Sound is generated by vibrations that travel through a medium, such as air or water. The human ear converts these physical vibrations into electrical signals that the brain interprets. The perceived highness or lowness of a sound is determined by its frequency, which is measured in Hertz (Hz). This measurement defines the limits of a person’s auditory range.
The Standard Human Auditory Range
The standard auditory range for a healthy young adult spans from 20 Hertz (Hz) at the low end to 20,000 Hertz (20 kHz) at the high end. This spectrum allows for the perception of sounds from the deepest musical notes to the highest-pitched whistles. The lower limit of 20 Hz represents a deep, rumbling sound. These slow vibrations are often perceived physically by the body rather than purely heard by the ear, such as the heavy bass registered at loud concerts.
The upper theoretical limit of 20 kHz represents the maximum frequency the inner ear structures can process. Sound waves at these high frequencies attenuate, or lose strength, faster in the air than lower frequencies. While 20 kHz is the standard for an ideal ear, the practical limit is often lower for many adults. The upper boundary of hearing is commonly restricted to around 15 kHz or 17 kHz, even in individuals who have not experienced significant hearing trauma. The ear is most sensitive between 250 Hz and 8,000 Hz, which is the range where human speech occurs.
Age and Noise Exposure Effects on Hearing
The standard auditory range is not static but degrades over time, primarily at the higher frequencies. This natural decline in hearing that occurs with age is medically termed presbycusis. Deterioration often starts early, with high-frequency loss detectable in the late teens or early twenties, though it becomes noticeable later in life. Presbycusis is characterized by a symmetrical, bilateral loss, meaning both ears are affected similarly, and the loss is most pronounced at the highest frequencies.
This age-related change is linked to cumulative wear and tear on the sensory structures inside the inner ear, specifically the cochlear hair cells. Hair cells responsible for processing high frequencies are located at the base of the coiled cochlea, making them more susceptible to damage. In contrast, noise-induced hearing loss (NIHL) is damage caused by repeated or sudden exposure to loud sounds. NIHL permanently restricts the frequency range by causing structural damage or metabolic exhaustion to these hair cells.
Loud noise exposure, particularly over 85 decibels for prolonged periods, causes physical trauma that can break or permanently bend the stereocilia atop the hair cells. Once these sensory cells are destroyed, they do not regenerate in humans, leading to a permanent reduction in frequency perception. NIHL often creates a characteristic notch or dip in hearing sensitivity, typically around the 4 kHz range. Both the gradual process of presbycusis and the preventable damage from NIHL contribute to the overall shrinkage of the audible frequency range.
Infrasound and Ultrasound
Frequencies outside the standard human auditory range are classified into two categories based on their position on the spectrum. Sounds lower than 20 Hz are known as infrasound. These low-frequency waves are often generated by natural events like earthquakes, volcanoes, and severe weather patterns. Although the human ear cannot perceive infrasound as a clear tone, the intense pressure waves can sometimes be felt as a deep vibration throughout the body.
Frequencies exceeding 20,000 Hz (20 kHz) are called ultrasound. These high-frequency waves are completely inaudible to humans but are widely used in technology, such as medical imaging like sonography. Certain animals, including bats and dolphins, produce and detect ultrasound for navigation and communication. The inability of humans to hear these sounds represents the physical limitations of the inner ear, which is not built to resonate at such rapid frequencies.