At louder volumes, a single instance can cause permanent damage, as it did with my grandmother. A gunshot a metre away can measure more than a hundred and forty decibels. A professor at the University of Texas at San Antonio told me that a major unrecognized cause of hearing loss is recreational shooting. Hunters often say they can’t wear ear protection because they need to be able to hear things like deer walking through dry leaves—although, of course, people who gradually deafen themselves can’t hear those things, either.

Members of the military face greater risks. Combat soldiers experience periods of intense gunfire, and also far louder explosive blasts, from bombs, rocket-propelled grenades, and improvised explosive devices. And military hearing threats aren’t limited to battle zones. James Henry, a research scientist at a U.S. Department of Veterans Affairs facility in Portland, Oregon, told me, “Aircraft carriers are a real problem—especially on deck, but really everywhere. Even when you’re sleeping on an aircraft carrier the noise can be above a damaging level.” Soldiers have access to effective sound-protection gear, but, like hunters, they’re often reluctant to use it. A fifth of all hearing aids sold in the United States are purchased by the V.A.

Hearing problems were part of Henry’s life before he began working with veterans. In the nineteen-seventies, he was the lead guitarist for Eli, a rock band that (after he quit) performed as a warmup act for Kiss. “I remember going home at night and having this roaring in my ears,” he said. “I didn’t realize that the roaring would eventually become a permanent condition.” That’s not what led him to his profession, though. Henry and his wife have a daughter who was born with virtually no hearing (because of genetic bad luck, not Eli concerts experienced in utero). In the early eighties, they moved to Portland so that she could attend the Tucker Maxon School, which specializes in teaching speech and listening skills to deaf children. That experience prompted Henry to earn a master’s degree in audiology, and after he went to work at the V.A. he got a Ph.D. in behavioral neuroscience.

Henry’s daughter is now thirty-eight. When she was twenty, she received a cochlear implant—a surgically placed electronic device that transmits sound impulses from a microphone near the ear to electrodes in the cochlea, bypassing the eardrum and directly stimulating the hair cells and the auditory nerve fibres. The Food and Drug Administration initially approved cochlear implants only for adults, but research has shown that they’re vastly more effective if they’re put in before the parts of the brain that process speech have developed fully. Henry’s daughter has a daughter, now five years old, also born deaf, who was fitted with cochlear implants in infancy. “The difference between my daughter and my granddaughter is that my daughter had great difficulty learning speech skills,” Henry said. “But my granddaughter can hear things and repeat them back without looking at the person who’s speaking.”

At the V.A., Henry’s specialty is tinnitus, which is both his own principal auditory problem and the leading cause of service-connected disability claims made by veterans. (Hearing loss is second.) Tinnitus is believed to be similar to the phantom-limb pain suffered by some amputees: as fewer impulses reach the cochlear nerve, the brain’s auditory circuitry compensates by becoming overactive, creating an illusion of sound. Soldiers who have both tinnitus and hearing loss often find the tinnitus more bothersome, since it’s an unceasing reminder of whatever horrifying incident caused it; in severe cases, sufferers sometimes require psychotherapy. Tinnitus treatment also often includes hearing aids, which can disguise the problem by bringing up the volume of everything else. For milder cases, air-conditioners, fans, and other masking devices can be helpful, especially at night. I sometimes pretend that the ringing in my ears is a sound I play on purpose to mask the ringing in my ears—a Zen-like switcheroo that works better than you might think.

A few weeks ago, David Corey, at Harvard, and his colleague Bence György showed me a sequence of videos in which three mice were dropped into a tank of water. The mouse in the first video paddled back and forth, trying to escape. “This is a normal mouse, and that’s the way a normal mouse swims,” Corey said. “He knows which way is up, and he always keeps his head above the water.” The second mouse had been bred with a specific genetic mutation, as a consequence of which it could hear nothing and had no sense of balance. (Balance is governed by a separate but connected part of the inner ear, and also depends on hair cells—a reason that hearing loss and balance problems sometimes occur together.) The second mouse thrashed wildly underwater, as though caught in a turbulent current. “He doesn’t know which way is up, and he just tumbles, and we have to rescue him,” Corey said. The third mouse had the same mutation, but had been given a functioning version of the faulty gene, delivered to its cochlea by a harmless virus. “He’s not quite as good a swimmer as the control mouse,” Corey said, “but he has enough of a balance system now to keep his head above the water.” The treated mouse was also able to hear, as it demonstrated by responding to a loud handclap. György and Corey said that although genetic mutations cause only a small percentage of hearing-loss cases, the viral delivery mechanism holds promise as a treatment for other types of hearing loss as well.

The inaugural breakthrough in the field of hearing restoration occurred in the late nineteen-eighties, when two researchers discovered, independently, that the ears of young chickens do something that human ears don’t: they rapidly regrow dead hair cells, restoring lost hearing within weeks. No mammal is known to share that enviable capability, but self-healing hair cells look enough like non-self-healing hair cells that scientists have been tantalized ever since by the possibility that human ears might be induced to repair themselves, too. In 2011, the Hearing Health Foundation, based in New York, created the Hearing Restoration Project, a consortium of fourteen scientists who agreed to work together toward that goal, partly with funding from the foundation. One of the originators of the project, Edwin Rubel, who was a co-discoverer of hair-cell regrowth in chickens, told me, “It’s potentially the best thing that ever happened, because it really does bring together a lot of different kinds of expertise.”

Four years ago, Albert Edge—a member of the consortium and a researcher at the Eaton-Peabody Laboratories, part of Massachusetts Eye and Ear—led a group of scientists who showed that young mice with noise-damaged ears could regenerate hair cells and recover some hearing if a drug was delivered into their inner ears shortly after they were deafened. It was the first time that mammals had proved able to regenerate hair cells. The drug, which had been developed for treating Alzheimer’s but turned out to be unsuitable for that, suppresses the activity of a protein that prevents hair cells from being created by so-called supporting cells—cells in the cochlea that function something like stem cells. “What that shows, beautifully, is that there is something there that can support regeneration,” Rubel said. “We just have to figure out how to goose it along.”

I visited Edge’s lab not long ago. A postdoctoral fellow there told me that she and her colleagues were currently able to improve the hearing of a deafened mouse by about fifteen decibels. “Which is good,” she said, “but we’d like to improve it further.” She took me up a flight of stairs to a small room containing a piece of equipment about the size of a washing machine. “This is the chamber we use to deliver high levels of noise, to kill off hair cells,” she said. On a black-and-white video monitor, I could see that the chamber contained a small cage with several mice inside. The mice appeared to be running around normally, but they were being subjected to two hours of steady noise at above a hundred decibels—enough to ruin their hearing, like being in a front-row seat at a Metallica concert.

Recently, Edge and several other researchers succeeded in causing supporting cells they’d extracted from normal mice to divide and differentiate into large clusters of hair cells. At the lab, Danielle Lenz, a co-author of the paper describing that experiment, put on latex gloves, washed her hands with alcohol, and removed two plastic trays from a shelf in an incubator, then placed them under a microscope. “In the second tray,” she said, “you can clearly see the organoids that have been formed from the single cells in the first tray, and you can see that they are multicellular.” The benefit right now is in the laboratory—having access to a big supply of living hair cells in dishes makes screening potential remedies easier—but there are hopes for bigger things. Edge told me, “The ear is maybe a little bit behind the eye, in terms of treatment, but there has been a lot of progress, and between the soldiers and the baby boomers there’s a lot of interest.”