## Introduction

MEM

_{AC}=AC_{TE}+ABG_{NTE}-IA,XUM

_{AC}=BC_{TE}+IA,MEM

_{BC}=BC_{TE}+ABG_{NTE},XUM

_{BC}=BC_{TE}+IA,

_{AC & BC}are the minimum effective masking levels for AC and BC, respectively, XUM

_{AC & BC}are the maximum usable masking levels for AC and BC, respectively, AC

_{TE}is the actual AC threshold of TE, ABG

_{NTE}is the actual air-bone gap (ABG) in the NTE, BC

_{TE}is the actual BC threshold in the TE, and IA is the actual interaural attenuation for AC [6]. By using these formulas, everyone can calculate the overall plateau width for the actual AC and BC thresholds of TE, ABG

_{NTE}and actual value of IA at each test frequency.

_{2}and N

_{1}) are equal to tones difference between them (T

_{2}and T

_{1}). As a result, the plateau width (PW) can be calculated between these two points according to the following equation:

_{1}and N

_{2}can be located in the plateau section of the masking diagram, the width of the plateau segment between these two points is less than the overall plateau width, and as a result the minimum plateau width is replaced by plateau width:

_{2}and N

_{1}are the noise levels (in dB HL) at point 2 and 1, respectively, on the X-axis of the diagram, and T

_{2}and T

_{1}are the pure tone levels (in dB HL) at point 2 and 1, respectively, on the Y-axis of the diagram.

_{AC}is the plateau width for the AC testing, IA is the actual interaural attenuation, and ABG

_{TE}and ABG

_{NTE}are the actual air-bone gap for TE and NTE, respectively.

_{NTE}is zero, and initial bilateral CHL that the plateau width is narrower.

## Subjects and Methods

#### Participants

_{AC}=2IA-ABG

_{TE}-ABG

_{NTE}), we conducted this research on persons with CHL (unilateral and bilateral). Two groups of adult subjects with unilateral and bilateral CHL, 20-45 years old, participated in this study: 29 subjects with unilateral CHL (12 men and 17 women) with a mean age and standard deviation of 34.14±5.42 years, and 30 subjects with bilateral CHL (23 men and 7 women) with a mean age and standard deviation of 38.13±5.03 years. These people were selected after performing otoscopy, immitance audiometry, and AC and BC pure tone audiometry. The experiment was approved by the Research Ethics Committee of Iran University of Medical Sciences (IR.IUMS.REC 1395. 9211303207). Informed consent letter were obtained for all participants.

_{TE}-BC

_{NTE}≥ 40 dB at most frequencies were enrolled. In these groups, BC thresholds were normal in both ears before masking.

#### Equipment

#### Experimental procedures

_{2}was considered equal to 100 dB HL.

#### Formula method:

_{NTE}+15 dB (the IML) in the NTE to determine N

_{1}and T

_{1}(The IML of this survey was selected according to Gelfand [9]).

_{NTE}-10 dB (or 100 dB HL), TE thresholds were obtained to get N

_{2}and T

_{2}.

_{2}-N

_{1})-(T

_{2}-T

_{1}), individually at 500 Hz to 4,000 Hz.

#### Plateau search method:

_{NTE}+15 dB (IML) in the NTE to determine the start point in the plateau curve.

_{NTE}-10 dB (or 100 dB HL). To determine the end of plateau part, this process was continued until at least with two steps of noise increment at NTE and the TE threshold also increased by two steps.

#### Statistical analysis

*p*<0.05 was considered to be a significant difference in this survey. With respect to the normal distribution of mPW data by Kolmogorov-Smirnov test, an independent sample t-test was performed to compare the mPWs in the two methods.

## Results

## Discussion

_{NTE}affects the width of the plateau: the wider ABG, the narrower the plateau [1]. Since clinical masking is started from IML, knowing the size of the plateau width after it is important. However, it is not necessary to determine the total amount of plateau width in clinical audiometry [6,10]. Plateau width is important for at least two reasons: confidence in the masked threshold in the wide plateau and inability to determine the masked threshold in the very narrow plateau [9].

_{AC}=2IA-ABG

_{TE}-ABG

_{NTE}) that was extracted from Liden, et al.’s equation for the minimum effective and maximum usable masking [6,9]. ABG

_{NTE}in the unilateral CHL group is ideally equal to zero but in the bilateral CHL group is greater than zero, and therefore, based on this equation, in the bilateral CHL group in addition to ABG

_{TE}, ABG

_{NTE}is also reduced from 2IA and therefore PW

_{AC}is smaller than the unilateral CHL group. In people with bilateral CHL, the mPW difference between the formula and plateau search method was only ±5 dB in more than 93% of cases.

_{NTE}is large, so it is likely the “Masking Dilemma” described by Naunton [12] had occurred. In fact, during IML delivery, if the previous AC or BC thresholds are repeated, the same thresholds are true and with an increasing noise level, overmasking will happen.

_{TE}+IA-IML). Of course, the mPW can be in any part of plateau of the masking diagram. According to Turner’s definition of AMR, for CHL, mPW and AMR are the same. In the other words, at least for CHL, AMR is equal to mPW: AMR=mPW.

_{2}is equal to or greater than the maximum usable masking. Therefore, the mPW is equivalent to the width from the maximum usable masking to IML, which is the same as AMR. In fact, based on the results of this study and the mPW formula, the IA value can be estimated by ±5 dB error in 90% of cases.

_{NTE}or IA is needed (after initial determination of binaural AC thresholds at the beginning of audiometry, the noise values can be selected for any value in the NTE). Therefore, it is easier to perform and takes less time in clinic. As already mentioned, the mPW formula has been derived from the masking diagram, and therefore it is possible to use the formula to describe the different sections of the masking diagram (for example, if the mPW value is equal to the noise changes (N

_{2}-N

_{1}), both points are at the plateau and each of the tones represents the threshold). Based on the results of this research, in unilateral hearing loss if masking for AC is needed, after determining the T

_{1}following IML in the NTE, the N

_{2}value can be taken from the difference between maximum output of the audiometer and T

_{1}in the TE. If there is not a plateau width required for masking (15-20 dB), this means there is a profound hearing loss in the TE.

_{2}is not in the overmasking part of the masking diagram, the T

_{2}value and thus the T

_{2}-T

_{1}values will be increased due to central masking and accordingly the mPW will be smaller. Therefore, the resulting mPW can be more reliable, especially in situations in which the plateau width decreases.