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 Table of Contents  
Year : 2022  |  Volume : 28  |  Issue : 2  |  Page : 153-155

The importance of considering the minimum interaural attenuation value in the masking procedure of pure-tone audiometry

Department of Audiology, School of Health Sciences, Universiti Sains Malaysia, Kelantan, Malaysia

Date of Submission16-Jan-2021
Date of Decision28-Apr-2021
Date of Acceptance04-May-2021
Date of Web Publication21-Sep-2022

Correspondence Address:
Dr. Mohd Normani Zakaria
Department of Audiology and Speech Pathology, School of Health Sciences, Universiti Sains Malaysia, 16150 Kubang Kerian, Kelantan
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/indianjotol.indianjotol_12_21

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In pure-tone audiometry (PTA) testing, air conduction (AC) and bone conduction (BC) thresholds are documented accordingly to obtain complete information on the hearing status. Nevertheless, due to the cross-hearing phenomenon, a masking procedure is typically implemented in PTA as an effort to obtain the exact (masked) AC and BC thresholds. As indicated by the respective masking rules, the decision to carry out the masking procedure is based on the “suggested” minimum interaural attenuation (IAA), which is 40 dB (for headphones), regardless of frequency. In this case report, the importance of considering the minimum IAA value based on research findings is highlighted. In particular, to produce more reliable PTA results, the masking procedure can be conducted by considering a lower IAA value (<40 dB).

Keywords: Air conduction, bone conduction, interaural attenuation, masking, pure tone audiometry

How to cite this article:
Zakaria MN. The importance of considering the minimum interaural attenuation value in the masking procedure of pure-tone audiometry. Indian J Otol 2022;28:153-5

How to cite this URL:
Zakaria MN. The importance of considering the minimum interaural attenuation value in the masking procedure of pure-tone audiometry. Indian J Otol [serial online] 2022 [cited 2022 Sep 25];28:153-5. Available from: https://www.indianjotol.org/text.asp?2022/28/2/153/356444

  Introduction Top

Obtaining an accurate hearing diagnosis is crucial in clinical settings.[1] To achieve this, pure tone audiometry (PTA) in conjunction with other audiological tests are conducted accordingly. In PTA, hearing test results are displayed on an audiogram that consists of air conduction (AC) and bone conduction (BC) thresholds.[2] The AC thresholds are determined using headphones or insert earphones, while a bone vibrator (that is typically placed on the mastoid area) is used to determine the BC thresholds. The severity of hearing loss and type of hearing loss is represented by AC and BC thresholds, respectively.[1],[2],[3] Of note, the AC and BC thresholds are calibrated in such a way that it is atypical for the BC thresholds to be worse than the AC thresholds.[3]

It is worth noting that the transducers such as headphones and insert earphones tend to vibrate, particularly at higher intensity levels. Consequently, a tone presented to one ear can travel to the other ear through the skull vibration (i.e., BC pathway).[3] In this regard, cross-hearing is said to occur and herein, the AC threshold of the test ear is not considered valid as there is the contribution from the nontest ear. To obtain the exact AC threshold, a masking noise is typically presented to the nontest ear (to prevent it from responding to the tone presented to the test ear). Similarly, the masking procedure is also required in BC testing due to the cross-hearing phenomenon.[3]

When the tone is transmitted to the nontest ear across the skull, its energy would be reduced.[3] This phenomenon, known as interaural attenuation (IAA), varies from person to person and is also influenced by tonal frequency and type of transducer.[4],[5],[6],[7],[8] In this regard, the tone would only be heard by the nontest ear if it is louder than the BC threshold (of the nontest ear) and after considering the amount of IAA.[3] For headphones, the IAA values were reported to range from 35 to 85 dB.[4],[5],[6] Other studies found the minimum IAA value to be higher (i.e., 45 dB).[7],[8] In clinical practice, the minimum IAA value of 40 dB has been considered as the basis for the need of masking.[3],[9]

To decide on the necessity of performing the masking procedure, masking rules have been established for different types of transducers.[3],[9] In particular, in the AC testing (with headphones), masking is indicated if the AC thresholds between ears differ by 40 dB or more at a similar frequency.[3] Similarly, masking is also required when the difference between the AC threshold of one ear and the BC threshold of the other ear is at least 40 dB. In the BC testing, masking needs to be conducted when the difference between AC and BC thresholds of the same ear is 15 dB or more.[3],[9]

When the masking rules are not fulfilled, there is no need to conduct masking. However, if a particular patient has a lower IAA value (<40 dB), the cross-hearing phenomenon would still occur (in which masking is needed to obtain the exact hearing threshold). In this case report, the importance of considering the minimum IAA value based on the research findings (<40 dB) is highlighted, particularly when the PTA results are not sensible. This information may guide clinicians to properly decide on the need of masking to have more reliable PTA results.

  Case Report Top

A 54-year-old man came to Audiology Clinic, University Hospital for hearing assessments. He complained of hearing difficulties in the left ear starting around 5 years ago. He felt that his hearing was getting worse gradually over time. The hearing difficulties were more prominent when listening to conversations in the presence of background noise. He also experienced continuous high-pitched tinnitus perceived only in the left ear. Starting around 5 years ago, the tinnitus was generally soft (and did not disturb his daily routines) but could become louder if he was paying attention to it. There is a history of noise exposure as he was involved in shooting activities (for >20 years, about 3–4 times per year without using hearing protection devices). No otalgia, otorrhea, aural fullness, or vertigo was reported. His general health was good. No other significant findings were reported in history.

As revealed by an otoscopic examination, the tympanic membrane looked intact with clear ear canal for both ears. The middle ear function was tested using a 226-Hz tympanometer (AT235H, Interacoustics, Denmark). A two-channel audiometer (GSI 61, Grason-Stadler Inc., USA) with the respective transducers (TDH-39 headphones and B-71 bone vibrator) were used in the PTA testing. [Figure 1] shows the results of PTA and tympanometry for this patient. As indicated, a type-A tympanogram was found for each ear suggestive of the normal middle ear function. For the right ear, PTA revealed a mild sensorineural hearing loss at high frequencies (hearing was within the normal limit at low and mid frequencies). On the other hand, a mild to profound sensorineural hearing loss (starting from 1000 Hz) was noted for the left ear. Of note, a notch at 6000 Hz was observed in both ears, consistent with the history of noise exposure. The response reliability of PTA testing was good. All clinical procedures were conducted in a dedicated soundproof room with calibrated instruments.
Figure 1: Pure-tone audiometry and tympanometry results of a patient

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In clinical practice, unmasked AC thresholds are obtained first in the PTA testing. If the masking rule is fulfilled, the AC masking is conducted to obtain masked AC thresholds. Then, unmasked BC thresholds are determined from either ear. Subsequently, the BC masking is carried out at specific frequencies (as indicated by the rule of masking). As shown in [Figure 1], based on the AC masking rules, masking was required at 3000 Hz, 4000 Hz, 6000 Hz, and 8000 Hz frequencies for the left ear. That is, it is imperative to find the exact AC thresholds at these frequencies. At 2000 Hz, the difference between left and right AC thresholds was only 30 dB. Furthermore, at this frequency, a difference of 35 dB was found when comparing left unmasked AC and right unmasked BC thresholds (i.e., masking was not required based on the two rules of AC masking).

Meanwhile, the BC masking was required at frequencies of 2000 Hz and 4000 Hz. Surprisingly, after the completion of masking, the left masked BC threshold was in fact much worse (65 dB HL) than the left unmasked AC threshold (50 dB HL) at 2000 Hz [Figure 1]. This was unexpected and further consideration was needed to address this issue. To ascertain the reliability of this BC threshold, the masking procedure was repeated, and a similar result was obtained (i.e., a clear “plateau” was achieved at 65 dB HL). It was then decided to proceed with the AC masking at this frequency (even though the rules of masking were not fulfilled). As shown, after the completion of AC masking, the AC threshold was indeed shifted (from 50 dB HL to 65 dB HL) and a more sensible result was obtained (i.e., similar AC and BC thresholds at 2000 Hz) for the left ear.

  Discussion Top

Masking is an important procedure in PTA testing. Due to the cross-hearing phenomenon, there is a clear need to present a masking noise to the nontest ear (while presenting a tone to the test ear) as an effort to determine the exact hearing threshold.[2] The needs of masking are based on the minimum IAA values.[3],[9] For the AC testing with headphones, 40 dB is the suggested minimum limit to indicate the need of masking, even though IAA has a bigger range with some differences at specific frequencies.[4],[5],[6],[7],[8] In this respect, if the difference between left and right AC thresholds is <40 dB, the cross-hearing phenomenon is considered insignificant (i.e., no contribution from the nontest ear) and masking is not required.

As illustrated in [Figure 1], it was unexpected to see the left masked BC threshold at 2000 Hz (i.e., 65 dB HL) to be worse than the left unmasked AC threshold (i.e., 50 dB HL). This pattern was not seen at other frequencies. Even though the BC thresholds can be worse than the AC thresholds in certain situations,[3] the 15 dB difference was considered significant and “unacceptable.” When the AC masking was performed at this frequency (even though the rules of masking were not fulfilled), the AC threshold did change and was shifted to 65 dB HL. This left masked AC threshold “fits” well with the left masked BC threshold [Figure 1]. In view of this, it seems that the cross-hearing may have occurred, and the nontest ear may have responded during the unmasked AC testing. This also indicates that the patient may have a lower amount of IAA (<40 dB) when tested with headphones. In line with this, as reported by Bunch,[4] the IAA value could be as small as 35 dB at 2000 Hz for headphones. Nevertheless, it is worth stating that the exact IAA value for this patient could not be determined since the IAA data are typically measured from those with unilateral severe or profound hearing loss.[4],[5],[6]

In addition, it was also of interest to know whether the 15 dB shift for the left AC threshold at 2000 Hz (after the completion of masking) was due to central masking. Central masking is defined as a small threshold shift in the test ear when the masking level is insufficient to produce overmasking.[3] As reported elsewhere, the central masking effect ranged from 5 to 15 dB.[10] Nevertheless, during the testing, no evidence on the occurrence of central masking was found at this frequency. Furthermore, even though the central masking could be large as 15 dB, a smaller value (5 dB) is more common in clinical practice.[3],[10]

In terms of case management, the patient reported that he was not yet keen to have a hearing aid. He was then referred to a specialized tinnitus clinic for further assessments and/or treatments. His progress will be monitored from time to time.

  Conclusions Top

This case report highlights the importance of considering an alternative IAA value when performing the masking procedure in the PTA testing. In addition to the established masking rules, masking should also be carried out by considering an IAA value that is lower than the suggested minimum value and based on the research findings (<40 dB), particularly when the PTA results are not sensible. This is because the IAA values would vary across individuals. The information gathered from this case report can be beneficial to guide clinicians to appropriately decide on the need of masking, so that more reliable PTA results can be obtained.

Declaration of patient consent

The author certifies that he has obtained the patient consent form. In the form, the patient has given his consent for his image and other clinical information to be reported in the journal. The patient understands that his name and initial will not be published, and due efforts will be made to conceal his identity, but anonymity cannot be guaranteed.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Wan Mohamad WN, Romli M, Awang MA, Abdullah R, Lih AC, Zakaria MN. The presence of unusual bone conduction thresholds in pure tone audiometry. Indian J Otol 2020;26:54-7.  Back to cited text no. 1
Romli M, Wan Mohamad WN, Awang MA, Aw CL, Zakaria MN. The clinical value of bilateral bone conduction testing in hearing diagnosis. Indian J Otol 2020;26:182-5.  Back to cited text no. 2
  [Full text]  
Katz J, Marshall C, English K, Hood L, Tillery KL, editors. Handbook of Clinical Audiology. Philadelphia: Lippincott Williams & Wilkins; 2015.  Back to cited text no. 3
Bunch CC. Auditory tests in cases of complete unilateral deafness. Ann Otol Rhinol Laryngol 1931;40:748-62.  Back to cited text no. 4
Snyder JM. Interaural attenuation characteristics in audiometry. Laryngoscope 1973;83:1847-55.  Back to cited text no. 5
Park YJ, Park KH, Jun GE, Oh SK, Lee CK. Evaluation of the interaural attenuation for pure tone audiometry in Korean. Korean J Audiol 2011;15:32-6.  Back to cited text no. 6
Liden G, Nilsson G, Anderson H. Narrow-band masking with white noise. Acta Otolaryngol 1959;50:116-24.  Back to cited text no. 7
Chaiklin JB. Interaural attenuation and cross-hearing in air-conduction audiometry. J Aud Res 1967;7:413-24.  Back to cited text no. 8
British Society of Audiology. Recommended Procedure: Pure-Tone Air-Conduction and Bone Conduction Threshold Audiometry with and without Masking; 2017. Available from: http://www.thebsa.org.uk/wp-content/uploads/2017/02/Recommended-Procedure-Pure-Tone-Audiometry-Jan-2017-V2-1.pdf. [Last accessed on 2020 Jan 15].  Back to cited text no. 9
Snyder JM. Central masking in normal listeners. Acta Otolaryngol 1973;75:419-24.  Back to cited text no. 10


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