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Monday, July 20, 2020

Minimum Plateau in Pure Tone Audiometry: A New Audiometric Symbol?

​By Hlologelo Ramatsoma, and Dirk Koekemoer, MBChB

Pure tone audiometry has been the gold standard for the measurement of hearing acuity for over 80 years.1 Despite some of its obvious limitations, pure tone audiometry remains the main diagnostic equipment for standard hearing assessment in many audiological practices.2

The American Speech-Language-Hearing Association (ASHA) first suggested the use of audiometric symbols to record results for pure tone audiometry in 1974, with the latest revised audiometric symbols adopted in 1990.3 The ISO 8253-1:2010 standard also specifies the audiometric symbols that should be used for the graphic presentation of hearing thresholds; these symbols are similar to those suggested by ASHA.3,4 Many audiology clinics use these symbols, making them universally accepted. The use of standard audiometric symbols allows for uniformity and efficiency in sharing audiometric information from one clinician to another, thus benefiting patients.

There is always a need to add to the exhaustive list of the ASHA audiometric symbols to benefit our patients, should a clinic/research discover an innovative way of recording results through audiometric symbols. In this article, we aim to start a conversation around the creation of a new audiometric symbol for audiometric findings for "minimum response levels" due to a non-patient behavior limitation, such as

  • a limitation of the audiometer,
  • a clinical decision to only test to a certain lower intensity,
  • or as a result of excessive ambient noise not permitting the testing of lower intensities.

One clear limitation of audiometry is the minimum and maximum output limit of the equipment.  The audiometric symbol to indicate a "no response" as a result of the equipment's maximum output limitations has been established and reported by ASHA.3 In essence if the true test ear threshold cannot be established due to the maximum output limitations of the audiometer, an audiometric symbol for the specific ear with a downward arrow should be plotted on the audiogram at the last presented (highest) pure tone intensity. This practice is common and standard among audiologists. In a sense, this audiometric symbol is important in indicating that the threshold for the individual, at the specific frequency, could not be found due to the limitations of the test equipment that was used. 

With the introduction of booth-less audiometry more than a decade ago, the need arose for mobile audiometers to record the minimum testable thresholds obtainable on a patient due to excessive ambient noise levels. Today, these audiometers typically measure ambient noise in real-time and have the potential to indicate when a minimum plateau was reached because of the noise levels. The first audiometer to have implemented a minimum plateau symbol was the KUDUwave in 2008. It then became apparent that there has always been a need for this annotation and that it should become standard practice in audiology.


An issue of clinical relevance (and one that begs for a new audiometric symbol) that is not making the most amount of noise in the clinical sphere is the minimum output limitation of an audiometer.

In this article, the term minimum plateau will be used to refer to the minimum intensity that one can test down to. This is widely known as the floor effect across the field of statistics. Specific to audiology, a floor effect may arise when the measurement tool has a lower limit thus not being able to measure lower hearing thresholds.

But why does this matter? For us to be able to answer this question, we first have to define what a hearing threshold is. A hearing threshold is the softest sound one can hear 50 percent of the time, and this is generally assessed using pure tone audiometry.  When a patient hears and responds 100 percent of the time to a specific intensity the clinician decreases the intensity further. But what if that is the lowest testable intensity?

The minimum intensity that clinicians test down to, or are able to test down to, is not always the testee's hearing threshold. It is inaccurate to refer to these patient responses as their hearing thresholds. The minimum-plateau responses are of clinical relevance as they indicate that the thresholds obtained at that moment are or could be "not as accurate" due to equipment limitations or the amount of ambient noise in the environment.

To be clear, the audiometer's minimum output levels can be influenced by a few things, namely:

  1. Equipment Audiometer Intensity Range
  2. Ambient noise levels
  3. Clinicians testing protocols
  4. Room certification limits

Equipment Intensity Range: Acoustic transducers, such as bone vibrators, insert earphones and supra-aural headphones, have their own maximum and minimum output limits. For example, some circumaural headphones have an intensity range of -20 to 100 dB HL for high frequencies, while a supra-aural headset has the intensity range of -10 to 120 dB HL in the conventional frequencies. This demonstrates the minimum output levels of the two acoustic transducers. Additionally, some audiometers have a minimum intensity of 0 dB HL.

Ambient Noise Levels: Ambient noise, whether transient or static, can affect the lowest intensity that you can present at. Various standards (i.e., ANSI S3.1, ISO 8253-1, SANS 10182) have prescribed maximum permissible ambient noise levels (MPANLs) to stipulate the acceptable noise levels in an audiometric assessment room or environment.5 For example, different levels of noise allow the audiologist to test either down to -10, 0, 10, or 25 dB HL. The lower the ambient noise levels in the environment, the lower the intensity one can test down to. As an example, many organizations have recommended that school-based hearing screening programs test down to 25 dB in developing countries (which may not have sound-treated booths) to mitigate ambient noise levels.

Clinicians Testing Protocols: The clinician's testing protocol or procedure may also define the lowest intensity tested down to. Using the same example as above, a clinician may choose to test down to 25 dB for school-based hearing screenings, whilst on the other hand, a clinician may choose to test down to 15 dB across the frequency spectrum when testing for percentage loss of hearing (PLH) in the occupational health setting.

Room Certification Limits: Certifications of sound booths and/or test environments that allow for compliance with standardized MPANLs for audiometric testing are conducted annually. Depending on the test setting (i.e., occupational settings vs clinical settings) and the frequency range tested, certain ambient noise levels are permitted when certifying the test environment to the standards mentioned previously. With clinical settings, ambient noise levels should be low to allow for testing down to -10 dB HL. Typically a sound booth is certified to allow testing to a minimum of 10 dBHL. What this means is that, if testing in those settings that do not allow for testing lower than 0 dB HL, one would technically always have marked the "threshold" using the recommended audiometric symbol if the patient could potentially have a lower threshold than 0 dB HL.


When conducting pure tone audiometry, audiologists should note the considerable difference between "hearing threshold" and "minimal response level," or as we refer to it here, the "minimum plateau." Thus, it must be indicated on the audiogram whether the response is a hearing threshold or a minimum plateau. This can be represented graphically or through notes written below one's audiogram. The symbol that we suggest is one that is almost similar to the "no response due to the maximum output of the audiometer" reported by ASHA3 and ISO. To record a minimum plateau for a specific ear, mark a threshold symbol for the ear with an upward arrow at the last presented (lowest) pure tone intensity. Table 1 is limited to air conduction audiometric symbols, however, the concept may be applied to any audiometric symbol. 

Table 1.JPG

Table 1. Recommended air-conduction audiometric symbols for continued-response at the audiometer's lower limits due to audiometric minimum output limitations. 

The issue of clinical relevance is the management of hearing loss based on minimum-plateau behavioral responses. In the worst-case scenario, medical referral and management of middle ear pathology can be made based on the "thresholds." For example, if the air conduction thresholds are 10 dB HL and the Bone Conduction was only tested to 10 dB HL as a minimum plateau, then the information cannot be used to exclude middle ear pathology.

Also, a no-response in audiology is many times tabulated as an "NR" in a table. We recommend that the letters "MP" be appended after the threshold levels to indicate that the threshold obtained is as a result of a minimum plateau.

Hearing care professionals must record and communicate the right patient results, and audiometric symbols assist us in doing so. We, therefore, propose these new symbols to assist in more accurate audiometric test recordings.


ABOUT THE AUTHORS: Mr. Ramatsoma, left, is a clinical and research audiologist whose research areas include advisory and validation of contextual audiological products to increase the availability of hearing health care. Dr. Koekemoer is a specialist in innovative telemedicine and medical practice automation. He is passionate about the development of medical devices to automate and improve the quality of primary health care examinations in ares with resource constraints.

Thoughts on something you read here? Write to us at [email protected].


1. Katz J, Chasin M, English K, Hood L, Tillery K. Handbook of clinical audiology. 7th ed. Philadelphia: Wolters Kluwer Health; 2015.

2. Martin FN, Champlin CA, Chambers JA. Seventh survey of audiometric practices in the United States. Journal of the American Academy of Audiology. 1998 Apr 1;9(2).

3. Audiometric Symbols [Internet]. 1990 [cited 11 June 2020]. Available from:

4. Acoustics — Audiometric test methods — Part 1: Pure-tone air and bone conduction audiometry. Geneva: ISO; 2010.

5. Frank T, Durrant JD, Lovrinic JM. Maximum permissible ambient noise levels for audiometric test rooms. American journal of audiology. 1993 Mar;2(1):33-7.