The ability to hear is a complex sensory process that allows humans to detect and interpret mechanical vibrations in the air. These vibrations, which we perceive as sound, are converted into signals the brain can understand, providing information about the surrounding environment. Defining “normal” hearing involves perceiving a specific range of sound intensity and pitch with a high degree of sensitivity. This standard range is established through scientific measurement and is fundamental to effective communication.
The Biological Process of Sound Perception
The physical journey of sound begins with the outer ear, including the visible pinna and the ear canal. The pinna’s funnel-like shape collects sound waves and directs them down the ear canal toward the eardrum, or tympanic membrane. This structure captures and mildly amplifies the air vibrations before they reach the middle ear.
The sound waves cause the eardrum to vibrate, setting the three tiny bones, the ossicles, into motion. These three bones—the malleus, incus, and stapes—mechanically amplify the vibrations. This amplification is necessary because the sound moves from the air-filled middle ear to the fluid-filled inner ear.
The stapes connects to the oval window, transmitting the amplified vibrations into the cochlea, a spiral-shaped structure within the inner ear. Inside the cochlea, the fluid ripples, creating a traveling wave along the basilar membrane. Specialized sensory cells, known as hair cells, convert the mechanical energy into electrical signals. These electrical impulses are carried by the auditory nerve to the brain, which processes and interprets them as recognizable sound.
Quantifying Normal Hearing: Decibels and Frequencies
Normal hearing is defined by two measurable physical properties of sound: intensity and frequency. Intensity, or loudness, is measured in decibels (dB) using a logarithmic scale. The softest sound an average young adult can hear is defined as zero decibels hearing level (0 dB HL).
A threshold between -10 dB HL and 20 dB HL across the frequency range is considered the boundary for normal hearing. Normal conversation typically occurs at approximately 60 dB, which is well within the comfortable listening range for someone with healthy hearing.
Frequency, which determines the pitch of a sound, is measured in Hertz (Hz), representing the number of sound wave cycles per second. The full range of human hearing spans from approximately 20 Hz up to 20,000 Hz. This wide range is necessary for perceiving the full spectrum of environmental sounds.
The most critical frequencies for daily function are those associated with human speech, which range from about 250 Hz to 8,000 Hz. Within this speech range, the lower frequencies (250–500 Hz) carry the power and rhythm of speech. The higher frequencies (4,000–8,000 Hz) are essential for distinguishing consonants like “s” and “f,” which determine speech clarity. A person with normal hearing can perceive these speech sounds easily at low decibel levels, ensuring they can understand conversation even in quiet settings.
Understanding the Audiogram and Hearing Classifications
The audiogram is the standardized visual chart used by professionals to map an individual’s hearing against normal thresholds of decibels and frequencies. This graph plots frequency (Hz) along the horizontal axis (low pitch to high pitch). The vertical axis represents intensity (dB HL), with the softest sounds at the top and the loudest sounds toward the bottom.
During a hearing evaluation, pure-tone testing is performed to determine the softest sound level, or hearing threshold, an individual can detect at specific frequencies. These thresholds are plotted on the audiogram using specific symbols for each ear and for both air conduction and bone conduction testing. Air conduction measures sound traveling through the outer and middle ear, while bone conduction bypasses those parts to test the inner ear directly.
A key feature on the audiogram is the “speech banana,” a curved area representing the approximate frequency and intensity levels of common human speech sounds. If a person’s plotted thresholds fall outside this area, they will likely struggle to hear or understand spoken language without amplification. Normal hearing is defined by thresholds that fall within the top section of the chart, typically at 20 dB HL or better across all tested frequencies.
Any threshold plotted below 20 dB HL indicates a deviation from the normal range and immediately places the hearing into a classification of loss. The mild category, for example, is used for thresholds between 21 dB HL and 40 dB HL. By visually representing the precise intersection of loudness and pitch where a person struggles to hear, the audiogram provides an objective measure of how far an individual’s hearing deviates from the established norm.