Hearing Thresholds: Measurement, Levels, and Influences
Discover how the quietest sound you can hear, your hearing threshold, is measured and what the results reveal about the nuances of your auditory health.
The ability to hear connects us to the world, and this connection depends on detecting a vast range of sounds. The quietest sound a person can perceive is known as their hearing threshold. This is not a single, fixed value but a concept that helps describe an individual’s unique hearing ability. Understanding these personal thresholds is the first step in identifying and addressing hearing difficulties.
What Are Hearing Thresholds?
Sound has two main properties: intensity and frequency. Intensity is perceived as loudness and is measured in decibels (dB), while frequency relates to pitch and is measured in Hertz (Hz). For hearing tests, clinicians use a scale called decibels Hearing Level (dB HL), based on the average hearing of young, healthy individuals. A hearing threshold is the intensity level, in dB HL, at which a person can just barely detect a sound of a particular frequency.
This threshold is formally defined as the lowest sound level a person can hear 50% of the time. It is a statistical measure because our ability to detect a faint sound can fluctuate. Hearing sensitivity is not the same across all pitches; humans are most sensitive to frequencies between 2,000 and 5,000 Hz. To establish these thresholds, audiologists use pure tones, which are sounds of a single frequency.
How Hearing Thresholds Are Measured
The standard method for determining hearing thresholds is a test called pure-tone audiometry. This test maps the quietest sounds an individual can hear across a range of frequencies from 250 Hz to 8000 Hz. The results are plotted on a graph called an audiogram, which provides a visual representation of a person’s hearing ability.
On an audiogram, the vertical axis represents sound intensity in dB HL, with lower numbers at the top indicating softer sounds. The horizontal axis represents frequency in Hz, from low to high pitches. During the test, a patient in a sound-treated room wears headphones and responds each time they hear a tone. An audiologist presents tones at different frequencies and adjusts the loudness to find the faintest level the person can detect for each pitch.
The procedure involves testing each ear separately using headphones, which measures air conduction—the pathway sound takes through the outer and middle ear. A bone vibrator may also be placed on the mastoid bone behind the ear to test bone conduction, sending vibrations directly to the inner ear. Comparing these results helps determine the degree of hearing loss and its potential location.
Interpreting Hearing Threshold Levels
Once an audiogram is completed, the plotted thresholds reveal the degree and pattern of a person’s hearing. Thresholds between -10 and 25 dB HL are considered within the normal range for adults, while thresholds higher than 25 dB HL indicate some degree of hearing loss. This loss is categorized by its severity based on the average of thresholds at key frequencies.
The common classifications for hearing loss are mild, moderate, severe, and profound. A mild hearing loss falls in the 26 to 40 dB HL range, where individuals may struggle with soft sounds or in noisy environments. Moderate hearing loss, from 41 to 55 dB HL, makes it difficult to follow normal conversation without amplification.
With severe hearing loss (71 to 90 dB HL), even loud sounds may be inaudible, and individuals often rely on hearing aids. Profound hearing loss is diagnosed when thresholds are 91 dB HL or higher, making communication difficult without assistance. The shape of the hearing loss on the audiogram can provide further diagnostic clues.
Influences on Hearing Thresholds
Several factors can influence hearing thresholds over a lifetime. The most common is age-related hearing loss, or presbycusis, a gradual decline in hearing that affects the higher frequencies first. This process is linked to the cumulative effects of aging on the delicate structures of the inner ear.
Prolonged or repeated exposure to loud noise is another factor that can lead to noise-induced hearing loss (NIHL). Sounds louder than 85 decibels can cause permanent damage to the sensitive hair cells within the cochlea. This damage can result from a single intense event or from chronic exposure to high-volume environments.
Beyond age and noise, hearing thresholds can be affected by medical conditions like middle ear infections, certain ototoxic medications, and diseases like otosclerosis. There is also a genetic component to hearing health. Genetic differences can account for variability in age-related hearing loss and can make some individuals more susceptible to noise damage.