than vowels, and high-frequency hearing is the first to be damaged by noise. (A voice in normal conversation is about 60 decibels.)

Further, the study found that the person who listened to 104-decibel music lost 25 decibels of hearing in one ear and 35 decibels in the other.

All the hearing losses had disappeared within 24 hours, but the researchers worried about the ability of the ear to stand up to repeated onslaughts, since the volunteers admitted using headsets up to six hours per day.

Another study, at Mount Sinai School of Medicine in New York, found that of 89 college students who used headset stereos, a third listened at levels louder than is considered safe under federal standards for industrial workers.

Those standards limit exposure to noise of 100 decibels to no more than two hours per day. The allowable time is halved for every five decibels above that; at 105 decibels, for example, the maximum is one hour.

When noise-induced hearing loss does occur, it is because of damage to the 15,000 “hair cells” in the inner ear. Sound vibrations are transmitted mechanically through the outer and middle ear, then converted to electrical impulses by hair cells. The signal travels through the auditory nerve to the brain for interpretation.

Loud noise causes hearing loss by making hair cells die or become stiff and brittle. They are never replaced. This process occurs slowly with normal aging but is accelerated by noise damage.

Speech and music send discernible sound up to about 8,000 hertz--a measure of sound-wave cycles per second--to the inner ear. It isn’t until a hearing loss occurs in the 3,000-hertz range that discerning speech becomes difficult, said Janet Endo, an audiologist at the House Ear Clinic in Los Angeles.

There are an estimated 21 million Americans who have a hearing loss in one or both ears. Problems coping with the hearing world often can lead to social isolation. Although technology has provided better hearing aids and even implantable devices, they are not as good as natural hearing, experts say.

Noise damage is more insidious in young children than in adults because their hair cells have not yet been affected by the normal aging process, Oregon’s Vernon said.

As a result, their noise-induced hearing losses would come first at very high frequencies that are not even included in normal audiology tests. Test equipment stops at about 8,000 hertz, but young children are sensitive to frequencies up to 20,000 hertz, he said.

Not only would this early noise-induced loss accelerate the normal aging process of the hearing apparatus, he said, but it also is known that, once a noise-related loss occurs, an individual becomes more sensitive to further losses.

After the early research into headset stereos’ effect on hearing, manufacturers responded by putting warnings on their packaging. Many also include a pamphlet from the Electronic Industries Assn. exhorting users to “use your head when you use your headset.”

Koss Corp. of Milwaukee earned praise from audiologists by marketing a unit for children that had a warning light when sound was turned up too loud. Koss has since stopped making stereo units.

In many cases, however, headset stereos now being marketed for children--particularly the less expensive ones--have no such features.

The exceptions are two bright red models in the My First Sony line sold by the Sony Corp. They feature a limiter switch that lowers the maximum amount of sound the unit can produce. However, the switch is easy for a child to flip off.

Marnix Van Gemert, president of Sony’s personal audio products division in the United States, said a parent can nonetheless monitor the child to see that the switch is being used. He also noted that the My First Sony line has radios and cassette players without headphones, which the parent might prefer.

Dean Gallea, an audio test engineer for Consumers Union, suggests the ideal goal would be for manufacturers to develop internal circuitry to automatically dampen sound when it exceeded dangerous levels.

This feature would have to be intelligent enough to allow short bursts of loud sound, such as when a cymbal is struck, or consumers would never accept it, he said.

“You want to be able to pass through enough of a dynamic range so that you’re not losing the peaks in the music,” Gallea said. “It has to be fairly benign so that it doesn’t cause damage to the music.”

Given that such technology is not on the horizon, and given that the Electronic Industries Assn. expects 23.6 million headset stereos to be sold in 1991, it is up to parents to figure out how to protect their children’s hearing.

Dr. John House, president of the House Ear Institute, has solved the problem in his home by encouraging his three sons, ages 11 to 18, to play their music on regular audio units.

“I discourage them from using headsets because it’s hard to have control over what the volume is--unless you can hear it clear across the room, then obviously that’s too loud,” House said.

But because some children simply must have headset stereos, experts devised a variety of home tests to determine the maximum safe loudness.

Tests of various brands at the House Ear Institute indicate that anything above the “5” setting on a 1-to-10 volume control is the maximum anyone should use, House said.

The Mount Sinai study, which tested three models marketed in 1984 by Sony, Toshiba and Sanyo, found that a “5” averaged 94 decibels. The federal government’s occupational standards say that level of sound is dangerous if experienced for more than four hours a day.

Every manufacturer has different loudness scales, however, so the setting offers only rough estimates. Headset audio also is difficult to gauge because the listener becomes accustomed to it and because environmental noise will cause a listener to turn the sound up to block the other sounds.

Gallea of Consumers Union suggests this test:

Play music on a conventional audio system at a level that allows you to carry on a conversation without shouting. Then switch the room audio off and the headset unit on with a similar kind of music.

Compare the sound levels between the two until the headset audio sounds no louder than the room unit does. Note the number on the volume level control of your headset unit. That is the maximum sound level at which you can safely use the player--even if traffic or other environmental noise tempts you to turn it up.

Once the maximum safe level is determined, Oregon’s Vernon advises finding a way to alter the volume dial so it cannot be turned beyond that.

This advice applies to adults as well as children, the experts noted.

It also applies to any kind of music, not just the rock music that children and teen-agers tend to prefer.

THE DAMAGE WITHIN Sounds higher than 80 decibels--a little louder than an alarm clock--can damage the inner ear’s hair cells, which convert sound vibrations into electrical signals that the brain can interpret. Often, the damage is temporary. But if prolonged noise destroys the hair cell, it does not regenerate.

Cranked up, a headset stereo can produce sound at around 115 decibels, the level at which hair cell damage can be expected after just 15 minutes.