Hearing is the perception of sound energy, characterized by intensity (loudness) and frequency (pitch). To define the “normal” range of human hearing, we focus on intensity, measured using the decibel (dB). Understanding the decibel scale is foundational to grasping the limits of what the human ear can safely perceive.
Understanding Decibel Measurement
The decibel (dB) is the standard unit for quantifying sound intensity, which we perceive as loudness. The decibel scale is logarithmic, meaning it does not increase linearly. This structure reflects how the human ear processes a vast range of sound pressures. Due to this scaling, an increase of 10 dB signifies a tenfold increase in sound pressure level. The starting point is the absolute threshold of hearing, where 0 dB represents the softest sound a healthy ear can detect.
The Standard Human Hearing Range
The complete audible range for human hearing is defined by both intensity (decibels) and frequency (Hertz, Hz). The intensity range spans from the softest perceptible sound at 0 dB to the threshold of pain, typically around 120 to 130 dB. Sounds above this level can cause immediate physical discomfort and damage. The corresponding frequency range for healthy hearing is generally from 20 Hz to 20,000 Hz. Clinically, “normal hearing” is defined as the ability to hear sounds below 25 dB across the full range of speech frequencies.
Decibels of Everyday Sounds
Placing the decibel scale into a real-world context helps clarify the potential for noise exposure and hearing damage. A soft whisper registers at approximately 30 dB, while a normal conversation typically occurs around 60 dB; louder mechanical sounds, such as a power lawnmower or leaf blower, often reach 90 dB. Sounds above 85 dB are recognized as potentially damaging to hearing with prolonged exposure. Louder environments, such as a rock concert, can easily exceed 110 dB, dramatically decreasing the safe exposure time. For every 3 dB increase above 85 dB, the safe listening time is cut in half.
How Hearing Loss is Classified by Decibel Level
Audiologists use the decibel scale to clinically diagnose and categorize the severity of hearing impairment. This classification is based on the quietest sound a person can hear, known as their hearing threshold, measured in decibels of hearing level (dB HL). A hearing threshold of 0 to 25 dB HL is considered within the normal hearing range.
Hearing loss is then categorized into several degrees based on how much sound intensity is required for the person to hear it. A Mild hearing loss is defined by thresholds between 26 and 40 dB HL, where soft speech and whispers become difficult to hear. Moderate loss occurs at 41 to 55 dB HL, making it challenging to follow normal conversation without amplification.
Severe hearing loss, spanning 71 to 90 dB HL, means a person cannot hear regular speech and requires powerful hearing aids or other assistance. Finally, a Profound hearing loss is diagnosed when the hearing threshold is 91 dB HL or greater, making even very loud sounds difficult or impossible to perceive. Monitoring exposure to intense sounds is a practical way to help preserve the ability to hear within the normal range.