AUDIOGRAM

Introduction and Definition

An audiogram stands as the definitive graphical representation utilized in audiology to chart an individual’s hearing sensitivity. Fundamentally, it is a clinical tool that maps the softest sounds (thresholds) a person can perceive across a range of frequencies, comparing these measurements against established norms for individuals with typical hearing. This comparison is critical, providing a precise, quantified visualization of any deviation from normal auditory function. The resulting graph offers a foundational diagnostic baseline for identifying, classifying, and monitoring various forms of hearing disorders and deficits. Its reliability and standardization make it indispensable in both research and clinical practice concerning the peripheral and central auditory systems.

The core function of the audiogram is to relate the patient’s measured pure-tone thresholds—specific points at which a tone is barely audible—to the threshold of normal hearing, often referred to as audiometric zero. This systematic plotting allows clinicians to immediately visualize the extent and nature of hearing impairment. Unlike subjective descriptions of hearing difficulty, the audiogram provides objective, quantifiable data that directs intervention strategies, such as the fitting of hearing aids or the necessity of further medical evaluation. The accurate derivation of the audiogram is contingent upon meticulously calibrated equipment, namely the audiometer, and standardized testing procedures to ensure the validity and replicability of the results across different clinical settings.

The concept underpinning the audiogram is the determination of sound pressure levels required for sound perception. When a patient requires a sound pressure level significantly higher than the normal baseline to perceive a tone, this difference is plotted on the graph, indicating a degree of hearing loss. For example, a person whose threshold is 30 dB above normal for a 4-kHz tone would have a measurement that shows a point plotted at 4 kHz and 30 dB HL. This is a highly specialized visualization, designed not merely to indicate that hearing loss exists, but specifically where along the frequency spectrum the deficits are most pronounced and, crucially, which parts of the auditory pathway (conductive or sensorineural) are primarily affected. This distinction is vital for accurate differential diagnosis.

The Axes and Measurement Units

The standardized structure of the audiogram relies on two primary axes, each representing a crucial dimension of sound measurement: frequency and intensity. The horizontal axis (x-axis) is dedicated to representing the frequency of the pure tone stimulus, measured in Hertz (Hz) or kilohertz (kHz). Frequency corresponds directly to the pitch of the sound. Standard clinical audiometry typically tests frequencies ranging from 125 Hz (very low pitch) up to 8000 Hz (very high pitch). Although the range of human hearing extends beyond these limits, these specific octave and half-octave intervals are deemed the most relevant for diagnosing communication-related hearing loss, as they encompass the critical speech frequencies.

The layout of the frequency axis is logarithmic, meaning that equal physical distances on the graph represent equal ratios of frequency increase, not equal absolute numerical increases. This design accurately reflects how the human ear processes pitch perception. Moving from left to right across the audiogram, the frequencies increase, starting with low bass tones and progressing toward high treble tones. A typical audiogram will display key test frequencies such as 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, and 8000 Hz, with intermediate frequencies often tested if specific pathologies or unusual threshold patterns are suspected, such as testing 3000 Hz or 6000 Hz.

The vertical axis (y-axis) represents the intensity of the stimulus, measured in decibels of Hearing Level (dB HL). Intensity corresponds to the loudness of the sound. Unlike the standard Decibel Sound Pressure Level (dB SPL), the dB HL scale is calibrated specifically for audiometric testing, where 0 dB HL represents the average threshold of hearing for young, healthy adults at each specific frequency. This standardization ensures that a loss of 30 dB HL at 1000 Hz means the individual requires a sound 30 decibels louder than the average normal listener to perceive it. Crucially, the intensity axis is inverted on the audiogram: 0 dB HL is positioned near the top, and intensity levels increase as one moves down the graph (e.g., 100 dB HL at the bottom), meaning that greater hearing loss is represented by points plotted lower on the chart.

The use of dB HL rather than absolute sound pressure levels is essential for clinical utility. If dB SPL were used, the normal threshold line would be curved, reflecting the ear’s natural sensitivity variations across frequencies. By utilizing dB HL, the normal hearing threshold is rendered as a straight line at 0 dB HL, simplifying the visual interpretation and allowing for immediate calculation of the degree of hearing impairment based on the vertical distance of the plotted points from this zero line.

Procedure: Pure-Tone Audiometry

The generation of an audiogram is accomplished through pure-tone audiometry, a behavioral test that requires active participation from the patient. The procedure determines the hearing threshold—defined clinically as the lowest intensity level at which a patient can correctly detect a pure-tone signal 50% of the time, typically demonstrated by raising a hand or pressing a response button. The testing environment must be acoustically controlled, typically within a sound-proof booth, to prevent ambient noise from contaminating the measurements, especially when testing hearing at low intensity levels close to 0 dB HL. The process involves presenting discrete, single-frequency tones to the patient via calibrated headphones, insert earphones, or a bone vibrator.

The actual threshold search follows a rigorous, standardized protocol, often utilizing a modified Hughson-Westlake procedure (ascending-descending technique). The audiologist begins by presenting the tone at an intensity clearly audible to the patient to ensure understanding of the task. The intensity is then decreased in steps (typically 10 dB) until the patient stops responding. Once the tone is inaudible, the intensity is increased in smaller steps (typically 5 dB) until the patient responds again. This bracketed search continues until the 50% criterion is met, usually confirmed by obtaining at least three responses out of six presentations at the same minimum intensity level. This specific intensity value is then meticulously recorded as the patient’s pure-tone threshold for that frequency in that specific ear.

Testing is conducted separately for each ear across the standard frequency range, starting usually at 1000 Hz, as it is highly repeatable and often used as a reference frequency. The testing proceeds to higher frequencies (2000 Hz, 4000 Hz, 8000 Hz), and then returns to test lower frequencies (500 Hz, 250 Hz). This systematic approach ensures maximum reliability and minimizes the effects of temporary threshold shifts or patient fatigue influencing the overall result. If the difference between the thresholds of the two ears is significant, a technique called masking is employed, where calibrated noise is introduced into the non-test ear to prevent cross-hearing, ensuring that the tone is truly being perceived by the ear under test. The data points collected are then plotted onto the audiogram graph, forming a unique threshold curve that characterizes the patient’s hearing status.

Interpreting the Audiogram Symbols

A fundamental aspect of reading the audiogram is understanding the standardized symbols used to denote the different types of measurements taken. These symbols are universally recognized in audiology and provide immediate information regarding the ear tested, the method of sound delivery, and whether masking noise was required. Consistency in plotting symbols is essential for clear communication among audiologists, physicians, and other healthcare professionals.

For Air Conduction (AC) thresholds, which test the entire auditory pathway (outer ear, middle ear, and inner ear), the primary symbols are: a red circle (O) for the Right Ear and a blue ‘X’ (X) for the Left Ear. These symbols denote the thresholds achieved using headphones or insert earphones. If these measurements are taken without introducing masking noise to the non-test ear, they represent the unmasked threshold. The connecting lines between the symbols—solid line usually for the right ear, dashed or separate line for the left—help visualize the overall configuration of the hearing loss across the frequency range.

When Bone Conduction (BC) thresholds are measured—which bypass the outer and middle ear to stimulate the cochlea directly—different symbols are used. The unmasked bone conduction threshold for the Right Ear is typically a bracket pointing left (). These symbols determine the integrity of the sensorineural mechanism (cochlea and auditory nerve). The absence of an air-bone gap (AC and BC symbols plotting closely together) confirms that the inner ear function is commensurate with the overall hearing loss measured.

Furthermore, when masking noise is introduced to the non-test ear to prevent the possibility of the tone crossing over and being heard by the better ear, the symbols are modified to indicate that the measurement is masked. Standard masked symbols include a square (□) or triangle (Δ) for air conduction, and square brackets ([ and ]) for bone conduction, often color-coded consistently (red for right, blue for left). Proper identification of these symbols is necessary to correctly determine if the measured threshold is reliable and to accurately classify the type of hearing loss present.

Types of Thresholds Measured (Air Conduction vs. Bone Conduction)

The diagnostic power of the audiogram lies in its ability to separate hearing deficits originating in the outer/middle ear (the conductive pathway) from those originating in the inner ear (the sensorineural pathway). This crucial distinction is achieved by comparing the thresholds obtained via Air Conduction (AC) and Bone Conduction (BC) testing. The relationship between the AC and BC thresholds dictates the clinical classification of the hearing loss.

Air Conduction testing reflects the sensitivity of the entire auditory system, as the sound must traverse the outer ear canal, vibrate the tympanic membrane and ossicles in the middle ear, and finally stimulate the cochlea in the inner ear. The resulting AC thresholds, plotted by the circles and X’s, represent the total magnitude of the hearing impairment, integrating any issues along the entire route. These thresholds form the basis for determining the overall degree of hearing loss, which is necessary for quantifying the functional impact on the individual.

Bone Conduction testing, conversely, provides a measure of cochlear function by bypassing the conductive mechanisms entirely. A small vibrator is placed directly onto the mastoid bone or the forehead. When activated, the vibrator sends mechanical vibrations directly to the skull, stimulating the cochlea and the auditory nerve directly. The BC threshold, represented by the brackets (), therefore reflects the true sensory capacity of the inner ear. If the BC thresholds fall within the normal range (generally 0 to 25 dB HL), it indicates that the cochlea is functioning normally, even if the AC thresholds show a loss.

The differential diagnosis relies on the presence or absence of the Air-Bone Gap (ABG). If AC thresholds show a loss, but BC thresholds are normal (an ABG of 10 dB or more exists), it signals a conductive hearing loss, meaning the pathology lies in the outer or middle ear (e.g., earwax blockage, fluid in the middle ear, or otosclerosis). If both AC and BC thresholds show loss and the gap between them is minimal or nonexistent, it signifies a sensorineural hearing loss, indicating a problem originating in the cochlea or auditory nerve (e.g., noise damage or presbycusis). If both thresholds show loss, and a significant air-bone gap exists, the diagnosis is a mixed hearing loss, implying simultaneous pathology in both the conductive and sensorineural mechanisms.

Configurations of Hearing Loss

The specific shape, or configuration, of the plotted thresholds across the frequency range provides essential information regarding the underlying cause and pathophysiology of the hearing loss. Audiograms rarely exhibit perfectly flat lines; instead, the thresholds typically vary, and these patterns are systematically categorized based on their overall slope and characteristics, assisting the audiologist and physician in narrowing the potential etiologies.

One of the most frequently observed patterns is the sloping configuration, where hearing sensitivity is significantly better in the low frequencies and gradually worsens as the frequency increases. This configuration is highly characteristic of presbycusis (age-related hearing loss), reflecting the progressive degeneration of hair cells in the basal turn of the cochlea, which processes high frequencies. A related, and highly diagnostic, pattern is the high-frequency notch, characterized by a sharp dip in hearing sensitivity, often centered at 4000 Hz, followed by a slight recovery at 8000 Hz. This specific pattern is considered pathognomonic of chronic, long-term exposure to loud noise or acoustic trauma (noise-induced hearing loss).

Conversely, a rising configuration, where thresholds are worse in the low frequencies and improve toward the high frequencies, is less common in sensorineural losses but often associated with conductive pathologies that disproportionately affect low-frequency transmission, such as stiffness in the middle ear system (e.g., otosclerosis or certain middle ear effusions). The flat configuration describes thresholds that are roughly the same across all tested frequencies, suggesting that the hearing loss affects all pitches equally. This pattern can be seen in certain types of congenital hearing losses, or severe, long-standing conductive pathologies.

Other complex configurations include the cookie-bite or U-shaped audiogram, where hearing is better in the low and high frequencies but worse in the mid-frequencies, often associated with genetic sensorineural hearing losses. The corner audiogram, where responses are only obtained at maximum intensity for the lowest frequencies, indicates a profound hearing loss across the entire spectrum, often requiring cochlear implantation rather than conventional hearing aids. Careful analysis of these configurations guides further testing and confirms the likely origin of the auditory impairment.

Classification and Clinical Significance

The raw data plotted on the audiogram must be translated into a clinically meaningful diagnosis by classifying both the type and the degree (severity) of the hearing loss. The degree of loss is typically determined by calculating the Pure Tone Average (PTA)—the arithmetic mean of the thresholds at the critical speech frequencies (usually 500 Hz, 1000 Hz, and 2000 Hz). This classification is crucial for determining the immediate impact on the patient’s ability to communicate and for guiding appropriate intervention strategies.

Hearing loss severity is categorized using standardized guidelines based on the PTA: Normal Hearing (0 to 25 dB HL), Mild Hearing Loss (26 to 40 dB HL), Moderate Hearing Loss (41 to 55 dB HL), Moderately Severe Hearing Loss (56 to 70 dB HL), Severe Hearing Loss (71 to 90 dB HL), and Profound Hearing Loss (91 dB HL and greater). An individual with a mild loss may struggle only with soft speech or speech in noisy environments, whereas those categorized with severe or profound loss typically require substantial amplification or assistive devices, such as cochlear implants, to access sound and spoken language effectively.

The audiogram serves as the paramount diagnostic map for the audiology team and related specialists. By identifying the type (conductive, sensorineural, or mixed) and degree of loss, the audiologist can formulate a targeted rehabilitation plan. For conductive losses, the patient is often referred to an otolaryngologist for medical or surgical intervention, as these losses are frequently reversible. For sensorineural losses, which are generally permanent, the audiogram dictates the precise frequency-specific gain and output limits required for the programming of hearing aids, ensuring that the patient utilizes their maximum residual hearing capacity. Furthermore, it acts as an essential longitudinal monitoring tool, allowing clinicians to track the progression or stability of hearing impairment over time, which is critical for managing conditions like sudden sensorineural hearing loss or progressive genetic disorders.

Related Concepts: Audiometric Zero and Audiometer

To properly understand the context and reliability of the audiogram, it is necessary to appreciate the concepts of audiometric zero and the crucial function of the audiometer. The audiogram is fundamentally a deviation graph, plotting a patient’s results relative to a highly standardized benchmark. Audiometric zero (0 dB HL) does not represent the absence of sound energy; rather, it is defined internationally as the average lowest sound intensity that young, healthy, normal-hearing listeners can perceive 50% of the time, meticulously calibrated across all tested frequencies based on international standards (ISO or ANSI).

The utilization of dB Hearing Level (dB HL) required the establishment of the reference equivalent threshold sound pressure levels (RETSPLs). This adjustment is necessary because the human ear is naturally more sensitive to sounds in the middle frequency range (around 1000 Hz to 4000 Hz) than to very low or very high frequencies. Consequently, the actual physical intensity (measured in dB SPL) required to reach the threshold of hearing changes significantly depending on the frequency. The dB HL scale mathematically adjusts for this non-linearity, ensuring that 0 dB HL at any frequency represents the same perceptual threshold for the average listener, thereby normalizing the baseline and simplifying the clinical interpretation of the audiogram.

The audiometer is the dedicated electronic instrument used to generate and control the presentation of calibrated pure tones and speech signals necessary for creating the audiogram. It is a highly specialized device capable of precisely controlling frequency, intensity, and stimulus routing (air conduction, bone conduction, and sound field). Regular calibration of the audiometer is legally and clinically mandated to ensure that the sounds presented correspond exactly to the intensity levels marked on the audiogram, thus preserving the clinical accuracy and diagnostic validity of the resulting graph. Without a properly calibrated audiometer and the standardized reference provided by audiometric zero, the audiogram would lose its reliability as a core diagnostic instrument in assessing hearing disorders.