Audiometer, though considered as the basic equipment to assess the function of hearing, has undergone significant transformation since its inception almost 100 years ago. The audiometer though emerged as an instrument to check hearing levels across a given range of frequencies, has today taken a vast spectrum of roles in determining various aspects of hearing function. Eventually as the role of Audiometer widened, there emerged the requirement of classifying the Audiometer based on the tests done & the test facilities made available. Accordingly, the audiometer which was initially introduced to measure & quantify the hearing level was sooner exposed to meet & explore various other related functions of hearing. The term Audiometer still prevailed for all these equipments though the Audiometer was categorized into different types. Now, all these were the events in history, in the pre-digital era. We are now into the digital era of this modern world ruled by technology, with technological advances influencing & penetrating into this field of Audiological Science & Audiological instrumentation with great success. Integration of various technologies led to the development of precision devices that would facilitate objective measurements of the functions related to hearing mechanism. The term Audiometer started loosing significance & marked the beginning of its disappearance in reference to the precision high end objective devices. Hence a renewed look (by this author) gave birth to a realization towards the need for a novel nomenclature in this digital era with a broader concept in terming & classifying the most sacred instrument – “The Audiometer”.
Audiometer – The prevailing definitions:
Audiometer is an electronic instrument designed for the measurement of hearing sensitivity, and for the calibrated delivery of supra-threshold stimuli. (Stach, B.A, 2003. Comprehensive Dictionary of Audiology, Illustrated, Delmar Cengage Learning; 2nd Revised edition.)
An Audiometer is an electronic instrument, used by an Audiologist to quantify hearing. (Stach, B.A, 1998. Clinical Audiology: An Introduction, Cengage Learning).
There are several types of Audiometers, that are classified primarily based on their functions. Initially, it was classified broadly into Clinical Audiometer & Screening Audiometer based on the extent of functions that the Audiologist might want to use for hearing assessment.
The Audiometer is an essential tool of the Audiologist for measuring hearing function. An Audiometer is an electronic instrument designed for the presentation of calibrated auditory stimuli to transducers. (DeRuiter, M., Ramachandran, V. 2016. Basic Audiometry Learning Manual, Plural Publishing, 2nd Edition)
An Audiometer is an electro-acoustic device that produces controlled levels of test tones and signals intended for use in conducting diagnostic hearing evaluations and assist in the diagnosis of possible otologic disorders. (Code of Federal Regulations, FDA, 1985-1999, Pg. 300)
An Audiometer is a primary tool used by the Audiologists in the assessment of peripheral hearing sensitivity. (Frank E.Musiek, Jain A Baran, Jennifer B Shinn, Raleigh Jones, 2011, Disorders of the Auditory System, Pg. 63, Plural Publishing)
Classification of Audiometers –The prevailing categories:
International Electro-technical Commission (IEC), 1980 classifies Audiometers into 5 types:
- Type -1 Audiometer
- Type -2 Audiometer
- Type -3 Audiometer
- Type -4 Audiometer
- Type -5 Audiometer
This classification is based on the range of features and complexity of the functions available within the equipment for the assessment of hearing function. The number of features and the extent to which it assesses the various hearing functions differs in each of these types and these features & functions generally decreases as we proceed from Type -1 Audiometer to Type -5 Audiometer. As such, these types are broadly classified as:
Type -1 Audiometer ®Advanced Diagnostic Audiometer.
Type -2 Audiometer ®Diagnostic Audiometer
Type -3 Audiometer ® Simple Diagnostic Audiometer
Type -4 Audiometer ® Screening Audiometer
Type -5 Audiometer ® Simplest Audiometer
A very brief description of these audiometer types is given as below:
Type-1 Audiometer: (Advanced Diagnostic Audiometer)
- 2 channel Audiometer, with testing facilitated with both tonal stimuli & speech stimuli.
- Hearing thresholds could be elicited both manually and by auto threshold tracking procedure as employed in Bekesy Audiometry.
- Provides facility for inputs from microphone or from other external devices like tape/CD.
- Stimuli includes Puretones, warble tones, pulsed tones, narrow band noise, broad band noise.
- Has facility for testing Right & Left ears independently using both air conduction mode as well bone conduction mode & free-field testing.
- Incorporates TDH 39/49 ear phones, BM71 Bone vibrator, insert earphones, high definition earphones for extended high frequency testing & insert masking.
- Includes octave frequency as well inter-octave frequency testing starting from 125Hz to 20,000Hz for air conduction & from 125Hz to 4KHz for Bone Conduction.
- Allows air conduction testing from -10dBHL to 120dBHL & Bone Conduction testing from -10dBHL to 70dBHL.
- Permits the administration of all special tests like, SISI, TDT, ABLB, MLB, Strenger, Lombard, MLD, SIN, Bekesy, TEN test etc.
- Allows communication & monitoring by way of ‘Talk Forward” & “Talk Back” systems.
Type-2 Audiometer: (Diagnostic Audiometer)
- This type of Audiometer is almost similar to Type-1, with the only difference seen in the limited extent to which the advanced tests could be possibly administered.
Type-3 Audiometer: (Simple Diagnostic Audiometer)
- It’s a single channel Audiometer, with only one source of signal generation.
- Facilitates testing for octave & inter-octave frequencies ranging from 125Hz to 8000Hz only for air conduction. (No provision for extended high frequency testing) & from 125Hz to 4000Hz for bone conduction.
- Extent to which the special tests could be administered would be limited.
- Permits speech audiometry.
Type-4 Audiometer: (Screening Audiometer)
- Permits only Puretone Audiometry (No provision for speech audiometry), under air conduction mode.
- Though frequency range includes test frequencies from 125Hz to 8000Hz, the intensity range would be limited to 90dBHL or 100dBHL.
- Would not have any provision for administering any of the special tests.
- No provision for administering Free-Field testing.
- No provision for pulsed tone & warble tone.
- Does not permit any external inputs.
Thus, its seen that Audiometers are classified according to the range of test signals they generate, according to the mode of operation or according to the complexity of the range of auditory functions they test. In such a classification, the Audiometer is not equipped to measure all the aspects & functions related to hearing mechanism.
Though Audiometers, in early 1920’s & 1930’s were mainly used to measure the hearing, confining the audiologic services to the measurement of hearing level to certify the degree of hearing handicap; developments in the field of hearing measurements for diagnostic, hearing conservation and rehabilitation purposes have resulted in the availability of a wide range of audiometers suited to assess more specialized functions of hearing mechanisms. In addition it is possible to consider the audiometer in terms of a set of functional units which can be specified independently. By specifying these functional units it is then possible to specify the performance of other audiometric equipment which uses these units.
Proposed Definition for Audiometer: (Proposed by this author)
An Audiometer is an electronic device that is capable of generating different calibrated acoustic stimuli, necessary for the examiner to assess various aspects of hearing mechanism.
Functions of Audiometer:
Functionally, Audiometer is used to:
- Quantify hearing.
- To determine the pathologic site in the ear.
- To determine the nature & degree of hearing impairment and thereby categorize the severity of hearing disability.
- To aid in the differential diagnosis of various auditory pathologies.
- To estimate the speech discrimination ability, thereby facilitating evaluation of communication competency that would reveal the extent of prevailing disability.
Applications with Audiometer:
Audiometer can be used as:
- A Hearing Screening Device to detect the presence/absence of hearing impairment.
- A simple equipment to measure the hearing levels across the given range of frequencies.
- A diagnostic equipment to assess various aspects of hearing & balance mechanism.
Components of Audiometer:
Although the micro-components of audiometer might vary with reference to the purpose with which it is manufactured to assess; any & all Audiometers basically consists of:
So, if Audiometer is what it was initially considered as an equipment that is used in the measurement of hearing & hearing disability, then there exists similar equipments which could be used for the assessment & measurement of hearing & hearing disability; which are known by the following names:
- Immittance Audiometer/Middle Ear Analyzer/Impedance Audiometer.
- Equipments to measure Otoacoustic Emissions known under various names. (OAE equipments or could also be termed as Otoacoustic Emission Audiometer)
- Equipments to measure Auditory Brainstem Activity, known under the names ABR, BERA, BSERA, AEP,.. etc. (could also be termed as Evoked Potential Audiometer)
According to World Report on Disability, prepared by WHO (World Health Organization), “disability is a complex, dynamic, multidimensional & contested one”. Hearing disability, which is regarded as one of the most invisible disability (hidden disability), is therefore a complex multidimensional experience that poses several challenges for measurement. As such any single equipment would not be sufficient enough to precisely assess & measure the hearing function to suit the wide range of clinical population, considering the high rate of associated disorders that prevail along with hearing impairment and the prevalence of hearing impairment right from birth. While the conventional so called ‘early audiometers’ need patient cooperation for the measurement of hearing function; very often Audiologists have to depend on sophisticated equipments (OAE equipments & Evoked Potentials (EP) equipments) to confirm the hearing function. This is very likely in case of newborns, difficult to test patients & patients who has limited access to competent communication. Hence the Audiologists need to have multiple access to equipments to measure the hearing and subsequently confirm the disability factor.
Going by the functional definition of Audiometer, all these devices (Immittance Audiometer, OAE equipment, EPequipment) are electronic devices that generates different acoustic signals necessary for the Clinical Audiologist to assess various aspects of hearing mechanism. This is well substantiated with the presence of basic components that are common in all these equipments. (i.e., Oscillator, Attenuator, Interrupter & Transducers; however in different forms).
Audiometer has changed its form & function since its inception almost 100 years back. It was initially a huge bulky equipment which comprised of only analog components in the making. Present day Audiometer (and perhaps all future audiometers) has witnessed the transition from its old form towards a new generation of Audiometers whose making is facilitated with exclusive digital technology, suited to perform multiple functions that would aid in the measurement of hearing and precisely categorize the disability factor. Thus the present day audiometer would be embedded with microprocessors and programmers that would aid in precise detection of various hearing functions.
Thus it is very much evident that various approaches need to be considered and adopted to accurately measure the hearing disability. A single measure with a single equipment would be too less to bring out this “Hidden Disability” into light, given the wide nature of circumstances that hides it.
Hence, the necessity to have a broader classification of Audiometer types arises.
Proposed Nomenclature for Audiometer: (Proposed by this author)
Here, Audiometers are classified based on the following factors:
- Based on the purpose of assessment.
- Based on the available range & limits within which the acoustic signals could be generated.
- Based on the hearing mechanism tested.
- Based on the behavioral & objective mode of test process.
- Type –I Audiometer ® Screening Audiometer (SA):
- Assess the Hearing Threshold levels using Puretone stimuli, Speech stimuli or any other desired stimuli with minimal navigation on frequencies & intensities. (Limited frequencies/frequency range & limited intensities/intensity range)
- Includes intensity fixed equipments, at specific test frequencies which would notify PASS/FAIL conditions on hearing functions.
- Includes OAE screeners using either TEOAE or DPOAE or both.
- Includes stand alone AABR devices or AABR feature coupled with OAE screener devices.
- As such all the screener equipments that evaluates the hearing function to facilitate a hearing screening objective could be categorized under this group.
- Type -II Audiometer ® Comprehensive Clinical Audiometer (CCA):
- Assesses the Hearing Threshold levels, using Puretone stimuli, Speech stimuli, & other desired Acoustic stimuli, by employing “behavioral mode” of response activity.
- Includes all facilities available with Puretone Audiometry as well as Speech Audiometry.
- Includes High frequency Audiometry & Free Field Audiometry.
- Includes all the available site of lesion tests like SISI, TDT, ABLB, MLB, Bekesy, STRENGER, LOMBARD, TEN, TINNITUS MATCHING & CAPD TESTS.
- Could also be moulded to accommodate any further additional tests that would serve similar purpose in assessing the site of lesion.
- Type – III Audiometer ® Immittance Audiometer (IA):
- Assesses the middle ear functions and exclusively meant to assess the status of middle ear condition.
- Includes test facilities like TYMPANOMETRY, REFLEXOMETRY, EUSTCHIAN TUBE FUNCTIONS, REFLEX DECAY.
- Could also be moulded to accommodate any further additional tests that would serve similar purpose in assessing the middle ear function.
- Type –IV Audiometer ® Advanced Diagnostic Audiometer (ADA):
- Assesses Hearing Threshold levels, using Puretone stimuli, Speech stimuli, or any other desired Acoustic stimuli by employing an highly “objective method” of recording.
- Includes all Auditory Evoked Potential (AEP) measures under the name “Evoked Potentials Audiometer” and Diagnostic measures of OAE under the name “Otoacoustic Emission Audiometer”.
- Type –V Audiometer ® Vestibular Audiometer (VA):
- Assesses all the vestibular functions and exclusively meant to assess the status of balance function using “acoustic stimuli”.
- Includes test facilities like Vestibular Evoked Myogenic Potential (VEMP).
- Can also be moulded to accommodate any other additional tests that would serve the purpose of assessing the vestibular related functions using acoustic stimulus.
Here, no attempt is made to make any changes in the ‘Standards’; rather this document focuses on a broader conception & classification of the term ‘Audiometer’; considering the multiple assessment procedures that could be utilized to include and evaluate various aspects & functions related to hearing mechanism. This classification would enable in categorizing all equipments belonging to hearing screening objective under Type –I Audiometer termed as “Screening Audiometer” (SA); all equipments that employ diagnostic measures in the assessment of hearing functions & rely on behavioral responses under Type -II Audiometer termed as “Comprehensive Clinical Audiometer” (CCA); all equipments related to the measurements of middle ear functions under Type –III Audiometer termed as “Immittance Audiometer” (IA); all equipments that employ an objective method of response recording during the process of hearing assessment under Type –IV Audiometer termed as “Advanced Diagnostic Audiometer” (ADA); all equipments that evaluate the balance function using acoustic stimuli as the input stimulus under Type –V Audiometer termed as “Vestibular Audiometer” (VA). Thus a clear cut distinction prevails in the classification of Audiometers in this new era of digital & advanced digital technology, thereby bringing in a wider dimension towards the conception of this sacred equipment –The Audiometer!
- ANSI S3.6 2004 Specifications for Audiometer.
- Arlinger S., 1991. Manual of Practical Audiometry, Vol.1
- Code of Federal Regulations, FDA, 1985-1999, Pg. 300
- DeRuiter, M., Ramachandran, V. 2016. Basic Audiometry Learning Manual, Plural Publishing, 2nd Edition
- Frank E.Musiek, Jain A Baran, Jennifer B Shinn, Raleigh Jones, 2011, Disorders of the Auditory System, Pg. 63, Plural Publishing
- Maureen Valente, 2009. Pure tone Audiometry & Masking, Plural Publishing.
- Silman S., Emmer M.B. 2011. Instrumentation for Audiology & Hearing Science: Theory & Practice, Plural Publishing.
- Stach, B.A, 1998. Clinical Audiology: An Introduction, Cengage Learning.
- Stach, B.A, 2003. Comprehensive Dictionary of Audiology, Illustrated, Delmar Cengage Learning; 2nd Revised edition.
WHO, 2011. World Report on Disability.