The eardrum and the three tiny bones in the middle ear amplify vibrations and carry them to the inner ear, where the bone nicknamed the stirrup sends them through a little opening into the fluid that fills the inner ear. The vibrations pass through that fluid in the cochlea, moving the tops of the hair cells located there, called a hair bundle, and sparking nerve impulses. Different sounds move the hair bundles in different ways, allowing the brain to distinguish between them.

Exposure to loud sounds damages both those sensitive hair cells and the hearing nerve. It's a process of destruction that can be kicked off by a single, loud impulse noise such as a gunshot or by too-long exposure to loud continuous noise. Think band saw. Or revving motors during a race. Or that ride on the power boat. Even a vacuum cleaner, used too long, can damage ears forever, depending on how loud it is.

Dr. Don Worthington, audiologist and director of the Intermountain Hearing and Balance Center, likens hearing damage to the path between friends' houses. Walk on the grass a couple of times, and the blades spring back up. But do it repeatedly, and the grass wears away.

Hearing experts say they can spot loss that's noise-induced by its pattern: Typically the high frequencies go first because the high-frequency hairs at the end of the cochlea are most susceptible.

Audiologist Alan Young said he's seeing the first signs in patients in their late teens and early 20s. Worthington says he diagnoses what will be a lifelong problem in lots of patients in their 30s and 40s.

Genetics also play a not well understood role in some hearing loss, audiologists say.

The Occupational Safety and Health Administration sets standards for how long someone can work in a noisy environment without ear protection. Damage is believed to start at 85 dB with daylong exposure. The louder things are, the less time you can safely be around them. At 100 dB, OSHA says you can't work more than two hours without hearing protection over the course of a day. Spend more than 15 minutes where the noise reaches 115 dB and you risk hearing loss. Interestingly, that's about where rock concerts fall on the noise-o-meter. And a loud explosion or the crack of a rifle close to the ear can cause instant, irreversible damage.

But the numbers are confusing. Decibel measurements are logarithmic, so pumping up the volume a few decibels brings exponentially more sound. A difference of 10 dB is actually 10 times louder. That means that a snowmobile, at 100 dB, is 10 times louder than a power mower at 90 dB. And a chainsaw or rock concert is 1,000 times louder than the power mower. Even heavy traffic — 85 dB — clocks in close to the danger level if you hear it for long periods of time.