Sound allows us to perceive the world through vibrations that create pressure waves traveling through the air to the ear. Our perception of these waves is defined by two primary characteristics: frequency and amplitude. Frequency determines the pitch of a sound, while amplitude dictates its perceived loudness, and the biological limits of the human ear constrain our experience of both.
The Limits of Human Frequency Perception
Frequency, measured in Hertz (Hz), describes how many sound wave cycles pass a point per second, translating directly to perceived pitch. For a healthy young person, the typical human hearing range spans from 20 Hz to 20,000 Hz (20 kilohertz or kHz). This spectrum allows us to hear everything from the deepest bass notes to the highest whistles.
The upper limit of this range declines noticeably with age; most adults find their upper frequency limit drops closer to 15,000 or 17,000 Hz. Sounds below the human limit of 20 Hz are classified as infrasound, which we cannot hear but may sometimes feel as a vibration. Natural examples of these extremely low frequencies include the rumbling of earthquakes, the churn of ocean waves, and communication between animals like elephants.
Conversely, sound waves with a frequency above 20,000 Hz are known as ultrasound. These high-frequency sounds are beyond the range of human perception but are used for navigation by animals such as bats and dolphins. Ultrasound technology is also employed extensively in medicine to create detailed images of internal body structures, such as a developing fetus during a sonogram. These inaudible sounds mark the absolute boundaries of the frequencies our auditory system can process.
Measuring Sound Intensity and Safe Exposure
The loudness or intensity of sound is measured using the Decibel (dB), a unit based on the amplitude of the sound wave. The decibel scale is logarithmic, meaning a small numerical increase represents a vast increase in sound power. For instance, a 10 dB increase signifies a sound perceived as roughly twice as loud to the human ear.
The quietest sound a healthy human ear can detect is defined as the threshold of hearing, which is 0 dB. At the other end of the spectrum, the threshold of pain, where sound becomes physically uncomfortable and potentially causes immediate damage, typically occurs around 120 to 130 dB. Common everyday sounds range widely across this scale; normal conversation is around 60 dB, while a lawnmower might reach 90 dB.
Exposure to loud sounds over time can permanently damage hearing, making understanding safe limits important. Experts generally consider sounds at or below 70 dB safe for prolonged exposure. However, continuous exposure to noise levels of 85 dB or higher, such as heavy city traffic or a noisy restaurant, can cause hearing loss over time. For example, 85 dB is the maximum level considered safe for an eight-hour workday, with the safe duration rapidly decreasing as intensity increases.
Common Causes of Hearing Range Reduction
An individual’s hearing range often shrinks over time due to two primary factors affecting the inner ear’s delicate structures. The first is Presbycusis, the progressive, age-related hearing loss that typically develops gradually. This condition usually affects the perception of higher frequencies first, often making it difficult to distinguish consonants in speech. Presbycusis is caused by the cumulative degeneration of the cochlea, including the loss of sensory hair cells and changes in the auditory nerve pathways.
The second major factor is Noise-Induced Hearing Loss (NIHL), which results from exposure to excessively loud sounds, either a single event or prolonged moderate exposure. This acoustic trauma mechanically damages the tiny hair cells (cilia) located within the cochlea. Once these hair cells are damaged or destroyed, they cannot regenerate in humans, leading to permanent hearing loss. Exposure to loud noise can also accelerate the effects of age-related hearing loss.
Testing and Mapping Your Hearing Range
Professionals determine an individual’s specific hearing capabilities through audiometry, a test that maps the full extent of a person’s hearing range. The results are plotted on an audiogram, a chart that visually represents the quietest sound a person can hear, known as the hearing threshold, across a range of frequencies.
The audiogram’s horizontal axis displays frequency (pitch), while the vertical axis shows intensity (loudness) in decibels. During the test, tones are presented at various frequencies to find the softest level the patient can perceive. This procedure involves two main methods: air conduction, where sounds travel through the outer and middle ear via headphones, and bone conduction, where sounds are transmitted directly to the inner ear via a vibrator placed on the mastoid bone. Comparing these results helps the audiologist determine the type and severity of any hearing loss present.