Sound is a phenomenon of vibrations that travel through a medium, such as air or water, reaching the ear and being interpreted by the brain. Frequency is the physical measurement determining perceived pitch, describing how rapidly those vibrations occur. Frequency is quantified in Hertz (Hz), where one Hertz equals one cycle per second. High frequencies correspond to high-pitched sounds, and low frequencies represent low-pitched sounds. The range of frequencies humans can detect is limited compared to the vast spectrum of vibrations in the natural world.
The Standard Range of Human Hearing
The accepted frequency range for a person with healthy hearing, typically a young adult, spans from 20 Hz to 20,000 Hz (20 kilohertz or kHz). Sounds below this range are considered infrasound, and those above are ultrasound. The low end, around 20 Hz, includes deep, rumbling bass tones. The upper limit of 20 kHz covers extremely high, whistling treble tones.
The human auditory system is not equally sensitive across this wide spectrum. The ear exhibits its greatest sensitivity, requiring the least amount of sound pressure to be detected, for frequencies between 2,000 Hz and 5,000 Hz. The quietest sound a healthy person can perceive is defined as the threshold of hearing, approximately 0 decibels (dB) at the most sensitive frequencies.
How the Ear Processes Different Frequencies
Frequency detection begins in the cochlea, a spiral-shaped structure within the inner ear. Sound waves transmitted from the middle ear cause fluid movement, generating a traveling wave along the flexible basilar membrane. The physical properties of the basilar membrane, which is narrow and stiff at one end and wide and flexible at the other, allow for frequency separation.
This arrangement creates tonotopy, a systematic mapping of frequency onto a physical location. High-frequency sounds cause maximum vibration near the base of the cochlea, close to the middle ear bones. Conversely, low-frequency sounds travel further into the spiral and cause maximum vibration near the apex, the innermost tip. Resting on the basilar membrane is the organ of Corti, which contains specialized sensory hair cells.
The mechanical movement of the basilar membrane shears the stereocilia atop these cells against the tectorial membrane. This bending motion converts the mechanical energy of the vibration into electrochemical signals. These neural signals are then transmitted along the auditory nerve to the brain, where they are interpreted as distinct pitches based on which section of the basilar membrane was activated.
Factors That Modify Frequency Detection
The full frequency range of 20 Hz to 20 kHz is rarely maintained throughout a person’s life, as various factors can narrow or shift this capability. The most common change is presbycusis, or age-related hearing loss, which involves the progressive loss of hearing sensitivity. This decline typically begins with the highest frequencies, often starting around 12 kHz to 16 kHz, because the hair cells at the base of the cochlea are the first to experience wear and tear.
Another factor is noise-induced hearing loss, which results from exposure to loud sounds, whether sudden and intense or prolonged over time. Sound levels exceeding 85 dB can irreparably damage the delicate inner hair cells. This type of damage often creates a characteristic ‘notch’ in hearing sensitivity, frequently centered around 4,000 Hz.
Genetic predispositions influence a person’s susceptibility to hearing loss. Certain medications, known as ototoxic drugs, can also cause temporary or permanent damage to the inner ear structures. The overall health of the auditory system is a dynamic state, constantly being modified by both internal biological processes and external environmental exposures.
Sounds Outside the Human Range
Frequencies that fall outside the typical human hearing range are generally categorized into two groups. Infrasound refers to sound waves with frequencies below 20 Hz, which are too slow for the human ear to perceive as distinct tones. Natural sources of infrasound include geological events, such as seismic activity and volcanoes, as well as weather phenomena and ocean waves.
While humans cannot hear infrasound, high-intensity waves in this range can sometimes be felt as a vibration or a pressure sensation within the body. At the opposite end of the spectrum is ultrasound, which consists of frequencies above 20 kHz. Animals like bats and dolphins use ultrasound for echolocation and communication.
For humans, ultrasound is primarily used in technological applications, such as medical imaging. High-frequency waves are transmitted into the body, and their returning echoes are used to create images of internal structures.