How hearing works and how noise damages it

Noise-induced hearing loss is one of the oldest recognized occupational diseases and still one of the most common. Blacksmiths, boilermakers and weavers were described as going deaf centuries ago, and the pattern has not changed much: a worker spends years around loud machinery, notices nothing dramatic, and one day realizes that conversations in a restaurant have become exhausting, that the television is always too loud for everyone else, and that a constant ringing has become part of every quiet evening. The damage is permanent. No surgery, medication or hearing aid restores the cells that noise destroys.

This course is about preventing that outcome. It is written for supervisors, safety coordinators, joint health and safety committee members, hearing conservation program administrators and workers who work in noisy places. It builds on the short overview of hearing protection in HSE-104 Personal Protective Equipment, lesson 5, and goes much further into how hearing is damaged, how noise is measured and assessed, how exposure limits work in your jurisdiction, how noise is controlled at the source, how hearing protectors are rated and fitted, how audiometric testing detects early damage, and how all of these fit together in a program that actually works.

The course does not make anyone a qualified noise surveyor, audiometric technician or audiologist. OSHA's occupational noise standard, 29 CFR 1910.95, requires audiometric tests to be performed by a licensed or certified audiologist, otolaryngologist or other physician, or by a technician who is certified by the Council for Accreditation in Occupational Hearing Conservation or who has equivalent competence, and complex noise problems usually need an industrial hygienist or acoustical engineer. Think of this course as the knowledge base that lets you read a noise survey intelligently, recognize a weak program and make sound decisions about controls.

The journey of a sound

Hearing turns tiny pressure fluctuations in the air into nerve signals the brain interprets as speech, music, alarms and machinery. The system has three main parts, and noise damages them in different ways.

The outer ear consists of the visible pinna and the ear canal, a tube about 2.5 centimeters long that ends at the eardrum. The canal is not just a passage. Its length makes it resonate most strongly at around 3,000 Hz, boosting sounds in that range by roughly 10 to 15 decibels before they reach the eardrum. That resonance helps us understand speech, but it also means the inner ear receives the most energy in the 3,000 to 6,000 Hz region, which is exactly where noise damage first appears.

The middle ear is an air-filled space behind the eardrum containing three tiny bones, the malleus, incus and stapes. They act as a lever and piston, converting the large, low-pressure movements of the eardrum into small, high-pressure movements at the oval window, the entrance to the fluid-filled inner ear. Two small muscles attached to these bones contract in response to loud sound, stiffening the chain and reducing transmission by a modest amount. This acoustic reflex takes tens of milliseconds to engage, so it offers almost no protection against sudden impulses such as gunshots or hammer blows, and it fatigues during long exposures.

The inner ear contains the cochlea, a snail-shaped spiral about the size of a pea. Inside the cochlea, the basilar membrane runs along its length and carries roughly 15,000 sensory hair cells in each ear: one row of inner hair cells, which send signals to the brain, and three rows of outer hair cells, which act as amplifiers that sharpen and boost quiet sounds. Different frequencies make different parts of the membrane vibrate most strongly. High frequencies stimulate the base of the cochlea, near the oval window, and low frequencies stimulate the apex.

How noise destroys hair cells

Hair cells take their name from the bundles of stiff projections, called stereocilia, on their upper surface. When the basilar membrane vibrates, the stereocilia bend, opening ion channels that trigger nerve signals. Loud sound bends them further and faster than they are built to handle. Two kinds of damage follow.

Mechanical damage happens with very intense sound, such as an explosion or a gunshot close to the ear. Stereocilia can fracture or fuse, hair cells can be torn from their supports, and in extreme cases the eardrum ruptures or the middle ear bones are disrupted. This is called acoustic trauma, and the hearing loss can be immediate.

Metabolic damage is the more common pathway at work. Hours of loud noise drive the hair cells to work at their limit. They produce damaging molecules known as reactive oxygen species, blood flow in the cochlea changes, and the cells exhaust their ability to repair themselves. Some cells die over the following hours and days. Outer hair cells are the most vulnerable, so the first effect is a loss of the cochlea's amplification: quiet sounds become harder to hear, and sounds lose their sharpness. Once a hair cell dies in a human ear, it is not replaced.

Research since about 2009 has added a third concern. Noise that causes only a temporary threshold shift, after which the audiogram returns to normal, can still permanently destroy some of the synapses connecting inner hair cells to the auditory nerve. This cochlear synaptopathy, sometimes called hidden hearing loss, does not show up on a standard audiogram but may explain why some noise-exposed people with normal test results struggle to understand speech in background noise. The research in humans is still developing, but it reinforces a simple message: a temporary dulling of hearing after a noisy shift is not harmless.

The noise notch

Hearing ability is measured with an audiogram, which records the quietest level a person can hear at each of several test frequencies, usually from 500 Hz to 8,000 Hz. Results are plotted in decibels hearing level (dB HL), where 0 dB HL is the average threshold of young people with normal hearing. Larger numbers mean worse hearing, so audiograms are drawn with higher numbers lower on the chart, and hearing loss appears as a line that dips downward.

Noise-induced hearing loss has a characteristic shape. Because the ear canal resonance and the mechanics of the cochlea concentrate damage in the 3,000 to 6,000 Hz region, the earliest change is a dip, or notch, centered around 4,000 Hz, with better hearing at 8,000 Hz. As exposure continues, the notch deepens and widens into the lower frequencies that carry much of the information in speech. Age-related hearing loss, by contrast, usually slopes steadily downward toward the highest frequencies without recovering at 8,000 Hz. Real audiograms often show both patterns combined, which is one reason why interpreting them is a job for trained professionals.

Why does a notch at 4,000 Hz matter if most speech energy is below 2,000 Hz? Because consonants such as s, f, th, t and k carry most of their information in the higher frequencies, and consonants carry most of the meaning in speech. A worker with a notch hears that someone is talking but cannot tell whether they said fifty or sixty, fit or sit, and struggles most when there is background noise. Many people with early noise damage say that others mumble.

Tinnitus

Tinnitus is the perception of sound with no external source. Noise-exposed workers often describe a high-pitched ringing or hissing after a loud shift, which fades overnight. With continued exposure, it can become constant. Constant tinnitus is linked to poorer sleep, difficulty concentrating, anxiety and depression, and for some people it is more disabling than the hearing loss itself. Tinnitus is often the first warning sign that a worker is overexposed, and workers should be encouraged to report it rather than dismiss it.

US data show how common these problems are. NIOSH has long estimated that about 22 million US workers are exposed to potentially hazardous noise at work each year, and hearing loss has for years been among the most commonly recorded occupational illnesses in manufacturing on OSHA logs. Surveys consistently find that a large share of noise-exposed workers do not wear hearing protection all the time, and that tinnitus is far more common among people with a history of noisy work than among those without.

Who is at risk

Noise hazards are not limited to factories. They occur wherever powerful machinery, impacts, compressed air, combustion, amplified sound or many people talking at once are found.

Within any workplace, exposure depends on the job, not just the location. A maintenance mechanic who moves between quiet offices and very loud equipment rooms may have a higher daily exposure than an operator who stays at one moderately loud machine. That is why later lessons emphasize personal dosimetry for mobile workers.

Why noise is underestimated

Noise is easy to underestimate for several reasons. The damage is painless and gradual, so workers rarely connect a difficulty hearing at age fifty with the shop floor at age twenty-five. People adapt to noise and stop noticing it. The decibel scale is logarithmic, so a machine that sounds only a little louder may deliver several times as much acoustic energy. Workers also often believe that they are tough, used to it, or that their ears have adjusted, when in fact the temporary dulling they feel is the first stage of damage.

Noise also has effects beyond the ear. High noise interferes with communication and can mask warning shouts, reversing alarms and approaching vehicles, contributing to injuries. Long-term noise exposure is associated with raised blood pressure, fatigue and stress, and noise in offices and call centers, though usually below hearing-damage levels, reduces concentration. This course focuses on hearing, but good noise control brings these other benefits too.

Noise exposure outside work

Hearing does not distinguish between work and leisure. Power tools at home, motorcycles, snowmobiles, recreational shooting, concerts and personal music players can all add to the dose. A worker who spends the day just under the workplace limit and the evening at a loud concert or on a firing range adds risk that no workplace program measures. Good programs include education on off-the-job noise, and many employers allow workers to take hearing protectors home. Audiometric results are also interpreted with leisure noise in mind, because a shift in hearing may come from either source.

What this course covers

Lesson 2 covers the sound basics needed for noise control: decibels, frequency, weighting and how levels combine. Lesson 3 sets out the exposure limits and laws in your jurisdiction. Lessons 4 and 5 cover how noise is measured and how exposure is calculated. Lessons 6 and 7 cover engineering controls, administrative controls and the structure of a hearing conservation program. Lessons 8 and 9 cover hearing protectors in depth, including ratings, derating and fit testing. Lesson 10 covers audiometric testing and recording. Lesson 11 covers impulse noise, substances that damage hearing and special workplaces. Lesson 12 brings everything together in a capstone program for an invented plant.

The physics of sound is covered in more depth, from a film and recording perspective, in FILM-310 Sound for Film, lesson 2. This course uses only the physics needed to protect hearing.