The normal range of human hearing is defined by two physical properties of sound waves: frequency and intensity. Frequency determines the pitch of a sound, measured in Hertz (Hz), ranging from low rumble to high-pitched whine. Intensity, or volume, dictates how loud a sound is perceived and is measured using the logarithmic Decibel (dB) scale. Understanding these boundaries is important for appreciating the ear’s sensitivity and protecting this delicate system from damage.
Defining the Limits of Frequency
The measurement of frequency, expressed in Hertz, quantifies the number of sound wave cycles that pass a point in one second. For a young, healthy adult, the accepted standard range of hearing spans from 20 Hz up to 20,000 Hz (20 kilohertz or kHz). Sounds at the lower end of this range are deep, rumbling tones, while those approaching 20,000 Hz are extremely high-pitched.
The human ear is not equally sensitive across this spectrum, demonstrating its greatest acuity for frequencies between 2,000 Hz and 5,000 Hz. This range is significant because it includes many consonant sounds important for speech clarity. Sounds that fall below the 20 Hz limit are classified as infrasound, and those above 20 kHz are termed ultrasound. These terms mark the boundaries of human perception, as many animals can hear frequencies well beyond the human range.
Understanding the Scale of Intensity
Sound intensity, interpreted as volume, is measured using the decibel scale. This scale is logarithmic, meaning that a small numerical increase represents a vast increase in sound energy. For instance, a sound 10 dB louder than another is ten times more intense.
The starting point of this scale, 0 dB, is defined as the threshold of human hearing, representing the quietest sound the average person can detect. Normal conversation typically registers around 60 dB, while exposure to sound above 85 dB is considered potentially damaging over prolonged periods. The threshold of discomfort generally begins around 100 dB, which is comparable to a motorcycle. The threshold of pain, where immediate and irreversible damage can occur, starts between 120 dB and 140 dB, equivalent to a jet engine at takeoff or a gunshot.
How Age and Environment Change the Range
The hearing range described for young adults is not static and changes significantly over a lifetime due to both physiological aging and environmental exposure.
The most common type of decline is age-related hearing loss, known as presbycusis, which is a progressive and irreversible sensorineural condition. This loss is typically symmetrical and begins in the high-frequency range, affecting the ability to hear sounds above 8 kHz first. The inner ear structure responsible for processing these high frequencies is located at the base of the cochlea, which is particularly susceptible to wear over time. High-frequency loss above 8 kHz can begin as early as the late 20s and 30s, making it harder to hear high-pitched sounds or certain consonants like “s” and “f”.
In addition to natural aging, chronic exposure to high intensity levels causes noise-induced hearing loss, which is a significant factor in reducing the overall range. Sounds consistently above 85 dB can cause permanent damage to the delicate hair cells within the cochlea. This damage is cumulative, and the louder the sound, the shorter the safe exposure time, with volumes exceeding 120 dB capable of causing immediate harm. Environmental noise exposure accelerates the decline, further restricting the ear’s ability to perceive both the highest frequencies and the softest sounds.