The human experience of sound begins with vibrations traveling through the air. The physical measure of these vibrations is frequency, quantified in Hertz (Hz), representing the number of cycles a sound wave completes per second. Frequency directly determines the pitch we perceive, with higher numbers corresponding to higher pitches and lower numbers to deeper tones. The human ear is specifically tuned to detect only a narrow band of these frequencies. This article explores the measurable limits of the human auditory system.
The Standard Human Hearing Range
The generally accepted spectrum of sound audible to the healthy human ear extends from a low of 20 Hz to a high of 20,000 Hz, or 20 kHz. Sounds below 20 Hz are classified as infrasound, while those above 20,000 Hz are known as ultrasound. This extensive range allows for the perception of a wide variety of sounds, from the deep rumble of thunder to the sharpest whistle.
The low end of the spectrum, around 20 Hz, is perceived as a deep bass or rumbling sensation, often associated with feeling the vibration more than hearing a distinct tone. Conversely, the upper limit of 20,000 Hz includes extremely high-pitched treble sounds. The ear is not equally sensitive across this entire range; human hearing is most acute and responsive to frequencies that fall between 2,000 and 5,000 Hz.
How Frequency Perception Works
The physical process of hearing begins when sound waves cause the eardrum to vibrate, which then transmits this mechanical energy through a chain of tiny bones in the middle ear to the cochlea. This snail-shaped structure in the inner ear contains fluid and the basilar membrane, which is lined with thousands of sensory receptors called hair cells. The movement of the fluid causes the basilar membrane to ripple, stimulating the hair cells.
The cochlea employs a system known as tonotopic organization to distinguish between different frequencies. High-frequency sounds cause maximum displacement of the basilar membrane near the base, which is the narrowest part of the cochlea. In contrast, low-frequency sounds travel further into the coil, stimulating the hair cells near the wider apex. This spatial mapping allows the brain to interpret which part of the cochlea is activated, thus determining the perceived pitch of the sound.
Factors That Alter Auditory Limits
While 20 Hz to 20,000 Hz represents the theoretical maximum, an individual’s hearing range changes significantly over a lifetime. The most common cause of altered auditory limits is aging, a condition termed presbycusis. This process typically results in a progressive loss of the ability to hear high-frequency sounds first. For many adults, the actual upper limit of hearing is closer to 15,000 or 17,000 Hz.
Exposure to loud noise is another common factor that permanently restricts the hearing range, leading to noise-induced hearing damage. Both aging and acoustic trauma damage the hair cells within the cochlea, which do not regenerate in humans. Once these sensory cells are damaged, the ability to perceive the corresponding frequencies is permanently diminished, often beginning with the highest pitches. Protecting the ears from excessive volume is necessary to maintain the widest possible frequency range throughout life.
Sounds Beyond Human Perception
Infrasound consists of vibrations below the 20 Hz threshold, such as those generated by earthquakes, large wind turbines, or large ocean waves. Instead of being heard as a pitch, these extremely low frequencies are often experienced as a physical vibration or a deep, internal pressure.
At the opposite end of the scale is ultrasound, comprising frequencies above 20,000 Hz. Humans cannot hear these sounds, but they are frequently used in technology and nature. Medical professionals use ultrasound for imaging internal structures, such as in prenatal scans, because the high-frequency waves create detailed echoes. Animals such as bats and dolphins use ultrasound for navigation and communication.