BEKESY AUDIOMETER

The Legacy of Georg von Békésy and the Bekesy Audiometer

The Bekesy Audiometer stands as a landmark achievement in the field of audiology, representing one of the first successful attempts at automating the process of hearing threshold measurement. This sophisticated screening instrument is specifically designed to determine the lowest intensity level at which a human subject can reliably detect pure tones across various frequencies. Developed by the prodigious Hungarian-born physicist Georg von Békésy in the 1950s—work for which he later received the Nobel Prize in Physiology or Medicine in 1961—the device revolutionized clinical and research audiology by introducing a novel, patient-controlled method for threshold determination. Its introduction marked a fundamental shift from strictly manual audiometry to a more efficient and objective methodology, establishing a foundation that continues to influence modern automated testing protocols used worldwide.

Prior to the development of this instrument, traditional pure-tone audiometry required constant intervention by a trained clinician who manually adjusted the stimulus intensity based on the patient’s verbal or signaled response. This manual method was often time-consuming and susceptible to observer bias, leading to inconsistencies in threshold measurements, particularly in large-scale screening environments or research settings requiring high precision. Von Békésy sought to eliminate this variability by creating a system where the patient directly controlled the stimulus intensity, thereby charting their own hearing sensitivity automatically. This ingenious self-tracking mechanism allowed for continuous, rapid documentation of thresholds, significantly improving the standardization and efficiency of audiological assessment.

The enduring legacy of the Bekesy Audiometer lies not only in its procedural efficiency but also in its profound diagnostic utility. The specific patterns generated by the patient’s continuous response—known as Bekesy tracings—provide crucial information beyond simply the numerical threshold. These tracing types are highly valuable indicators for differentiating between various forms of hearing loss, particularly in distinguishing cochlear (sensory) pathologies from retrocochlear (neural) pathologies. Thus, what began as an engineering solution for automation quickly became a powerful diagnostic tool, cementing the Bekesy Audiometer’s role as a cornerstone of audiological testing for more than six decades.

Core Principles of Automatic Audiometry

The operational foundation of the Bekesy Audiometer is rooted in the psychoacoustic method of limits, adapted into an adaptive tracking procedure. Unlike fixed-level testing, which presents tones at preset intensities, Bekesy audiometry employs a dynamic process where the sound intensity continuously sweeps up and down based directly on the patient’s real-time perception. The core principle involves the patient maintaining the sound at their perceptual threshold by continuously interacting with a control mechanism, typically a simple push-button or switch. When the patient perceives the sound, they depress the button, signaling detection; when the sound fades below their hearing threshold, they release it, signaling non-detection. This reciprocal action creates a closed-loop feedback system central to the tracking methodology.

When the patient depresses the response button, the audiometer’s attenuation circuit automatically initiates a decrease in sound intensity, typically at a fixed rate, often set between 2.5 dB/second and 5 dB/second. Conversely, when the button is released, the intensity automatically increases at the same predetermined rate. This continuous adjustment process generates a characteristic zig-zag pattern, or tracing, on a synchronized recording chart. The midpoint of these excursions—the peaks and valleys of the tracing—represents the true hearing threshold for that specific frequency. By constantly oscillating around the threshold, the device captures the subject’s ability to detect the presence or absence of the stimulus with high precision, minimizing the need for manual judgment or interpretation during the test itself.

A crucial element in Bekesy audiometry is the presentation mode of the stimulus. The device is capable of presenting tones in two primary modes: continuous tone presentation and pulsed tone presentation. In the continuous mode, the pure tone is presented without interruption, testing the auditory system’s ability to maintain detection over time and measure auditory adaptation. In the pulsed mode, the tone is rapidly interrupted, often at a rate of 1 to 2 pulses per second (e.g., 250 milliseconds on, 250 milliseconds off). Comparing the thresholds obtained using these two modes forms the basis of the differential diagnostic power of the Bekesy test, particularly concerning auditory fatigue and physiological adaptation phenomena.

The adaptive nature of the Bekesy tracking procedure offers significant advantages in isolating the true threshold. Because the patient is actively controlling the signal intensity, they are less likely to employ rigid response criteria (e.g., waiting until the sound is definitely loud) compared to passive manual procedures. Furthermore, the test can be conducted using either fixed frequencies (discrete frequency sweep) or, more commonly, a continuous frequency sweep, where the frequency automatically changes across the audible spectrum (e.g., from 250 Hz to 8000 Hz) while the patient tracks the threshold. This continuous sweep capability allows for rapid and detailed mapping of the entire audiogram, often completed in a fraction of the time required for a traditional manual assessment.

Design and Key Components of the Instrument

The physical structure of the Bekesy Audiometer system is meticulously designed to facilitate accurate and reproducible measurements by rigorously controlling the acoustic environment and the stimulus delivery. The system is fundamentally composed of three integrated elements: the sound-attenuating environment, the control and stimulus generation console, and the patient response mechanism. The audiometric booth or sound-treated room is absolutely essential; its primary function is to minimize external, ambient noise that could interfere with the perception of low-level test tones, ensuring that the measured thresholds accurately reflect the subject’s physiological hearing sensitivity rather than being masked by environmental interference.

The control console serves as the central hub for the audiometer, housing the signal generator, the attenuator, and the recording mechanism. The signal generator produces highly stable, pure tones across the required frequency range (typically 125 Hz to 8000 Hz). The motor-driven attenuator is the key mechanism that automatically increases or decreases the sound intensity (measured in dB Hearing Level, or HL) based on the patient’s input. Crucially, the console contains the electromechanical or electronic circuitry that links the patient’s push-button response to the attenuator motor and simultaneously drives the recording pen. This linkage ensures that the pen accurately charts the intensity level in real-time corresponding to the frequency being tested, thereby generating the graphic tracing.

Output delivery is achieved either through standard audiometric headphones (supra-aural or insert) for measuring air conduction thresholds, or via a bone vibrator placed on the mastoid or forehead for assessing bone conduction thresholds. The ability to switch between these transducers allows the clinician to determine not only the degree of hearing loss but also its type (conductive, sensorineural, or mixed), making the Bekesy Audiometer a versatile tool for comprehensive audiological assessment. The patient interface remains intentionally simple, ensuring that the test is accessible even to individuals with limited cognitive or motor skills, provided they understand the basic tracking instruction.

Standardized Procedures: Bekesy Tracking Methodology

Execution of the Bekesy test requires precise patient instruction to ensure valid and reliable tracings. Before the test commences, the patient is seated comfortably within the sound-attenuating booth and fitted with the appropriate transducers. The instructions emphasize the need for continuous tracking: the patient is told to keep the button depressed only while the tone is audible and to release it immediately upon perceiving the tone fade or disappear. This instruction is vital because the amplitude of the tracing excursions (the difference between the peaks and valleys) reflects the patient’s sensitivity, reaction time, and ability to track the threshold dynamically, all of which are critical factors for accurate threshold determination.

The standard procedure typically involves a continuous frequency sweep testing protocol. The frequency changes automatically, usually covering the range from 250 Hz to 8000 Hz in a controlled amount of time, depending on the chosen sweep rate. The intensity changes continuously, generating the characteristic zig-zag tracing. The clinician monitors the recording to ensure the tracing is stable and consistent, noting the average excursion width, which usually falls between 5 dB and 15 dB for normal listeners. An unusually large excursion (e.g., 20 dB or more) might indicate difficulty in patient comprehension, motor control issues, or perhaps functional hearing loss, requiring re-instruction or consideration of response validity.

When measuring air conduction thresholds, the procedure is repeated for both the right and left ears independently. For diagnostic purposes, the comparison of continuous tone tracings versus pulsed tone tracings is a critical step. The patient completes the frequency sweep once using a steady, continuous tone and then repeats the process using an interrupted (pulsed) tone. The relationship between these two resulting tracings—specifically, the degree to which the continuous tone tracing shifts downwards (or adapts) relative to the pulsed tone tracing—provides the crucial diagnostic information necessary for classifying the hearing loss pattern based on the site of lesion.

To complete the comprehensive assessment, bone conduction thresholds are measured next using the same tracking methodology. A bone vibrator is placed on the mastoid process or the forehead. The procedure mirrors the air conduction test, utilizing both continuous and pulsed tones across the relevant frequency range. By comparing the air conduction tracings to the bone conduction tracings, the clinician can calculate the air-bone gap, confirming whether a conductive component exists. The systematic application of this protocol—air conduction (continuous/pulsed) followed by bone conduction (continuous/pulsed)—ensures that the resulting tracings provide a rich set of data for highly accurate differential diagnosis.

Interpretation of Bekesy Tracings and Classification

The true diagnostic utility of the Bekesy Audiometer stems from the interpretation of the resultant tracings, which are classified into distinct types based primarily on the separation observed between the continuous tone threshold and the pulsed tone threshold. This separation reflects the extent of auditory adaptation or fatigue present within the auditory system. Generally, the pulsed tone tracing represents the standard pure-tone threshold, as the intermittent nature of the stimulus minimizes the effects of adaptation. The continuous tone tracing, however, is highly sensitive to pathologies that cause excessive auditory fatigue or loudness recruitment.

Type I Tracing: Normal or Conductive Loss. In a Type I tracing, the pulsed tone and the continuous tone thresholds overlap or are separated by no more than 10 dB. This pattern is typically observed in individuals with normal hearing or those with a purely conductive hearing loss (e.g., middle ear pathology). Since conductive losses affect the transmission of sound mechanically without damaging the inner ear or neural pathways, the auditory system adapts normally, resulting in minimal or no separation between the two tracings. The excursion width is usually standard, confirming a well-established tracking ability.

Type II Tracing: Cochlear Pathology. A Type II tracing is characterized by a significant separation between the pulsed and continuous tracings, usually only above 1000 Hz. The pulsed tone tracing remains relatively stable, but the continuous tone tracing drops significantly below the pulsed tracing, often by 15 dB or more, particularly at higher frequencies. Crucially, the continuous tone tracing frequently remains at the pulsed tone tracing level at 250 Hz and 500 Hz, showing adaptation primarily in the high frequencies. This pattern is strongly correlated with cochlear lesions, specifically sensorineural hearing loss originating in the inner ear (e.g., Meniere’s disease or noise-induced hearing loss). The steep drop-off of the continuous tracing reflects abnormal rapid auditory fatigue or the phenomenon known as recruitment.

Type III Tracing: Retrocochlear Pathology. This tracing is highly indicative of a retrocochlear lesion, meaning pathology affecting the auditory nerve (VIIIth cranial nerve) or brainstem, such as an acoustic neuroma. In Type III, the continuous tone tracing begins to drop precipitously below the pulsed tone tracing immediately at the lowest tested frequencies (e.g., 250 Hz). The separation rapidly increases, often exceeding 20 dB across the entire frequency range, indicating profound auditory adaptation or decay. This severe separation, evident even in the low frequencies where cochlear adaptation is minimal, is a key diagnostic marker signaling a potentially serious neural disorder and necessitates immediate medical follow-up.

Type IV Tracing: Advanced Cochlear or Specific Retrocochlear Pathology. Type IV is similar to Type III but shows the continuous tone tracing tracking below the pulsed tone tracing by at least 20 dB across all frequencies, yet the continuous tracing remains parallel to the pulsed tracing, rather than exhibiting the extreme, rapid decay seen in Type III. While historically associated with specific severe cochlear damage, Type IV is now often considered a variation of Type III, particularly when observed in cases of large acoustic neuromas or severe neural compromise. A final, non-standard pattern, sometimes designated Type V, occurs when the continuous tone tracing tracks above the pulsed tone tracing, often indicative of non-organic hearing loss (malingering) where the patient is attempting to manipulate the test results by responding inaccurately.

Clinical Applications and Advantages Over Manual Audiometry

The widespread adoption of the Bekesy Audiometer in clinical settings stemmed directly from its marked advantages over traditional, manual threshold determination methods. Perhaps the most significant advantage is efficiency; the continuous frequency sweep capability allows a complete audiogram (air conduction, pulsed and continuous) to be generated in under ten minutes per ear, a fraction of the time required for a meticulous manual assessment. This speed makes the Bekesy test ideal for large-scale industrial screening programs, military assessments, and busy clinical environments where rapid, reliable data acquisition is crucial for initial triage and diagnosis.

Furthermore, the Bekesy method provides inherent objectivity and reliability. By automating the intensity control and recording process, the subjective influence of the audiologist—such as presentation timing, intensity step size, and recording interpretation—is significantly minimized. The patient’s response directly dictates the tracing output, leading to a highly reproducible measurement. This objectivity enhances the reliability of the threshold determination and makes longitudinal monitoring of hearing loss progression more accurate, as changes over time are less likely to be attributable to inter-tester variability or procedural inconsistencies.

The most powerful clinical advantage lies in its ability to facilitate differential diagnosis based on the tracing patterns. The comparison between continuous and pulsed tones provides immediate, non-invasive indicators of the site of lesion—distinguishing between the sensory organ (cochlea) and the neural pathway (retrocochlear system). This capability provides crucial early information that guides subsequent, often more complex, diagnostic testing (like Auditory Brainstem Response or Otoacoustic Emissions), streamlining the diagnostic pathway for patients presenting with sensorineural hearing loss. Recognizing a Type III tracing, for instance, often serves as a red flag, mandating immediate referral for specialized neurological and radiological examination.

Finally, the versatility of measuring both air and bone conduction thresholds is essential for a comprehensive audiological workup. By integrating both capabilities into the automated tracking format, the clinician obtains a rapid assessment of the air-bone gap, confirming the presence and degree of any conductive component. This comprehensive data set, generated quickly and reliably through a patient-controlled mechanism, solidified the Bekesy Audiometer’s position as an indispensable tool for decades, providing a critical balance of procedural simplicity, measurement accuracy, and unparalleled diagnostic insight into auditory pathology.

Conclusion and Future Directions

In summation, the Bekesy Audiometer represents a pivotal technological advance in the history of audiology, transforming the often laborious process of threshold measurement into an efficient, automated, and diagnostically rich procedure. Its enduring success is attributable to its ingenious application of patient-controlled adaptive tracking, which generates a unique graphic representation of hearing sensitivity—the Bekesy tracing. This tracing provides not merely a numerical threshold, but a pattern that offers critical clues regarding the underlying site of auditory pathology, allowing clinicians to distinguish reliably between normal hearing, conductive loss, and various forms of sensorineural damage.

Although modern audiology clinics now frequently utilize sophisticated computerized audiometers that incorporate digital versions of adaptive tracking (often using modified staircase procedures rather than continuous sweeps), the fundamental principles established by Georg von Békésy remain central to contemporary testing protocols. The concepts of self-controlled intensity modulation and the comparison between continuous and interrupted stimuli continue to inform automated audiometry systems designed for rapid screening and specialized diagnostic environments. The Bekesy method laid the groundwork for modern adaptive procedures like the Hughson-Westlake technique iterations, which prioritize efficiency and minimal tester bias.

Despite the rise of fully digitized systems, the classic Bekesy Audiometer procedures are still taught in audiology programs globally due to their foundational importance in understanding auditory adaptation and fatigue. The historical significance of the Type I through Type IV classifications cannot be overstated, as they offered audiology its first highly reliable, standardized method for localizing the source of sensorineural hearing impairments without requiring invasive procedures. This diagnostic paradigm shift empowered clinicians to provide more targeted referrals and interventions.

Looking forward, while dedicated Bekesy units are less common in modern practices than they once were, the methodology has been successfully integrated into software applications and portable screening devices, ensuring its continued influence. Future innovations will continue to leverage automation and adaptive algorithms to enhance accuracy and speed. Ultimately, the Bekesy Audiometer will forever be recognized as the instrument that ushered audiology into the era of automated, objective, and differential threshold measurement, ensuring its status as a vital chapter in the advancement of hearing science and clinical practice.

References

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• Kirkwood, M. W., & Stredney, D. (2000). Audiology: Diagnosis (2nd ed.). St. Louis, MO: Mosby.
• Moore, B. C. J. (2012). An introduction to the psychology of hearing (6th ed.). Amsterdam, The Netherlands: Academic Press.
• Shelley, P., & Marlow, M. (2003). Clinical audiology: An introduction. London, UK: Whurr Publishers.