The lowest level of sound detectable by the human ear is defined by the absolute threshold of hearing (ATH). This physiological limit represents the minimum sound intensity required for an average person with healthy hearing to detect a pure tone. The threshold is statistically defined as the sound level a listener can perceive 50% of the time under ideal, quiet conditions. Understanding this threshold requires knowing how sound is measured and how the ear’s sensitivity changes across different pitches.
Quantifying Sound Levels
Scientists use the decibel (dB) scale to measure sound intensity, which is a logarithmic ratio scale. The logarithmic nature means that every increase of 10 dB represents a tenfold increase in sound power. This system allows for the comparison of the softest detectable sound to the loudest, most damaging sounds without using unmanageably large numbers.
To standardize measurements, the Sound Pressure Level (SPL) scale compares a measured sound pressure to a specific reference pressure. For sounds in air, this reference pressure is set at 20 micropascals (µPa). This tiny fluctuation was chosen because it approximates the quietest sound a healthy young person can hear, defining 0 dB SPL.
The Absolute Threshold of Hearing
The theoretical lowest level of sound that can be heard is 0 dB SPL, which corresponds to the reference pressure of 20 micropascals. This value is used as the foundational benchmark for normal hearing in young adults. It is important to realize that 0 dB does not represent the complete absence of sound energy, but rather the standardized reference point for the hearing scale.
The absolute threshold is based on the average responses of young, healthy listeners, often tested in a soundproof chamber. Some individuals with exceptional hearing might detect sounds below 0 dB SPL, sometimes reaching as low as -9 dB SPL in certain frequency ranges. Measuring this threshold requires psychophysical techniques, where a listener indicates whether they perceive a pure tone presented at varying, extremely low intensities.
The Role of Frequency in Sound Detection
The human ear’s sensitivity is not uniform across all frequencies, which determines a sound’s pitch. The true absolute threshold follows a U-shaped curve, meaning detection depends heavily on frequency, measured in Hertz (Hz). The ear is most sensitive and requires the least amount of sound pressure in the mid-frequency range, specifically between 2,000 and 5,000 Hz, where the theoretical 0 dB SPL is established.
Sensitivity sharply drops off at the high and low ends of the audible spectrum, which typically ranges from 20 Hz to 20,000 Hz. Sounds below 20 Hz (infrasound) and those above 20,000 Hz (ultrasound) are generally undetectable by the average human, regardless of their intensity. Therefore, a low-frequency tone, such as 50 Hz, would need to be significantly louder, perhaps 30 dB higher, to be perceived.
Real-World Practical Limits
While 0 dB SPL is the theoretical minimum, it is rarely experienced in daily life due to the presence of a noise floor in any real environment. Even a quiet room in a secluded house typically has an ambient sound level between 20 and 30 dB SPL. This background noise masks sounds near the theoretical threshold, making the practical limit of detection much higher than 0 dB.
Sounds that approach the theoretical limit include a soft whisper at seven feet (about 15 dB) or the gentle rustling of leaves (around 20 dB). The practical detection limit also degrades with age, a condition known as presbycusis. This age-related hearing loss typically affects the high-frequency range first, requiring older adults to have a higher sound pressure level to perceive high-pitched sounds.