Sound surrounds us, conveying information through vibrations that travel through the air. These vibrations have various characteristics, including frequency. Frequency describes the rate at which sound waves vibrate, dictating a sound’s perceived pitch. Humans can hear only a specific range of these frequencies.
Understanding Sound Frequency
Sound frequency refers to the number of sound wave cycles per second, measured in Hertz (Hz). One Hertz equals one cycle per second. A higher frequency corresponds to a higher-pitched sound, while a lower frequency results in a lower-pitched sound. For example, the deep rumble of thunder has a much lower frequency than the high-pitched chirping of a bird.
The Human Ear’s Lowest Limit
The lowest frequency a typical human ear can perceive is around 20 Hertz (Hz). Sounds below this threshold are known as infrasound. These extremely low-pitched sounds are generally imperceptible to human hearing. While 20 Hz is the commonly accepted lower limit, some individuals might detect sounds as low as 12 Hz under ideal laboratory conditions.
How We Perceive Low Frequencies
The human ear is structured to detect sound waves. Sound waves first enter the outer ear and travel through the ear canal, causing the eardrum to vibrate. These vibrations are then transmitted through three tiny bones in the middle ear—the malleus, incus, and stapes—to the inner ear.
Within the inner ear, the cochlea, a fluid-filled, spiral-shaped structure, plays a central role in processing sound. The cochlea contains the basilar membrane, which vibrates in response to sound waves. The wider, more flexible end of the basilar membrane, located at the apex of the cochlea, is particularly responsive to low-frequency sounds. This movement stimulates hair cells along the membrane, which convert these vibrations into electrical signals. These electrical signals are sent to the brain via the auditory nerve for interpretation as sound.
Variations in Low Frequency Hearing
While 20 Hz is a general benchmark, the ability to hear low frequencies varies significantly among individuals. Age is a prominent factor, as hearing sensitivity diminishes over time. Although high-frequency loss (presbycusis) is more common with aging, low-frequency hearing can also be affected.
Individual differences in ear anatomy, genetics, and overall health also influence a person’s low-frequency hearing threshold. Exposure to loud noises can damage the delicate hair cells in the inner ear, potentially impacting low-frequency sound perception. Certain medical conditions, such as Ménière’s disease or otosclerosis, can also lead to low-frequency hearing loss.
Sounds Beyond Our Hearing
Sounds below the human hearing threshold, known as infrasound, are not consciously heard as distinct tones. However, humans can perceive these very low-frequency vibrations as physical sensations, such as pressure or unease, particularly at high intensity levels.
Infrasound originates from natural phenomena, including earthquakes, volcanic eruptions, severe weather systems, and ocean waves. Human activities also generate infrasound, such as large machinery, wind turbines, and sonic booms. Many animals, such as elephants, whales, giraffes, and pigeons, can perceive and utilize these low-frequency sounds for communication or navigation over long distances.