We live in a sonic world, immersed in vibrations that stimulate microscopic hair cells deep inside our ears. This unseen energy influences our mood, our learning, even our health. We experience it as comforting music, as information-laden speech, or—all too often—as irritating noise, a by-product of our increasingly mechanized world. Despite all the ways sound affects us, we often let it slip unnoticed into the background of our lives. Hoping to understand it better, I set out to explore the mysteries of sound in the course of one day.
At 6:50 a.m., my alarm clock begins the assault on my ears as the groggy gray matter between them is rudely yanked toward consciousness. My eyes shoot open, and as awareness slowly crystallizes, a single idea crowds out all others: Make the noise stop. My right hand knows just what to do and immediately puts an end to the awful blare.
The formal term for the unpleasant shock that jolts me awake is acoustic startle response. Loud, sudden noises can trigger movements involving the limbs, torso, and eyelids, as well as increases in heart rate and blood pressure. This stress reaction comes in handy when noise indicates danger from, say, a wild animal or a deadly explosion. It is less useful when the enemy is a clock.
The rapid movement of an object (such as the speaker in my clock) throws surrounding air molecules into a frenzy. That disturbance produces waves of high and low pressure traveling at about 760 miles per hour, which we experience as sound.
At the receiving end, the outer ears collect the waves’ energy, causing vibrations in the tympanic membrane—the eardrum. The membrane wiggles the three middle-ear bones, which transmit fluctuations to the fluid-filled, snail-shell-shaped cochlea of the inner ear. Inside this chamber, delicate hair cells relay the sound to the auditory nerve, which carries electric signals to the brain, where they are interpreted as a familiar voice, an expressive instrumental melody, or the jarring beep that starts my day.
Our perception of loudness is subjective, but sound has an intensity, independent of our hearing, that is measured in decibels (dB). Throughout this story, decibels are measured using a dBA filter, which closely matches how the human ear absorbs sound. (Acousticians sometimes use other scales, such as dBC, which includes low and very high frequencies that human ears cannot register.) On the dBA scale, 0 dB corresponds with the threshold of hearing (10-12 watts/square meter), the level that can just barely be detected by most people. The quiet inside a sound-insulated recording studio registers around 20 dB, while a loud telephone ring is about 70. A jet engine or loud rock concert can hit 120 dB, sometimes described as the threshold of pain for our ears. The U.S. Army Medical Department ominously lists 180 dB as the threshold of death, the point at which sound can cause fatal injuries. Pressure waves generated by explosions or weapon noise, for instance, can rupture air-filled organs, such as lungs.
Shortly before 8 a.m., as I stand on the platform awaiting my commuter train to Manhattan, I pull out a sound-level meter. Vehicle traffic is light, and the most conspicuous noise emanates from a cooling unit on the station roof. The sound level hovers around 64 dB but bumps up to 75 dB as the train approaches.
Once in the city, the subway ride downtown from Grand Central Terminal does not seem loud, but the deep rumbling hovers above 75 dB, peaking at 86 dB, when joined by a large group of 7th graders on a field trip. At 14th Street, where I exit, the high-pitched squeal of arriving trains is excruciating. My sound meter registers 95 to 98 dB—above my personal threshold of pain, apparently. Psychoacoustician William Hartmann, who studies sound perception at Michigan State University, says that the high-pitched sound of the subway probably has components in the range of 3,000 to 4,000 hertz (Hz), or vibrations per second. Such frequencies transmit efficiently to the inner ear. “Brakes tend to have a pure-tone sound with only a few frequencies contributing to the power,” Hartmann says. “By contrast, the rumble of the train is broadband—lots of frequencies. Intense pure-tone sounds are more annoying.” I escape the subway and walk into DISCOVER’s lobby a few minutes before 9 a.m., relieved to measure a peaceful 55 dB, the quietest reading of the day so far.
Habitual exposure to sound levels of 85 dB or above will cause hearing loss for many, according to the American Hearing Research Foundation. Loud sounds put too much pressure on the cochlear hair cells, which can damage or kill them. Once dead, they do not regenerate, so hearing is permanently affected. One in 10 Americans has hearing loss, including more than one in three over the age of 65. Impairments range from an inability to hear certain frequencies—for example, the high frequencies of women’s and children’s voices—to complete deafness. On the bright side, the World Health Organization (WHO) estimates that 50 percent of deafness and hearing impairment is preventable. Newly sensitive to the dangers of my loud morning, I stop by Manhattan’s Center for Hearing and Communication for a screening. Director of audiology Ellen Pfeffer Lafargue administers a series of beeps, ranging from 500 Hz to 4,000 Hz, and asks me to respond to each. She soon reassures me that my hearing is within the normal range. That does not necessarily mean it has not deteriorated, though. Young adults can pick up frequencies ranging from as low as 20-Hz bass notes to 20,000-Hz squeaks. But as we age, we tend to lose some of our high-frequency hearing. For most people, peak sensitivity occurs for frequencies around 1,000 to 4,000 Hz.
Curious whether I still have my full high-frequency range, I check out an informal Web-based hearing test from the University of New South Wales. To my surprise, I can make out the lowest (30 Hz) and highest (16,000 Hz) pitches. But in another batch of online tones that includes even higher frequencies, I hear nothing at 18,000 Hz or above. In comparison, dogs can hear up to 45,000 Hz and mice to 91,000 Hz. Bats top out at 110,000 Hz, while dolphins and porpoises can reportedly detect frequencies at 150,000 Hz.
The surrounding bath of urban sound has mental as well as physical effects, according to environmental psychologist Arline Bronzaft, who has spent more than 30 years studying how people perceive and respond to noise. Currently with the Council on the Environment of New York City, she has addressed noise problems as diverse as a neighbor’s too-loud TV, the banging of a bed during sex, and a mysterious whistling in the middle of the night that disturbed Brooklynites in the 1990s. (It turned out to be a signal used by a ring of drug dealers.) I stopped by Bronzaft’s home on the Upper East Side to learn more about why noise matters. As we sat among bookcases filled with decades of research, she explained that not everyone reacts to noise the same way, but the negative effects of unwanted sounds on health are real and measurable. Even soft noises, well below the level that can damage hearing, have been linked to stress and cardiovascular disease. “A sound that is interpreted as noise, as something we don’t want to hear, causes stress,” Bronzaft says, “which causes problems with the heart, the gut, and other organs.”