To understand the lowest sound a person can perceive, two distinct measurements must be considered: frequency and intensity. Frequency (pitch) is measured in Hertz (Hz), representing the number of sound wave cycles per second. Intensity (loudness) is measured in decibels (dB), quantifying the acoustic pressure of the sound wave. The question of how low the human ear can hear is answered by exploring the absolute lowest frequency we can detect and the minimum intensity required for perception.
The Absolute Lowest Frequency (Hertz)
The generally accepted lower boundary of human hearing is 20 Hz, which is a deep, rumbling bass tone. This frequency represents the lowest pitch the auditory system typically processes as a distinct sound. Sound waves that oscillate below this 20 Hz threshold are known as infrasound. Although some individuals may perceive tones as low as 12 Hz, 20 Hz serves as the standard cutoff for the nominal human range.
Infrasound is generated by powerful natural and man-made sources, such as severe weather events, volcanic eruptions, earthquakes, and large machinery. Since these low-frequency waves have long wavelengths, they can travel over great distances and penetrate physical barriers, including buildings. Although the eardrum and cochlea do not effectively translate these extremely low frequencies into recognizable auditory signals, the body can still sense the vibrations.
Exposure to powerful infrasound manifests as a physical sensation rather than a distinct sound. People may experience feelings of pressure in the chest or head, dizziness, or a general sense of unease. This physical perception is caused by the sound wave mechanically interacting with the body’s tissues and organs, including the vestibular system in the inner ear. Therefore, while we may not “hear” infrasound, our physiological systems still react to its presence.
The Quietest Sound at Low Frequencies (Decibels)
The human ear’s sensitivity is not uniform across the entire frequency spectrum. The threshold of hearing is an intensity measurement that varies significantly depending on the sound’s frequency. For mid-range frequencies, typically 1,000 to 5,000 Hz, the auditory system is most efficient. The quietest sound a healthy young person can detect in this range is standardized at 0 dB.
As the frequency drops toward the lower end of the spectrum, the ear becomes dramatically less sensitive, requiring a much higher sound intensity for perception. To perceive a tone at the 20 Hz limit, the sound pressure level must be significantly elevated. The intensity required to just barely hear a 20 Hz tone can be 60 to 70 dB or higher. This non-linear relationship explains why deep bass notes require tremendous amplification to achieve the same subjective loudness as sounds in the middle-frequency range.
This phenomenon is graphically represented by equal-loudness contours, which illustrate the sound pressure levels needed at different frequencies to produce the same perceived loudness. The steep curve at the low-frequency end demonstrates that a small increase in sound level near the perception threshold causes a disproportionately large change in perceived loudness. Consequently, a low-frequency sound inaudible to one person might be perceived as loud by another, even with slight differences in their individual hearing thresholds.
Factors That Shift the Hearing Threshold
An individual’s ability to hear the lowest sounds can deviate from established standards due to several biological and environmental factors. Age is a primary influence, though the most noticeable effect is the loss of high-frequency hearing. While the low-frequency threshold is often better preserved, older adults may still show reduced sensitivity compared to younger individuals. Accumulated damage from long-term noise exposure can also raise the hearing threshold across all frequencies. Temporary changes, known as temporary threshold shifts, can occur after short-term exposure to loud noise, or be caused by the masking effect of environmental noise or changes in middle-ear pressure.