ISO TC 43/SC Reference number ISO 8253 1 2010(E) © ISO 2010 INTERNATIONAL STANDARD ISO 8253 1 Second edition 2010 11 01 Acoustics — Audiometric test methods — Part 1 Pure tone air and bone conduction[.]
General
Hearing threshold levels are assessed through air conduction and bone conduction audiometry, with air conduction involving test signals delivered via earphones and bone conduction using a bone vibrator placed on the mastoid or forehead It is recommended to start with air conduction measurements before proceeding to bone conduction tests Threshold levels can be determined using fixed-frequency audiometry or sweep-frequency audiometry, with detailed methods provided in specific sections During testing, thresholds for each ear should be measured separately, and masking noise may be applied to the non-test ear under certain conditions using suitable earphones to prevent cross-hearing.
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Standard reference zero for the calibration of audiometric equipment
The standard reference zero for air conduction audiometers is outlined in ISO 389-1, ISO 389-2, ISO 389-5, and ISO 389-8, while for bone conduction audiometers, it is specified in ISO 389-3, based on reference equivalent threshold sound pressure levels (RETSPL) and vibratory force levels (RETVFL) at specific frequencies Different RETVFL values are applicable depending on the vibrator's placement, whether at the mastoid or forehead ISO 389-3:1994 provides the RETVFL values for mastoid placement, with Annex C detailing the corresponding difference values for forehead location of the vibrator.
Requirements on audiometric equipment
Audiometers must be built following IEC 60645-1 standards and calibrated according to ISO 389 specifications to ensure accuracy and reliability In occupational audiometry and schoolchild hearing testing, a type 4 audiometer (IEC 60645-1:2001) is permitted, with some models limiting the frequency range to 500 Hz and above for suitable assessments.
Qualified tester
A qualified tester is an individual who has completed appropriate training in both the theoretical and practical aspects of audiometric testing, ensuring proficiency and adherence to industry standards This qualification may be recognized and specified by national authorities or accredited organizations dedicated to maintaining testing quality It is essential that audiometric tests are conducted solely by, or under the supervision of, a qualified tester to ensure accurate and reliable results, as emphasized in ISO 8253 standards.
The audiometric tester must make critical decisions during the test, including which ear to test first—typically the one deemed more sensitive—whether masking is necessary, and if the test subject's responses accurately reflect the test signals Additionally, the tester should assess for external noise interference or behavioral responses that could compromise the test validity, as well as determine when to interrupt, terminate, or repeat parts or the entirety of the test Proper decision-making ensures accurate and reliable audiometric results.
Test time
To ensure accurate test results, it is important to avoid over-fatiguing the test subject Providing rest breaks after approximately 20 minutes helps prevent fatigue, which can compromise the reliability of the data collected.
Conditions for audiometric test environments
Ambient sound pressure levels in an audiometric test room shall not exceed the values specified in Clause 11
The test subject and the tester shall be comfortably seated during audiometric testing and shall be neither disturbed nor distracted by unrelated events nor by people in the vicinity
Air temperature in the audiometric test room should be in the range permitted for offices by local authorities The audiometric test room should allow for sufficient exchange of air
When operating a manual audiometer, the examiner must have a clear view of the test subject while ensuring the subject cannot see any changes in audiometer settings or when test tones are presented or interrupted For automatic recording audiometers, the recording mechanism should remain hidden from the test subject to maintain unbiased results Proper concealment of settings and testing processes is essential for accurate and reliable audiometric assessments.
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When conducting audiometric tests from outside the test room, the subject must be visually monitored via a window or closed-circuit TV system to ensure their safety and compliance Additionally, acoustic monitoring should be implemented to accurately assess the subject's responses and maintain testing integrity.
Measurement uncertainty
The accuracy of hearing threshold levels determined through ISO 8253 procedures depends on various factors, including the performance of audiometric equipment, the type and fitting of transducers, and the frequency of test tones Test environment conditions, especially ambient noise, as well as the qualification and experience of the tester, play a critical role Additionally, the cooperation and reliability of responses from the test subject, along with the use of properly optimized masking noise, significantly influence the results.
Measuring uncertainty in audiometric tests is complex due to factors like personal behaviors of both the test subject and the tester While a single, universally applicable figure is challenging to determine, a comprehensive assessment of measurement uncertainty offers valuable insights into the reliability of audiometric results This detailed evaluation helps ensure accurate interpretation and confidence in audiometric testing outcomes across various applications.
The measurement uncertainty according to ISO 8253 should be assessed following ISO/IEC Guide 98-3 standards When reported, the expanded uncertainty, along with its coverage factor for a specified coverage probability, must be provided Guidance for determining the expanded uncertainty is detailed in Annex A of the standard.
5 Preparation and instruction of test subjects before audiometric testing and positioning of transducers
Preparation of test subjects
Recent noise exposure can temporarily raise hearing threshold levels, so it's important to avoid significant noise before audiometric testing or to note any exposure To prevent errors caused by physical exertion, test subjects should arrive at least 5 minutes prior to their hearing assessment Proper preparation ensures accurate audiometric results and reliable hearing evaluations.
An otoscopic examination by a qualified professional should always precede an audiometric test to ensure accurate results If earwax obstructs the ear canal, it must be properly removed, which may delay subsequent audiometry to allow the ear to recover Additionally, the ear should be assessed for collapsing ear canals, and appropriate measures should be taken if necessary to ensure reliable testing conditions.
NOTE 1 Preliminary information about the type of hearing loss and masking requirements can be obtained by performing tuning fork tests
Qualified persons are designated based on criteria established by national authorities or other appropriate organizations It is important to note that a qualified person does not have to be the same individual as the qualified tester referenced in section 4.4 Ensuring proper qualifications and clear distinctions between roles helps maintain compliance with regulatory standards.
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Instruction of test subjects
To obtain accurate and reliable test results, clear and unambiguous instructions for the test procedure must be provided, ensuring that the test subject fully understands the process.
Effective audiometry instructions should clearly inform the listener to understand the response task, emphasizing the importance of responding whenever a tone is heard in either ear regardless of faintness; they should also instruct the listener to respond immediately when a tone is detected and to cease responding once it stops, while providing details about the pitch sequence of the tones and specifying which ear will be tested first.
The response from the test subject indicating when the tone is heard and when it is no longer heard shall be clearly observable Examples of commonly used responses are:
⎯ pressing and releasing a signal switch;
⎯ raising and lowering the finger or hand
Test subjects should be instructed to minimize unnecessary movements to prevent extraneous noise during testing After providing these instructions, confirm the subject's understanding and inform them that they may halt the test if they experience discomfort If any uncertainties arise, repeat the instructions to ensure clarity and proper compliance.
Placement of transducers
Prior to testing, remove spectacles, head ornaments, and hearing aids as necessary to ensure accurate results Move hair away from the ears to facilitate proper placement of sound transducers such as earphones and bone vibrators The tester must correctly position the transducers, with the earphone's sound opening facing the ear canal entrance and the bone vibrator placed to maximize contact with the skull, typically behind the pinna near the mastoid without touching it Subjects should be instructed not to touch the transducers after placement to prevent measurement interference.
6 Air conduction hearing threshold level determinations using fixed-frequency audiometry
General
The audiometric test may be carried out using a manual audiometer or an automatic-recording audiometer The procedures are specified in 6.2, 6.3, and 6.4
The order of presentation of test tones when the audiometer settings are performed manually shall be from
1 000 Hz upwards, followed by the lower frequency range, in descending order A repeat test shall be carried out at 1 000 Hz on the ear tested first
Vibrotactile sensations may occur at low frequencies and high hearing levels; care, therefore, shall be taken that such sensations are not misinterpreted as hearing sensations
Preferably, automatic-recording audiometers should present test tones in the same sequence as in manual audiometry
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Manually controlled threshold determination
6.2.1 Presentation and interruption of test tones
The test tone should be continuous and presented for a duration of 1 to 2 seconds During testing, the interval between tone presentations must be varied but not shorter than the tone duration Unless specified otherwise, all references to tone presentation in ISO 8253 pertain to this method, ensuring consistent and accurate audiometric testing.
Automatically pulsed tones are sometimes used as an alternative stimulus However, correlative data are not currently available The use of such stimuli should be noted on the audiogram
Prior to threshold determination, the test subject should be familiarized with the task by presenting a signal of sufficient intensity to evoke a clear response This familiarization step ensures that the test subject understands the task and can reliably perform the required response, leading to more accurate and consistent testing results.
To familiarize with the tone, present a clear 1,000 Hz sound at a comfortable hearing level, such as 40 dB for individuals with normal hearing Gradually reduce the tone's volume in 20 dB steps until the subject no longer responds Then, increase the volume in 10 dB steps until a response is observed again Finally, present the tone once more at the previous response level to confirm detection, following a systematic method for hearing assessment.
If the responses are consistent with the tone presentation, the familiarization is complete If not, it should be repeated After a second failure, the instructions should be repeated
In cases of profound deafness, these procedures may not be applicable
6.2.3 Hearing threshold measurements with and without masking
Section 6.2.3.2 details the test procedures for audiometric assessments conducted without masking noise on the non-test ear In contrast, section 6.2.3.3 outlines the procedures for tests that include masking to ensure accurate results Additionally, section 6.2.4 provides the standardized method for calculating hearing threshold levels, ensuring precise and consistent audiometric measurements.
6.2.3.2 Procedures for testing without masking
Two audiometric test procedures utilizing a manual audiometer are the bracketing method and the ascending method, which are essential techniques in hearing assessments These methods primarily differ in the sequence of test tone levels presented to the test subject, with the bracketing method involving systematic adjustments around threshold levels, while the ascending method gradually increases stimulus intensity to determine hearing thresholds Both procedures are vital for accurate audiometric testing and diagnosing hearing impairments.
In the ascending method, consecutive test tones having ascending levels are presented until a response occurs
In the bracketing method, consecutive test tones having ascending levels are presented until a response occurs, after which test tones having levels in a descending sequence are presented
When properly carried out, both methods result in substantially the same hearing threshold levels
Measurements using the ascending method differ from those of the bracketing method only in step 2 of the measurements presented below
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When hearing threshold levels are 40 dB or higher in either ear at any frequency, these results should be interpreted cautiously due to cross-hearing In such cases, contralateral masking may be necessary to ensure accurate assessment of hearing sensitivity.
Begin by presenting the first test tone at a level 10 dB below the lowest response level of the test subject during familiarization If the subject fails to respond, increase the tone by 5 dB increments until a response is observed This stepwise increase helps accurately determine the auditory threshold during hearing assessments.
To determine the hearing threshold level, gradually decrease the volume in steps of 10 dB until no response is detected Then, increase the volume in 5 dB steps, monitoring for responses Continue this process until three responses are recorded at the same level out of a maximum of five ascents The level at which these responses are consistently obtained is defined as the hearing threshold level (refer to section 6.2.4.2).
If fewer than three responses out of five ascents are achieved at the same level, present a test tone 10 dB above the level of the last response Continue with the standard testing process: reducing the volume by 10 dB after a response, then increasing it by 5 dB until another response occurs This method ensures accurate audiometric threshold determination by systematically adjusting the tone intensity based on patient responses.
A shortened version of the ascending method can achieve nearly equivalent results and may be suitable in certain situations This approach involves continuing testing until at least two responses occur at the same level out of three consecutive ascents, making the process more efficient without significantly compromising accuracy.
Begin the test by increasing the tone level by 5 dB to assess the subject's response Gradually decrease the tone in 5 dB steps until no response is observed, indicating the hearing threshold Continue lowering the tone beyond this point to confirm the minimum level at which the subject detects the sound, ensuring accurate audiometric measurement.
5 dB and begin the next ascent at this level This should be continued until three ascents and three descents have been completed
Shortened versions of the bracketing method can be effective in certain situations This approach involves omitting additional descent of 5 dB after no response is observed, or limiting the procedure to two ascending and two descending steps in sequence These modifications are suitable when the four minimal response levels differ by no more than 5 dB, streamlining the testing process while maintaining accuracy.
Proceed to the next test frequency at an estimated audible level, as indicated by the previous responses, and repeat step 2 Finish all test frequencies on one ear
NOTE For any frequency, the familiarization, or an abbreviated form of it, can be repeated
Repeat the hearing measurement at 1,000 Hz and compare the results with the initial test for the same ear If the difference is within 5 dB, proceed to test the other ear However, if there is a change of 10 dB or more in hearing threshold, retest at additional frequencies in the same sequence until the results agree within 5 dB or less.
Proceed until both ears have been tested
6.2.3.3 Procedures for testing with masking
To prevent the test tone from being heard in the non-test ear, it is essential to apply masking noise This masking noise is delivered through earphones during the procedure, ensuring accurate audiometric testing results by isolating each ear effectively.
While experience significantly influences the selection of procedures and masking noise levels, it is recommended to follow a standardized method to accurately determine the hearing threshold with masking.
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Hearing threshold determination with an automatic recording audiometer
Automatic-recording audiometers often have no masking facilities and this procedure is therefore limited to air-conduction audiometry and to cases where no masking is required
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Pulsed tones are preferred for threshold determination, providing more accurate results, while continuous tones may also be used When both types of tones are employed in testing, pulsed tones should be presented first to ensure optimal assessment accuracy.
The temporal characteristics of the pulsed tone are specified in IEC 60645-1
NOTE 1 Continuous tones are used only for some specialized audiological purposes
NOTE 2 The increment in level can vary between instruments but is typically less than 1 dB The attenuation rate is often 2,5 dB/s (IEC 60645-1:2001, 8.4.2)
Prior to measuring hearing threshold levels, the test subject must undergo familiarization with the test tones and response tasks This process begins by activating the attenuation system at the initial test frequency, ensuring the subject is comfortable and understands the test procedures.
To assess the test subject's understanding, observe their performance during a 20 to 30-second practice session If they demonstrate comprehension, initiate the recording process; if not, repeat the instructions to ensure proper instruction comprehension before proceeding.
After the recorder mechanism has been started, the test shall be continued until both ears have been tested once
6.3.5 Calculation of hearing threshold level
The test results are processed by ignoring the initial reversal after a change in frequency and any reversals linked to minimal trace excursions of 3 dB or less The peaks and valleys of the tracing for each frequency and ear are then averaged separately The mean of these two averages is calculated, and this value is rounded up to the nearest whole number in decibels to determine the hearing threshold level for that specific frequency and ear.
An audiometric recording should be considered of doubtful reliability and should be repeated if either of the following conditions apply:
⎯ the peaks deviate by more than 10 dB from each other and/or the valleys deviate by more than 10 dB from each other;
⎯ less than six reversals remain after a)
NOTE 1 When the trace excursions are regular, results very close to those obtained by the procedure given above can be obtained more simply by “visual averaging”
On average, automatic audiometers tend to record hearing threshold levels that are approximately 3 dB lower than those obtained through manual audiometry This difference, typically around 3 dB, is considered in ISO 8253, which assumes that automatic audiometry measurements are slightly more sensitive, recording thresholds with 5 dB steps Understanding this discrepancy is essential for accurate hearing assessment and consistent audiometric testing standards.
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Computer-controlled threshold determination
The programming and operation of computer-controlled audiometric equipment shall be done in such a way that results are equivalent to those obtained by the methods described in this part of ISO 8253
7 Air conduction hearing threshold level determinations using sweep-frequency audiometry
General
In sweep-frequency audiometry, the frequency range is swept automatically at a given rate of change (normally in the range of 0,5 octave/min to 2,0 octave/min) The normal sweep direction is from low to high, but the reverse direction may also apply
Sweep-frequency audiometers often have no masking facilities and this procedure is therefore limited to air-conduction audiometry and to cases where no masking is required.
Presentation of test tone
The test tone can be delivered either as a pulsed or continuous signal, with pulsed tones preferred for threshold determination When both types are utilized, it is recommended that pulsed tones be presented first to ensure accurate assessment.
Familiarization
To ensure accurate hearing threshold level measurements, subjects should undergo familiarization with test tones and response tasks This involves starting the attenuation system without activating the recording mechanism at the lowest required frequency and observing the subject's performance during a 20 to 30-second practice session to confirm understanding If the subject demonstrates comprehension, the recording mechanism is activated; otherwise, instructions should be repeated to ensure reliable results.
Hearing threshold level measurement
After the recording mechanism has been started, the test shall be continued until both ears have been tested.
Calculation of hearing threshold level at a specified frequency
For a specified frequency, the hearing threshold level is determined by averaging from tracing the three peaks and averaging the three valleys closest to the frequency in question
The mean of these two averages, rounded to the nearest whole number in decibels, is the hearing threshold level for that frequency and ear
The hearing threshold level can be accurately estimated as a semicontinuous function of frequency by calculating a running average of three consecutive pairs of peaks and valleys This approach involves taking the arithmetic mean of six level values, resulting in a precise hearing threshold measurement The threshold is assigned to the frequency corresponding to the geometric mean of the six frequencies where these peaks and valleys are observed, ensuring a smooth and reliable representation of hearing sensitivity across different frequencies.
NOTE 1 If the three peaks or valleys used to obtain an average deviate by more than 10 dB from each other, the threshold determination is less reliable
When trace excursions are consistent, similar results can be achieved by averaging each peak-valley and valley-peak pair or through simple “visual averaging,” ensuring accurate and reliable measurements with minimal complexity.
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8 Bone conduction hearing threshold audiometry
Method of audiometry
Hearing threshold levels for air conduction are influenced by the audiometric test method, but this has not been thoroughly studied for bone conduction audiometry Currently, no quantitative adjustments are recommended for different bone conduction testing techniques, such as manual or automatic recording Therefore, the same standardized procedures should be used for both air and bone conduction audiometry to ensure consistency and accuracy.
For precise monaural hearing threshold level determination, bone conduction audiometry requires masking of the non-test ear at all levels
NOTE Where a precise monaural bone conduction hearing threshold is not required, bone conduction audiometry may be undertaken without masking.
Occlusion
The ear being tested by bone conduction should be unoccluded If the ear is occluded (see Note 1 to 8.3), it shall be noted on the audiogram.
Airborne sound radiation from the bone vibrator
To ensure accurate bone conduction testing, any airborne sound emitted by the bone vibrator during contact with a test subject's head should be kept at a low enough level This minimizes the risk of air conduction artifacts and maintains a clear distinction between the true bone conduction hearing threshold and false air conduction signals Proper control of airborne sound levels is essential for reliable assessment of hearing thresholds in individuals with normal outer and middle ear function.
To effectively eliminate unwanted sound radiation at frequencies above 2,000 Hz, inserting an ear plug into the outer ear canal of the test subject is recommended if the condition is not directly met However, it is important to consider the potential occurrence of the occlusion effect at these higher frequencies, which can influence sound perception and measurement accuracy.
NOTE A detailed test procedure is described in IEC 60645-1.
Vibrotactile sensation
The mastoid location of the bone vibrator typically produces vibrotactile thresholds that correspond to hearing levels of about 40 dB at 250 Hz, 60 dB at 500 Hz, and 70 dB at 1,000 Hz However, significant individual variations can occur, so it is important to ensure that vibrotactile sensations are not mistaken for actual hearing sensations during testing.
NOTE If the audiometer is calibrated for forehead placement of the vibrator, the values quoted above are approximately 10 dB lower.
Procedures for testing with masking in bone conduction audiometry
Although experience, to a large extent, dictates the procedure used and the choice of the masking level, the following procedure is recommended
When positioning the bone vibrator on the subject, ensure the masking earphone is properly placed on the non-test ear Care must be taken to prevent the headbands of the two transducers from interfering with each other during testing Hearing threshold levels should be measured without masking noise, following the simplified procedures outlined in section 6.2.3.2 for accurate and reliable results.
Note that the measurement results may not accurately reflect the true non-masked bone conduction threshold due to potential occlusion effects in the non-test ear, which can influence the accuracy of the assessment.
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To ensure accurate audiometric testing, present the masking noise to the non-test ear at an effective level equal to its air conduction hearing threshold Gradually increase the masking noise until the test tone becomes inaudible or the noise level surpasses the test tone level by 40 dB This process helps prevent cross-hearing and ensures the test results accurately reflect the test ear's hearing thresholds Proper masking is essential for reliable audiometric assessment and diagnosis of hearing impairments.
To determine the hearing threshold, identify the point at which the tone remains audible even when the noise level is 40 dB higher than the test tone If the test tone is masked by background noise, gradually increase its level until it becomes audible again, ensuring accurate assessment of hearing sensitivity.
To determine the correct masking level, gradually increase the noise level by 5 dB and adjust the test tone accordingly until it becomes audible again if it was inaudible Repeat this process, raising the masking noise level by more than 10 dB, until the test tone remains audible without needing further level increases The masking level at which the tone remains audible despite increased noise indicates the appropriate masking level, reflecting the true hearing threshold at the test frequency Record this masking level for accurate hearing assessment.
NOTE 1 This is the plateau-seeking method In some cases where the plateau is short, the above procedure can give false results
Overmasking occurs when masking noise unintentionally masks the test tone in the ear being evaluated, potentially affecting audiometric results To minimize this effect, it is recommended to deliver the masking noise through appropriate insert earphones, which help reduce overmasking and ensure more accurate hearing assessments.
NOTE 3 The masking plateau can have a slope greater than zero due to central masking
NOTE 4 In certain cases, it is appropriate to increase the noise level in steps of 10 dB
General
In screening audiometry, the test tones at the screening level are either audible or inaudible to the subject
The test result shows whether hearing threshold levels are lower (better), or the same, or higher (worse) than the screening level used
Screening audiometry may be combined with hearing threshold measurements at those frequencies where the test subject fails the screening test A procedure in accordance with Clause 6 should then be used
For the preparation and instruction of the test subjects before audiometric testing, see Clause 5.
Procedure for the screening test
Procedures for manual audiometry are specified in 9.2.2 and for computer-controlled audiometry in 9.2.3
The test involves presenting one or more test tones of preset frequencies and levels, and recording the responses of the test subject
Present the test frequencies in increasing sequence from 1 000 Hz, followed by the range below 1 000 Hz in decreasing sequence
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To ensure the test subject understands the instructions, present a 1,000 Hz tone at 40 dB hearing level to the right ear If the subject does not respond, re-instruct and repeat the tone Should there still be no response, gradually increase the volume until the subject provides a response, confirming comprehension.
Adjust the signal level to the specified screening level and present two tones lasting 1 to 2 seconds, separated by an interval of 3 to 5 seconds If the subject perceives both tones, they pass the screening at this frequency If only one tone is heard, a third tone is presented, and if the subject perceives it, they pass; if not, they fail the screening test at 1,000 Hz The procedure is then repeated at other test frequencies and for the left ear to complete the screening.
The programming and operation of computer-controlled audiometric equipment shall be such that the results are consistent with those obtained by the methods specified in 9.2.2
Hearing threshold levels can be depicted either in a table or graphically through an audiogram In audiograms, one octave on the frequency axis corresponds to a 20 dB change on the hearing level axis When presenting hearing thresholds graphically, specific symbols from Table 1 should be used for clarity To illustrate data accurately, continuous straight lines connect adjacent air conduction points, while broken lines are used for bone conduction thresholds.
Table 1 — Symbols for the graphical presentation of hearing threshold levels
Air conduction — Unmasked Example of no-response symbols Air conduction — Unmasked Air conduction — Masked Bone conduction — Unmasked, mastoid Bone conduction — Masked, mastoid
Bone conduction — Masked, forehead Bone conduction — Unmasked, forehead
NOTE Where symbols ({, ¯) are used for masked air conduction as well, the use of masking should be noted in the audiogram
If no response is detected at the audiometer's maximum output level, an arrow should be added to the appropriate symbol, pointing vertically or positioned at the lower outside corner—to the right for left-ear symbols and to the left for right-ear symbols—and drawn downward at a 45° angle This no-response symbol must be placed on the audiogram at the corresponding hearing level that reflects the audiometer's maximum output.
If colour is used, red shall be used for the right-ear symbol and connecting lines, and blue for the left-ear symbol and connecting lines
Results obtained from screening audiometry shall be clearly indicated as such
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Permissible ambient noise for threshold determinations
Ambient sound pressure levels in an audiometric test room must not exceed specified maximum permissible levels (L S,max) in one-third octave bands to prevent masking of test tones, ensuring accurate hearing assessments These limits are established for various conditions, including a lowest hearing threshold of 0 dB, and account for threshold shift uncertainties of +2 dB and +5 dB at the lowest test tone level Measurements are conducted through both air conduction using earphones and bone conduction via a bone vibrator, covering three frequency ranges for air conduction (125 Hz to 8,000 Hz, 250 Hz to 8,000 Hz, and 500 Hz to 8,000 Hz) and two ranges for bone conduction (125 Hz to 8,000 Hz and 250 Hz to 8,000 Hz) These guidelines ensure controlled ambient noise levels to maintain the accuracy and reliability of audiometric testing.
Table 2 provides the L S,max values for air conduction audiometry using typical supra-aural earphones, with the average sound attenuation detailed in Table 3 based on experimental data for two commercial earphone models When using different earphone types, any variation in sound attenuation should be added to the L S,max values specified in Table 2 to ensure accurate assessment Additionally, Table 4 outlines the L S,max values for pure-tone bone conduction audiometry, essential for comprehensive hearing evaluations.
When measuring minimum hearing threshold levels beyond 0 dB, alternative values for L S,max should be used These sound pressure levels are determined by adding the specified minimum hearing threshold to the values listed in Table 2 and Table 4, depending on the context.
Ambient noise level measurements should be conducted under conditions representative of those during audiometric testing, including operation of ventilation systems if they are normally active Measurements must be taken at the test subject’s head position within the test room, with the subject absent, ensuring accurate representation of environmental noise The measurement devices used should meet IEC 61672-1 and IEC 61620 Class 1 sound level meter standards, featuring a noise floor at least 6 dB below the sound pressure levels being assessed for precise and reliable results.
Psycho-acoustic check on ambient noise
If sound pressure level measurements are not feasible, a psychoacoustic assessment of ambient noise can be performed by conducting audiometric tests on at least two individuals with stable audiograms and consistently better (lower) hearing thresholds across all frequencies than those used in standard testing The hearing thresholds obtained through this method can serve as an alternative indicator of ambient noise levels, provided they are higher than the predefined thresholds for regular assessment.
A noise reduction of 5 dB or more is required in the room to ensure accurate audiometric testing When bone conduction audiometry is performed, the assessment must be conducted using bone conduction methods The audiometric tests should be scheduled during the standard testing timeframe to ensure consistency and reliability of results.
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Table 2 outlines the maximum permissible ambient sound pressure levels (L S,max) in one-third-octave bands for air conduction audiometry, ensuring accurate hearing threshold measurements down to 0 dB These levels are specifically for use with typical current supra-aural earphones, providing guidelines to maintain optimal testing conditions and prevent ambient noise from affecting audiometric results Proper adherence to these permissible sound pressure levels is essential for reliable hearing assessments and accurate diagnosis of hearing thresholds.
Maximum permissible ambient sound pressure levels a
Mid-frequency of one-third-octave band
125 Hz to 8 000 Hz 250 Hz to 8 000 Hz 500 Hz to 8 000 Hz 31,5 56 66 78
According to ISO standards, the lowest hearing threshold level to be measured is 0 dB, with a maximum uncertainty of +2 dB caused by ambient noise If a higher uncertainty of up to +5 dB is acceptable, the threshold values can be increased by 8 dB to ensure accurate measurements These guidelines are based on ISO 389-4 and ISO 389-7, which specify testing procedures and measurement standards for hearing assessments Proper understanding of ambient noise impact is essential for reliable audiometric evaluations and adherence to international standards.
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Table 3 — Average sound attenuation, in decibels, for different earphones
Typical current supra-aural earphone abc dB
8 000 24 43 44 a The values given are based on measurements using pure tones in a free sound field and using Telephonics TDH39 d with
This article discusses attenuation data for MX 41/AR cushions and Beyer DT48 earphones, emphasizing the importance of measuring attenuation in a diffuse sound field using narrow-band noise for more accurate results Attenuation values in real-world diffuse fields may be slightly lower than standard data, though current data are limited The measurements are based on sources referenced in the document, with data valid for artificial diffuse fields per ISO 4869-1 standards Additionally, the article notes that some products mentioned are commercially available, and their inclusion is for user convenience without implying ISO endorsement References supporting these findings are cited throughout the article.
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Table 4 — Maximum permissible ambient sound pressure levels in one-third-octave bands, L S,max , for bone conduction audiometry for hearing threshold level measurements down to 0 dB a
Maximum permissible ambient sound pressure levels
Mid-frequency of one-third octave band
125 Hz to 8 000 Hz 250 Hz to 8 000 Hz 31,5 55 63
The lowest hearing threshold level to be measured is 0 dB, with an uncertainty of up to +2 dB caused by ambient noise If a larger uncertainty of +5 dB is acceptable, the threshold values may be increased by 8 dB to account for ambient noise interference.
NOTE 2 With most of the current sound level meters, it is difficult to measure sound pressure levels below 5 dB a Sources: ISO 389-4 [1] , ISO 389-7 [2]
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12 Maintenance and calibration of audiometric equipment
General
Proper calibration of audiometers and associated equipment is crucial for ensuring accurate and reliable hearing test results To achieve this, audiometric devices must be calibrated in accordance with ISO 389 standards and must meet the specifications outlined in IEC 60645-1 Regular calibration in line with these international standards guarantees precise measurements and dependable diagnostics in audiology.
To ensure optimal performance, a three-stage check and calibration scheme should be implemented: Stage A involves routine checks and subjective tests, Stage B includes periodic objective evaluations, and Stage C comprises fundamental calibration tests, forming a comprehensive process for maintaining accuracy and reliability.
It is recommended that stages A and B be performed on the equipment in its normal working position.
Intervals between checks
The recommended intervals for various checks serve as a general guide and should be followed unless there is clear evidence that a different schedule would be more appropriate Adhering to these suggested timeframes ensures optimal maintenance and safety, but adjustments can be made based on specific circumstances or findings Regularly reviewing and updating check intervals helps maintain efficiency and compliance with best practices.
For optimal safety and maintenance, it is essential to perform comprehensive Stage A check procedures weekly on all equipment in use Additionally, the daily checks outlined in sections 12.3.2.2 to 12.3.2.6 should be diligently followed each day the equipment is operated Regular inspections ensure equipment reliability and compliance with safety standards.
Periodic objective checks for stage B should ideally be conducted every three months to ensure optimal equipment performance However, depending on equipment type and usage conditions, longer intervals may be acceptable if stage A checks are consistently performed with diligence It is essential that the maximum interval between these checks does not exceed 12 months to maintain safety and reliability.
Routine calibration tests at stage C are unnecessary if regular stage A and B checks are performed Stage C procedures should only be conducted in the event of a serious equipment fault or suspected performance deviation after a long period of use It is advisable to perform a stage C calibration after approximately five years of operation if the equipment hasn't undergone such testing in that time.
Stage A — Routine checking and subjective tests
Routine equipment checks are essential to verify proper functionality, ensuring that calibration remains accurate and that attachments, leads, and accessories are free from defects that could impact test results These simple tests, which do not require measuring instruments, help maintain equipment reliability and accuracy.
The most important elements in stage A are the subjective tests and these tests can only be successfully carried out by an operator with unimpaired and preferably very good hearing
The ambient noise conditions during the tests should not be substantially worse than those encountered when the equipment is in use
12.3.2.1 The following tests and checks should be carried out to fulfil the requirements of stage A
It is recommended that the tests given in 12.3.2.2 to 12.3.2.6 tests be carried out on the equipment on each day of use
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To ensure accurate audiometric testing, the procedures outlined in sections 12.3.2.2 to 12.3.2.10 should be performed with the audiometer in its normal operating condition When using a booth or separate test room, verify that the equipment is checked as installed, possibly requiring an assistant to facilitate the process These checks should include the interconnections between the audiometer and the testing environment, as well as inspecting connecting leads, plug and socket connections at the junction box, and identifying any potential sources of intermittency or incorrect connections to maintain testing reliability and compliance with safety standards.
When performing subjective bone conduction threshold tests with a normally-hearing operator, air-conducted sound emitted from the bone vibrator can be audible at levels that invalidate the results, particularly at frequencies above 2000 Hz To ensure accurate testing, sufficient attenuation of this air-conducted sound can be achieved by wearing disconnected air conduction headphones or earplugs during the test at frequencies of 2000 Hz and higher.
Regularly clean and inspect the audiometer and all associated accessories to ensure optimal performance Examine earphone cushions, plugs, main leads, and accessory leads for signs of wear or damage, and replace any damaged or worn components promptly to maintain safety and accuracy.
Ensure to switch on the equipment and allow the recommended warm-up time; if not specified by the manufacturer, a 5-minute warm-up is sufficient for circuits to stabilize Perform any necessary setting adjustments as outlined by the manufacturer For battery-powered devices, verify the battery status using the designated method Additionally, check that the serial numbers of the earphones and bone vibrators match the instrument serial number when possible to ensure proper identification and functionality.
Ensure the audiometer output is approximately accurate for both air and bone conduction by sweeping through frequencies at a hearing level of 10 dB or 15 dB, listening for “just audible” tones This verification should be performed across all relevant frequencies and using both earphones and the bone vibrator to confirm proper functionality.
Perform a comprehensive high-level check by assessing hearing levels (e.g., 60 dB on air conduction and 40 dB on bone conduction) across all relevant functions and both earphones at all frequencies, ensuring proper operation, absence of distortion, and no extraneous noises Verify that all earphones, including masking transducers and bone vibrators, are free from distortion and intermittency, and inspect plugs and leads for consistent connectivity Additionally, confirm that all switch knobs are securely attached and that lamps and indicator lights function correctly.
12.3.2.6 Check that the signal system of the test subject operates correctly
When conducting audio tests, listen at low levels to detect any noise, hum, or unwanted sounds such as breakthrough signals caused by channel interference Ensure that all attenuators properly reduce signal levels across their entire range and confirm they operate quietly without electrical or mechanical noise when used during tone delivery Additionally, verify that switch keys function silently and that no noise emanates from the instrument that could be audible to the test subject Proper testing and verification of these components are essential for maintaining optimal audio performance and accurate assessments.
12.3.2.8 Check subject communication speech circuits, if appropriate, applying procedures similar to those used for pure-tone function
Regularly check the tension of the headset headband and bone vibrator headband to ensure optimal comfort and performance Verify that swivel joints move freely and return without being excessively slack, preventing potential damage Inspect noise-excluding headsets for signs of wear, strain, or metal fatigue in the headbands and swivel joints to maintain durability and safety.
Ensure the proper functioning of automatic recording audiometers by inspecting the marking pen, mechanical operations, and the functionality of limit switches and frequency switches Verify that no extraneous instrument noise is audible at the test subject's position, ensuring accurate and reliable audiometric testing.
Stage B — Periodic objective checks
Periodic objective checks involve measuring and comparing key performance parameters against established standards, such as test signal frequencies and sound pressure levels generated by earphones in an acoustic coupler or ear simulator, to ensure optimal device performance and compliance.
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The article discusses key factors affecting auditory testing, including the vibratory force levels produced by the bone vibrator on a mechanical coupler, which are essential for accurate calibration It also highlights the importance of controlling masking noise levels to prevent interference during audiometric assessments Additionally, the significance of attenuator steps—particularly below 60 dB—ensures precise stimulus adjustment across a broad range Furthermore, the consideration of harmonic distortion is crucial for maintaining the integrity of auditory signals during testing procedures.
NOTE 1 Complete checks of attenuator range and levels of masking noise are not possible with the equipment recommended below
NOTE 2 With sweep frequency audiometers, standardized calibration data are available only at discrete frequencies specified in ISO 389-1, ISO 389-2, ISO 389-3, ISO 389-5, and ISO 389-8
The following equipment is recommended as a minimum for periodic objective checks:
⎯ class 1 sound level meter, comprising a pressure-calibrated condenser microphone of a type suitable for the ear simulator, designated as being in compliance with IEC 61672-1;
⎯ one-third-octave-band filter set, complying with IEC 61260;
⎯ ear simulators or acoustic couplers, designated as being in compliance with IEC 60318-1 [4] ,
⎯ mechanical coupler, designated as being in compliance with IEC 60318-6 [8] ;
⎯ contact thermometer for checking the operating temperature (23 °C) of the mechanical coupler
If frequencies or test tone levels are out of calibration, they can usually be adjusted; otherwise, the equipment should be sent for basic calibration It is important to record both sets of measurements—before and after calibration—to ensure accurate documentation of the adjustments made.
Recording measurement results from equipment allows for the detection of calibration drift over time Monitoring these trends helps determine appropriate intervals for objective testing, ensuring ongoing accuracy and reliability of the equipment Regular calibration checks are essential to maintain measurement integrity and prevent measurement errors.
It is recommended that a calibration check label be attached to the equipment, giving the date on which the next objective test is due.
Stage C — Basic calibration tests
A basic calibration of audiometric equipment must be conducted by a qualified laboratory to ensure accurate performance The calibration process should verify that the equipment meets the standards specified in IEC 60645-1, ensuring compliance with relevant audiometric requirements Proper calibration guarantees reliable test results and maintains the integrity of audiometric assessments.
When the instrument is returned after basic calibration, it should be checked in accordance with procedures outlined in 12.3 or 12.4 before being put back into service
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The ISO/IEC Guide 98-3 provides a standardized format for expressing measurement uncertainties, emphasizing the importance of establishing a functional relationship between the measurand and input quantities In the context of ISO 8253, the measurand is the frequency-dependent hearing threshold level of a test subject, influenced by various factors Each input quantity is characterized by its estimate, probability distribution, and standard uncertainty, which are compiled into an uncertainty budget This budget enables the calculation of the combined standard uncertainty and the expanded uncertainty of the measurement results, ensuring accurate and reliable audiometric assessments.
Currently, scientifically verified data required to build a comprehensive uncertainty budget for measurements using ISO 8253 procedures are unavailable at the time of publication However, it is possible to identify relevant sources of uncertainty and describe their characteristics based on empirical knowledge The annex illustrates the general approach to calculating uncertainties in accordance with ISO/IEC Guide 98-3, providing an approximate method for determining uncertainties under specific assumptions.
The expression for the determination of the hearing threshold level, L HT , at a certain frequency is given by Equation (A.1):
HT HT eq tr n m te su pr
L′HT is determined through hearing threshold level assessments following specific procedures outlined in ISO 8253 Various input quantities, such as δ eq, δ tr, δ n, δ m, δ te, δ su, and δ pr, are incorporated to account for deviations and uncertainties in the testing process These factors ensure accurate and reliable audiometric measurements by considering equipment performance, environmental conditions, transducer fitting, masking noise, tester qualification, subject cooperation, and challenging measurement situations.
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In sound level analysis, each of the δ input quantities is typically assigned an estimate of 0 dB, indicating no correction is applied to the measured hearing threshold level However, as detailed in section A.3, these input quantities are associated with inherent uncertainties that must be considered Importantly, none of these quantities are significantly correlated with each other, ensuring independent variability in the analysis Incorporating these uncertainties and independence assumptions is essential for accurate and reliable hearing threshold assessments.
Input quantities outlined in sections A.3.2 to A.3.6 are essential for nearly all audiometric applications, ensuring accurate hearing assessments In contrast, the parameters described in sections A.3.7 to A.3.9 should only be considered in exceptional cases, relying on the personal judgment of the tester Proper selection of these input quantities is crucial for reliable audiometric testing and accurate diagnosis.
A.3.2 Determined hearing threshold level, L′ HT
During routine audiometry, a person's hearing threshold at a specific frequency is typically measured once per ear; however, empirical data suggest standard uncertainties for repeated tests under identical conditions For air conduction audiometry, the measurement uncertainty is approximately 2.5 dB at frequencies up to 4 kHz and increases to 4 dB at frequencies above 4 kHz Similarly, for bone conduction audiometry, the expected variability is about 3 dB up to 4 kHz and 5 dB at higher frequencies, reflecting the inherent measurement uncertainties in audiometric testing.
The probability distribution of probable values of L HT ′ can be assumed to be normal; its estimate is designated L′ HT,est (see Table A.1)
When using audiometric equipment that complies with IEC 60645-1 standards for type 1 or type 2 audiometers, the primary source of measurement uncertainty is typically the deviation of output levels from their nominal values IEC 60645-1 specifies maximum deviations of ±3 dB for air conduction at frequencies up to 4 kHz, and ±5 dB at frequencies above 4 kHz For bone conduction measurements, the standard permits deviations of ±4 dB up to 4 kHz and ±5 dB at higher frequencies Ensuring that equipment stays within these tolerances is crucial for accurate audiometric assessments.
When specific performance data for the equipment is unavailable, the output levels are assumed to follow a rectangular probability distribution This assumption leads to standard uncertainties calculated as half the maximum range of possible values divided by the square root of three.
If the step size of the hearing level control is 5 dB, this introduces another non-negligible uncertainty contribution with a rectangular probability distribution and a standard uncertainty of 2,5/√3 dB
The two contributions result in an approximate overall standard uncertainty, e.g for air conduction and frequencies up to 4 kHz of √[(3/√3) 2 + (2,5/√3) 2 ] dB = 2,3 dB
In specialized testing scenarios, factors such as significant fluctuations in a subject's hearing threshold across different frequencies, deviations of test tone frequencies from their nominal values, and harmonic distortion of the test tones can all increase the uncertainty of results Additionally, equipment performance issues may further impact the accuracy of hearing test outcomes, emphasizing the importance of precise calibration and reliable testing equipment.
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A.3.4 Transducers and their fitting, δtr
RETSPL and RETVFL values for various transducers, including supra-aural, circumaural, insert earphones, and bone vibrators, are defined in ISO 389 but are not entirely equivalent Although the precise differences are not fully known, it is reasonable to estimate a standard uncertainty of 1.5 dB up to 4 kHz and 2.5 dB for frequencies above 4 kHz, impacting accurate sound level measurements.
Different types of transducers deliver sound pressure or vibratory forces to the ear or bone, with their effectiveness influenced by individual anatomical and physiological characteristics, placement accuracy, and headband tension deviations For bone vibrators, additional uncertainties may stem from radiated airborne sound and vibrotactile sensations, though precise quantification of these effects remains challenging In the absence of detailed data, a standard uncertainty of 2.5 dB up to 4 kHz and 3 dB above 4 kHz can be assumed to account for these variabilities.
The two effects together result in an approximate standard uncertainty of √(1,5 2 + 2,5 2 ) dB = 2,9 dB at frequencies up to 4 kHz and √(2,5 2 + 3 2 ) dB = 3,9 dB at frequencies above 4 kHz
When ambient noise conditions meet the specified requirements, the standard uncertainty of δ n can be assumed to be 2 dB, modeled with a normal probability distribution, especially for test subjects with hearing thresholds near 0 dB For individuals with hearing thresholds significantly above 0 dB, the impact of ambient noise on measurement uncertainty is typically negligible.
On the other hand, the maximum permissible ambient sound pressure levels might often be exceeded during routine testing, resulting in a considerably larger uncertainty contribution
Measured hearing threshold levels can be influenced by the use of non-optimized masking noise, which may introduce measurement uncertainty While specific figures on this effect vary, a standard uncertainty of approximately 2 dB is generally considered acceptable when masking noise is applied This uncertainty follows a normal probability distribution, highlighting the importance of proper masking techniques to ensure accurate audiometric results.
A.3.7 Experience of the tester, δte
In typical test scenarios, a qualified tester with sufficient experience can have their personal judgement uncertainties incorporated into the standard uncertainty for repeated measurements (refer to A.3.2) However, in certain special circumstances, it may be necessary to allocate an additional uncertainty to δ te to ensure accurate assessment.
A.3.8 Responses of the test subject, δsu