Absolute threshold of hearing

Measurement and units

Thresholds are determined using psychophysical methods such as the method of limits or the method of constant stimuli and are most often reported in decibels sound pressure level (dB SPL). The reference for 0 dB SPL is a sound pressure of 20 micropascals (20 µPa = 2×10^-5 pascal), a value commonly cited as the threshold of hearing for a young, healthy ear near 1 kHz. Measurements may be made in an open free field or with earphones; the test tone is typically a pure tone at a specific frequency.

Physiology and variation

Detection at threshold depends on the outer ear, tympanic membrane, middle ear bones, and the inner ear’s sensory hair cells in the cochlea, along with neural processing in the auditory pathway. Sensitivity varies greatly with frequency: humans are most sensitive in the mid‑frequencies (around 2–5 kHz), and less sensitive at very low or high frequencies. Thresholds also change with age, exposure to noise, and health factors, and differ markedly between species.

Factors affecting thresholds

• Age and hearing loss due to noise or disease
• Background or masking sounds during testing
• Test method, earphone type, and calibration
• Individual attention, motivation, and response criteria

Applications, significance and examples

The absolute threshold is a foundational concept in audiology and acoustics. It provides the reference for sound level meters, the definition of 0 dB SPL, and baseline data for hearing tests used to fit hearing aids or assess hearing impairment. It also informs regulations and product standards for audible alarms and environmental noise limits. Simple analogies compare the ear to a sensitive pressure detector like a microphone, although biological transduction is more complex.

History and notable facts

Early psychoacoustic research formalized the notion of a threshold as investigators measured detection limits for different frequencies and developed standard procedures. The commonly quoted 20 µPa reference has become widespread in engineering and audiology. For further technical detail or standards references see specialized sources (sound measurement) and contemporary audiology texts.