Sound is a form of mechanical energy that travels through a medium as a wave; the frequency of that wave determines the pitch we perceive. Frequency is measured in Hertz (Hz), representing the number of wave cycles that pass a point per second. The process of hearing begins when sound waves are funneled into the ear canal, causing the eardrum and tiny middle ear bones to vibrate. These vibrations are converted into hydraulic energy within the cochlea, where specialized sensory cells translate the physical movement into electrical signals for the brain to interpret. This allows us to perceive sound only within a defined range of frequencies.
The Standard Range of Human Hearing
For a young, healthy human, the standard range of hearing frequencies spans from 20 Hertz to 20,000 Hertz, or 20 kilohertz (kHz). The lower limit of 20 Hz marks the boundary for infrasound, the deepest rumbling sounds a person can perceive. Conversely, the upper limit of 20 kHz is the maximum frequency that stimulates the inner ear’s sensory apparatus, beyond which lies the realm of ultrasound.
This broad 20 Hz to 20 kHz range represents the absolute physiological threshold, but the human ear is not equally sensitive across all frequencies. The most acute hearing sensitivity occurs in the mid-range, specifically between 1,000 Hz and 4,000 Hz. This frequency band encompasses most of the frequencies found in human speech.
Frequency (pitch) is distinct from amplitude, which is the measure of a sound’s intensity, or loudness, typically expressed in decibels (dB). A sound must reach a minimum loudness threshold to be heard at any given frequency. For example, a 20 Hz tone needs to be significantly louder than a 3,000 Hz tone to be perceived, illustrating the ear’s reduced sensitivity at the extremes of its range.
Factors That Limit Hearing Range
The pristine 20 Hz to 20 kHz range is an ideal standard that often degrades over a person’s lifetime due to several cumulative factors. The most common cause of this limitation is age-related hearing loss, medically known as presbycusis. This condition is a progressive and irreversible sensorineural loss that primarily affects the ability to detect high-frequency sounds.
The deterioration is linked to wear and tear on the delicate sensory hair cells in the cochlea, particularly those located at the basal turn, which process higher pitches. This natural decline often begins subtly in the late teens or early twenties, with the upper frequency limit gradually dropping from 20 kHz to 15 kHz or lower in average adults. The result is a bilateral and symmetrical loss that makes high-pitched sounds, such as consonants like ‘s’ and ‘f,’ increasingly difficult to distinguish.
Noise exposure represents another major mechanism that limits an individual’s hearing range, accelerating age-related changes. Prolonged exposure to loud sounds, or even sudden bursts of high-intensity noise, physically damages the stereocilia, the tiny hair-like projections on the sensory cells. This damage leads to permanent hearing loss, which frequently manifests as a specific dip in the audiogram around the 4,000 Hz frequency.
Genetic predisposition, certain diseases, and exposure to ototoxic medications contribute to a restricted hearing range. These factors cause damage to the cochlea or the auditory nerve. They contribute to the individual variation in hearing thresholds, meaning few people maintain the perfect 20 Hz to 20 kHz range throughout life.
Frequencies Beyond Human Perception
The spectrum of sound extends far beyond the human audible range into regions known as infrasound and ultrasound. Infrasound consists of frequencies below the human limit of 20 Hz, originating from sources like earthquakes, volcanic activity, and large ocean waves. While humans cannot hear these ultra-low frequencies, they may sometimes be perceived as a physical sensation or internal pressure.
Many large animals, such as elephants and whales, utilize infrasound to communicate over vast distances. At the opposite end of the spectrum is ultrasound, which includes all frequencies above 20 kHz. These high-frequency waves are used extensively in medical imaging and industrial applications.
Animals like bats and dolphins rely on ultrasound for echolocation, emitting sounds that can reach up to 100 kHz or 200 kHz to navigate and hunt. The human auditory system evolved to largely ignore these very high frequencies, as they hold little survival value for a terrestrial species. The narrow band of sound frequencies to which human perception is specifically tuned.