The concept of a hearing threshold defines the quietest sound an individual ear can detect across a range of pitches. It represents the absolute boundary between silence and an audible sound. Measuring this threshold is a fundamental practice in audiology, providing a precise map of auditory sensitivity. This measurement is the first step in diagnosing potential hearing problems and understanding the nature of any hearing impairment.
The Science of Sound Measurement
The measurement of sound relies on two distinct scales to describe its physical properties: intensity and frequency. Sound intensity, which we perceive as loudness or volume, is measured in decibels, abbreviated as dB. The decibel scale is logarithmic, meaning that every 10 dB increase represents a tenfold increase in sound energy, which our ears perceive as roughly twice the loudness.
In audiology, hearing is measured using the decibel Hearing Level (dB HL), which standardizes the softest sound an average young, healthy ear can detect as 0 dB. This standard reference point allows for the consistent comparison of a person’s hearing sensitivity against that of the normal population. The second scale, frequency, which we perceive as pitch, is measured in Hertz (Hz) and represents the number of sound wave cycles per second.
Humans can typically hear frequencies ranging from 20 Hz (very low pitch) up to 20,000 Hz (very high pitch). However, the most critical range for communication, containing the sounds of human speech, falls between approximately 500 Hz and 3,000 Hz. Measuring the threshold across several key frequencies in this range is necessary because hearing sensitivity is not uniform, and loss can affect high or low pitches differently.
Determining Individual Hearing Thresholds
Individual hearing thresholds are determined through a diagnostic process called pure-tone audiometry, which is considered the gold standard for testing hearing sensitivity. The test uses an audiometer to deliver pure-tone signals, or sounds of a single frequency, to the patient through headphones or insert earphones. The audiologist searches for the threshold by adjusting the volume until the patient signals that they can just hear the sound.
This process employs a specific descending/ascending technique, where the sound level is initially decreased in 10 dB steps after the patient responds, and then increased in 5 dB steps until the sound is heard again. The true hearing threshold is recorded as the minimum intensity at which the person responds correctly at least 50% of the time during the ascending presentations. This ensures the measurement is reliable and accurately captures the quietest audible sound for that frequency.
The results are charted on an audiogram, which is a graph plotting frequency (Hz) on the horizontal axis and intensity (dB HL) on the vertical axis. The test measures two distinct pathways of hearing: air conduction and bone conduction. Air conduction tests the entire auditory system, from the outer ear through the middle ear to the inner ear, using headphones.
Bone conduction testing bypasses the outer and middle ear by sending vibrations directly to the inner ear, typically using a small vibrator placed behind the ear on the mastoid bone. Comparing the thresholds for air and bone conduction is essential because it reveals the location of any hearing problem. A significant difference between the two, known as an air-bone gap, points to a problem in the outer or middle ear.
What Hearing Thresholds Reveal About Hearing Health
The completed audiogram provides a detailed picture of a person’s hearing health by linking the measured decibel levels to clinical classifications of hearing loss severity. Normal hearing is defined by thresholds that fall at or below 25 dB HL across the tested frequencies. Any measured threshold above this 25 dB level indicates some degree of hearing loss.
Classifications categorize the degree of hearing loss based on the softest sound a person can hear. These categories translate directly into the functional challenges a person experiences every day.
- Mild hearing loss (26–40 dB HL): Individuals may struggle to hear soft speech or follow conversations in noisy environments.
- Moderate hearing loss (41–55 dB HL): A person often needs conversational speech to be louder for understanding.
- Severe hearing loss (71–90 dB HL): Normal conversation is inaudible without significant amplification.
- Profound hearing loss (91 dB HL and greater): A person may only be able to hear very loud sounds, even with hearing devices.