Occupational Soundscapes – Part 5: Audiometry

     Audiometry is the measurement of individuals’ hearing sensitivity using finely-regulated sound inputs.  It is a crucial component of a hearing loss prevention program (HLPP) with an emphasis on the range of frequencies prevalent in speech communication.  To be valid, audiometric testing must be conducted under controlled conditions and the results interpreted by a knowledgeable technician or audiologist.
     This installment of the “Occupational Soundscapes” series provides an introduction to audiometry, requirements for equipment, facilities, and personnel involved in audiometric testing, and the presentation and interpretation of test results.  It targets, primarily, those enrolled in – as opposed to responsible for – an HLPP.  Its purpose is to develop a basic understanding of a critical component of hearing conservation efforts to, in turn, engender confidence in the administration of procedures that may be foreign to many who undergo them.

The Audiologist
     Audiometric testing is conducted by an audiologist, audiometric technician, audiometrist, or physician.  Distinctions among these roles are not important to the present discussion; therefore, the term audiologist will be applied to any competent administrator of audiometric tests.
     Demonstration of competency in audiometric testing is typically achieved by attaining certification from the Council for Accreditation in Occupational Hearing Conservation (CAOHC) or equivalent body.  Physicians, such as otolaryngologists, are certified by their respective medical boards.
     The Occupational Safety and Health Administration (OSHA) requires that audiometric testing be administered by a licensed or certified audiologist, physician, or technician capable of obtaining valid audiograms (see “The Results,” below) and maintaining test equipment in proper working order.  OSHA does not require technicians operating microprocessor-controlled (i.e. automated) audiometers (see “The Equipment,” below) to be certified, but the National Institute for Occupational Safety and Health (NIOSH) rejects this exemption.
 
The Facility
     Audiometric testing is typically conducted in one of three types of test facility – onsite, mobile, or clinical.  Each has unique characteristics that must be considered to determine which is best-suited to an organization and its HLPP.
     An onsite test facility utilizes dedicated space within an organization where an audiometric test booth is permanently installed.  An onsite facility is typically feasible only for large organizations with more than 500 noise-exposed employees enrolled in an HLPP at a single location.  Dedicated facilities often require full-time professional staff, further limiting the range of organizations for which onsite facilities are appropriate.
     Mobile test facilities may be provided by a third-party contractor to support an organization’s HLPP.  This may be an appropriate solution for an organization with multiple operations throughout a region when the number of employees enrolled in the HLPP at each location is relatively small.
     A clinical test facility is an independent medical or occupational health practice.  Employees schedule audiometric tests as they would an eye exam, annual physical checkup, or other outpatient procedure.  For smaller entities or programs, this is often the most practical choice.  Administration by an independent brick-and-mortar medical practice may also increase employees’ confidence in the HLPP, providing a psychological benefit that is difficult to quantify.
     The facility, regardless of the type chosen, must be sufficiently isolated to prevent interference with audiometric testing.  Vibrations, ambient noise, and distracting sounds must be minimized to ensure a valid audiogram.  Maximum Permissible Ambient Noise Levels (MPANLs) are defined in standards and regulations (e.g. ANSI S3.1, CFR29 Part 1910.95) for various types of test equipment.  It is important to note that sounds below the required MPANL, such as phone alerts, conversation, or traffic, can still be distracting and should be avoided.
 
The Equipment
     The two pieces of equipment most relevant to this discussion are the audiometer and the earphone.  There are three types of audiometer that may be encountered in an HLPP – manual, self-recording, and computer-controlled.  In the context of occupational hearing conservation, pure-tone, air-conduction audiometers are used; other types (e.g bone-conduction) may be utilized for advanced analysis and diagnosis.
     Using a manual audiometer, the audiologist retains control of the frequency, level, and presentation of tones and manually records results.  This is the least sophisticated, thus least expensive, type of audiometer.  It is also the most reliant upon an audiologist’s skill and concentration.
     A self-recording, or Békésy audiometer (named for its inventor) controls the frequency and level of tones, varying each according to test-subject’s responses; test results can be difficult to interpret.  This type of audiometer is no longer in common use in occupational HLPPs; its use is more common in research settings where its finer increments of tone frequency and level control are advantageous.
     Computer-controlled audiometers are prevalent in modern practice.  Continually-advancing technology has improved reliability and added automated functions, such as data collection, report generation, and test interruption for excessive ambient noise.  Stand-alone units may be called microprocessor audiometers; they also perform automated tests, but have fewer capabilities and cannot be upgraded as easily as software residing on a PC.
     There are also three types of earphone available for audiometric testing – supra-aural, circumaural, and insert.  A more-precise (“technical”) term for an earphone is “transducer;” “headset” or “earpiece” is more colloquial.
     Supra-aural earphones consist of two transducers, connected by a headband, that rest on the test subject’s outer ears; no seal is created.  Therefore, little attenuation is provided, requiring increased diligence in control of ambient sounds.
     Circumaural earphones consist of two transducers, housed in padded “earmuffs” that surround the ears, connected by a headband.  The seal provided by the earmuffs, though imperfect, provides significantly greater attenuation of ambient sound than supra-aural earphones.
     Insert earphones consist of flexible tubes attached to foam tips that are inserted in the ear canal.  The foam tip seals the ear canal, providing the greatest attenuation of ambient sound.  Test tones are delivered directly to each ear via the flexible tubes; the lack of physical connection between the transducers reduces the opportunity for transmission of tones from the tested ear to the “silent” ear.
     Some test subjects may experience discomfort, particularly when using insert earphones, which could lead to distraction that influences test results.  Recognizing signs of discomfort, distraction, or other interference is among the required skillset of an effective audiologist.
     Evidence suggests that the choice of earphone does not significantly affect test reliability.  However, earphones and audiometers are not interchangeable; an audiometer must be calibrated in conjunction with a paired earphone to provide valid test results.
 
The Test
     A typical audiometric test does not evaluate the entire frequency range of human hearing capability (~20 ~ 20,000 Hz).  Instead, the focus of testing is on the range of critical speech frequencies introduced in Part 2 of the series.  Specific test frequencies used are 500, 1000, 2000, 3000, 4000, and 6000 Hz.  Testing at 8000 Hz is also recommended for its potential diagnostic value; testing at 250 Hz may also be included.
     Each ear is tested independently by delivering pure tones at each frequency and varying levels, usually in 5 dB increments.  The minimum level at which a subject can hear a tone a specified proportion of the times it is presented (e.g. 2 of 3 or 3 of 5) is the person’s hearing threshold at that frequency.  Consecutive tests indicating thresholds within ±5 dB are typically treated as “consistent,” as this level of variability is inherent to the test.
     A single audiometric test may identify a concern, but multiple tests are needed to identify causes and determine appropriate actions.  The first test conducted establishes the subject’s baseline hearing sensitivity.  The subject should limit exposure to less than 80 dB SPL for a minimum of 14 hours prior to a baseline test, without the use of hearing protection devices (HPDs).  Some test protocols reduce the quiet period to 12 hours minimum or allow use of HPDs, but an extended period of “unprotected rest” is preferred.
     A baseline test is required within 6 months of an employee’s first exposure to the loud environment, though sooner is better.  Ideally, a baseline is established prior to the first exposure, thus eliminating any potential influence on the test results.
     Monitoring tests are conducted annually, at minimum.  They are often called, simply, annual tests, though more frequent testing is warranted, or even required, in some circumstances.  Monitoring tests are conducted without a preceding “rest” period, at the end of a work shift, for example.  Doing so provides information related to the effectiveness of HPDs, training deficiencies, etc.
     A retest is conducted immediately following a monitoring test indicating a 15 dB or greater hearing loss in either ear at any of the required test frequencies.  This is done to correct erroneous results caused by poor earphone fitment, abnormal noise intrusions, or other anomaly in the test procedure.
     A confirmation test is conducted within 30 days of a monitoring test indicating a significant threshold shift (discussed further in “The Results,” below).  Confirmation test protocols mimic those of a baseline test to allow direct comparison.
     Exit tests are conducted when an employee is no longer exposed to the loud environment.  This may also be called a transfer test when the cessation of exposure is due to a change of jobs within the organization, rather than termination of employment.  Exit test protocols also mimic those of a baseline test, facilitating assessment of the impact of working conditions over the course of the subject’s entire tenure.
 
The Results
     The results of an audiometric test are recorded on an audiogram; a blank form is shown in Exhibit 1.  Tone frequencies (Hz) are listed on the horizontal axis, increasing from left to right.  On the vertical axis, increasing from top to bottom, is the sound intensity level scale (dB).  This choice of format aligns with the concept of hearing sensitivity; points lower on the chart represent higher intensity levels required for a subject to hear a sound and, thus, lower sensitivity to the tested frequency.

     The audiogram shown in Exhibit 2 places examples of familiar sounds in relative positions of frequency and intensity.  Of particular interest is the “speech banana” – the area shaded in yellow that represents typical speech communications.  Presented this way, it is easy to see why differentiating between the letters “b” and “d” can be difficult.  These letters hold adjacent positions at the lower end of the speech frequency range, where several other speech sounds are also clustered.  This diagram also reinforces the idea that the ability to hear chirping birds and whispering voices are among the first to be lost; they are high-frequency, low-intensity sounds.
     Visual differentiation of data for each ear is achieved by using symbols and colors.  Each data point for a subject’s left ear is represented by an “X,” while each data point for the right ear is represented by an “O.”  Colors are not required; when they are used, the convention is to show left-ear data in blue and right-ear data in red.  The increased visual discrimination facilitates rapid interpretation of test results, particularly when all data for a subject are shown in a single diagram.  When baseline data are shown on a monitoring audiogram, the baseline data is typically shown in grey to differentiate between historical and current test data.
     The vertical scale represents a person’s hearing threshold – the minimum sound intensity level required for the test tone to be heard.  An example audiogram, representing “normal” hearing using the formatting conventions described above, is shown in Exhibit 3.  Sound stimuli above the line on the audiogram are inaudible; only those on or below the line can be heard by the subject.  Widely-accepted definitions of the extent of hearing loss are as follows:

• normal hearing:  < 20 dB hearing level;
• mild hearing loss:  20 – 40 dB hearing level;
• moderate hearing loss:  40 – 70 dB hearing level;
• severe hearing loss:  70 – 90 dB hearing level;
• profound hearing loss:  > 90 dB hearing level.

     The example audiogram in Exhibit 4 also demonstrates the use of symbols and colors to differentiate data, though the dual-chart format makes it less critical.  The data is also tabulated to provide precise threshold levels for each frequency.
     A significant drop in sensitivity, in both ears, at 4000 Hz is depicted in Exhibit 4.  This is the infamous “4K notch,” indicative of noise-induced hearing loss (NIHL).  The appearance of this notch or other deviation from normal hearing should elicit an appropriate response.
     The presence of a notch in a baseline audiogram suggests that permanent hearing loss has already occurred.  Appropriate measures must be taken to ensure that no further damage occurs.  Furthermore, additional assessments may be necessary to ensure that the subject’s abilities are compatible with the work environment.  If diminished communication abilities creates a hazard for the subject or others, for example, an appropriate reassignment should be sought.
     The appearance of a notch or other decline in hearing sensitivity in a monitoring audiogram should trigger follow-up testing.  A retest is conducted to ensure the validity of the data by verifying that the facility and equipment are operating within specifications and the test was conducted properly by both the subject and audiologist.  NIOSH recommends retesting when a monitoring audiogram indicates a 15 dB or greater increase in hearing level, relative to the baseline audiogram, at any frequency from 500 to 6000 Hz.
     If the monitoring and retest audiograms are consistent, two parallel paths are followed.  On one path, the subject undergoes a confirmation test to determine if the indicated hearing loss is permanent.  Appropriate follow-up actions are determined according to the results of this test.
     On the other path, HPD use and effectiveness is reviewed to determine necessary changes to the individual’s work process or to the HLPP more broadly.  Other changes to the work environment may also be necessary; noise-control strategies will be discussed further in future installments of this series.
     The decline in hearing sensitivity represented by a lower line on an audiogram is called a threshold shift.  When the arithmetic average of the differences between the baseline and monitoring audiograms at 2000, 3000, and 4000 Hz exceeds 10 dB in either ear, a standard threshold shift (STS) has occurred.  An STS is depicted in the comparative audiogram of Exhibit 5; calculation of the shift’s magnitude is shown in the table.

     If the change in hearing sensitivity is shown by confirmation testing to be irreversible, a permanent threshold shift (PTS) has occurred.  Some level of hearing loss is recoverable with “rest” in a quiet setting.  This change is called a temporary threshold shift (TTS).  Appropriate action must be taken to prevent a TTS from becoming a PTS.
     A baseline audiogram represents a person’s “best-case” hearing or maximum sensitivity.  Therefore, if subsequent testing results in a “better” audiogram than the baseline, the baseline is replaced by the new audiogram.  This can occur if influences on the baseline test were not noticed or properly addressed.  Examples include an insufficient rest period preceding the test, intrusive noise or vibration in the test chamber, and suboptimal earphone fitment.
     Results other than a pronounced 4K notch can also prompt additional testing.  The series’ focus remains on NIHL; therefore, only a brief overview will be provided here.  Interested readers are encouraged to consult other sources for additional information.
     Bone-conduction testing is performed with transducers placed behind the ears.  This type of test may be warranted to diagnose an occlusion of the ear canal, which can include impacted cerumen (“earwax”), or other condition of the outer or middle ear that limits air-conducted hearing.  Conductive hearing loss is suggested by differences between air- and bone-conduction thresholds of greater than 10 dB.  An example audiogram depicting this condition in one ear is shown in Exhibit 6.

     A positively-sloped audiogram, such as that shown in Exhibit 7, depicts higher sensitivity to higher frequencies, often indicative of a disorder of the middle or inner ear.  In the case of Meniere’s disease, for example, audiometric testing may be used to validate a medical diagnosis, whereas the reverse is often true for other conditions.
     A negatively-sloped audiogram, such as that shown in Exhibit 8, depicts lower sensitivity to higher frequencies, often indicative of the advancement of presbycusis (age-related hearing loss).  Guidance on the appropriate use of an audiogram of this nature in the context of an HLPP varies.  A non-mandatory age-adjustment procedure remains in the OSHA standard (CFR 29 Part 1910.95 Appendix F), though NIOSH has rescinded support for the practice of “age correction”.  Organizations utilizing age-adjusted audiograms should consider that OSHA regulations tend to follow NIOSH recommendations; the lag on this specific matter has been quite long already.

The Bottom Line
     Noise-induced hearing loss (NIHL) is the accumulation of irreparable damage to the inner ear, particularly the fine hairs of the cochlea (see Part 2).  Hearing loss usually occurs in higher frequencies first.  The focus of audiometric testing on speech communication leads us to define “high frequency” as the 3000 – 6000 Hz range, where the 4K notch is of particular concern.  Hearing loss in frequencies above 8000 Hz often go undiagnosed as the highest frequencies in the audible range are rarely tested.
     NIHL is one of several possible causes of hearing impairment.  Other causes include hereditary conditions, exposure to ototoxic substances, and illness (i.e. infection).  The various audiometric tests are valuable tools beyond the scope of NIHL; they can also aid diagnosis of several other conditions.  For example, a baseline audiogram may confirm the presence of a congenital disorder, or a confirmation test may reveal that an STS was caused by an illness from which, in the interim, the subject had recovered.
     A thorough, well-crafted health and wellness program will include audiometric testing.  In addition to the direct benefits of an HLPP, information about other conditions may also be obtained, further improving the work environment.  Psychological well-being of employees can be improved via increased effectiveness of verbal and nonverbal communication, in addition to the physical health benefits that participation in such a program can provide.


     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).
 
References
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Noise – Measurement And Its Effects.”  Student Manual, Occupational Hygiene Training Association; January 2009.
[Link] “Hearing Protection.”  Laborers-AGC Education and Training Fund; July 2000.
[Link] “Criteria for a Recommended Standard - Occupational Noise Exposure, Revised Criteria 1998.”  Publication No. 98-126, NIOSH, June 1998.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] “OSHA Technical Manual (OTM) - Section III: Chapter 5 - Noise.”  Occupational Safety and Health Administration; July 6, 2022.
[Link] ”29 CFR 1910.95 - Occupational noise exposure.’  OSHA.
[Link] Noise Control in Industry – A Practical Guide.  Nicholas P. Cheremisinoff.  Noyes Publications, 1996.
[Link] “Pediatric Audiology:  A Review.”  Ryan B. Gregg, Lori S. Wiorek, and Joan C. Arvedson.  Pediatrics in Review, July 2004.
[Link] “Familiar Sounds Audiogram:  Understanding Your Child’s Hearing.”  Hearing First, 2021.
[Link] “Hearing and Speech.”  University of California – San Francisco, Department of Otolaryngology – Head and Neck Surgery.
[Link] “Audiograms.”  ENT Education Swansea.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com